Arctic sea ice melts each summer, reaching its minimum extent sometime 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 media speculation that we would soon see a summer which was “ice-free” (meaning a year with less than 1 million km2 of sea-ice).

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 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 other speculation that a recovery was underway. Is this claim warranted?

The figure below shows 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.

Ed Hawkins/University of Reading

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. As with global temperature, these erratic changes are what we expect to see.

Bouncing towards an ice-free summer

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. Previous studies have suggested that natural climate variations (or “bounces”) play a key role in how sea ice evolves, and suggested that some of the rapid melt in the early 2000s was a temporary acceleration.

A new study I co-authored with a team of Canadian and American scientists, published in Nature Climate Change, highlights that the recent slower melt is a temporary, but not unexpected, deceleration. The complex climate models used to make projections of future climate also exhibit similar periods of little change and more rapid change in Arctic sea ice. The recent trends are well within the range of these expectations. 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.

An ice-free future?

When will the Arctic be ice-free – or equivalently, when will the ball reach the bottom of the hill? The IPCC concluded it was likely that the Arctic would be reliably ice-free in September by 2050, assuming high future greenhouse gas emissions (where “reliably ice-free” means five consecutive years with less than 1 million km2 of sea ice).

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.

Even when it reaches the bottom of the hill the ball will continue to bounce. Similarly, not every future year will be ice-free in summer. But if global temperatures continue to increase the bounces will become smaller and the ice-free periods will spread from late summer into autumn and early summer.

Commercial Arctic shipping is already increasing to exploit shorter journey times from Europe to Asia, while oil, gas & mineral extraction possibilities are being explored and Arctic tourism is growing. Decisions about such activities need to assess both the risks and opportunities. The important role of natural sea ice fluctuations needs to be considered in such assessments.