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Tracking Change, Predicting Trouble//

By century’s end, Arctic will feel like US South


Winter is getting warmer, spring is coming earlier, and plants are enjoying an extended growing season in northern areas. But that is not good news.

“It’s the initial gold rush,” says Ranga Myneni, a College of Arts & Sciences professor of earth and environment, but what will follow will not be pleasant. As vegetation flourishes, it could draw down the water supply, bringing on drought, insect infestations, and forest fires. What was once green, lush land could become brown and barren.

In an article published in March in Nature Climate Change, Myneni and 21 collaborators describe how seasonal temperatures and vegetation north of the US-Canada border have shifted over the past 30 years, to what is typically experienced 4 to 7 degrees latitude to the south. Should global warming continue at its current pace, Bruce Anderson, a CAS associate professor of earth and environment, who worked with Myneni on the paper, predicts a further latitudinal shift of as much as 20 degrees south by the end of the century. That means arctic and boreal regions of Canada would look and feel much more like the southern United States.

Myneni has been sounding the alarm on rising temperatures and increased vegetation in these regions for decades, but with this article he hopes to push the envelope by framing change in terms of seasonal shifts.

Seasons are determined by two factors: the Earth’s tilt, and the planet’s orbit of the sun. Both change only in small, barely perceptible increments over tens of thousands of years. But, Myneni says, “if you change the solar radiation distribution on our planet, you can change the climatic character of the seasons—most importantly, the temperature difference between winter and summer.” So, he asks, are seasons changing because of a shift in solar radiation, or could it be something else?

The work of American scientist Charles David Keeling helps answer that question. Every year from 1958 until his death in 2005, Keeling measured the level of carbon dioxide in the atmosphere from points in Hawaii and Alaska. His data showed that levels of the gas have been increasing and that they fluctuate in an annual cycle as plants absorb carbon dioxide and release oxygen (or photosynthesize) during the northern hemisphere’s growing season, and reverse modes (or respire) during the dormant season. Keeling noticed that swings in this cycle, now called the Keeling Curve, were getting larger and were most pronounced during photosynthetic periods. More photosynthesis meant more green, leafy vegetation around the globe. Myneni compared a decade of satellite data about greenness with carbon dioxide levels that Keeling had been plotting over time. The two data sets overlaid nicely.

In 1997, Myneni published his findings in Nature, concluding that plant growth in the northern hemisphere had increased by as much as 10 percent over the previous decade, that the increase was driven by a warmer growing season, and that spring was arriving earlier.

Myneni thought the rising levels of carbon dioxide and other greenhouse gases like methane were absorbing thermal radiation reflected from the Earth, preventing it from escaping the atmosphere. The sum effect is a positive feedback loop that continues to warm the Earth, shifting seasonal temperatures.

He noticed that winter temperatures were increasing faster than summer temperatures, leading to earlier springs and a longer growing season. Compton Tucker, a senior scientist at the Goddard Space Flight Center and another contributor to the Nature Climate Change paper, says one-third of the northern landscape—a patch roughly the size of the United States—shows vigorously productive vegetation similar to what’s typical farther south. In May, scientists at the National Oceanic and Atmospheric Administration reported that the level of carbon dioxide in the atmosphere had reached an average daily level above 400 parts per million, higher than it has been for at least three million years, long before humans evolved.

All of this hardly means “the tropics are approaching,” Myneni says. “There’s no way mangoes will grow in the Arctic.” It does mean that global migration patterns, water supplies, and relations among plant and animal species will alter radically—threatening the existence of cold-tolerant plants and animals that depend upon them.

Anderson points out that as permafrost thaws in the Canadian tundra, greater amounts of organic matter will decompose, releasing additional methane into the atmosphere and adding to the greenhouse effect. “Where we go from here is entirely within our control,” he says. That’s because the biggest uncertainty is not how our atmosphere will respond to more greenhouse gas concentrations, he says, but how much humans can—or will—control carbon dioxide emissions.

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