Boreal forests were supposed to soften the blow of climate change, soaking up more carbon dioxide from the atmosphere as they expand north into the warming Arctic. But as a new study suggests, these forests are more likely to make like a tree and leave.
Rising temperatures will help boreal woodlands move north, according to research by Lawrence Berkeley National Laboratory, although not the way scientists had expected. Rather than expanding north, the forests will shift north — and relinquish more trapped CO2 back into the atmosphere than most current climate models predict.
"I found that the boreal ecosystems ringing the globe will be pushed north and replaced in their current location by what's currently to their south," study author Charles Koven says in a statement. "In some places that will be forest, but in other places it will be grassland."
That's an important detail, Koven adds, since forests and grasslands behave differently in the context of climate change. Both can store large amounts of carbon absorbed from the air, but grasslands accumulate it much less quickly than dying forests release it.
Berkeley Lab has produced a series of animations to illustrate how climates will move around as temperatures rise. The one below shows North American prairie shifting north to fill the void left by boreal forests, which will move deeper into Canada and Alaska:
These forecasts use a new method of measuring global warming's influence on various climates, based on the premise that it forces warm climates to "flow" toward cooler areas, eventually making it warmer everywhere. This approach allowed Koven to predict where a climate will go in the future and where its successor will come from.
He applied the technique to 21 different climate models, using simulations of a middle-of-the-road scenario that assumes a range of warming between 1 and 2.6 degrees Celsius (1.8 to 4.7 Fahrenheit) above a 1986-2005 baseline. Each model already divides the planet into climate zones that cover dozens to hundreds of square miles each, so Koven identified which of those have neighboring zones with a similar climate in terms of average monthly temperature and precipitation. He then calculated the speed at which one zone will supplant its matching neighbor over the next 80 years, as well as how that transition will affect the carbon stored by plants growing in the original zone's old climate.
Overall, he found that climates flow toward the poles and up mountains. But the most extreme ecological exodus will apparently unfold at high latitudes, where boreal forests must scramble to keep up with their fleeing climates. A forest in Alberta, Canada, for example, will need to move 100 miles north in the next 90 years to maintain its current climate, before another climate that's now 100 miles to the south takes its place.
The problem is that few forests can adapt that quickly, Koven explains, so they'll likely be stressed and weakened in the short term. That could make them more vulnerable to natural threats like fire, drought and insects, setting the stage for stronger southern vegetation — including grasslands — to overrun their historical homeland.
"Most Earth system models don't predict this, which means they overestimate the amount of carbon that high-latitude vegetation will store in the future," he adds. "[Their] approach misses the fact that the whole forest might shift to a different place."
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