Decoupling of seasonal events will have unforeseen consequences
By Summit Voice
SUMMIT COUNTY — Global warming is reshaping the Arctic on vast scale by reducing the temperature differences between seasons and spurring vast areas of new plant growth on more than a third of the northern landscape. The changes cover about 9 million square kilometers, where the Arctic is greening up dramatically, according to a new study published in the journal Natural Climate Change.
“The … increased greenness in the Arctic … is visible on the ground as an increasing abundance of tall shrubs and tree incursions in several locations all over the circumpolar Arctic,” said study co-author Prof. Terry Callaghan of the Royal Swedish Academy of Sciences and the University of Sheffield, UK. He said greening in the adjacent boreal areas is much less conspicuous in North America than in Eurasia.
Much of the change has happened in just the past 30 years, said the scientists from seven countries, explaining that temperature over the northern land mass has increased at different rates during the four seasons, causing a reduction in temperature and vegetation seasonality in this area. In other words, the temperature and vegetation at northern latitudes increasingly resembles those found several degrees of latitude farther south as recently as 30 years ago.
“These changes will affect local residents through changes in provisioning ecosystem services such as timber and traditional foods,” said Prof. Bruce Forbes, University of Lapland, Rovaniemi, Finland.
The NASA-funded studyuses improved ground and satellite data to explore the relationship between changes in temperature and vegetation productivity in northern latitudes.
“A greenhouse effect initiated by increased atmospheric concentration of heat-trapping gasses—such as water vapor, carbon dioxide and methane—causes the Earth’s surface and nearby air to warm,” said Prof. Ranga Myneni, with the Department of Earth and Environment at Boston University. “The warming reduces the extent of polar sea ice and snow cover on the large land mass that surrounds the Arctic ocean … This sets in motion a cycle of positive reinforcement between warming and loss of sea ice and snow cover, thus amplifying the base greenhouse effect,” he said.
The colder seasons are warming faster than summer, leading to a longer ground-thaw season that increases the total amount of heat available for plant growth.
“This created during the past 30 years large patches of vigorously productive vegetation totaling more than a third of the northern landscape (an area about the size of the U.S.) resembling the vegetation that occurs further to the south,” said Dr. Compton Tucker, Senior Scientist, NASA’s Goddard Space Flight Center, Greenbelt, Maryland.
To standardize their findings, the researchers used latitude as a yardstick to quantify changes over time.
“Arctic plant growth during the early-1980s reference period equaled that of lands north of 64 degrees north. Today, just 30 years later, it equals that of lands above 57 degrees north—a reduction in vegetation seasonality of about seven degrees south in latitude,” said co-author Prof. Terry Chapin, Professor Emeritus, University of Alaska, Fairbanks.
The changes may lead to a decoupling of growing season warmth and vegetation productivity in some parts of the North, said Hans Tømmervik, a senior researcher with the Norwegian Institute for Nature Research in Tromsø. Other consequences from the amplified Arctic greenhouse effect could include ermafrost thawing, frequent forest fires, outbreak of pest infestations, and summertime droughts, Tømmervik said.
“The way of life of many organisms on Earth is tightly linked to seasonal changes in temperature and availability of food, and all food on land comes first from plants,” said Dr. Scott Goetz, deputy director and senior scientist with the Woods Hole Research Center, Falmouth, USA. “Think of migration of birds to the Arctic in the summer and hibernation of bears in the winter: Any significant alterations to temperature and vegetation seasonality are likely to impact life not only in the north but elsewhere in ways that we do not yet know.”