This is a climate for early bloomers

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Anyone concerned about human-induced global warming will know of the increasing damage and disruption directly linked to rising temperatures. Sea levels have risen dramatically, causing more frequent flooding in coastal areas, especially in coincidence with hurricanes. More frequent and prolonged droughts affected some regions, including the southwestern United States. Episodes of extreme temperatures have become more frequent, triggering deadly heat waves and stoking wildfires.

For the most part, these are the expected direct effects of warming. But it’s fair to wonder if we focus primarily on these direct effects just because they’re the easiest to see and understand. On a planet far from any type of thermodynamic equilibrium, dominated by rich geophysical and biological feedbacks, the most serious consequences of warming may be indirect. Many are only beginning to appear.

For example, scientists are closely monitoring ongoing and accelerating changes in the timing of key biological events – the spring flowering of trees, nest building and egg hatch, leaf color change at ‘fall. Early evidence suggests that many are not changing fast enough to keep pace with current warming. It is also possible that such changes could disrupt the delicate biological coordination of the activities of millions of interacting and interdependent species on which the world’s ecosystems depend. These effects are much more subtle and unpredictable than the direct consequences of a warmer Earth, but can be much more significant.

Many of us see global warming as something difficult to detect, evident only in sophisticated estimates of global average sea surface temperatures, atmospheric CO2 concentrations, or fine statistical patterns of extreme atmospheric events. A whole science of attribution now focuses on identifying individual events that can be linked to warming in a statistically significant way. Yet the warming is also evident in other changes happening literally all around us.

Phenology is the technical term for the study of the timing of key events in biological life cycles, such as flowering, first flights, egg laying, and migrations. In Kyoto, Japan, records of the first cherry blossoms date back to the year 812. These data, and a variety of other records from around the world (Y. Speed ​​et al., Nat. Air conditioning To change 12, 300–302; 2022) indicate that the dates of flowering and first leaf of plants changed little during the 19th century, then started to progress between 1900 and 1950 following the rise in global temperatures. Phenological changes have accelerated since then, advancing up to 30 days in some places. In 2021, Kyoto cherries bloomed on the first date seen in 1,200 years.

It’s no surprise, of course, that nature reacts to rising temperatures – species have to adapt, changing their behavior to adapt to new conditions. Do they adapt fast enough? Worryingly, some studies suggest that many are not.

A recent study (Y. Song et al., AGU Adv. 2, e2021AV000431; 2021) examined the rate of phenological change for plant species in mid- and high-latitude regions of the Northern Hemisphere, comparing observed changes to local temperature changes over the past 30 years. The study used remote sensing to look at many species together, recording changes in greening dates in spring and plant senescence in autumn. The study found a significant lag between plant responses and temperature change patterns, with most plants lagging behind the pace of recent warming, with some even shifting their pace in the wrong direction. The study also found that these shifts were more pronounced in landscapes degraded by agriculture and other human activities.

The cause of this inability to respond is not entirely clear, but how plants respond to temperature changes is not straightforward. It might seem obvious that higher temperatures would always lead to earlier growth, but some plants need sufficient cooling in winter to prepare for growth, and higher temperatures can ruin their preparation. There is also another possibility, because species react not only directly to temperatures, but also to the changing activities of other species on which they depend. Therefore, species in undisturbed regions may react faster because they are exposed to more cues about the changing environment. In contrast, slower-responding plants in areas degraded by human impacts may lag behind because they lack access to normal signals that would alert them to ongoing changes.

Phenology is not only a matter of time, but also of space, as plants and animals can adapt to a changing environment by changing location, although plants obviously do so more slowly than animals. This is another aspect related to nature’s indirect responses to climate change, and which should be of considerable concern to us. Ecologists are also seeing plenty of evidence that many species are gradually drifting poleward as the planet warms. A consequence of this (CJ Carslon et al., Nature https://doi.org/10.1038/s41586-022-04788-w; 2022) is the likely mixing in habitats of species that have never encountered each other before.

Of course, migratory species carry their parasites and pathogens with them, creating myriad opportunities for pathogens to jump from host to host, finding new reservoirs for further transmission and evolution. With approximately 10,000 distinct viruses capable of infecting humans currently circulating in wild animal populations, we can expect the natural migrations of species in response to rising temperatures to create conditions for many future pandemics, some on the scale of the COVID-19 pandemic. Carlson et al. projected likely geographic shifts in the range of some 3,139 mammal species in response to warming, and in combination with expected changes in land use. Their analysis predicts more than 4,000 probable cross-species virus transmissions, many of which have a good chance of spreading to humans.

Global warming is of course going to cause things like more frequent and severe floods, fires and droughts, sea level rise, and so on. But viewing these direct consequences of higher temperatures as the most problematic aspects of warming can only reflect our collective lack of imagination. Far more significant may be the indirect consequences, as the entire ecosystem that surrounds and supports us begins to change in unpredictable ways.

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Correspondence with Mark Buchanan.

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Buchanan, M. This is a climate for early bloomers.
Nat. Phys. 18 years old, 607 (2022). https://doi.org/10.1038/s41567-022-01632-w

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