2008-05-17

Human-caused climate change has impacted a wide range of Earth's natural systems, from permafrost thawing to plants blooming earlier across Europe to lakes declining in productivity in Africa.

Cynthia Rosenzweig of NASA's Goddard Institute for Space Science in New York and scientists at 10 other institutions have linked physical and biological impacts since 1970 with rises in temperatures during that period, including changes to physical systems, such as glaciers shrinking, permafrost melting, and lakes and rivers warming. Impacts also included changes to biological systems, such as leaves unfolding and flowers blooming earlier in the spring, birds arriving earlier during migration periods, and ranges of plant and animal species moving toward the poles and higher in elevation. In aquatic environments such as oceans, lakes, and rivers, plankton and fish are shifting from cold-adapted to warm-adapted communities.

"This is the first study to link global temperature data sets, climate model results, and observed changes in a broad range of physical and biological systems to show the link between humans, climate, and impacts," said Rosenzweig, lead author of the study.

Rosenzweig and colleagues also found that the link between human-caused climate change and observed impacts on Earth holds true at the scale of individual continents, particularly in North America, Europe, and Asia.

Photograph of a forest When permafrost melts, the layer of loose soil deepens and trees lose their foundations and tip over. Similar impacts across Earth are likely due to human-caused climate change.

To arrive at the link, the authors built and analyzed a database of more than 29,000 data series pertaining to observed impacts on Earth's natural systems, collected from about 80 studies each with at least 20 years of records between 1970 and 2004.

The team conducted a "joint attribution" study in which they showed, first, that at the global scale, about 90 percent of observed changes in diverse physical and biological systems are consistent with warming. Other driving forces, such as land use change from forest to agriculture, were ruled out as having significant influence on the observed impacts.

Next, the scientists conducted statistical tests and found that the spatial patterns of observed impacts closely match temperature trends across the globe, to a degree beyond what can be attributed to natural variability. So, the team concluded that observed global-scale impacts are very likely due to human-caused warming.

"Humans are influencing climate through increasing greenhouse gas emissions and the warming is causing impacts on physical and biological systems that are now attributable at the global scale and in North America, Europe, and Asia," said Rosenzweig.

An unexpected consequence of rising temperatures may be its effect on long-dead prehistoric life.

For thousands of years animal waste, and other organic matter left behind on the Arctic tundra, have been sealed off from the environment by permafrost. Now climate change is melting the permafrost and freeing mass quantities of prehistoric “ooze” from its state of suspended animation.

Russian scientist, Sergei Zimov, has been studying climate change in Russia's Arctic for 30 years now. He is worried that as this organic matter becomes exposed to the air it will drastically accelerate global warming predictions even beyond some of the most pessimistic forecasts.

"This will lead to a type of global warming which will be impossible to stop," he said.

According to Zimov, when the organic matter left behind by mammoths and other wildlife is exposed to the air by the thawing permafrost, microbes that have been dormant for thousands of years will spring back into action. They’ll begin once again to emit carbon dioxide and methane gas as a by-product. Zimov says thought the microbes are tiny, they will start emitting these gases in enormous quantities simply because there will be a lot of them.

Yakutia is a region in the north-eastern corner of Siberia, where a belt of permafrost contains the mammoth-era soil. It covers an area roughly the size of France and Germany combined. There is even more of it elsewhere in Siberia.

"The deposits of organic matter in these soils are so gigantic that they dwarf global oil reserves," Zimov said. U.S. government statistics show mankind emits about 7 billion tons of carbon a year."Permafrost areas hold 500 billion tons of carbon, which can fast turn into greenhouse gases," Zimov added. "If you don't stop emissions of greenhouse gases into the atmosphere ... the Kyoto Protocol (an international pact aimed at reducing greenhouse emissions) will seem like childish prattle."

On other continents, including Africa, South America, and Australia, documentation of observed changes in physical and biological systems is still sparse despite warming trends attributable to human causes. The authors concluded that environmental systems on these continents need additional research, especially in tropical and subtropical areas where there is a lack of impact data and published studies.

The study, published May 15 in the journal Nature, concludes that human-caused warming is resulting in a broad range of impacts across the globe.

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