Fuente: REALscience
Expuesto el: martes, 31 de julio de 2012 17:23
Autor: Michael Bradbury
Asunto: Human Activity is Melting Arctic Sea Ice Faster than Nature
| One of the big areas of uncertainty in climate science is natural variability. The Earth’s climate is a complex system and one that allows for natural and cyclical shifts from warmer to cooler and back to warm again. Since the beginning of the Industrial Revolution human activity – primarily the burning of fossil fuels that have increased the amount of carbon dioxide in the atmosphere – have begun to add to what was happening naturally already.
Arctic Sea Ice Extents, (L) September 1979, (R) September 2003, Composite Image Courtesy of NASA In the last 40 years we have begun outpacing the natural climate variability and human activity is becoming visible as a primary contributor to climate change. But the question still remains; how much human activity is causing climate change? Scientists from University of Reading and the Japan Agency for Marine Earth Science and Technology (JAMSTEC) believe that human activity has led to much of the loss of Arctic sea ice since the 1970s.
University of Reading Climate Scientist Jonny Day Lead researcher Jonny Day says, “The debate over how much the change observed in Arctic sea ice can be attributed to humans and how much is due to natural variability in the climate is an important one. Our study shows that while natural changes play a significant role, the majority of sea ice loss – between 70 and 95 percent – is likely to be due to man-made greenhouse gas emissions.” They found that some of the reduction in ice since 1979 – between 5 and 30 percent – may be linked to the Atlantic Multi-decadal Oscillation (AMO), a natural cycle of warming and cooling in the North Atlantic, which repeats every 65-80 years. The AMO has been in a warming phase since the mid 1970s. Day and his colleagues were excited to be able to show such a clear natural signal that would account for a significant part of the warming. He says, “That’s a fair amount to be attributed to natural causes.” The team also found that Arctic Oscillation, the wind pattern that persists over the North Pole, allowing ice to thin in some areas and pile up in others, has surprisingly little influence on the overall loss of sea ice. Day says, “The Arctic Oscillation moves the sea ice around but it doesn’t affect its extent very much – like the total amount of ice.” However, when they studied climate models and the observed data the human activity signal really stood out. The AMO accounts for up to 30 percent of the sea ice loss, which means the rest – at least 70 percent – can be attributed to human activity. Day says, “On the other hand, it implies that 70-95 percent of the changes are essentially caused by human-induced global change.” The research appears online in the journal Environmental Research Letters.
Monthly September Sea Ice Extent for 1979-2010 Shows a Decline of 12 Percent Per Decade Concerned that the sea ice extent record lacked current data in the literature, Day and his team set out to study the natural variability component, which they viewed as one of the three reasons the Third Coupled Model Intercomparison Project (CMIP3) doesn’t show the consistent decline in Arctic sea ice every September. Since the short observation records of sea ice extent began in the 1970s scientists have watched the decrease in sea ice extent accelerate every year since the 1990s. Yet, most climate models don’t have a way to capture that trend. Day and his team determined the reason for that data gap was one (or more) of three reasons: observational uncertainty, physical model limitations and vigorous natural climate variability. Seeing the least information about vigorous climate variability, that’s what he decided to tackle. Using complicated statistical techniques that compare satellite data going back to 1979 with computer simulations run on supercomputers he helped answer an important and nagging climate question and in the process is providing a better estimate of the importance of natural climate variability on the reduction in sea ice, and how much could be attributed to human activity. Day and his colleagues believe their work will provide more accurate predictions of changes in sea ice extent. It will also allow those working in science, policy, industry and those living throughout the polar region to have a better understanding of what the Arctic will look like in the future. According to two 2009 studies – one by Muyin Wang and James Overland and the other by Julien Boé, Alex Hall and Xin Qu – September ice extent in the Arctic is predicted to continue shrinking by about one-third every year until the Arctic is seasonally ice-free at some point later this century. Now we know why.
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