April 10, 2013

A greener Arctic

As higher temperatures create more temperate environments across parts of the Arctic, vegetation is creeping northward. Places like Denali National Park in Alaska may soon see forests encroaching on what are currently tundra biomes. Photo credit: Timothy Wildey (http://www.flickr.com/photos/timothywildey/4964905772/)
If gardening in icy Greenland sounds unbelievable, think again. A chef in Kangerlussuaq has started growing tomatoes, peppers, herbs, and even strawberries for his restaurant, and Greenland’s potato production doubled between 2008 and 2012. These agricultural feats would have been impossible only a hundred years ago. The Arctic has been warming over the past several decades at a rate twice as fast as the rest of the globe. As the region warms, several teams of scientists have been tracking changes in Arctic vegetation, trying to see if higher temperatures mean more green.

Tracking a trend

Scientist Liang Xu and his colleagues studied how green the Arctic has become by looking at several decades of data, from 1982 through 2012. Focusing on the May through September growing season, they compared greenness and temperature across the Northern Hemisphere. They found that the increasing warmth fostered a longer growing season, allowing more types of vegetation to thrive further north. In fact, they found that in some areas, vegetation had crept more than 400 miles north—a dramatic shift in such a short time. Shrubs and trees are not only replacing low-lying tundra across the Arctic’s lower latitudes; they’re thriving in some areas. Another team of scientists, led by Laia Andreu-Hayles, studied white spruce trees along the northern edges of Alaska’s tundra, expecting to find the trees stunted and drought stressed by warmer temperatures. Instead, the trees flourished, growing faster and more vigorously during the longer growing season.

Of the 10 million square miles (26 million square kilometers) of northern vegetated lands, 34 to 41 percent showed increases in plant growth (green and blue), 3 to 5 percent showed decreases in plant growth (orange and red), and 51 to 62 percent showed no changes (yellow) over the past 30 years. Satellite data in this visualization are from the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS). Image credit: NASA's Goddard Space Flight Center Scientific Visualization Studio

Feeding into a feedback

A greener Arctic with longer growing seasons might be great for growing vegetables, but like any dramatic environmental change, there can be far-reaching repercussions. Scientists are only beginning to discern what those might be. For instance, during the long Arctic winters, white snow and ice typically cover the landscape. This bright blanket reflects sunlight and helps keep land and air temperatures lower. A warmer climate has already reduced sea ice extent, and the exposed dark ocean absorbs more heat from the sun. As trees and leafy shrubs start populating the Arctic, many researchers expect the Arctic landscape to also start absorbing more heat. Scientists use the term albedo to refer how much heat or light a surface reflects. Xu said, “If the snow is replaced by vegetation, the albedo will change, and it will absorb more solar radiation. A lower albedo will further amplify the warming.” Scientists also wonder how this vegetation shift may influence the how the Earth stores and releases carbon. They are especially concerned about the release of trapped organic matter in permafrost. A warmer Arctic thaws permafrost, which releases the greenhouse gases methane and carbon dioxide. But a new, leafy canopy may also increase the Arctic’s ability to store carbon. Researchers are only now beginning to investigate whether more vegetation will help offset some of that outgassing from underground.

Warmer and greener?

To study potential scenarios, scientists use climate models to help them analyze possible outcomes. Scientist Richard Pearson and his colleagues used models to look at vegetation shift over the next several decades. They found that at the current rate, vegetation across the Arctic could increase nearly 50 percent by 2050. Trees and shrubs will likely continue shifting north as the lower latitudes of the Arctic become more temperate. Xu and his team also used seventeen different computer climate models to look at future Arctic changes. Although the results indicated that temperatures would continue to increase across the Arctic, Xu pointed out that does not necessarily mean the entire Arctic will continue greening at its current rate. “Unlike temperature, which can increase as greenhouse gases increase, vegetation has hard limits, like solar radiation and precipitation,” Xu said. The Arctic’s long and dark winters may still only afford trees a relatively short growing season each summer. For some species, that may not be sufficient to survive, even if temperatures warm enough to allow them to spread northward. Currently, it is too soon to tell how green the Arctic may become. Scientists are only now beginning to understand the far-reaching implications of this environmental shift.

Related Icelights posts:

Modeling the Arctic climate


Andreu-Hayles, L., R. D’Arrigo, K. J. Anchukaitis, P. S. A. Beck, D. Frank, and S. Goetz. 2011. Varying boreal forest response to Arctic environmental change at the Firth River, Alaska. Environmental Research Letters 6, doi:10.1088/1748-9326/6/045503.

Pearson, R. G., S. J. Phillips, M. M. Loranty, P. S. A. Beck, T. Damoulas, S. J. Knight, and S. Goetz. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate Change doi:10.1038/nclimate1858.

Xu, L., R. B. Myneni, F. S. Chapin III, T. V. Callaghan, J. E. Pinzon, C. J. Tucker, Z. Zhu, J. Bi, P. Ciais, H. Tommervik, E. S. Euskirchen, B. C. Forbes, S. L. Piao, B. T. Anderson, S. Ganguly, R. R. Nemani, S. J. Goetz, P. S. A. Beck, A. G. Bunn, C. Cao, and J. C. Stroeve. 2013. Temperature and vegetation seasonality diminishment over northern lands. Nature Climate Change, doi:10.1038/nclimate1836.