Natural variability could account for as much as half (30-50%) of the overall decline the area of September Arctic sea ice since 1979, reports a paper published online this week in Nature Climate Change. The study shows that changes in atmospheric circulation, which are primarily related to natural internal variability, influence the extent of Arctic summer sea ice.
Qinghua Ding and colleagues investigate how atmospheric circulation in the summer months - June, July and August - influences the sea-ice extent in September. They analyse three factors that affect atmospheric circulation and, therefore, sea ice - temperature, humidity and downward longwave radiation - by combining an atmospheric general circulation model with an ocean sea-ice model and reanalysis data. They find that circulation changes are contributing to up to 60% of the sea-ice decline.
The authors then focus on whether the atmospheric circulation changes are natural or caused by human influence, and find that about 70% of the atmospheric circulation changes are due to natural internal variability. These findings highlight that high latitude circulation affects sea ice and that gaining understanding of decadal trends may increase the ability to predict sea-ice coverage over seasonal to decadal timescales.
In an accompanying News & Views, Neil Swart writes: “Recent changes in Arctic sea ice are driven by two main components: an overall long-term loss of ice in response to external forcings such as increasing greenhouse gases, and shorter term, random changes due to internal climate variability. The challenge, until now, is that there has been no clear understanding of the relative contributions of human-induced warming versus internal variability to the observed long-term decline in Arctic sea ice. Qinghua Ding and colleagues illustrate that around half of the observed summer-time Arctic sea-ice loss has been driven by naturally induced changes in large-scale atmospheric circulation.
The results of Ding et al. do not call into question whether human-induced warming has led to Arctic sea-ice decline - a wide range of evidence shows that it has. Rather, the implication is that Arctic sea-ice is less sensitive to human-induced forcing than if one assumes that all loss observed to date is anthropogenically driven.”