Are scientists conservative about sea ice?

The U.S. Coast Guard Cutter Healy encountered only small patches of sea ice in the Chukchi Sea when this photograph was taken on July 20, 2011. (Courtesy NASA Goddard Space Flight Center)

The U.S. Coast Guard Cutter Healy encountered only small patches of sea ice in the Chukchi Sea when this photograph was taken on July 20, 2011. (Courtesy NASA Goddard Space Flight Center)

Guest post by Walt Meier, NSIDC Scientist

Arctic sea ice set a record minimum extent in September 2012, far below the previous record low in 2007. Summer extents have been far lower than average for the last decade, with several record or near-record years. Looking at the numbers, one is tempted to think that the Arctic Ocean may reach nearly sea ice-free conditions within just a few years. But most expert analyses indicate that we’re likely at least a couple decades away from seeing a blue Arctic Ocean during the summer.

So what is going on here? Readers have asked if scientists are being too conservative in their assessment of the recent ice loss. We asked Walt Meier, NSIDC scientist, to address this question. Following is his response.

Conservative science, or complex systems?
Scientists by their nature tend to be conservative when viewing new evidence. While the recent years have been surprising, most scientists are not willing to accept that ice-free conditions are imminent. But that is not because they’re being too conservative. The Arctic sea ice system is complex and there are many aspects that are not yet well understood.

A variety of feedbacks promote ice growth or ice loss. For example, sea ice has a high albedo, meaning it reflects the sun’s energy back into space. Darker ocean water has a low albedo, meaning it absorbs more of the sun’s energy, and thus more heat. As sea ice extent decreases, the change in albedo spurs a well-known feedback that enhances summer melt because the ocean absorbs more of the sun’s energy than the ice.

Likewise, there are also negative feedbacks that will slow the loss of ice. One of these results from the fact that ice grows more rapidly when there is no ice or thin ice than when thick ice is present under the same air temperatures. Thus in fall when the sun goes down and the atmosphere gets cold, open water areas grow ice quickly allowing such regions to “catch up” to thicker ice regions. These feedbacks and many other factors, such as ocean and air temperatures, wind, and weather patterns, prevent an easy assessment of a complex system.

This graph shows a time series of modeled and observed September sea ice extent from 1900 to 2100. The red line indicates the observations, and individual ensemble models from Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) are included as dotted colored lines, with their individual model ensemble average in solid color lines. The black line is the model average. The inset image shows multi-model ensemble average from both the IPCC AR5 and IPCC AR4; shading indicates the range of model estimates. The IPCC AR5 will be published later this year. (Courtesy Stroeve, et al., Geophysical Research Letters)

This graph shows a time series of modeled and observed September sea ice extent from 1900 to 2100. The red line indicates the observations, and individual ensemble models from Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) are included as dotted colored lines, with their individual model ensemble average in solid color lines. The black line is the model average. The inset image shows multi-model ensemble average from both the IPCC AR5 and IPCC AR4; shading indicates the range of model estimates. The IPCC AR5 will be published later this year. (Courtesy Stroeve, et al., 2012, Geophysical Research Letters)

Models, averages, and trends

Climate models indicate that sea ice will decline more slowly than recent observations. Some models suggest ice-free summers by around mid-century, while others are even more conservative and don’t indicate such conditions until late in the century. Scientists looking at the trend recognize that the models may underestimate the rate of ice decline.

Sea ice models, though far from perfect, are the best tools we have to understand and project the future changes in sea ice. While the models on average show a slower trend, a closer look provides a more subtle view. Looking at averages can mask important variations in the sea ice that occur in the real world. Individual model simulations do show periods of rapid ice loss lasting several years, but they also show periods of stasis, with little or no trend, over several years.  When averaging over several model simulations (and several different models), the extremes tend to get averaged out and one sees only the slower, steady long-term trend. Even in the historical observations, it is clear that there are large variations in ice conditions from year to year.

Thus the observations that we are seeing may be a period of rapid ice decline that models indicate will happen from time to time. And we may be due to experience a period of slow down. There is no certainty of this and scientists have been surprised by the dramatic record lows in 2007 and 2012. Nonetheless, given the complexity of the sea ice system, the large year-to-year variations observed, and potential negative feedbacks that can act to slow the ice loss, most scientists feel a conservative estimate of the future ice changes is probably warranted.

One thing that all scientists who study sea ice agree upon is that under increasing temperatures, the overall long-term declining trend will continue and some summer in the future, we will look down on the North Pole and see a blue Arctic Ocean. It’s not a matter of if, but when.

References

Holland, M. M., C. M. Bitz, and B. Tremblay. 2006. Future abrupt reductions in the summer Arctic sea ice. Geophysical Research Letters 33, L23503, doi:10.1029/2006GL028024.

Kay, J. E., M. M. Holland, and A. Jahn. 2011. Inter-annual to multi-decadal Arctic sea ice extent trends in a warming world. Geophysical Research Letters 38, L15708, doi:10.1029/2011GL048008.

Stroeve, J. C., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W. N. Meier. 2012. Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations. Geophysical Research Letters 39, L16502, doi:10.1029/2012GL052676.

Wang, M., and J. E. Overland. 2012. A sea ice free summer Arctic within 30 years: An update from CMIP5 models. Geophysical Research Letters 39, L18501, doi:10.1029/2012GL052868.

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