-Contributed by Alexandra Jahn, University of Colorado, Boulder, USA
The ups and downs of minimum Arctic sea ice extent have captured the attention of many people in recent years. The Arctic sea ice loss between 2000 and 2007 was unprecedented in the satellite record, with a record minimum sea ice extent reached in 2007, followed by an even lower minimum in 2012. However, due to a recovery in the sea ice extent during 2013 (which continued in 2014), the more recent 7-year period (2007 to 2013) showed a small positive trend, indicating a slowdown in extent loss (see Figure 1). How likely is such a slowdown and what does it mean for the future of Arctic sea ice? These questions were addressed in a recent commentary in Nature Climate Change by Swart et al. (2015), which presented an analysis of observed and simulated Arctic sea ice trends over the satellite period (1979 to 2013).
Their analysis showed that in the CMIP5 climate models and in a large ensemble from the Community Earth System Model (CESM), the strong negative sea ice extent trend observed from 2000 to 2007 had only a 5% chance of occurring. This makes this decline a rather rare event, but within the range of expectations based on the climate models that reflect our best understanding of the climate system. On the other hand, a positive or neutral 7-year trend as observed more recently had a one-in-three (or 34%) chance of occurring, despite strong anthropogenic forcing. And with the same likelihood as the rapid ice loss between 2000 and 2007 (5%), this period of little change could last for 20 years or more according to the model simulations.
The cause for these reduced and enhanced short-term sea ice extent trends is climate variability due to internal variations in weather patterns, which can attenuate or amplify the forced response of September Arctic sea ice extent for years to decades. However, these short-term trends have little predictive skill for the future evolution of the Arctic sea ice during the 21st Century. Over the long term, Arctic sea ice extent continues to decline in all model simulations during the 21st century, due to the strong anthropogenic forcing that is warming the Arctic.
Due to the strong influence of internal variability, no exact date can be given for when the Arctic will become ‘ice-free’. Indeed, the model simulations analyzed by Swart et al. suggest that internal variability could lead to the intermittent re-appearance of late summer sea-ice for several decades after the first ‘ice-free’ summer.
Citation: Swart, N. C., J. C. Fyfe, E. Hawkins, J. E. Kay & A. Jahn, 2015, Influence of internal variability on Arctic sea-ice trends, Nature Climate Change, 5, 86-89, doi: 10.1038/nclimate2483.