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For each tonne of carbon dioxide (CO2) that any person on our planet emits, three square metres of Arctic summer sea ice disappear.
This is the finding of a study that has been published in the journal Science by Dirk Notz, leader of a Max Planck Research Group at the Max Planck Institute for Meteorology and Julienne Stroeve from the US National Snow and Ice Data Centre.
The figures let us grasp – for the first time – the individual contribution to global climate change. The study also explains why climate models usually simulate a lower sensitivity than can be detected in observations.
It concludes that the two degrees Celsius global warming target agreed on in the most recent UN Climate Conference will not allow Arctic summer sea ice to survive.
‘The observed numbers are very simple. For each tonne of carbon dioxide that a person emits anywhere on this planet, three square metres of Arctic summer sea ice is lost.’
50% ice loss
The rapid retreat of Arctic sea ice is one of the most direct indicators of ongoing climate change on our planet.
Over the past 40 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 the models might not be the most suitable tools to quantify the future evolution of the ice cover.
CO2 and ice
To address this issue, a new study in the journal Science now derives the future evolution of Arctic summer sea ice directly from the observational record.
To do so, the authors examine the link between carbon dioxide emissions and the area of Arctic summer sea ice, and find that both are linearly related.
‘So far, climate change has often felt like a rather abstract notion. 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 San Francisco causes about five square metres of Arctic sea ice to disappear.’
While climate models also simulate the observed linear relationship between sea ice area and CO2 emissions, they usually have a much lower sensitivity of the ice cover 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 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.’
2ºC limit not enough
Regarding the future evolution of Arctic sea ice, the internationally agreed objective to limit global warming to two degree Celsius is not sufficient to 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 volume of emissions is usually taken as a rough estimate of the allowable emissions to reach the two degree Celsius global-warming target.
The authors conclude that the Arctic summer sea ice’s only realistic chance of long-term survival is if global warming is kept below 1.5 °C.
Click here to read ‘Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission’.