Amplified Greenhouse Effect Shifts Growing Seasons In The North
March 11, 2013

Researchers Analyze Amplified Greenhouse Effect’s Impact On Seasonality

redOrbit Staff & Wire Reports - Your Universe Online

A warming climate and reduced temperature seasonality has resulted in increased vegetation production in northern latitudes, which have begun to resemble the conditions found in regions located several degrees of latitude further south as recently as three decades ago, an international team of researchers have discovered.

Writing in the journal Nature Climate Change, 21 authors from 17 different institutions in seven countries report that diminishing snow and ice in the northern latitudes has caused temperatures in the region to increase at different rates during each of the four seasons.

Essentially, the amplified warming in the circumpolar area in North America is making it so that the typically colder months are warming more quickly than the hotter summer ones, the NASA-funded study — entitled “Temperature and vegetation seasonality diminishment over northern lands” — asserts.

“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,” Boston University professor and study lead co-author Ranga Myneni said Sunday in a statement. “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.”

“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,” added Dr. Compton Tucker, a senior scientist at the NASA Goddard Space Flight Center in Maryland. “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.”

In their study, the research team used latitude as a measuring stick to gauge changes in seasonality. To start with, they defined reference latitudinal profiles for each of the quantities they were studying. They then tracked changes in them from one year to another as those shifts occurred.

For example, arctic plant growth during the reference period of the early 1980s equaled that of locations north of 64 degrees northern, explained co-author and University of Alaska, Fairbanks professor emeritus Terry Chapin. In 2013, that same plant growth is equal to that of lands above 57 degrees north, meaning that vegetation seasonality has decreased by roughly seven degrees south in latitude.

“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,” co-author Terry Callaghan, a professor at the Royal Swedish Academy of Sciences and the University of Sheffield in the UK, said in a statement.

The study also determined that there is an accelerated greening rate in the Arctic and a decelerating one in the adjacent Boreal areas, even though the temperature seasonality rate in both locations has diminished at a consistent rate over the past 30 years.

That has led one of the researchers, Hans Tømmervik of the Norwegian Institute for Nature Research, to suggest that the phenomenon could indicate a potential “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.”

The authors also paint a bleak picture of the future. According to their analysis of 17 advanced climate model simulations, the diminishment of temperature seasonality in these regions could top 20 degrees in latitude by the end of the century.

However, co-author Bruce Anderson, Professor of Earth and Environment at Boston University warns, “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.”

Image 2 (below): 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