Global warming is not just difficult for humans and animals. It also destroys plants.
The movement of spring shows the awakening of nature. Prompted by the warming air and the stronger sunlight, the flowers unfold on the cherry trees and impatient green buds burst from the horse chestnuts. Little hope returns as the bees hum and the birds build nests. This year it happened a little earlier – and the reason is not difficult to find.
In Washington, the city’s famous cherry trees – originals donated by Japan in 1912 – reached their peak of flowering on March 21, less than a century ago. In Kyoto, where the cousins of these trees live, records show that the first blooms progressed by a week in the last century, along with rising temperatures of more than 5 degrees Fahrenheit. Planetary warming is leading to a similar trend worldwide, displacing the time not only of the first leaves and flowers, but also of bird migrations and egg hatching. These changes have accelerated over the last 20 years.
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We often think of global warming as something that only becomes apparent through difficult scientific measurements of CO2 levels in the atmosphere or average sea surface temperatures. But the signs of warming are all around us in distortions of the historical rhythms of the natural world, adding ominous shades to the joyful spring greenery. All of these changes reflect nature under increasing pressure – and have unpredictable consequences for our well-being and the resilience of global ecosystems in the coming decades.
The study of the time of important biological events is known as “phenology” and much of what we know about it comes from careful observations made over the centuries. The longest time series recorded anywhere is for cherry trees in Kyoto; remarkably, it dates back to 812. These data and a number of other records – including data in the United Kingdom covering more than 250 years – show that the dates of flowering and the first leaf of the plant remained fairly stable in the 19th century. this, together with rising planetary temperatures, began to creep earlier in the first half of the 20th century.
Since then, change has accelerated, while showing significant regional differences: spring phenology has advanced by six days in China over the past 35 years and by 30 days in Switzerland. In Kyoto in 2021, cherry trees bloom at the earliest date of 1,200 years of careful record keeping.
Adapting to the pace and unpredictability of global warming is difficult – not only for animals and humans, but also for plants. A recent study examined changes in the beginning and end of the growing season in the northern hemisphere over the past 30 years, comparing changes in temperature with plant reactions. Researchers have found that most plants lag behind the pace of recent warming. Some have even shifted their time in the wrong direction – they bloom later, not earlier. And some fell asleep earlier in the fall, although you’d think warmer autumns could extend their growing season.
These discrepancies are more pronounced in landscapes dominated by human activities such as intensive agriculture. It is not clear why, but here is a possible reason: species change their behavior not only in direct response to climate variables – temperatures, precipitation patterns and so on – but also in response to changing activities of other species with which they interact. Faster species responses in undisturbed regions could reflect exposure of these species to a wider range of changing environment signals coming from many other plant species. In contrast, slow-growing plants in areas degraded by humans can adapt slowly because they do not receive signals from other plants.
Whatever the reason, the lagging response of plants is worrying. The consequences can be unpleasant for both nature and humanity. It may not be so important when exactly the flowers bloom or the birds hatch, but the integrity of the natural world depends on millions of such events, taking place in delicate coordination and synchronicity. When flowers bloom, it affects when bees can pollinate them, which later determines when birds and other animals find fruit to raise their young, which potentially provide food for other animals – including us. In nature, nothing happens in isolation.
How will global warming disrupt these delicate relationships, accelerate species extinction and increase the cost of human agriculture? Scientists do not know. No one knows. We are conducting a large-scale and dangerous experiment, and it would be much, much better if we never had to find out.
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