The planet is undergoing one of the biggest changes in climate since the dinosaurs went extinct, but what could be of even more concern to humans, plants and animals is the speed of change.
Climate scientists at Stanford University warn that the likely rate of change over the next century will be at least 10 times faster than any climate change in the past 65 million years.
If the trend continues at its current rapid pace, it will place significant stress on terrestrial ecosystems around the world, and many species will need to make behavioral, evolutionary, or geographic adaptations to survive.
Although some of the changes that the planet will experience in the coming decades are already "built into the system", how the climate looks in the late 21st century will largely depend on how humans respond.
The findings come from a review of climate research by Noah Diffenbaugh, associate professor of environmental earth system sciences, and Chris Field, professor of biology and environmental earth system science and director of the Department of Global Ecology at the Carnegie Institution.
The work is part of a special report on climate change in the current issue of the journal Science.
Professor Diffenbaugh and Professor Field, both Stanford Woods Institute for the Environment fellows, conducted a specific but comprehensive review of the scientific literature on aspects of climate change that can affect ecosystems, and investigated how observations and Recent projections for the next century with past events in Earth's history.
For example, the planet experienced a temperature rise of five degrees Celsius 20,000 years ago, when Earth emerged from the last ice age. This is a comparable change to the high-end projections for warming during the 20th and 21st centuries.
The geological record shows that 20,000 years ago, as the ice sheet that covered much of North America receded northward, plants and animals recolonized areas that had been under ice. As the weather continued to warm, those plants and animals moved north to colder climates.
"We know from past changes that ecosystems have responded to some degrees of global temperature change over thousands of years," said Professor Diffenbaugh.
“But the unprecedented trajectory we are on now is forcing that change to happen for decades. That is an order of magnitude faster, and we are already seeing that some species are challenged by that rate of change ”, explained the scientist.
Some of the strongest evidence for how the global climate system responds to high levels of carbon dioxide comes from paleoclimatic studies.
Fifty-five million years ago, carbon dioxide in the atmosphere rose to a level comparable to today. The Arctic Ocean was ice-free in the summer, and the nearby land was warm enough to support alligators and palm trees.
Diffenbaugh argued that “there are two key differences for ecosystems in the coming decades compared to the geological past. One is the rapid pace of modern climate change. The other is that today there are multiple human stressors that were not present 55 million years ago, such as urbanization and air and water pollution ”.
Professor Diffenbaugh and Professor Field also reviewed the results of two dozen climate models to describe possible climate outcomes from the present to the end of the century.
In general, extreme weather events, such as heat waves and heavy rains, are expected to become more severe and frequent.
For example, the researchers note that, with continued greenhouse gas emissions at the upper end of the scenarios, annual temperatures in North America, Europe, and East Asia will increase by two to four degrees Celsius by 2046-2065.
With that amount of warming, the hottest summer in the past 20 years is expected to occur every two years, or even more often.
By the end of the century, if current greenhouse gas emissions are not controlled, temperatures in the Northern Hemisphere will drift by 5.0 ° C to 6.0 ° C higher than current averages. In this case, the hottest summer in the last 20 years becomes the new annual norm.
"It is not easy to intuit the exact impact of the annual warming of temperatures by 6.0 ° C, but this would present a novel climate for most land areas," said Professor Diffenbaugh.
"Given the impact these types of seasons currently have on terrestrial forests, agriculture and human health, we are likely to see considerable stress due to extreme heat conditions," he added.
According to Diffenbaugh, the variety of climate projections offered in the report could inform decision-makers about the risks that different levels of climate change pose to ecosystems.
"There is no doubt that a climate in which each summer is warmer than the warmest in the last 20 years poses real risks to ecosystems around the world," said the scientist.
"However, there are opportunities to reduce these risks, while ensuring access to the benefits of energy consumption," he closed.