The rapid retreat of Arctic sea ice is one of the most direct indicators of the ongoing climate change on our planet. Over the past forty years, the ice cover in summer has shrunk by more than half, with climate model simulations predicting that the remaining half might be gone by mid-century unless greenhouse gas emissions are reduced rapidly. However, a number of studies have indicated that climate models underestimate the loss of Arctic sea ice, which is why these models might not be the most suitable tools to quantify the future evolution of the ice cover. A new study explains the underlying issues and allows for the first time to calculate individual contributions to Arctic's shrinking sea ice.

Arctic sea ice has shrunk by more than half over the last 40 years. This will have a big impacts on climate and Arctic wildlife such as polar bears. Hence it is essential to understand future sea ice developments. Picture: Katja Riedel
Arctic sea ice has shrunk by more than half over the last 40 years. This will have a big impacts on climate and Arctic wildlife such as polar bears. Hence it is essential to understand future sea ice developments. Picture: Katja Riedel

For each tonne of carbon dioxide that an individual on our planet emits, three square metres of Arctic summer sea ice disappear. This is the finding of a study that has been published recently in the journal Science by Dirk Notz, scientist at the Max Planck Institute for Meteorology and Julienne Stroeve from the US National Snow and Ice Data Centre. The researchers compared model calculations with data from satellite measurements, and discovered that climate models underestimate the loss of Arctic sea ice. To address this issue, they derived the future evolution of Arctic summer sea ice directly from the observational record. The authors examined the link between carbon-dioxide emissions and the area of Arctic summer sea ice, and found that both are linearly related. "The observed numbers are very simple", explains lead author Dirk Notz. "For each tonne of carbon dioxide that a person emits anywhere on this planet, three square metres of Arctic summer sea ice is lost".

Scientists are taking samples of the Arctic sea ice at Spitzbergen. Examining these samples helps them to better understand the factors that influence the development of sea ice and ultimately improve climate models predicting future sea ice development. Picture: Dirk Notz
Scientists are taking samples of the Arctic sea ice at Spitzbergen. Examining these samples helps them to better understand the factors that influence the development of sea ice and ultimately improve climate models predicting future sea ice development. Picture: Dirk Notz

"So far, climate change has often felt like a rather abstract notion. Our results allow us to overcome this perception", says co-author Julienne Stroeve. For example, it is now straight-forward to calculate that the carbon dioxide emissions for each seat on a return flight from, say, London to San Francisco causes about five square metres of Arctic sea ice to disappear."

The study gives the public and policymakers for the first time a concrete illustration of the consequences of burning fossil fuels. For instance, a U.S. family of four would claim nearly 200 square meters of sea ice a year, based on U.S. emissions in 2013. Over 3 decades that family would be responsible for destroying more than an American football field’s worth of ice.

The daily Arctic sea ice extent for 2016 is shown in blue, the minimum extent for 2012 in green, The 1981 to 2010 average is depicted in dark grey. The grey area around the average line shows the two standard deviation range of the data. Sea Ice Index data. Picture: National Snow and Ice Data Center
The daily Arctic sea ice extent for 2016 is shown in blue, the minimum extent for 2012 in green, The 1981 to 2010 average is depicted in dark grey. The grey area around the average line shows the two standard deviation range of the data. Sea Ice Index data. Picture: National Snow and Ice Data Center

While climate models also simulate the observed linear relationship between sea ice area and carbon dioxide emissions, they usually have a much lower sensitivity of the ice cover than has been observed. The new study finds that this is most likely because the models underestimate the atmospheric warming in the Arctic that is induced by a given carbon-dioxide emission. "It seems that it's not primarily the sea ice models that are responsible for the mismatch. The ice just melts too slowly in the models because their Arctic warming is too weak", says Stroeve.

Depicted is the Arctic sea ice extent in September. Comparing observations with computer model runs scientists at the National Snow Ice Data Center (NSIDC) and the National Center for Atmospheric Research (NCAR) found that satellite and other observations show the Arctic ice cover is retreating more rapidly than estimated by any of the eighteen computer models used by the Intergovernmental Panel on Climate Change (IPCC) in preparing its 2007 assessments. Picture: NSIDC updated with September 2007 minimum by Sagredo.
Depicted is the Arctic sea ice extent in September. Comparing observations with computer model runs scientists at the National Snow Ice Data Center (NSIDC) and the National Center for Atmospheric Research (NCAR) found that satellite and other observations show the Arctic ice cover is retreating more rapidly than estimated by any of the eighteen computer models used by the Intergovernmental Panel on Climate Change (IPCC) in preparing its 2007 assessments. Picture: NSIDC updated with September 2007 minimum by Sagredo.

Regarding the future evolution of Arctic sea ice, the researchers concluded, that the 2°C global-warming-target agreed on in the most recent UN Climate Conference will not allow Arctic summer sea ice to survive. Given the observed sensitivity of the ice cover, the sea ice will be gone throughout September once another 1000 gigatonnes of carbon dioxide have been emitted. This amount of emissions is usually taken as a rough estimate of the allowable emissions to reach the two degree Celsius global-warming target. Only for the much lower emission scenario, that would keep global warming below 1.5 °C, as called for by the Paris agreement, Arctic summer sea ice has a realistic chance of long-term survival, the study authors Dirk Notz and Julienne Stroeve concluded.

Reference: Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission, Dirk Notz and

Julienne Stroeve, Science, Science 03 Nov 2016, DOI: 10.1126/science.aag2345

Arctic sea ice will only have a chance of long-term survival if global warming can be kept below 1.5 °C. This will require to limit carbon dioxide emissions considerably. Picture: Katja Riedel
Arctic sea ice will only have a chance of long-term survival if global warming can be kept below 1.5 °C. This will require to limit carbon dioxide emissions considerably. Picture: Katja Riedel

Source: Max Planck Society