by Sara Rimer, Boston University
March 21, 2013 (TSR) – As snow and ice cover shrink in northern latitudes, researchers say temperatures and vegetation increasingly resemble those found farther south.
The findings, published in the journal Natural Climate Change, are based on newly improved ground and satellite data sets.
“As a result of the enhanced warming over a longer ground-thaw season, the total amount of heat available for plant growth in these northern latitudes is increasing,” says Compton Tucker, senior scientist at NASA’s Goddard Space Flight Center.
“This created during the past 30 years large patches of vigorously productive vegetation, totaling more than a third of the northern landscape—over 9 million square kilometers, which is roughly about the area of the USA—resembling the vegetation that occurs further to the south,” adds Tucker.
On the amplified greenhouse effect, Ranga Myneni, professor of earth and environment at Boston University and lead co-author of the study, says, “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.
“The warming reduces the extent of polar sea ice and snow cover on the large land mass that surrounds the Arctic ocean, thereby increasing the amount of solar energy absorbed by the no longer energy-reflecting surface.
“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.”
Liang Xu, a Boston University doctoral student and lead co-author of the study, says the increased warming in the area roughly above the Canada-USA border is “reducing temperature seasonality over time because the colder seasons are warming more rapidly than the summer.”
Past 30 years
The international team of scientists measured seasonality changes using latitude as a yardstick. They first defined reference latitudinal profiles for the quantities being observed and then quantified changes in them over time as shifts along these profiles.
“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,” says co-author Terry Chapin, professor emeritus at the University of Alaska, Fairbanks.
“This manner of analyses suggested a decline in temperature and vegetation seasonality of about four to seven degrees of latitude during the past 30 years,” says co-author Eugenie Euskirchen, research professor at the University of Alaska, Fairbanks.
“The reduction of vegetation seasonality, resulting in 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,” says co-author Terry Callaghan, a professor at the Royal Swedish Academy of Sciences and the University of Sheffield. He notes that the greening in the adjacent Boreal areas is much less conspicuous in North America than in Eurasia.
Forecasting the future
A key finding of this study is an accelerating greening rate in the Arctic and a decelerating rate in the boreal region, despite a nearly constant rate of temperature seasonality diminishment in these regions over the past 30 years.
“This may portend a decoupling between growing season warmth and vegetation productivity in some parts of the North as the ramifications of amplified greenhouse effect—including permafrost thawing, frequent forest fires, outbreak of pest infestations, and summertime droughts—come in to play,” says co-author Hans Tømmervik, a senior researcher at the Norwegian Institute for Nature Research.
According to the authors, the future looks troubling: based on analysis of 17 state-of-the-art climate model simulations, decline of temperature seasonality in these regions could be more than 20 degrees in latitude by the end of this century relative to the 1951-1980 reference period.
The projected temperature seasonality decline by these models for the 2001-2010 decade is actually less than the observed decline.
“Since we don’t know the actual trajectory of atmospheric concentration of various agents capable of forcing a change in climate, long-term projections should be interpreted cautiously,” says co-author Bruce Anderson, professor of earth and environment at Boston University.
“These changes will affect local residents through changes in provisioning ecosystem services such as timber and traditional foods,” says Bruce Forbes, a research professor at the University of Lapland.
They could also affect the global community through changes in regulatory ecosystem services relating to emissions of greenhouse gases.
“The soils in the northern land mass potentially can release significant amounts of greenhouse gases which are currently locked up in the permanently frozen ground. Any large-scale, deep-thawing of these soils has the potential to further amplify the greenhouse effect,” says co-author Philippe Ciais, associate director of the Laboratory of Climate and Environmental Science in Paris, France.
“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,” says Scott Goetz, deputy director and senior scientist at the Woods Hole Research Center.
“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.”
NASA funded the study, which was authored by 21 researchers from 17 institutions in seven countries.