quinta-feira, julho 31, 2008

Earth Impacts Linked to Human-Caused Climate Change

A new NASA-led study shows that 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. 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. "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. Credit: Jon Ranson 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. Observed impacts included 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. 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. Satellite image of Siberia Impacts from warming are evident in satellite images showing that lakes in Siberia disappearing as the permafrost thaws and lake water drains deeper into the ground. Credit: NASA Earth Observatory > Larger image "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.

Summer Storms Could Mean More Dead Zones

Ocean dead zones are regions where water becomes stripped of its dissolved oxygen. These “hypoxic” (low oxygen) and "anoxic" (no oxygen) conditions can prove lethal for many marine species, changing the biology and chemistry of the ocean. In a few places throughout the world, dead zones occur throughout the year, but summer storms can intensify agricultural runoff, resulting in big phytoplankton blooms that trigger the phenomena. Phytoplankton are critical to life and are particularly important marine organisms. These microscopic aquatic plants are dispersed throughout the world’s oceans. The plants’ distribution is driven by available light, the presence of nutrients, and physical processes like ocean circulation and upwelling. "It’s safe to say, with a few exceptions, that all life in the ocean ultimately depends on phytoplankton for its nutrition," notes NASA oceanographer Gene Feldman, Goddard Space Flight Center, Greenbelt, Md. Under certain conditions, excessive phytoplankton growth can result in a dead zone. Dead zones form when large quantities of organic matter, present from the excessive blooms of phytoplankton at the surface, sink to the bottom. After the organic matter or dead phytoplankton sink, bacteria break the dead or decaying phytoplankton down in a process known as aerobic decomposition, releasing carbon dioxide but absorbing oxygen as they work. The resulting low oxygen conditions can cover expansive areas, killing the oxygen-dependent aquatic species, such as fish, that cannot escape their reach, or driving them out of their habitat. Summer storms intensify the runoff of fertilizers from lawns and farmland, which seep into the rivers and streams that comprise a local watershed and provide a jolt of nutrients to phytoplankton that live along the shore. Excessive nutrients from human activity are one reason many dead zones occur at the mouth of large rivers and in the bays along the shore.