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 AVHRR and MODIS instruments, which contribute to a vegetation index that allows researchers to track changes in plant growth over large areas. (Credit: NASA's Goddard Space Flight Center Scientific Visualization Studio) Enlarge

Mar. 10, 2013 — An international team of 21 authors from 17 institutions in seven countries has just published a study in the journal Natural Climate Change showing that, as the cover of snow and ice in the northern latitudes has diminished in recent years, the 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.

The NASA-funded study, based on newly improved ground and satellite data sets, examines critically the relationship between changes in temperature and vegetation productivity in northern latitudes.

On the amplified greenhouse effect, Prof. Ranga Myneni, Department of Earth and Environment, Boston University and lead co-author 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."

"The amplified warming in the circumpolar area roughly above the Canada-USA border is reducing temperature seasonality over time because the colder seasons are warming more rapidly than the summer," says Liang Xu, a Boston University doctoral student and lead co-author of the study.

"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. 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 km2, which is roughly about the area of the USA — resembling the vegetation that occurs further to the south," says Dr. Compton Tucker, Senior Scientist, NASA's Goddard Space Flight Center, Greenbelt, Maryland.

The authors 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.

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