By Ed Hawkins
Arctic sea ice melts each summer, reaching its minimum extent some time in September, before refreezing through the winter. Over the past 35 years, the September sea ice extent has reduced by about 35% overall and this decline is projected to continue as global temperatures increase.
In 2007 and 2012 the summer ice extent was dramatically lower, causing some some speculation that we would soon see a summer which was “ice-free” (meaning a year with less than 1 million square kilometres of sea-ice). However, most climate scientists were more cautious. The weather in 2007 and 2012 was warmer than usual and the winds were particularly favourable for melting sea ice. Although a human influence on Arctic sea ice has been detected, there was no evidence that these weather patterns would continue each year.
In contrast, 2013 and 2014 had more sea ice than 2012, causing speculation that a recovery was underway. Is this claim warranted?
Figure 1 below shows that Arctic sea ice extent (the black line) has undergone a long-term decrease, with the dashed line representing a linear trend. But there have also been shorter periods of rapid melt, no change, and apparent increases in extent during this decline – represented below by coloured trend lines for some deliberately chosen eight year periods.
Figure 1. Arctic sea ice extent in September, 1979-2014, million sq km
The most recent eight-year period, starting from the extreme low of 2007, shows an upward trend. This does not mean that the Arctic sea ice is recovering – these erratic changes are what we expect to see.
As an analogy, imagine a ball bouncing down a bumpy hill. Gravity will ensure that the ball will move downwards. But if the ball hits a bump at a certain angle it might move horizontally or even upwards for a time, before resuming its inevitable downward trajectory. This bouncing ball is an analogy for the changing Arctic sea ice.
The hill represents the long-term downward trend in Arctic sea ice due to increasing global temperatures and the bumps introduce changes from this smooth trajectory. These erratic bounces could be in either direction, causing an apparent acceleration or temporary reduction in melt rate. By only examining a small part of the trajectory you might conclude that the ball was moving against gravity. A longer term view would see it as a bounce.
There is no expectation that sea ice, or any other aspect of the climate, will change smoothly over time. The climate system simply does not work that way. The recent trends are well within the range of expectations from our climate model projections. We might even see a decade or more with little apparent change in sea ice.
The causes of these fluctuations in melt rate are still being explored. One suggestion is that slow variations in Atlantic sea surface temperatures are involved. More observations of the Arctic ocean, atmosphere and sea ice would help answer this question.
We expect the long-term decline in Arctic sea ice to continue as global temperatures rise. There will also be further bounces, both up and down. Individual years will be ice-free sometime in the 2020s, 2030s or 2040s, depending on both future greenhouse gas emissions and these natural fluctuations.
A version of this post was first published at The Conversation
Link to paper: http://www.readcube.com/articles/10.1038/nclimate2483