9/22/2014 6:23:49 PM

Short-term Sea Ice Gains Don't Eliminate Long-term Threats

Natural systems are complex, and the Arctic system is no exception. Given this complexity, the human body and an analogy between sea ice and dieting may be a more intuitive place to start in understanding short-term changes within long-term trends.

Let's say you want to lose weight. To accomplish your weight loss goal, you exercise more. If you consistently exercise more over a long time period, you burn more calories and you lose weight. Why? Just like increased greenhouse gases are an external forcing that causes sea ice loss, increased exercise is an external forcing that causes weight loss. 

Simple right? Well... anyone who has tried to lose weight knows it is not always that simple. Why? Just like natural systems, dieting is complex. Weight loss is affected both by consistently applied external forcings and by internal variability.

Can you gain weight even if you exercise more? Sure! What happens if you temporarily start eating a lot of cookies? Even though you are still applying the external forcing of exercising more, the internal variability caused by your increased cookie consumption may temporarily stop or even reverse your weight loss! What's the solution? Well, for dieting, the answer is clear. To lose weight, you need the external forcing of increased exercise to be larger and longer lasting than the internal variability caused by eating too many cookies.

So How Does This Apply to Sea Ice?

2014 will not be a record-breaking year for Arctic sea ice loss, but, just as with a dieter steadily keeping up an exercise program, the amount of sea ice continues on its long-term downward trajectory. According to the latest data from the National Snow and Ice Data Center in Boulder, Colorado, the Arctic sea ice extent today (September 18, 2014, ~5 million square kilometers) is very similar to what it was in 2013.

Recent claims that Arctic sea ice is bouncing back, and therefore we needn't worry about future ice loss, are all unfortunately, to our best knowledge, wrong. It is true that the trend of September ice extent data from 2007 to 2014 has slightly increased over the last eight years, but cherry-picking short-term trends does not change observed and projected Arctic sea ice loss on climate timescales.

Whether optimism, willful ignorance, or limited understanding is behind false claims that such short-term increases imply long-term recovery, these bouncing back claims are important to debunk. The importance of sea ice for polar bears and for the Arctic system in general, require that managers and policy makers understand the reality of what scientists understand about sea ice trends. To us, the key to understanding observed and projected sea ice trends is recognizing the difference between short-term variability and the long-term imprint of externally forced human-caused climate change.

Completing the analogy between weight loss and sea ice loss, we see that sea ice loss in response to increased greenhouse gases (external forcing) can temporarily stop or even reverse when the influence of internal variability temporarily dominates (e.g., due to noisy atmospheric circulation patterns such as the Arctic Oscillation). On the other hand, if greenhouse gas forcing increases, persists, and dominates over internal variability, the fate of sea ice is inevitable: it melts.

Knowing that Arctic sea ice trends are influenced by both internal variability and external forcing, how do we interpret recent sea ice trends and the projections for an ice-free Arctic over our lifetimes? Since reliable hemispheric estimates of Arctic sea ice cover began in 1979, statistically significant declining sea ice trends have occurred in all months, but especially in September.

Observed September Arctic sea ice loss cannot be explained by natural internal variability alone. Our greenhouse gas emissions are in large part responsible for the loss of Arctic sea ice.

A Matter of Scale

When you look at trends over short time periods, the influence of external forcing on sea ice trends is hard to detect. Why? The shorter the timescale, the larger the influence of internal variability. We are not at all surprised that sea ice can temporarily increase in a warming world. In our modeling experiments, we predicted pauses in sea ice loss lasting up to 20 years. But what we also know is that if greenhouse gases continue to increase, sea ice loss on longer timescales is inevitable.

Our best guess is that the Arctic will be ice-free in late summer by the middle part of this century. A reporter from Discovery News made a great musical analogy when writing about our 2011 study on sea ice trends in a warming world (Kay, Holland, and Jahn, 2011, GRL): "The Arctic's summer ice coverage could hold its ground or even bounce back slightly, even as global average temperatures rise. But like a Guns 'n' Roses tour, the comeback will be short lived and eventually doomed to disappear." 

What does short-term variability and long-term sea ice loss mean for polar bears? Summer is the limiting season because polar bears depend on sea ice as a platform to hunt seals. With summer and early fall sea ice losses, polar bears have less time on the ice, eat less, and become skinnier—something that polar bear scientists like Steve Amstrup are currently observing. Any temporary increases in sea ice cover won't diminish the threat of long-term sea ice loss to polar bears.  

As Dr. Amstrup warns: "Polar bears can effectively feed only from the surface of the sea ice, and a progressively warmer world will hold less sea ice. In the long term, without greenhouse gas mitigation, the news for polar bears can only be bleak. But shorter-term variations can result in transient benefits to polar bears and buy us time to change our ways." 

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