How Much Warming Can We Avoid?

Stabilization of Carbon Dioxide (2)

In the previous blog I highlighted a couple of papers which addressed the issue of stabilizing the amount of carbon dioxide in the atmosphere. Two points were that 1) each year we delayed in reducing carbon dioxide would lead to an additional 9 parts per million (ppm) of carbon dioxide in the atmosphere, and 2) that the warming associated with excess carbon dioxide would be around for more than a 1000 years even if the carbon dioxide levels were stabilized.

The figure below, from IPCC, shows a set of stabilization curves. This was in the 2001 IPCC report . The thick red line is the emissions for a business as usual scenario. The other curves show a future in which we start to reduce emissions below business as usual. We are already well beyond the 350 ppm curve, and we are approaching the time where we have to start reducing emissions to stay less than 550 ppm. And business as usual? Until the recession came upon us, we were increasing our emissions.




Figure 1: Stabilization of Atmospheric Carbon Dioxide as a Function of Emissions (from IPCC). (Note 20130117: This figure is flawed, and in fact, suggests stabilization more quickly and at smaller values as revealed by future research.)

We are, therefore, committed to significant warming. It makes sense to ask the question - what would be the impact of reducing our emissions? Can we do something that matters?

It has often been posed that in order to avoid “dangerous” climate change that we need to keep the average warming at the surface to less than 2 degrees centigrade above the pre-industrial values. We have already used half, or more, of those 2 degrees in the 20th century. So, with this criterion, we need to keep the warming in this century to 1 degree or less. As we start to think about the reality of this challenge, then we have to consider not only carbon dioxide, but other greenhouse gases such as nitrous oxide and methane. And, of course, we have to consider the greenhouse effect of water vapor which will increase as the atmosphere warms.

In a recent paper, Warren Washington and colleagues posed the question - How much climate change can be avoided by mitigation? They performed a series of experiments that compared business as usual to a reduction of carbon dioxide emissions by 70% in 2100. The numerical simulations were done with the Community Climate System Model . This paper is summarized with a good color figure in a press release from University Corporation for Atmospheric Research.

The most basic result from this paper is that with this reduction about one half of the expected warming would be avoided. The biggest absolute impact would be at high latitudes, consistent with the global warming being strongest at the poles. Following through, the impacts in Canada and Siberia are amongst the largest; hence, the reduction of greenhouse gases by 70% in 2100 would save a substantial amount of permafrost. Similarly, it would be possible to stabilize and maintain some Arctic sea ice. Very important, the sea level rise due to thermal expansion of water is reduced by more than one third.

This study suggests that if we were reduce the greenhouse gas emissions by 70% it would be enough to make a difference. It would stop what seem to be runaway impacts in the Arctic with melting permafrost and sea ice. The impact on the melting of ice sheets around the perimeter of Greenland was not explicitly calculated, but it is possible to determine that it would take many centuries for the ice sheets to stabilize. There would still be enormous ecological changes.

Going back to the previous blog … The amount of carbon dioxide in atmosphere in these stabilization experiments is about 450 ppm. We saw in the previous blog that with an aggressive reduction in carbon dioxide emissions starting today we would see 475 ppm. Therefore, our ability to keep warming at the level indicated by the Washington paper is already in doubt. And the reduction used in the Washington study is the minimum that gives us a chance of avoiding the 2 degree global average temperature rise. Given our commitment to economic growth, the relation of the economy to energy, and the low cost of fossil fuels, I conclude that we have to develop other ways of managing carbon dioxide than simply through the reduction of emissions. Note, I chose managing carbon dioxide, rather than managing the heat budget of the planet.


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Ye Olde Stabilization Blog: A Strange Urgency
Previous Blog: Warm for a 1000 Years