Tuesday, December 21, 2010

On Thin Ice

When it comes to charismatic megafauna, polar bears are right up there in terms of cute stuff people like to like. Lately they have also been the poster-bear against climate change. That makes sense, considering that polar bears (Ursus maritimus) occur only in the Northern Hemisphere and are dependent on sea ice for access to their marine mammal prey, mainly seals. Declines in summer sea ice have been associated with declining physical stature, declining physical condition, poorer survival rates, and declining population sizes for these bears. This sea ice decline has been linked to these declines.

A new paper this week in Nature uses projections of twenty-first century global mean surface air temperature (GMAT) and data from the Community Climate System Model (CCSM3) to test the hypothesis that a tipping point will lead to irreversible loss of seasonal ice habitat as GMAT increases. Basically, that there are some elements/variables within a system that, when changed enough, cause habitats that support cold-dependent species to disappear abruptly and irreversibly. Namely when a particular GMAT is exceeded.
In a nutshell, they tested whether mitigating the rise in greenhouse gases could improve the outlook for polar bears.
Now, a USGS study in 2007 concluded that two-thirds of the world's polar bears could disappear by 2050 if atmospheric temperatures continue to increase due to greenhouse gases. Their model was a general circulation model (GCM) that projected losses of Arctic sea ice based on the Special Report on Emissions Scenarios (SRES) and a "business as usual" greenhouse gas emissions scenario, where emissions continue to increase and the carbon dioxide concentration reaches 689 parts per million (ppm) by the end of the century. However, they did not consider the possible benefits of greenhouse gas mitigation. Think about that in terms of tipping points. If you mitigate too little and/or too late then you get no conservation benefits for polar bears as their ice would already be gone. If you mitigate more and/or soon enough then you save the ice and the bears. That's the thought anyway. So the researchers modeled 5 different scenarios that ranged from "business as usual" all the way to aggressive cuts that reduce carbon dioxide concentrations to 368 ppm, those seen in the year 2000.
The study concludes that mitigating the rise in greenhouse gases will result in substantially more sea ice habitat being retained. The business-as-usual-model shows a 50% loss of sea ice by 2050 whereas the aggressive-mitigation-model shows only a 20% loss. They also show that this habitat retention, in turn, will allow polar bears to persist throughout the century in greater numbers and in more areas. The business-as-usual-model shows a 50-80% chance of polar bears disappearing from these habitats whereas the aggressive-mitigation-model shows only a 25-50% chance. However, the models did not give the thresholds or tipping point values that will lead to irreversible ice loss. They found that sea ice will decline at a steady rate as global mean annual temperature rises.
The paper also addresses positive feedback in this system. Its all about albedo, or how strongly a surface reflects light. Its really a quite logical scenario. Ice reflects light very well, warmer temperatures cause ice to melt, retreating ice means less reflective surface, retreating ice also means more exposed water, the darker water absorbs more sunlight, more absorbed light increases temperatures, increased temperatures melt more ice. And so on and so on. In this paper they test models that might counter this feedback mechanism. They specifically refer to rapid ice-loss events (RILEs). These rapid freezes result from going from open water to cold conditions reappearing in the Fall, and these compensate for the effects that are working to provide the potential tipping points.
Polar bears are not out of the woods yet. After all, we are still running the business-as-usual-model. And, in the past, models predicting sea ice loss have fallen short. But this paper shows us some good news, that with proper mitigation we can potentially slow down the decline.
Here's the paper:
Amstrup, Steven C. et al. (2010) Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence. Nature: 468, 955-958. (DOI: 10.1038/nature09653)

Additionally, I suggest reading this paper on polar bear and grizzly hybridization:
Kelly, Brendan P., Andrew Whiteley, and David Tallmon (2010) The Arctic Melting Pot. Nature: 468, 891. (DOI:  10.1038/468891a)
Story links:
(image from metro.co.uk)

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