Month: December 2024

How did your garden grow in 2024? Was it a lush playground full of beautiful flowers and plentiful produce? Or was it a sere landscape of brown, wilted foliage? How your own garden fared in 2024 was certainly dependent on where you live, what you had planted, and how you took care of it, but it was also subject to the variations in weather and climate in your area. This week we will take a look back at the climate conditions in 2024 and look forward to what it might mean for 2025. Now is the time to look at your seed catalogs and dream!

What controlled the climate this year around the world?

With just a few days to go in 2024, it is quite clear that this will be the warmest year we have ever measured since official global records began in 1880. There are three main factors that controlled the climate in 2024, although of course there are also local variations due to smaller-scale weather events. The contributing factors are the warming trend across the world caused by greenhouse warming of the planet, the El Niño that dominated the Eastern Pacific Ocean in the first half of the year, and the unusual warming of the Atlantic Ocean this year which provided fuel for the growth of Atlantic tropical cyclones this year as well as raising the global temperature.

Impacts of the rising temperature trend

Rising temperatures for the world are well-documented by scientists across the globe and are generally linked to increases in the amount of greenhouse gases like carbon dioxide and methane from sources like the burning of fossil fuel. This is not a new concept and can be found in scientific literature going back to at least 1856 when Eunice Foote discovered that carbon dioxide trapped heat in her home laboratory. Many scientists since then have corroborated that effect and others have shown that the primary source of greenhouse gases like carbon dioxide is the burning of fossil fuel, although there are other sources as well.

A timeline of global temperature for January through November (we don’t have the values for December 2024 yet since it is not quite over) shows that this is almost certain to be the warmest year on record so far. But since temperatures are still rising, we can expect to see more record-setting warm years in the future. The rise in global and regional temperature that is occurring now may not be apparent on a day-to-day basis due to short-term variations caused by passing weather systems, but the changes are reflected in increases in normal temperatures when they are updated every ten years. Longer-term changes like the drift in Plant Hardiness Zones also reflect this upward trend. Heat waves across Northern Europe and in South America in 2024 have also been attributed in part to the warming trend. Of course, winter will still occur and we will continue to get cold periods, just fewer of them than in the past.

El Niño to neutral conditions

The second major impact on the climate in 2024 was the lingering El Niño that was occurring as 2024 began and lasted until early June. The warm water in the Eastern Pacific Ocean associated with El Niño helped raise the global temperature during the first half of the year and affected the climate around the world. In North America, an El Niño is associated with warm dry conditions at high latitudes and wet cool conditions in southern latitudes as the jet stream is shifted to the south, bringing storms, clouds, and rain along with it. Once the El Niño ended in June, neutral conditions prevailed until the end of 2024, although the last few months we almost reached the threshold for the opposite phase, La Niña. Climate patterns associated with La Niña were starting to appear late in 2024, leading to dry conditions across southern parts of the United States and wet conditions farther to the north. In fact in the Southeastern US most areas were in drought for a good part of the summer except areas that were hit by tropical cyclones like Beryl, Debby, Francine, Helene, and the remains of Rafael. By the end of 2024, over 87% of the lower 48 states were covered by drought or abnormally dry conditions, a big change from early in the year.

Notable droughts also occurred in Brazil and other parts of South America and in northern Europe. These droughts were also associated with record-setting warm temperatures as high pressure over those areas tamped down any development of rainstorms and caused clear skies which increased temperatures. You can look at maps and timelines of specific areas of the world or country at https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/. The map above shows how the drought status in the US changed over the year with some areas getting much wetter and others drying out. Note that areas with tropical storms and atmospheric rivers this year experienced a lot of changes from month to month but it did not affect the total change over the year by much.

Record-setting warm temperatures in the Atlantic

The third impact on this year’s climate was the abnormally warm temperatures in the Atlantic and Gulf of Mexico. These record-setting temperatures have been linked to decreases in aerosols from burning of fossil fuels by ships crossing the ocean and in more recent literature, to decreases in low clouds over the ocean. Both of these can lead to more sunlight reaching the surface of the ocean and increasing its temperature. Those warm sea surface temperatures led to a larger number of tropical systems than usual in the Atlantic Ocean by providing a source of fuel that helped them develop into full-fledged tropical storms and hurricanes. There were 11 hurricanes and 18 named storms in the Atlantic Basin this year, the 5^th largest number in the satellite era. The number and intensity of tropical cyclones in other parts of the world like the western Pacific are also attributed in part to warmer ocean water. The Philippines experienced five different typhoons in just a few weeks, causing tremendous damage there, and other areas of Southeast Asia also felt the impacts of tropical systems.

What do we expect in 2025?

By January I expect that the La Niña will be officially declared. Whether or not it is, though, we can predict that the early part of the year will show the characteristic pattern of a weak La Niña, including a shift to the north for the jet stream over the United States. That will bring cloudy and colder weather to the northern states and warmer and drier conditions to the southern states since the jet stream is what is pushing our precipitation-producing systems around. These conditions will likely be reflected in the soil moisture present during the spring planting season, so I expect dry conditions in the Southeast that could affect germination of seeds. Wetter conditions in the North should not have this problem but farmers could have trouble getting into the fields to plant if it is really wet and cool, leading to delays in establishing crops. This La Niña is likely to be fairly weak, so it may only last for a few months before we return to neutral conditions. NOAA’s predictions are that the neutral conditions are likely to last for most of the summer. That means we are likely to get another active Atlantic tropical season. The South could be fairly dry except where the tropics bring storms to the area again in 2025. If you are in other countries, you can find more information here or check with your local authorities for how your region usually responds to a La Niña in winter and later in the year.

How did your garden do in 2024? What are you looking forward to in 2025? Let us know in the comments. We are happy to get your questions, too, as we plan for our blog posts in 2025. Many thanks for the comments we have received in this past year and for your support of our blog! We appreciate it!

We at the Garden Professors stress the importance of accurate diagnoses of plant “problems.” Often, problems aren’t due to pests or disease, and sometimes they aren’t problems at all. That got me thinking about a tree oddity I saw earlier this year when I was visiting my daughter in Walla Walla. Near her office at Whitman College stands a mature box elder (Acer negundo), whose lower crown has large swaths of pale yellow leaves (Figure 1). From a distance, one might think of several reasons these chlorotic leaves have developed: lower crown leaves and branches are routinely less productive than upper canopy leaves and as a result receive fewer resources from the tree. Lack of water in particular can cause early fall color change.

But that’s not what’s going on here. A closer examination of the leaves (Figure 2) also reveals a lumpy trunk (Figure 3) – and venturing around the trunk we find multiple trunks that have fused together (Figure 4). What become obvious through careful observation is that those pale yellow leaves are associated with the smaller diameter stems wrapped around the main trunk.

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Unfortunately, Whitman College does not have an online tree database for me to access, so I don’t know when this tree was planted or what variety it is. But I am fairly confident that it is Acer negundo f. auratum Schwer. (That “f.” stands for forma, which is synonymous with subspecies.) Anyway, this natural variation was recorded in 1893 in Gartenflora 42:202. More current references to this variant erroneously call it a cultivar (‘Auratum’). It’s identified as having golden-yellow leaves with smooth undersides.

Box elder is widely regarded as a fast-growing, weedy tree with challenged aesthetics. The auratum form, with its chlorotic leaves, is less vigorous and would be better suited for a college campus landscape. It’s reported to only reach 25’ at maturity, with its yellow leaves becoming nearly white in the summer.

And that life history characteristic is what spelled the near-demise of the originally planted tree. Varieties, subspecies, and cultivars are often grafted to species rootstock, which is a faster method of propagation than by seed. Careful management of this young tree would have included removal of suckers as they appeared at the base. Left unchecked, a vigorous sucker rapidly outgrew the scion, which then became embedded in the new, dominant trunk.

The take-home message here reflects the importance of onsite field diagnostics. In this case, a photo of a chlorotic leaf does not tell the whole story and would likely result in a misdiagnosis.

Happy holidays to you and your trees!