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U.S. and German researchers calculate individual contribution to climate change


The National Snow and Ice Data Center (NSIDC) is part of the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder.
How much can a single person affect Earth’s changing climate? According to researchers in the United States and Germany, 3 square meters (1.15 square miles) of summer sea ice disappear in the Arctic for every metric ton of carbon dioxide (CO2) that a person directly or indirectly produces.
How can one person produce 1 metric ton of CO2? That’s about a roundtrip flight from New York to Europe per passenger. Or, a 4,000-kilometer (2,485.5-mile) car ride.
Researchers Dirk Notz and Julienne Stroeve hope the findings offer the public a better grasp of their individual contribution to global climate change. The study also explains why climate models usually simulate a lower sensitivity to CO2 emissions and therefore underestimate how quickly the Arctic is transforming to a seasonally ice-free Arctic. Notz is a senior researcher at the Max Planck Institute for Meteorology in Hamburg, Germany. Julienne Stroeve is a senior researcher at the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, as well as a professor of polar observation and modeling at the University College London in the United Kingdom.
The paper, titled “Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission,” will be published online by the journal Science on THURSDAY, NOVEMBER 3.
The findings come more than a month after sea ice in the Arctic reached an annual minimum extent of 4.14 million square kilometers (1.60 million square miles), a statistical tie with 2007 as the second lowest minimum in the satellite record.
The rapid retreat of Arctic sea ice is one of the most direct indicators of ongoing climate change on our planet. Over the past forty years, the Arctic’s summer ice cover has shrunk by more than half. Climate model simulations predict that the remaining half might be gone by mid-century unless greenhouse gas emissions are reduced rapidly.
However, a number of studies have found that climate models underestimate the loss of Arctic sea ice, which is why the models might not be the most suitable tools to quantify the future evolution of the ice cover.
To address this issue, Notz and Stroeve derived the future evolution of Arctic summer sea ice directly from the observational record, namely 1953 to 1978 data from the U.K. Met Office’s Hadley Centre Sea Ice and Sea Surface Temperature data set and 1979 to 2015 data from the NSIDC Sea Ice Index. The researchers examined the link between CO2 emissions and the area of Arctic summer sea ice, and found that both are linearly related.
“The observed numbers are very simple,” Notz said. “For each ton of carbon dioxide that a person emits anywhere on this planet, 3 square meters (1.15 square miles) of Arctic summer sea ice disappear.”
“So far, climate change has often felt like a rather abstract notion,” Stroeve said. “Our results allow us to overcome this perception. For example, it is now straightforward to calculate that the carbon dioxide emissions for each seat on a return flight from, say, London to New York cause about 3 square meters (1.15 square miles) of Arctic sea ice to disappear.”
The study also explains the linear relationship between CO2 emissions and sea ice loss. “Put simply, for each ton of carbon dioxide emission, the climate warms a little bit. To compensate for this warming, the sea ice edge moves northward to a region with less incoming solar radiation. This then causes the sea ice area to shrink. Simple geometric reasons cause these processes to combine to the observed linearity,” Notz said.
Climate models also simulate the observed linear relationship between sea ice area and CO2 emissions. However, ice cover in climate models have a much lower sensitivity than has been observed. The Science study finds that this is most likely because the models underestimate the atmospheric warming in the Arctic that is induced by a given CO2 emission.
“It seems that it’s not primarily the sea ice models that are responsible for the mismatch. The ice just melts too slow in the models because their Arctic warming is too weak,” says Stroeve.
Regarding the future evolution of Arctic sea ice, the new study finds that the international target of 2 degrees Celsius of global warming is not sufficient to allow Arctic summer sea ice to survive. To stay below 2 degrees Celsius (3.6 Fahrenheit) of warming, the world can emit no more than 1,000 gigatons of additional carbon by 2100. However, given the observed sensitivity of the Arctic ice cover, an additional 1,000 gigatons of CO2 will likely result in ice-free Septembers in the Arctic.
The study concludes that Arctic summer sea ice has a realistic chance of long-term survival only in a scenario of lower emissions, such as a global warming target of below 1.5 degrees Celsius (2.7 degrees Fahrenheit) as called for by the Paris Agreement.


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