Climate Change
Wednesday, 27 March 2013 10:37

When green is not good

WEBSITE: Amplified greenhouse effect shifts northern latitudes’ growing seasons, according to a new study published in the journal Nature Climate Change.

Vegetation growth at the earth’s northern latitudes increasingly resembles lusher latitudes to the south, according to a National Aeronautics Space Administration (Nasa)-funded study based on a 30-year record of land surface and newly-improved satellite data sets. The study was published on 10 March in the journal Nature Climate Change.

An international team of university and Nasa scientists examined the relationship between changes in surface temperature and vegetation growth from 45º north latitude to the Arctic Ocean. Results show temperature and vegetation growth at northern latitudes now resemble these found 4º to 6º of latitude further south as recently as 1982.

Temperature hikes

“Higher northern latitudes are getting warmer, Arctic sea ice and the duration of snow cover are diminishing, the growing season is getting longer and plants are growing more,” said Ranga Myneni of Boston University’s Department of Earth and Environment. “In the north’s Arctic and boreal areas, the characteristics of the seasons are changing, leading to great disruptions for plants and related ecosystems.”

Quantification
Myneni and colleagues used satellite data to quantify vegetation changes at different latitudes from 1982 to 2011. Data used in this study came from the National Oceanic and Atmospheric Association’s (NOAA) advanced very high resolution radiometers (AVHRR) onboard a series of polar-orbiting satellites and Nasa’s Moderate Resolution Imaging Spectroradiometer (Modis) instruments on the Terra and Aqua satellites.

A changing landscape
As a result of enhanced warming and a longer growing season, large patches of vigorously-productive vegetation now span a third of the northern landscape, or more than 9-million km2. That is an area about equal to the contiguous United States. This landscape resembles what was found 400km to 700km to the south in 1982.

Taking green to the next level
The Arctic’s greenness is visible on the ground as an increasing abundance of tall shrubs and trees in locations all over the circumpolar Arctic. Greening in the adjacent boreal areas is more pronounced in Eurasia than in North America.

An amplified greenhouse effect is driving the changes, according to Myneni. Increased concentrations of heat-trapping gasses, such as water vapour, carbon dioxide and methane, cause the earth’s surface, ocean and lower atmosphere to warm. Warming reduces the extent of polar sea ice and snow cover, and, in turn, the darker ocean and land surfaces absorb more solar energy, thus further heating the air above them.

“This sets in motion a cycle of positive reinforcement between warming and loss of sea ice and snow cover, which we call the amplified greenhouse effect,” Myneni said. “The greenhouse effect could be further amplified in the future as soils in the north thaw, releasing potentially significant amounts of carbon dioxide and methane.”

The future
To find out what is in store for future decades, the team analysed 17 climate models. These models show that increased temperatures in Arctic and boreal regions would be the equivalent of a 20-degree latitude shift by the end of this century relative to a period of comparison from 1951-1980.

However, researchers say plant growth in the north may not continue on its current trajectory. The ramifications of an amplified greenhouse effect, such as frequent forest fires, outbreak of pest infestations and summertime droughts, may slow plant growth.

Also, warmer temperatures alone in the boreal zone do not guarantee more plant growth, which also depends on the availability of water and sunlight.

“Satellite data identifies areas in the boreal zone that are warmer and dryer, and other areas that are warmer and wetter,” said co-author Ramakrishna Nemani of Nasa’s Ames Research Centre in Moffett Field, California. “Only the warmer and wetter areas support more growth.”

“We found more plant growth in the boreal zone from 1982 to 1992 than from 1992 to 2011, because water limitations were encountered in the later two decades of our study,” said co-author Sangram Ganguly of the Bay Area Environmental Research Institute and Nasa Ames.

Data, results and computer codes from this study will be made available on Nasa Earth Exchange (NEX), a collaborative supercomputing facility at Ames Research Centre. NEX is designed to bring scientists together with data, models and computing resources to accelerate research and innovation and provide transparency.

Full acknowledgement and thanks are given to www.nasa.gov for providing the information to write this article.