Author: Pam Knox

This month has been an exciting one for climatologists around the United States with the November 14 release of the Fifth National Climate Assessment (NCA5), a massive project that is undertaken every four years to capture our current understanding of climate change based on recent research. I was a chapter author for the Southeast and spent the last two years working with over 700 authors around the United States to gather and document how the climate is changing and how it is affecting all of us. This week I will explain how NCA5 was put together, what it says about climate, and what gardeners can do to help reduce the future impacts of global warming and other climate changes. But this month was also exciting because USDA just released an updated Plant Hardiness Zone map, just a few weeks after my post in October about how the 2012 map was outdated. I guess they were listening (just kidding!). I will discuss that briefly at the end of this post, too.

What is the National Climate Assessment?

The National Climate Assessment (NCA) is a report mandated by Congress to compile the latest scientific findings on how climate is changing so that we can respond to reduce its future impacts. It is published every four years, and the last one (the 4^th NCA) was released on the day after Thanksgiving in 2017. While the underlying message has not changed, each assessment focuses on the newest scientific research that has been published since the last assessment was done. The document is divided into chapters so that the authors of each chapter could concentrate on that topic.

NCA5 starts with a review of the general scientific principles of how the climate is changing. That is followed by seventeen chapters focused on national topics such as agriculture, water, energy, and transportation as well as specific groups that are being especially affected by climate change such as indigenous peoples. Following the national topics, chapters address changes that are happening in ten different regions of the country . These address how we need to reduce future greenhouse gas emissions that are driving the warming of the earth as well as how we can adapt to the changes that are already happening now and may get worse in the future.

How was NCA5 produced?

There is a long process involved in producing a national climate assessment. Teams of scientists from an array of disciplines were chosen as authors for each chapter to write the initial text of the document. To keep the authors on task and within tight word limits, there were lead chapter authors and technical advisors who moderated group meetings where the key messages for our chapter were identified. Initial figures to include in each chapter were drafted by a graphics team or requested from scientific journals. After the first draft was complete it was first reviewed by federal agencies to make sure that their concerns were addressed and then by the public, who provided many additional comments. All of these comments were provided to the chapter authors so they could refine their text and figures for the next draft. In all, the document when through six different reviews and all comments were addressed.

Where can I read NCA5 and learn more about what it says?

The NCA contains a vast amount of information in its 32 chapters, five appendices, and special topics, so it is hard to summarize. I encourage you to explore the document online to see what it says about your region and special topics of interest like agriculture, land, and ecosystems. A good starting place is the introductory website https://www.globalchange.gov/our-work/fifth-national-climate-assessment, which explains how the report was written and provides links to read the report, attend a webinar on an individual chapter, and see where the figures came from. I also encourage you to explore the excellent interactive atlas developed in conjunction with the report. Many other resources such as podcasts are available, too.

What are some steps that gardeners can take to respond to climate change?

There are two approaches that gardeners (and all of us) need to take to respond to the challenges of a changing climate. We are already dealing with the consequences of trends towards warmer temperatures and more extreme swings in the water cycle such as increases in floods and droughts. Gardeners are adapting to these changes in climate by planting different plants that are better suited to the warmer climate and changing how they manage their gardens using rain gardens, drip irrigation, and other techniques. Adaptation is a key approach that gardeners will continue to need to follow as the climate continues to get warmer and more variable.

In addition all of us, including gardeners, have a responsibility to cut the emissions of additional fossil fuels which are driving most of the warming (mitigation is reducing the inputs to prevent future harm). This will reduce the impacts that our world will have to navigate in the future. Even a small decrease in the emission of greenhouse gases now can prevent the worst outcomes. A prime target for gardeners is the elimination of gasoline-powered equipment like blowers, mowers, and trimmers. These small tools have highly inefficient engines that emit a lot of greenhouse gases as well as air pollutants (and a lot of noise, too). Switching to electric tools and vehicles, composting, adding solar power to houses and businesses, and conserving energy and water (which often uses energy to purify it) through carefully chosen plantings as well as through other methods can also help reduce future warming.

What about the new USDA Plant Hardiness Zone map?

I was surprised last week that the USDA had just produced an update to the 2012 map that I discussed last month. In that post, I noted that the 2012 map was already outdated due to the increasing temperatures we have seen in the 21^st century. The new 2023 map uses data from 1991-2020, the current 30-year normal period, to identify the current plant hardiness zones for the United States. You can see the new map and zoom to your city at https://planthardiness.ars.usda.gov/. By comparing it to the 2012 map, you will see that more than half the country has increased by half a zone, which correlates to about a 5-degree F increase in the average lowest minimum temperature a location experiences each year. I asked USDA for a map that showed the changes of zone and was provided one by Chris Daly of the PRISM group that put together the 2023 map (below). Areas in tan experienced a half-zone change since the 2012 map. (There are a few areas in the Mountain West where the zones got colder, as shown in green, but these are mostly linked to new datasets that were available for the analysis rather than any changes to the local climate there.)

Science has made it clear that the earth’s climate is changing and that most of the warming we are experiencing is due to burning of fossil fuels. We must learn to adapt to these changes and make sure that all groups can be protected from the worst impacts of the more extreme weather we are likely to experience. But we can also make changes now to reduce those future impacts, and I know gardeners will be part of that solution.

UPDATE: As of 11/15/2023, the USDA has published an updated Plant Hardiness Zone map that covers the 1991-2020 period, which includes a lot of the warmest years on record for the US. This map shows more detail than the old map and generally increases the zones in most areas by maybe a half-category. It also now includes Canada and Mexico. You can see it and read about it at USDA Plant Hardiness Zone Map | USDA Plant Hardiness Zone Map.

One of the first questions a gardener should ask when they are considering adding new plants to their garden is whether the plants can survive and thrive in the weather and climate conditions in their yard. One of the most useful tools for this is the USDA Plant Hardiness Zone designation. It provides a quick snapshot of the coldest weather the location is likely to experience, a key factor for how well the plants will survive in that area.

What are plant hardiness zones?

Plant hardiness zones are based on the average annual minimum winter temperature at a location. For simplicity the zones are based on 10-degree Fahrenheit ranges. Each zone is further subdivided into “a” and “b” categories for the colder and warmer halves of the range. You can see the temperature ranges listed on the USDA Plant Hardiness Zone Map website, which also includes a link to an interactive map that will help you determine what zone your location is in. My home in Athens GA is listed as being in zone 8a, which has an average annual minimum temperature range of 10-15 degrees F. Linda provided good descriptions of how to use the zones in this blog in 2019 in A Gardener’s Primer to Cold Hardiness, Part 1 and Part 2.  

How accurate is the USDA Plant Hardiness Zone map?

The latest official version of the map was published in 2012 and showed that most areas had experienced a half-zone change to a warmer zone from the previous map because of rising temperatures. There has been no new map since that time but as temperatures have continued to rise it seems pretty clear to me that the current map is outdated. And in fact, even back in 2012 shortly after it was published, Bert Clegg posted an article in this blog showing that the 2012 map was likely already outdated when it was published because it was based on a 30-year average in an era when temperatures are rising and minimum temperatures are rising much faster than maximum temperatures due to increases in humidity and urbanization.

We need to be a little bit careful with this comparison because the average minimum temperature is not the same thing as the average annual minimum temperature. The average minimum temperature is the average of all the daily minimums in a specified time period, while the average annual minimum temperature is the average of the single lowest daily temperature that occurred each year. You can have a fairly warm winter which still experiences an extreme cold outbreak that has a very low minimum temperature on one or two days. In fact, December 2022 had exactly that situation with the fiercely cold outbreak that occurred right around Christmas across a lot of the eastern United States. The extremely cold air was barely seen in the winter average temperature at all since February 2023 was extremely warm for most of the month and washed out the impact of the extreme cold since it occurred over just a few days in the average. But it certainly caused a lot of damage to plants that were exposed to the frigid air on those few icy days! If the 2012 map was outdated when it was published, it is surely more out of date now after an additional decade with some of the warmest years on record.

How will the plant hardiness zones change in the future?

As global warming continues, the average annual minimum winter temperature is expected to continue to rise. This will result in a northward movement of plant hardiness zones over time. For example, areas that are currently in Zone 6 may become Zone 7 or 8. The rate of change will depend on how fast the earth warms and that depends on how much and how quickly humans respond to minimize greenhouse warming. It would not surprise me if our hardiness zones in most parts of the United States now are at least a half-zone warmer than what is shown on the 2012 map and it could be even greater in some locations. Not all areas of the country (and the world, for that matter) are warming at the same rate, and areas closer to the poles tend to be warming more quickly because of the loss of snow and ice in winter, especially in the Northern Hemisphere.

How will the shift in plant hardiness zones affect gardens?

This shift will have a significant impact on gardening and agriculture. Plants that are not adapted to warmer temperatures may struggle to survive. For example, some fruit trees that are currently grown in Zone 6 may not be able to produce in Zone 7 because they require a certain amount of cold weather to set a good flush of blossoms that form the fruit. Warmer winter temperatures will increase the chance of insect pests and diseases surviving over the colder months, leading to more problems in the next growing season. The last spring frost is likely to come earlier and the first fall frost later in the year. This might make some gardeners happy, since they can get out and start planting earlier, but has implications for pollination since the pollinators may not be able to adapt to the changes in the timing of flowering. That would result in less fertile crops and potentially lower yields of vegetables and other crops.

Gardeners and growers will need to adapt to the changing climate by selecting plants that are suited to warmer temperatures. You may already be doing this by choosing varieties and species for your gardens that are listed as being suitable for a warmer Plant Hardiness Zone than the 2012 map suggests. Gardeners may also need to change their planting practices, such as planting earlier in the spring or providing more shade for plants. In addition, changes in precipitation (which are not included in the Plant Hardiness Zones)  also affect what kind of plants you need to put in your garden since drought is likely to increase in warmer conditions at the same time that individual storm events may drop more rain than in previous years.

Of course, this does not negate the effects of local climate variation across your plot of land. Variations in shade, soil, and drainage will continue to affect variations in the microclimate across your garden, as I discussed in my first blog post, The Weather Where You Are. However, the local variations will occur on top of the changes to the overall plant hardiness in your region and global temperature increases are likely to cause much bigger changes to your local climate in the long term.

National Arboretum in October, DC Gardens, Commons Wikimedia.

Earlier this spring, I posted an article about seasonal climate forecasting and noted that we expected to see the development of an El Niño after three years of La Niña conditions ended in March 2023. And sure enough, an El Niño was declared in August 2023 and has been strengthening ever since. It has a 71% chance of becoming a strong event by December or January before starting to weaken, as they usually do in spring or early summer. In today’s post, I will remind you all what El Niño is and how it is expected to affect our climate this (Northern Hemisphere) winter and spring. That will affect how our gardens survive the colder conditions and prepare for next year’s growing season.

Refresher: What is El Niño?

If you are a new reader of this blog, you may be wondering what El Niño is. El Niño and its companion, La Niña, are two opposite phases of an oscillation in atmospheric and oceanic weather patterns linked to the water temperature in the Eastern Pacific Ocean (EPO). When the ocean there is warmer than usual as it is now, rising air over the warm water creates thunderstorms which can affect the movement of global air currents that bring stormy weather to parts of the earth while leaving other areas high and dry. When the ocean there is cooler than normal in the La Niña phase of the pattern those currents shift, changing the expected weather pattern to something quite different resulting in a different pattern of temperature and precipitation than in El Niño . The maps below show how the climate changes globally in an El Niño in the December-February and June-August periods, corresponding to Northern and Southern Hemisphere winters, respectively. The El Niño pattern is linked to a variety of unusual weather phenomena around the globe. The variations in temperature and rainfall are what affect how our gardens grow or rest in preparation for the next growing season.

What is the current state of El Niño and how is it changing?

Right now, ocean temperatures in the EPO are from 1 to 3 degrees C ( 2-5 degrees F) warmer than normal all the way from the west coast of South America all the way west to the International Date Line. This is a large area of very warm conditions that are being heated even more by the trend towards warmer temperatures due to increases in greenhouse gases in the atmosphere. The heated water provides a lot of water vapor to the atmosphere that helps fuel thunderstorms and tropical systems. Normally in an El Niño year the number of tropical cyclones in the Eastern Pacific is larger than the number in the Atlantic because of that pool of warm water. This year the Atlantic has near record-setting sea surface temperatures which are helping to produce one named tropical storm after another (today we are have “Philippe” and “Rina” active in the Atlantic but only through the name “Kenneth” in the Eastern Pacific). Global warming has affected our climate patterns to such an extent that what used to be established El Niño and La Niña patterns are less likely than in previous decades, although there have always been variations from one event to the next.

What will happen from N. H. winter through spring?

According to the predictions of how the current El Niño will evolve, we can expect the current pool of warm water and the associated global weather patterns to last through at least the April-June period. Since El Niño seldom lasts for longer than a year we are likely to go back into neutral conditions after that and neither El Niño nor La Niña will dominate. This means that over the next few months we can expect the southern part of the United States to be cooler and wetter than normal since in El Niño years the jet stream is positioned over that part of North America. As storms are pushed through the region rainy and cloudy conditions keep daytime temperatures cool as precipitation in the form of rain or sometimes snow or ice falls. In northern parts of the United States extending north into Canada, warm and dry conditions are likely to lead to a lack of snow cover and shorter ice coverage on what are usually frozen lakes. Warmer than normal conditions are also likely to occur in most of Southeast Asia stretching from India to Japan. Drier than normal conditions are likely in the Western Pacific Ocean and in southern Africa as well as South America, leading to the possibility of droughts in those areas.

What does this all mean for our gardens the next few months?

In the parts of the world that are under the jet stream, cooler and wetter than normal conditions should lead to high levels of humidity and increases in soil moisture over the winter since evaporation will be low in the cold winter months. That means gardens in those areas should be fairly wet going into spring. That means a spring or summer drought there will be less likely than after a La Niña winter, but it could be muddy working in your garden areas next planting season. Since El Niño is already strong and getting stronger this wet winter pattern may start early this year so don’t dawdle in preparing your fall garden for winter since it might be hard to work in those wet conditions. Soil temperatures may stay cool later in the spring, delaying planting of seeds and vegetables or flowers that require warm ground to germinate and grow.

If you are in northern parts of the United States and up into Canada, you can expect warmer and drier conditions than usual. That could mean a lack of snow cover and loss of some plants that need insulation provided by the snow to survive the winter. Even though temperatures will be overall warmer than in non-El Niño years, there are still going to be cold outbreaks that can cause damage to plants that are over-wintering. The lack of precipitation could also lead to dry soil conditions in spring that could require increased irrigation or hinder the growth of seeds or new seedlings you might plant. The lack of soil moisture could also contribute to the development of drought later in the growing season.

Of course, even though El Niño and La Niña are the most reliable predictors for climate several months ahead, there are always other factors that can affect climate patterns too. There is no guarantee we will see these exact patterns this winter and there are sure to be some surprises that we don’t expect.

Since my last post, the news has been full of one weather disaster after another. Wildfires in Maui. The remains of Hurricane Hilary moving north into California and other parts of the western USA with moisture even streaming as far east as Wisconsin. Record-breaking heat and humidity across most of the continental USA and severe weather outbreaks in the Midwest and Northeast. This does not even include the typhoons, floods, droughts, and heat waves occurring in other parts of the world at the same time. And today I am watching the development of a tropical system in the northwestern Caribbean that is likely to become Tropical Storm or Hurricane Idalia (pronounced ee-DAL-ya) by early next week, bringing rain and strong winds to parts of Florida and southeastern Georgia.

Each of these weather events can impact our gardens and properties. Not all the impacts are bad since tropical rainfall is an important source of summer moisture in many areas. In the Southeast as much as 40% of our summer precipitation comes from tropical systems and if we don’t get that rain, we can go into a drought quickly in our hot summertime temperatures. The rain from the remains of Hurricane Hilary helped provide some needed rain to help increase reservoir levels in the desert Southwest USA, which desperately needs the water. But if we get too much very strong rain or winds the damage to our homes and yards can be severe. This week I want to discuss some ways you can prepare your gardens and landscaping for the severe weather that will inevitably occur in your area at some point in the future (and that future may be nearer than you think).

Awareness is key to proper preparation

To properly prepare your gardens and homes for severe weather, you need to know what kinds of weather to expect and how it will impact your plants and buildings. The types of weather you are likely to experience will drive how you prepare. If you live in the Pacific Northwest, you are not too likely to experience hurricanes, but you certainly can experience extreme rain storms in winter and wildfires in summer especially if you have a heat wave like you did last year. If you live in the Southeast then hurricanes and tropical storms are much more likely but damage from straight-line winds can be just as important, as I found out in late July when strong thunderstorms knocked down so many trees in my neighborhood that we lost power for 44 hours. A friend of mine lost the roof of her house to two pine trees that toppled over in the strong winds. We can even experience ice and snow storms here in the South from time to time; this means you should not just prepare for the most common disasters but for any extreme event that could occur there. Of course, getting ready for the most common natural hazards is the best way to save your homes and gardens because those are the most likely to occur where you live. But you should also think about rare events like floods even if you are not in a flood plain because the consequences of an event are so severe.

Once you have identified the natural hazards that affect your area, then you need to think about what kinds of weather conditions are likely to occur in each of those events. In a hurricane or a strong winter storm on the West Coast, heavy rain and high winds are both likely weather conditions your garden will experience. In a heat wave, high evaporation rates and dangerous outdoor working conditions are likely to be the major dangers. Those are the impacts you will need to consider when protecting gardens and gardeners.

Look at your landscape and home to identify potential problems

Once you have determined what hazards are likely to affect your property, you need to do an assessment of where your risks are. Take a walk around your garden and look at the trees and plants you have. Are there dead trees that could fall over or broken limbs that could snap off in high winds from hurricanes or thunderstorms and become wind-borne missiles? Do you have garden decorations like garden gnomes or mirror balls that could also blow into the sides of cars or buildings? Do you live in an area that is prone to frequent flooding? How will you keep that water away from your house and out of your garden plots? Are there a lot of plants close to your home where they could spread wildfire in a drought under gusty winds?

After you have determined the risks take steps to minimize or remove those risks where you can. There are a number of ways that you can storm-proof your house and garden, many of which should be done anyway to maintain your garden’s health.

I was interested to read the story of the lone house in Lahaina, Hawaii, that survived the recent fires that destroyed much of the town because they had a large area around the foundation covered by river rock, which did not burn and helped keep fire from igniting the house (although I am sure there was some luck involved there too). One of my favorite books, The Control of Nature by John McPhee, also describes the increase in debris flows in western landscapes that occur due to fires that burn waxy plants. This creates perfect conditions for rapid land movement downhill after rain events following the fires, often right through people’s yards and houses.

After the storm is over and you are safe, then it is time to assess your garden and house for damage and take care of your plants, lawns, and buildings. Be careful since there may be downed power lines and dangling tree limbs that could be hazardous. You may need professional help to prune or remove trees or clean out contaminated soil after flooding. Once the immediate hazards are taken care of, then the longer-term work of repairing drainage, eliminating the effects of erosion, rebuilding beds, and other work can begin.

Take care of your family and pets too

Of course, all this planning for your garden and property should not take the place of emergency planning for your own family. If extreme weather does occur, you need to have a plan already in place to determine where to shelter in your home if you can and how to evacuate safely if you can’t stay. Every minute saved makes a difference although in the worst cases even that may not be enough time. FEMA has a good website that provides a lot of information on how to plan for both natural hazards and other emergencies like chemical spills. Many states also have excellent resources for dealing with emergencies, such as the Resident’s Handbook To Prepare for Natural Hazards in Georgia, which covers all kinds of severe weather and how to prepare for it in Georgia and beyond.

My news feed has been filled in recent weeks with many stories about the unusual heat that has affected many parts of North America, Europe, and the Atlantic Ocean. High temperature records are being broken at an amazing rate, and while we don’t expect every day or season to break a new heat record, the trend towards warmer global temperatures overall from greenhouse warming makes periods of extreme heat more likely.

Today I want to talk about the term “heat dome” and explain what they are, how they affect gardeners (and other humans) as well as the plants in our gardens and agricultural fields. We had a great introduction to the physiological impacts of heat a few weeks ago in Jim Downer’s post “I’m hot! So are my plants!” on how high temperatures affect plant growth and respiration. If you have not already done so please read that for more details about heat’s impacts on garden plants.

What is a “heat dome?”

A “heat dome” is an expansive stationary area of high pressure that is associated with unusually hot temperatures. While slow-moving high-pressure areas are often seen in summer months, the size and unmoving nature of a heat dome coupled with the extremely high temperatures in the area beneath them make “heat domes” especially dangerous for humans and animals. They can also cause detrimental effects on many garden and agricultural plants because they are also associated with long dry spells along with high rates of evapotranspiration. The term “heat dome” is a phrase that has been popularized by the news media as a way to explain extreme heat events across large regions and is more jargon than a true scientific term, but it is certainly descriptive!

Any high-pressure center in the atmosphere is characterized by sinking air, light winds, and relatively cloud-free skies. The sinking air heats up as it compresses near the surface, and the rising temperatures are enhanced by the lack of clouds, which lead to more incoming sunlight and even warmer air in summer. Pavement also absorbs sunlight and radiates it out at night, leading to overnight low temperatures that can be in the 90s in extreme cases.

When the center of high pressure is locked in place over one area over a long time period, it tends to divert cooler, moister air away. That makes it appear as though there is a glass dome overlying the area blocking rain from falling. The hot and dry conditions tend to get worse quickly, leading to temperatures that in the worst cases can break records. This week many daily high temperature records were broken in the western United States as well as in parts of Europe. If the air is already warmer than average to begin with, that makes it even easier to reach record high temperatures. Often, a very wavy but stagnant pattern in the upper-level winds causes a series of heat domes to form across the globe, leading to heat waves at several locations at the same time.

What danger does a heat dome cause to gardeners?

Air in the center of the high pressure that makes up the heat dome tends to trap pollutants in a shallow layer near the surface of the earth. This can lead to episodes of smog and high surface ozone that can cause health effects on people, especially children and people with poor lung function. It can also trap smoke from wildfires, as I discussed last month. The trapped pollution makes it hard to breathe and can cause lung damage as well as leaf damage on plants. Temperatures that stay in the 80s or higher overnight do not allow human and animal bodies to cool down to their normal temperature, resulting in health issues that build up over several days as the heat wave continues. Phoenix has experienced every day this July with a maximum temperature of 110 F or higher, and has seen deaths due to heat increase, especially in homeless populations that have little access to cooling. Deaths from heat spells increase after several days of extreme temperatures when the body is not able to cool itself down and the heat causes physiological changes that can lead to severe impacts. While drinking a lot of water is important for hydration, it is not enough to fend off the impacts of the high temperatures. You must find a way to cool off or your health will suffer.

Humidity is also a factor in heat-related illnesses. Humans cool their bodies off by sweating. The moisture is evaporated from the skin, leading to an energy transfer that makes the skin cool off as the sweat is changed to water vapor. But if the humidity is too high, evaporation is so slow that it can not provided the needed cooling, and the body stays hot. That can eventually lead to death. In fact, it is not just humans and terrestrial animals to suffer. The water temperature near southern Florida this week was near 100 F, leading to predictions that the coral reefs in the ocean there would soon die because it is too hot for them to survive, much less thrive. This is also happening in other ocean locations around the world. Since coral reefs provide food and shelter for many marine species like fish, it is likely to cause dire consequences for the oceanic food chain, including those of us who eat shrimp, fish, and other seafood.

What do gardeners need to do when a heat dome is forecast?

When a heat wave or heat dome is predicted, gardeners and farmers should recognize that a protracted period of extremely hot, oppressive conditions is likely to occur. The first action they should take is to make sure they are protected from the effects of the heat. That means working outside early in the day when it is cooler, drinking plenty of water, wearing light-colored clothing to reflect sunlight, and taking plenty of breaks in the shade or air conditioning. They should take care of pets and livestock as well and consider keeping them inside or in shady places with plenty of water for drinking. They should monitor their plants carefully for signs of heat stress and water regularly to reduce the impacts of the hot, dry conditions. Trees should also be watered, especially if they are not well established or if the dry period is especially lengthy. If the humidity is high, watering incorrectly can increase the likelihood of fungal diseases, so read Linda Chalker-Scott’s article Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases – The Garden Professors™ to learn the best ways to water your plants and preserve soil moisture. Saving Your Trees From Drought! – The Garden Professors™ also provides information about watering trees during dry periods.

Eventually, all heat domes and dry spells do pass, but it is always good after one is over to assess how your garden did and to plan ahead to protect soil moisture using some of the techniques mentioned in the links above.

Over the last month, I have seen many stories related to smoke from Canadian wildfires drifting down into the eastern United States, causing muted sunsets as well as terrible air quality. Even my mom up in Michigan told me how bad the air is up there this week and friends in Wisconsin have told me that they can’t go outside without donning N95 masks to cut down on breathing in all the smoke particles. Of course, our readers in the western U. S. may be rolling their eyes since they have gone through severe wildfire seasons in past years with little attention from the eastern press, and poor air quality from wildfires and pollution is also a frequent problem in other parts of the world. But since it is in the news, I thought I would address aerosols and their impact on the atmosphere, human health, and our gardens.

What is an aerosol?

Aerosols are very small particles that float in the atmosphere. They can be from natural sources like salt from breaking ocean waves or pollen from blooming plants or can produced by humans through burning coal, construction, or poor agricultural practices. Saharan dust, volatile organic compounds emitted by trees, wildfire smoke, and volcanic ash can all add to the dust burden in the atmosphere. Some aerosols attract water vapor, causing them to expand in size and reducing the visibility of the atmosphere even more than the particles alone. Aerosols can be toxic, too, and areas with a lot of atmospheric pollution can cause severe problems for vulnerable people and pets when aerosols get deep into lungs.

Impacts depend on where they are in the atmosphere

The impacts that aerosols have on humans and the environment near the ground depends on how high up the aerosols are concentrated. If the particles were lifted above the surface due to the heat from burning forests or trash, the main effects that the aerosols might have are optical, reducing the amount of incoming sunlight but not significantly affecting the air we breathe near the ground. Some acidic particles that attract water vapor might also contribute to acid rain that falls to earth. But if the dirty air is mixed down to the ground or is produced locally, the aerosols can cause significant issues for human and animal health because of their irritating effects on lungs and sometimes skin and eyes. They can also provide hazards to transportation if visibility gets too low. Acidic particles can also cause damage to plant tissues or change the pH of the soil if they affect an area over a long time period.

How do aerosols affect climate?

Aerosols affect climate by reducing incoming solar radiation. Volcanic ash and sulfuric acid droplets from volcanic eruptions can cut enough sunlight to reduce global temperatures for several years after a large volcanic eruption, especially if they occur in the tropics. This year’s unusually warm Atlantic Ocean temperatures can be linked in part to a lack of the usual plume of Saharan dust blowing off the west coast of Africa, which has allowed more sunlight to warm the surface water. The so-called “warming hole” in the Southeast has been linked to aerosol emissions from power plants upwind in the Midwest and Western U. S., which caused reductions in sunlight over the Southeast until the passage of the Clean Air Act of 1970 reversed that effect. Since then, the temperature in the Southeast has risen in concert with rising temperatures across the rest of the world. Aerosols contribute to the development of clouds, too, and that has the potential for affecting climate at larger spatial scales.

How do aerosols affect health?

Aerosols affect human and animal health when they are inhaled into the lungs, irritating tissues and causing swelling and producing fluid as the lungs try to clear the aerosols out. According to estimates from the World Health Organization (WHO), particle pollution contributes to approximately 7 million premature deaths each year, making it one of the leading causes of worldwide mortality. Fine particles that are smaller than 2.5 micrometers (called PM2.5) are the most damaging because they are so small that they can make it deep into the lungs where they are deposited on the lung tissue. Because of this, gardeners and others who spend a lot of time outside need to be aware of the current air quality measurements and minimize time outside when the air quality is bad. You can find current air quality information in the United States at AirNow. Many state health agencies also post air quality information and the National Weather Service also puts out alerts on days with bad air quality. When the plumes of smoke from the Canadian wildfires moved over the Midwest and the Northeast, some U.S. cities had the worst air quality of any metropolitan areas in the world while the smoke was present.

How do aerosols affect gardens?

Aerosols have several impacts on plants and gardens. Aerosols provide benefits for gardeners since clouds and rain form from water that is collected into water droplets on aerosol particles known as Cloud Condensation Nuclei (CCN). No doubt if you collect rain or snow water, you have seen the dirt that remains after the water is gone. But aerosols also have detrimental effects. Aerosols aloft can reduce incoming sunlight, leading to slower plant growth, especially for plants like corn that are sensitive to the amount of sunlight they receive. Aerosols at ground level can cover the plants with a layer of dust that decreases photosynthesis by blocking incoming sunlight and clogging pores. If the aerosols are acidic or contain toxins, they can damage the plants or increase the acidity of the soil, especially over long time periods. In the case of smoke from wildfires, the smoke particles can also affect the taste of grapes or other food products they interact with. Smoke taint on wine grapes, caused by compounds from aerosols that are absorbed by the grapes, can impart an ashy flavor to the wine made from those grapes, making it unsellable, as producers in California and Europe have found in recent years.

If you are experiencing air quality issues in your community, we encourage you to monitor the weather forecasts closely and stay inside when the aerosol count gets too high, especially if you have asthma or other lung conditions that may be made worse by poor air quality. If you have noticed other impacts of the wildfire smoke or other air quality issues on your garden plants, please feel free to share them in the comments.

I saw an article describing an atmospheric phenomenon called the “pneumonia front” this week and it made me start thinking about local kinds of wind and their names. No matter where you live, in the United States or elsewhere in the world, you have wind patterns that are related to your local geography. These winds can affect gardens, especially if they are persistent over time, but I enjoy hearing about the different names for wind too.

What causes the wind to blow?

Wind is the movement of air from one place to another. The air movement is driven by differences in air pressure from one place to another—the atmosphere tries to even out the pressure so air molecules are always moving from areas with higher density and pressure to areas with lower density and pressure. Since density and pressure are related to temperature (remember your ideal gas law from high school chemistry?) and temperature frequently changes as the sun moves across the sky or lakes and oceans warm and cool, the air is nearly always moving except where there is locally no variation in pressure such as the center of a high-pressure area.

Two common types of local winds

Winds are often linked to specific geographic features. For example, sea or lake breezes are located along the shores of large water bodies and are driven by pressure differences related to the relative temperatures of the land and water. When the water is colder than the land (for example, on a hot summer day), air pressure over the hot land is lower than over the cold water due to rising air over land (you can often see clouds where this is occurring). Air from over the water blows onshore in response to the lower pressure on land, leading to a cool breeze flowing over the hot land, cooling things off. At night when the land cools off more quickly than the water, the flow reverses and becomes a land breeze. Monsoons like the ones in India, the Southwest US, and other places are the largest-scale version of a sea breeze over thousands of miles and develop over weeks instead of hours.

Another geography-linked local wind is the katabatic wind. Katabatic winds are related to differences in elevation that cause temperature variations that result in density differences in the air. In a katabatic wind, air at upper elevations cools off at night, creating a pool of very dense air that rushes down the sides of the mountains to pool in the valleys, creating pockets of very cold air. Vineyard owners know this and plant vines on the sides of hills so that the vines are not exposed to the coldest air (and to take advantage of sunlight, too). The recent frost in New England caused severe losses of apple blossoms in the bottom of valleys while orchards in higher elevations were less affected. In your gardens, this occurs on a small scale with frost pockets that can form in the lowest-lying areas of your yards and garden plots. Antarctica has some of the strongest katabatic winds, with shallow winds that can reach up to 200 mph due to extreme temperature and elevation differences in that continent.

Other local wind names

There are many other location-specific winds and weather patterns linked to wind that occur in other parts of the world. Some are driven by elevation differences, with wind blowing through gaps in mountain ranges (the mistral in France and the tehuantepecer in Mexico, for example). Others blow in specific directions where mountains prevent air movement in some directions, funneling the air into channels that bring characteristic weather to the local area. In northeast Georgia, for example, we have frequent incursions of cold air from the northeast, with air pushed south due to high pressure in northern latitudes that is prevented from spreading to the west by the Southern Appalachian Mountains. We call that phenomenon “the Wedge” due to the shallow and dense wedge of surface air that is pushed by the wind flow into our region numerous times a year. Areas with very persistent topography-driven winds often have trees with most of their limbs on the downwind side of the trunk.

How does wind affect gardens?

Wind causes many effects on gardens. It can blow frigid air into a region from the poles towards the equator, leading to advective frost which causes damage to fruit blossoms in spring. If the humidity of the wind is low, it can quickly remove soil moisture and desiccate plants where irrigation is limited or unavailable. When strong, it can rustle leaves, break limbs, and even topple entire trees, especially where wet ground weakens the anchoring of tree roots. In fact, one measure of wind speed, the Beaufort Scale, uses an empirical scale related to the appearance of waves (on the sea) and tree movement (on land) to categorize wind strength. Some wind is a good thing for many plants because it provides stresses that help strengthen the stems and trunks, but too much can cause a lot of damage from wind-blown debris or direct force on the plants.

What local winds do you see and what impacts do they have on your gardens?

This blog has reached 194 different countries with many thousands of unique visitors a year, so the variety of local winds you experience must be amazing. Some of them are variations on the winds described above, either topography-driven winds like katabatic or anabatic (the opposite of katabatic, with up-valley winds during the day) or foehn winds. Others may develop due to unique geographic features of your area such as the Columbia River Gorge with winds so strong it is a haven for windsurfers. We’d love to see a comment on your local winds and how they affect your gardens!

I close by quoting the famous poem from Christina Rossetti that provided our title for this blog post, one of my favorites:

Who Has Seen the Wind?

Who has seen the wind?
Neither I nor you:
But when the leaves hang trembling,
The wind is passing through.

Who has seen the wind?
Neither you nor I:
But when the trees bow down their heads,
The wind is passing by.

This time of year, I often get asked for a forecast for the coming growing season. Will we have a drought? Will it be warmer or colder than normal? Will we have any tropical storms in our area? All of these things affect how farm crops (and gardens) will perform over the next few months and how big the yield might be when it comes time to harvest. In this week’s blog, I will look at some of the factors that go into seasonal forecasting and how it all comes down to numbers.

How are seasonal forecasts made?

Seasonal forecasts use computer models to look for large-scale patterns in weather that affect what the climate is likely to be on monthly to seasonal time scales, but also use statistical methods based on observations from previous years to determine what the most likely climate conditions are. The predictions are usually based on several factors, including current conditions, long-term trends, and other regional variations like El Niño Southern Oscillation (ENSO), which we’ve discussed before.

Current conditions are important because they define the starting point of the prediction. If you are starting from a drought, for example, you would not expect that next month would be much wetter than normal because the dry conditions would make it hard for rain clouds to form unless you have an unusual event like a tropical storm or atmospheric river that comes along and changes conditions on the ground quickly. Long-term trends are important because they define the base state of how the climate is behaving. If you are on an upward trend towards warmer temperatures, as we are now with global warming, it is more likely that you will observe a month or season that is above normal than below normal. If there were no trend in temperature, then seasons that were colder or warmer than average would be equally likely. You can find climate trends for your local area using the Climate at a Glance tool and picking your own country, state or county.

Knowledge about regional variations like ENSO also contributes to seasonal forecasts because the atmospheric and oceanic climate conditions which help define those oscillations can last for months or sometimes even years, making long-term prediction easier. The pattern of the atmospheric waves associated with those oscillations defines where warm and cold air are located as well as the position of the jet streams that blow storm systems around. In some respects it is like putting a rock into a river—when you have warm water in the Eastern Pacific Ocean as we do now in advance of the coming El Niño, thunderstorms develop vertically over that warm water, and those towers of rising air divert the flow of air currents that push weather systems around just as a rock diverts the flow of water in a stream. If the warm water were not there, the atmosphere would likely have a very different pattern.

Oirase Mountain Stream – Towada, Aomori, Japan, Daderot, Commons Wikimedia

Forecasts based on ENSO work best when it is at one extreme or the other and in areas where the differences between El Niño and La Niña conditions are most pronounced. That makes ENSO more useful for making seasonal forecasts in the Southeast United States and in northern states where statistical relationships are well-defined than in the central U.S., where there is a weaker statistical relationship between the ENSO phase and the local climate. Climatologists look at similarities between different El Niño years using statistics to identify recurring patterns that can help to predict the climate the next time an El Niño occurs, although other factors can come into play that throw off the forecasts.

How do seasonal forecast maps depict the future climate?

For the United States, seasonal forecasts are presented as maps with probabilities of above, near, or below normal temperature or precipitation. If there was no hint in any of the predictors of what would happen, each of the three categories would have equal weight, or a 33% chance of that climate category occurring. Those forecasts are based on current conditions, what they expect to happen in the months after the prediction made, the long-term trends that are pushing the temperatures or precipitation upward or downward, and the expected state of oscillations like ENSO. You can see the whole suite of 3-month forecasts for the next year for the United States at https://www.cpc.ncep.noaa.gov/products/predictions/90day/.

In the maps above, the outlook for December 2023 through February 2024 is shown. In the temperature map, the climate has a higher probability of being warmer than normal in the northern part of the country, especially the Northeast. This is consistent with the expected pattern of temperature in an El Niño winter. But the prediction of warmer than normal weather in the Southeast is not what we expect if we look only at El Niño. It also includes the effects of greenhouse warming, which is likely to overcome the cooler conditions that would be expected in the Southeast from El Niño alone. On the other hand, since there is not much long-term trend in precipitation, the precipitation outlook on the right shows a clear El Niño signal with wet conditions along the Gulf and South Atlantic coasts and dry conditions in the Northwest and to a lesser extent, in the Upper Great Lakes. In the Southeast, El Niño winters are expected to be cooler than normal because all the clouds associated with the rainy conditions keep daytime temperatures down.

What do we expect for this year?

Because the El Niño is coming on strong, I expect it to drive a lot of this year’s growing season climate. While it is still weak, it won’t have much impact on our regional climate variations, which makes summer difficult to predict. In that case, we look more towards the current conditions and trends to conclude that areas that are already experiencing drought are likely to get worse, and areas that are starting out wet may get a reprieve from drought conditions for a few months.

The biggest impact of an El Niño in the growing season is in how it affects developing Atlantic and Eastern Pacific hurricanes. In El Niño years, tropical systems in the Atlantic tend to be fewer in number and weaker because the strong jet stream aloft keeps tropical cyclone circulation from strengthening into tropical storms or hurricanes. In the Eastern Pacific, it is just the opposite, with more storms forming than usual. That can help feed moisture into the Southwest. Of course, it only takes one storm coming over your home and garden to cause tremendous damage, so you need to prepare for storms if you live in a hurricane-prone part of the United States or other Northern Hemisphere location.

By fall, the El Niño is expected to be well developed and areas where El Niño usually brings rain could see the wet conditions start earlier than usual. For farmers, that means harvest could be difficult if conditions are wetter than usual, and it might be hard to get that last cutting of hay. Winter could be drier than normal in the northern states since many of the rain-bearing storms will be farther south than usual, although you will certainly see some rain.

Last April 30, 2022, I wrote a post about what a third year of La Niña meant for gardens. La Niña, you might remember, is an atmosphere/ocean phenomenon driven by unusually cold water in the Eastern Pacific Ocean (EPO) that shifts the jet streams that steer storm systems around the world. In La Niña winters in North America, it is often linked statistically to colder and wetter than normal conditions in northern parts of the United States and north into Canada and warmer and drier conditions in the southern tier of states in the US. Just a couple of weeks ago, NOAA announced that the long La Niña has finally ended and that we are now in a neutral period. In last year’s post, I described the difference between La Niña and the opposite climate phase, El Niño, and how they affect climate conditions around the world. So now you might be wondering how this switch to neutral conditions and then potentially to an El Niño later in the year will affect your gardens in the coming growing season.

Elfen-Krokus (Crocus tommasinianus), AnRo0002, Commons Wikimedia

How good was last year’s forecast?

A discussion of how accurate the forecast for last winter was can be found in NOAA’s ENSO blog. As you can read, the temperature forecast was much better than the precipitation forecast (as it usually is). That is not surprising because many causes of rain and snow occur on very small spatial scales from a variety of physical processes that are not always well-captured in our current climate models. You can hear a more detailed discussion of historical ENSO patterns by David Zierden, the Florida State Climatologist, in this 15-minute video for the March 2023 Southeast Climate Monthly Webinar starting at minute 28:35).

As a result of the cold and snowy conditions in northern parts of the country, spring there has been delayed, and my friends in the Upper Midwest have only started to see daffodils and other early spring flowers, while in the Southeast, our azaleas and dogwoods are already in decline about a month earlier than usual (you can track this at the National Phenology Network website we’ve discussed before).

What happens next?

Now that La Niña has ended, we are in neutral conditions. That means it is difficult to predict the climate during the next few months because without La Niña (or El Niño) to give us statistical guidance on what climate to expect, almost anything can happen. The variations in climate in neutral seasons are caused by local variations, other interactions in climate on regional or global scales, and other factors that are not always well understood. In addition, we are in spring, which historically gives us the least trustworthy predictions for the coming seasons due to what is called the “spring predictability barrier.” That means that while we think we are likely to swing into an El Niño in the next few months, the atmosphere might have other ideas and could keep us in neutral conditions for quite a while before we switch to an El Niño.

This year, we are already getting signs in the Eastern Pacific Ocean that a switch to El Niño will come quickly. The water near the coast is already quite warm and the warm pool is starting to stretch out to the west. In other words, El Niño-like conditions are already present in the EPO, but have not lasted long enough yet for an official El Niño to be declared. That usually takes several months of monitoring to make sure this is not just a short-term change.

What do neutral and El Niño conditions mean for the Northern Hemisphere growing season?

Usually, ENSO conditions do not strongly affect the NH summer climate. That is because both La Niña and El Niño are strongest in the winter months and tend to weaken or disappear in the summer. Local conditions including soil moisture variations such as drought, ocean temperature variations, and local weather systems have a much bigger impact on growing season weather than ENSO does in most areas.

However, the ENSO state does have one strong influence. That is in the tropical activity in the Atlantic and Eastern Pacific Oceans. When neutral conditions or La Niña conditions occur, the jet stream aloft is weak and it is easier for tropical waves to develop into tropical storms and hurricanes, so neutral and La Niña seasons tend to be more active and have more named storms. When an El Niño occurs, the jet stream is unusually strong and that keeps tropical waves growing vertically into strong storms, so the number of tropical storms and hurricanes in the Atlantic is usually lower in years when an El Niño is occurring. Of course, it only takes one storm (Hurricane Andrew in 1982 was in an El Niño year) to cause tremendous damage if it hits somewhere vulnerable. In contrast, the storm activity in the EPO increases in El Niño years due to the pool of warm water there.

This year, the likely storm activity may be more tied than usual to the ENSO state. If we see a quick switch to El Niño after just a few months of neutral conditions, the Atlantic may be most active early in  the season, while storms later in the season will be suppressed. By comparison, in the western US where some moisture enters the country through EPO storm activity, you may see an increase in thunderstorms that are fed by the water vapor entering the country from the storms in the EPO.

What does this mean for your gardens?

If you live in an area that normally gets rain from Atlantic tropical activity, even if it is not from actual tropical cyclones or hurricanes, you can probably expect drier conditions this year, or at least more variable rainfall from less organized systems as more precipitation will be produced from local influences. El Niño does tend to delay the onset of the Southwest Monsoon, so that could be something to watch next year, although it may not have much impact this year. In Texas, the switch to neutral conditions means rain in April through June is more likely, which would be great for avoiding drought. If you live in an area that does not receive much moisture from the tropics, it will be hard to make a good forecast because the statistics just don’t give much guidance. We do know that globally, El Niño years tend to be very warm, so it is likely that 2023 may be one of the warmest, if not THE warmest, since global records began in 1880 due to the long-term rise in temperature from greenhouse warming. Warmer weather will mean a longer growing season, more hot days and nights, more humid conditions (unless a drought occurs), and more diseases and pests that thrive on the warmer conditions.

What do we expect next winter?

If you like to plan far ahead, you can see seasonal forecasts from NOAA’s Climate Prediction Center at Climate Prediction Center – Seasonal Outlook (noaa.gov). If you are not from the United States, your own country’s weather service may provide similar outlooks for your region. Here in the US, we are likely to see wetter and cooler conditions in the southern US as the jet stream steers storm systems over us (days will be cooler because of the clouds, but nights won’t necessarily be colder than usual since clouds trap nighttime heat). In northern states, warmer and drier conditions will be more likely, and that could mean an earlier start to the growing season next year. Now that something to look forward to if you are a gardener!

Tulipa “El Niño” 2015, Retired electrician, Commons Wikimedia.

If you’ve been following the national weather this week, you might be wondering if the groundhog has developed a split personality this year. Is winter over or are we in for six more weeks of cold? While the eastern half of the United States is feeling the effects of record-setting high temperatures and one of the earliest springs on record, the western U. S. is observing cold and snowy conditions all the way down into Southern California. I’ve heard reports of snow reaching all the way down to Tucson in southern Arizona.

What is an atmospheric wave?

Some of you may be wondering how we can have some tremendous differences between the eastern and western U. S. at the same time. The reason we see such differences is linked to the wavy pattern of the upper atmosphere, which helps direct warm or cold air into different regions. Today I will discuss atmospheric waves and how they affect the surface weather.

Since air is a fluid, it should not be surprising that it has waves in it. In fact, there are many different types of waves that can occur in the atmosphere, ranging from small-scale pressure waves downwind of mountain ranges to the largest planetary-scale oscillations in pressure. Sometimes you can see some of those smaller-scale waves in clouds occurring above you.

Where do atmospheric waves come from?

What ultimately drives the atmospheric circulation is the temperature difference between the equator which is warm due to direct sunlight and the poles which are cold because whatever little sun they get, especially in winter, is of low angle and there is not much solar energy reaching the surface to heat things up. This temperature difference causes a difference in atmospheric pressure that makes winds blow to try and equalize those differences.

Variations in land versus sea, the rotation of the planet, and differential heating from other causes like drought all contribute to the atmospheric developing waves that look like large swings in the pressure patterns at mid-levels in the atmosphere. Areas where the pressure is low are called troughs and areas where it is high are called ridges, like the patterns we see in topographic maps. Surface fronts are located near the boundary between troughs and ridges where the contrast in temperature and humidity is often the greatest.

How is the current wave pattern driving these big temperature swings?

This week there is a tremendous ridge of high pressure located in the eastern US while a very deep trough is located over the western part of the country. Warm air blowing up from the south under the ridge has brought record-setting high temperatures to many parts of the East, while cold air blowing in from the north behind a strong surface low pressure center has caused winter storm and blizzard warnings all the way south to the mountains near San Diego. Record-setting low temperature and wintry conditions are occurring in those areas.

The warm temperatures in the East are of special concern for gardeners since they have caused the first leaves and blossoms to occur as much as a month early in some locations, according to the National Phenology Network. Since the average frost date for those areas is a month or two later, the likelihood of damage to fruit crops is high since freezing temperatures could significantly reduce the production of fruit in gardens and orchards if they occur in the next month or two. It could also cause damage to many other flowering plants that are fooled into blooming early due to the unusual warmth.

Will atmospheric wave patterns change in a warmer world?

Is the unusual warmth due to global warming? This is a hotly debated topic (if you will excuse the pun) with both proponents and skeptics weighing in. Many scientists believe that as the arctic regions warm up faster than the equator (a process known as “Arctic amplification”), the atmospheric wave patterns will become more amplified, with both deeper troughs and stronger ridges occurring. That could mean more extremes in both hot and cold weather as these waves occur. Other scientists caution, however, that the computer models used to study the large-scale wave patterns don’t always agree with global warming as a root cause. If we do have generally warmer conditions that are punctuated by significant cold outflows our gardens will have to be able to survive the increasing variability of the weather, especially in winter and spring when there is still a significant pool of cold, dry air near the poles available to flow south into mid-latitudes.

Please feel free to share how this crazy weather has impacted your own garden. Are your trees blooming early, or has snow covered everything? If you are not in the United States, what do you see going on in your own neighborhood? We are interested to know!