Nick Carne
The Arctic has started to transition from predominantly frozen to an entirely different climate, according to a new report.
Writing in the journal Nature Climate Change, scientists from the US National Centre for Atmospheric Research (NCAR) say the planet’s north has warmed so significantly that its year-to-year variability is moving outside the bounds of any past fluctuations, signalling the move to a new normal.
Sea ice has melted to the extent that even an unusually cold year will no longer have the amount of summer sea ice that existed as recently as the mid-20th century.
“The rate of change is remarkable,” says lead author Laura Landrum. “It’s a period of such rapid change that observations of past weather patterns no longer show what you can expect next year.”
The report suggests the changes are not uniform: the far north is warming more rapidly than lower latitudes due to Arctic amplification, which occurs because light-coloured sea ice, which reflects heat back into space, is replaced by darker ocean water, which traps heat.
They are dramatic, however. The average extent of sea ice in September, when it reaches its annual minimum, has dropped by 31% since the 1980s, the first decade of the satellite era.
For their study, Landrum and colleagues Marika Holland analysed multiple simulations from the five climate models used for the international Coupled Model Intercomparison Project 5 (CMIP5).
From this they assembled a picture of Arctic climate, allowing them to differentiate year-to-year natural climate variability from a transition to a new climate. They then compared the model output to observations, confirming, they say, that the models were accurately capturing past climate and therefore could reliably simulate future climate.
They identified a different climate as emerging when the 10-year average was at least two standard deviations away from the average of the climate in the decade 1950-59.
In other words, if the sea ice extent changed so much that the average in, say, the 1990s was lower in 97.7% of all cases than the sea ice extent for any year in the 1950s, then the 1990s were defined as a new climate.
Applied to sea ice extent, each of the five models showed sea ice retreating so dramatically that a new climate for sea ice had emerged in the late 20th and early 21st centuries.
“All five… simulate mean ice-free summer by 2100, and three of these ensemble projections (the three with the closest ice extents to the observations during the satellite era) suggest that the Arctic will remain completely ice free for 3-4 months,” Landrum and Holland write in their paper.
“Not only will the warming exceed that of lower latitudes, but daily fall–winter temperatures will increase by 16–28 degrees C for most of the Arctic Ocean. Rainfall will replace snowfall, with an extension of the rainy season by 2-4 months. These changes have extreme consequences for Arctic communities and local ecosystems.”