Arctic ecosystems are getting greener

August 20, 2018

In recent decades, some scientists have noted a surge in Arctic plant life as a symptom of climate change. But without observations showing exactly when and where vegetation has bloomed as the world’s coldest areas get warmer, it’s hard to predict how vegetation will respond to future warming. Now, researchers at UC Berkeley and the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and have developed a new approach that may paint a more accurate picture of our climate’s past—and future.

In a study published online in Nature Climate Change, the researchers used satellite images taken over the past 30 years to track—down to a pixel—the ebb and flow of plant populations in cold areas north of the Equator, such as Alaska, the Arctic region of Canada, the Tibetan Plateau, and the Patagonia region of South America, where winter temperatures are as low as minus 35 degrees Celsius.

The satellite data showed that trees and plants were “greening” more ground as these climates warmed. What they discovered next surprised them.

“We found that the state-of-the-art climate models overestimated how much of the land surface in these areas is actually covered by vegetation,” said first author Trevor Keenan, an assistant professor in the Department of Environmental Science, Policy, and Management, and a faculty scientist in Berkeley Lab’s Earth & Environmental Sciences Area. Furthermore, the satellite data also showed that these models underestimated the temperature at which it is no longer cold enough to stop plants from growing to their maximum potential.

Scientists collaborating through the World Climate Research Program developed these models for CMIP5—the Coupled Model Intercomparison Project Phase 5—to help researchers around the world gain a better understanding of the relationship between carbon emissions and global warming. International consortiums such as the IPCC (International Panel on Climate Change) have also used them to help inform policy decisions. Keenan said that while the CMIP5 models provided researchers with a broad overview of the problem, they do not always accurately represent the important role plants play in reflecting light back into the atmosphere, sending water back into the atmosphere, and absorbing carbon dioxide.

To correct past projections, Keenan and co-author William Riley, a senior scientist in Berkeley Lab’s Earth & Environmental Sciences Area, used satellite data to build a new benchmark estimate of how much greener these cold areas will get as they warm.

Their findings reveal that by the year 2100, only 20 percent of the land surface in the northern hemisphere will still be under the same cold conditions that have been in place there for centuries. The remaining 80 percent will warm, and without snow, arctic wind chills, and frigid winter temperatures dipping below zero degrees, trees can grow faster, and encroach on other trees and plants. “Already, 16% of vegetated land that was limited by temperature three decades ago no longer is. Although the greening might sound like good news, it represents a major disruption to the delicate balance in cold ecosystems,” Keenan said.  

“No one has looked at high-latitude systems from this angle before as they are very complex, but they’re necessary if you want to get a better understanding of how plants’ sensitivity to temperature may affect our ecosystems in the future,” added Riley.

Now that Keenan and Riley have established a standard approach for assessing climate models, they plan to explore how they can use more advanced machine learning techniques to quantify how soil quality, carbon dioxide in the atmosphere, fire frequency, and nutrients in the soil factor overall in the climate equation.

Read the story on the Berkeley Lab website.