On September 10th 2016 Arctic sea-ice reached its minimum extent of 4.14 million square kilometres, making it the second lowest minimum on record. The record low is still retained by 2012, when the ice extent fell to an incredible 3.39 million square kilometres. But predicting exactly when the Arctic will see its first ice-free summer may be more difficult than previously believed, according to the results of new research.

Arctic sea-ice plays an important role in the Earth’s climate system. It keeps the Polar Regions cool and helps moderate global climate. Sea-ice has a bright surface; 80 percent of the sunlight that strikes it is reflected back into space. As sea ice melts in the summer, it exposes the dark ocean surface. Picture: Katja Riedel
Arctic sea-ice plays an important role in the Earth’s climate system. It keeps the Polar Regions cool and helps moderate global climate. Sea-ice has a bright surface; 80 percent of the sunlight that strikes it is reflected back into space. As sea ice melts in the summer, it exposes the dark ocean surface. Picture: Katja Riedel

How well are we able to predict the timing of when the Arctic will first reach ice-free conditions?

Using climate models researchers under the lead of Alexandra Jahn, an assistant professor at the University of Colorado Boulder’s Department of Atmospheric and Oceanic Sciences, tried to answer this question. They examined different model scenarios with high- and medium-level carbon dioxide emissions, but found that it is not possible to exactly predict when the Arctic will be ice-free in summer for the first time. The uncertainty could not be reduced to a period of less than 25 years due to the chaotic nature of the climate system and the uncertainty how fast humans will tackle climate change.

The graph shows the Arctic sea ice extent along with daily ice extent data for four previous years. 2016 is shown in blue, 2015 in green, 2014 in orange, 2013 in brown, and 2012 in purple. The 1981 to 2010 average is depicted in dark grey. The grey area around the average line shows the two standard deviation range of the data. Picture: National Snow and Ice Data Center
The graph shows the Arctic sea ice extent along with daily ice extent data for four previous years. 2016 is shown in blue, 2015 in green, 2014 in orange, 2013 in brown, and 2012 in purple. The 1981 to 2010 average is depicted in dark grey. The grey area around the average line shows the two standard deviation range of the data. Picture: National Snow and Ice Data Center

What exactly is an “ice-free” Arctic summer? Scientists typically define an “ice-free Arctic” as having fewer than 1 million square kilometres worth of ice cover, which would leave the Arctic Ocean virtually clear while some pockets of ice would remain in the northern reaches of Canada and Greenland. “When it comes to predicting the timing of an ice-free Arctic, climate models show a large spread of over 100 years. Many studies have attempted to narrow this wide range to as little as five years in some cases,” said Alexandra Jahn.

After reaching its annual peak extent at the end of winter, Arctic sea-ice melts as temperatures rise through spring and into summer. It reaches its minimum extent in September. The graph shows the trend line of the average monthly sea-ice extent in September between 1979, when satellite observations began, and 2016. It currently declines with 13.3% per decade. Picture: National Snow and Ice Data Center
After reaching its annual peak extent at the end of winter, Arctic sea-ice melts as temperatures rise through spring and into summer. It reaches its minimum extent in September. The graph shows the trend line of the average monthly sea-ice extent in September between 1979, when satellite observations began, and 2016. It currently declines with 13.3% per decade. Picture: National Snow and Ice Data Center

There are three principal sources of uncertainty in predictions of an ice-free Arctic summer: natural variability in the climate system, how quickly humans will tackle climate change by reducing greenhouse gas emissions, and model skill. For the new study, scientists tested how natural variability affects the long-term predictability of an ice-free summer. They used a collection of 40 simulations of 21st century climate in an earth system model. The results showed that the uncertainty for Arctic sea-ice predictions caused by natural variability amounts to around two decades. This means that natural variability alone limits how close scientists can get to a prediction for an ice-free summer to a window of about 20 years.

Age is another indicator of the state of sea ice because older ice is generally thicker ice. The image shows sea-ice ages for the week of the 2016 sea-ice minimum. The bar chart shows the extent of each multi-year age category (in millions of square kilometres); the green lines on the bar chart are the high values in the satellite record for the minimum week. Picture: NASA Scientific Visualization Studio
Age is another indicator of the state of sea ice because older ice is generally thicker ice. The image shows sea-ice ages for the week of the 2016 sea-ice minimum. The bar chart shows the extent of each multi-year age category (in millions of square kilometres); the green lines on the bar chart are the high values in the satellite record for the minimum week. Picture: NASA Scientific Visualization Studio

But the speed of sea ice melt in the Arctic will also depend on how quickly we are able to cut global emissions and slow the pace of rising temperatures. Examining a high and moderate emission scenario the study also found that this added another five years to the uncertainty around predicting an ice-free summer. Considering two of the three main uncertainties of model predictions resulted in a prediction uncertainty of around 25 years.

The Arctic sea ice not only is important for the climate, but is also is an important ecosystem. Life in the Arctic depends on sea-ice. That includes organisms living in and under the ice, like zooplankton and phytoplankton, fish and marine mammals, birds, land animals, plants and humans. Picture: Katja Riedel
The Arctic sea ice not only is important for the climate, but is also is an important ecosystem. Life in the Arctic depends on sea-ice. That includes organisms living in and under the ice, like zooplankton and phytoplankton, fish and marine mammals, birds, land animals, plants and humans. Picture: Katja Riedel

But while natural fluctuations of weather and climate will affect exactly when an Arctic summer will first be ice free, we can be fairly certain that it will happen well before the end of this century without significant cuts to greenhouse gas emissions. The simulations in the new study put the first ice-free Arctic summer at some point between 2032 and 2053 for the high emission pathway, and between 2043 and 2058 for medium emission scenario. Under the high emission scenario consecutive ice-free summers could become common after 2060 while remaining the exception in the medium emission scenario. “Overall, these results serve as a sort of caution against over-narrowing the long-term sea-ice predictions from climate models” said Jahn.

Source: CU Boulder Today and Alexandra Jahn in CarbonBrief