A firefighter watches a controlled burn in Sequoia National Forest in August 2015.Max Whittaker / Reuters

If climate change continues unabated, nearly every ecosystem on the planet would alter dramatically, to the point of becoming an entirely new biome, according to a new paper written by 42 scientists from around the world.

They warn that the changes of the next 200 years could equal—and may likely exceed—those seen over the 10,000 years that ended the last Ice Age. If humanity does not stop emitting greenhouse-gas emissions, the character of the land could metamorphose: Oak forest could become grassland. Evergreen woods could turn deciduous. And, of course, beaches would sink into the sea.

“Anywhere on the globe, the more you change climate, the more likely you are to see major ecological change,” says Stephen Jackson, an author of the report and the director of a climate-adaptation center at the U.S. Geological Survey.

“Having this kind of change occur at such a massive scale in such a short period of time is going to create unprecedented challenges for natural-resource management,” he told me.

The paper, published Thursday in Science, tries to find clues about the world of the future by examining the ecology of the past. Between the peak of the last Ice Age, about 20,000 years ago, and 1800 A.D., the world warmed by between 4 and 7 degrees Celsius. This warming transfigured the landscape: It erased the mile-high plateau of ice that sat on Manhattan Island, for instance. By melting this and other continent-sized ice sheets, that ancient warming—which was caused by minute shifts in the Earth’s orbital path—raised global sea levels by almost 400 feet. If that sounds fun, it could happen again, within the lifetime of babies born today: Earth could experience 4 to 5 degrees Celsius of warming by 2100 if humanity does not slow the emission of heat-trapping gases.

So what did that kind of shift look like that last time?  Jackson and two of his colleagues—Jonathan Overpeck and Connor Nolan—addressed that question to dozens of researchers around the world. They asked them to consult what are called “paleoecological records”: special objects in the Earth that have captured evidence of ecosystems over time. The mud at the bottom of a lake is a paleoecological record, for instance. Every year, plant pollen in the air falls into the lake, laying down minute layers as decades pass. Scientists can examine that pollen under the microscope—identifying it, sometimes, to its individual species—to piece together what an ecosystem looked like thousands of years ago. For this study, researchers looked at local paleoclimatic records from two windows of time: the height of the last Ice Age, 20,000 years ago; and the last moment, roughly 200 years ago, before modern global warming began.

Slowly, over years, those squadrons of researchers around the world sent back their data. A global picture began to take shape: nearly everywhere on Earth where the temperature changed dramatically, and the vegetation wholly transformed.

Consider, for a second, the scale of these changes. Jackson asked me to think of the hot, humid, marshy plain of Washington, D.C.

“If you took a walk through Rock Creek Park today, you’d see almost entirely deciduous hardwood forest—oak trees, and beech, and tulip poplar, and things like that,” he told me. “But if you were to walk through Rock Creek Valley 20,000 years ago, you would be walking through boreal forest.” It would resemble the forests in the far north of Quebec, the mighty evergreen stands of the Canadian Shield.

It’s the same story out west. “Five miles from where I sit is the middle of the Sonoran Desert and Saguaro National Park,” he told me, from his desk in Tuscon. “Today, there’s big saguaro cacti, mesquite trees, ironwood trees. If we were to roll back the calendar 20,000 years, and we went to the same place, we would find a woodland of evergreen trees.”


How Ecosystems Changed Between the Last Ice Age and 1800 A.D.

These two maps depict roughly 20,000 years of ecosystem change, as captured by nearly 600 lake beds, cave burrows, and other natural records around the world. The red points at left show how much ecosystems changed in their species composition: that is, whether one species (such as white oaks) came to be more abundant than another (such as blue spruce). The green points show how ecosystems changed in structure: a dramatic transition from a forest biome to grassland biome. During the transition out of the last Ice Age, only ecosystems in the tropics avoided huge changes in structure. (Nolan, et al / Science)

The same warming-induced changeovers of the land—which transpired over at least five millennia the last time they struck—are now happening again. “Over the next 100 years, we could see temperature changes that are similar to that,” says Connor Nolan, an ecologist at the University of Arizona and an author of the paper.

Just how much would vegetation change worldwide? Nolan and his colleagues found a relationship in their bevy of worldwide records between how much temperatures rose and how significantly ecosystems change. When this relationship is projected forward, they find that Earth’s entire land surface is more than 75 percent likely to switch over its biome entirely.

It’s hard to state what that scale of change would mean for just about everyone who works with or depends on the land. “If you’re a wildlife manager and your ecosystem changes, if you’re a forest manager trying to respond to wildfires, if you’re a water manager who is responsible for converting rainfall estimates into reservoir levels,” Jackson warned, “then the old rules are not necessarily going to apply.”

Future changes may be even more dramatic than those predicted by the paper. That’s because the climate of the past—and the end of the last Ice Age—cannot tell us everything about our future. “It’s a very, very crude analog,” Jackson told me. “The future will not be like the past. Going into a greenhouse world will not be the same—is not the same—as going from the glacial world to the pre-industrial world.”

“But it’s an instructive analog,” he said. “It provides another way of telling us—beside the models and our limited array of ecological observations—it tells us that terrestrial ecosystems are sensitive to temperature change.”

Outside scientists agreed. “The paper’s findings are not surprising, but they are notable because of the approaches the authors took,” said Margaret Frasier, an associate professor of ecology at the University of Wisconsin at Milwaukee, in an email.

Dorothy Peteet, a senior research scientist at NASA, told me that “the wild cards” of modern climate change made it hard to know the shape of that future vegetation change. “The nonlinearity of drought and rainfall,” as well as extensive wildfires or floods, “may affect vegetation greatly,” she said in an email.

“These are notable effects of climate warming we are seeing today ... and they will probably be much more exacerbated in the future,” she added.

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