Previous periods of rapid warming millions of years ago drastically altered plants and forests on Earth. Now, scientists see the beginnings of a more sudden, disruptive rearrangement of the world’s flora — a trend that will intensify if greenhouse gas emissions are not reined in.
Some 56 million years ago, just after the Paleocene epoch gave way to the Eocene, the world suddenly warmed. Scientists continue to debate the ultimate cause of the warming, but they agree on its proximate cause: A huge burst of carbon dioxide entered the atmosphere, raising Earth’s average temperature by 7 to 14 degrees Fahrenheit. The Paleocene-Eocene Thermal Maximum (PETM), as this event is known, is “the best geologic analog” for modern anthropogenic climate change, said University of Wyoming paleobotanist Ellen Currano.
She studies how the PETM’s sudden warmth affected plants. Darwin famously compared the fossil record to a tattered book missing most of its pages and with all but a few lines obscured. The PETM, which lasted roughly 200,000 years, bears out the analogy. Wyoming’s Bighorn Basin is the only place on Earth where scientists have found plant macrofossils (visible to the naked eye, that is) that date to the PETM. The fossil leaves that Currano and her colleagues have found there paint a vivid portrait.
Before the PETM, she said, there lived a forest of cypress, sycamores, alders, dogwoods, walnuts and other species, all of them suggestive of a temperate climate — a bit swampy, perhaps not unlike that of the southeastern United States. Then, with the onset of the PETM, that forest disappeared, its trees vanishing from the fossil record. “During the climate event you have a nearly complete turnover of plants,” Currano said. A new forest appeared, this one consisting of palms, heat-tolerant members of the bean family, and other plants evocative of the semi-arid tropics.
It is a story repeated throughout the fossil record: When the climate changes, so does the arrangement of the world’s plants. Species move back and forth toward the poles, up and downslope. Some species grow more common, others rarer. Species arrange themselves together in new combinations. The fossil record reveals plants for what they are, as mobile beings. For plant species, migrating in response to climate change is often a matter of survival.
Warmth-loving plants are growing more common, from the middle of the Amazon to the middle of Nebraska.
As human-generated greenhouse gas emissions cause the world to rapidly warm, this movement is once again under way. Scientists have observed plants shifting toward the poles and upslope. They’ve noted old ecosystems suddenly replaced by new ones, often in the wake of fire, insect outbreaks, drought or other disturbances. They’ve observed an increase in the number of trees dying and watched as a growing number of the world’s biggest and oldest plants, including the baobabs of Africa and the cedars of Lebanon, have succumbed. Just this month, scientists announced that the Castle Fire, which burned through California’s Sierra Nevada last year, singlehandedly killed off more than 10 percent of the world’s mature giant sequoias.
So far, many of these changes are subtle, seemingly unrelated to one another, but they are all facets of the same global phenomenon — one that scientists say is likely to grow far more apparent in the decades to come.
The climate is currently warming at least 10 times faster than it did at the onset of the PETM. Under its worst-case scenario, the Intergovernmental Panel on Climate Change projects that, over the next 100 to 150 years, Earth’s average temperature could rise by roughly the same amount as it did during the PETM. Dramatic vegetational shifts could arrive not in a matter of centuries or millennia, but decades; a 2019 study, for example, projected that Alaska’s vast interior forests will shift from being dominated by conifers to being dominated by broadleaf trees as soon as the middle of this century.
Scientists debate what this floral rearrangement will look like. In some places, it may take place quietly and be easily ignored. In others, though, it could be one of the changing climate’s most consequential and disruptive effects. “There’s a whole lot more of this we can expect over the next decades,” said University of Wisconsin-Madison paleoecologist Jack Williams. “When people talk about wildfires out West, about species moving upslope — to me, this is just the beginning.”