Friday, January 28, 2011
Climate Change and Risk Assessment
This is a draft that I will be revising.
There is a scientific consensus that mankind's activities is causing our climate to change. The consensus starts to thin out when how much, how soon and what to do are discussed. The main culprit in Anthropogenic Global Warming (AGW) are the so called greenhouse gases with carbon dioxide being in the main role. I use the terms AGW and greenhouse gases, because Svante Arrhenius developed the theory around the turn of the 20th century.
Arrhenius theorized that atmospheric carbon dioxide concentration change was the driving force behind the cyclic glacial and interglacial periods in our history. Since he lived in the current Holecene Interglacial period, he calculated that halving atmospheric CO2 would decrease global temperature by about 4.5 degrees C and that doubling would increase temperature by about 5.5 degrees C. He defined the relationship between radiative forcing and carbon dioxide concentration as F=alpha*ln(C/Co). The constant, alpha, is derived via the Stephan-Bolzman law. His equation is still used today.
Another scientist of the day, Knut Angstrom, criticized Arrhenius' results based on the absorption bands of carbon dioxide (there was a mistake in Angstrom's experiment by the way). A decade later, Arrhenius revised his estimate for a doubling of CO2 to 1.6 degrees. While many skeptics of AGW use this revision to indicate that current climate sensitivity to CO2 doubling is over estimated, it is more likely that 1.6 is what Arrhenius considered to be the lower bound of sensitivity. Mistakes by both Arrhenius and Angstrom actually lead to a very reasonable range of climate sensitivity from 1.6 to 5.5 degrees C.
There is an extremely high probability that actual climate sensitivity lies in that range. That range though is too large to accurately determine the risk climate change places on us and our planet. The number of attempts to more accurately define sensitivity is large and growing. The current consensus is 2 to 4.5 degrees C. That consensus still leads to a wide range of uncertainty in calculating risk.
Dr. James Annan, is a scientist that studies the risk and potential economic impacts of climate change mitigation. (There is probably a better way of describing his work, but that is mine.) Using Bayesian Statistics, he has implied a sensitivity range of 1.3 to 3.7 degrees C. I use the term implied because the range and methods are far from definitive. Dr. Annan, very wisely, is calling for a multi-discipline approach to determining the most accurate estimate of climate sensitivity.
No statistical method can predict the unpredictable, so there will always be the possibility that sensitivity lies outside that range. There is always the possibility that the atmosphere will respond in a matter that will limit the impact of carbon dioxide doubling. For purposes of risk management, that range does illustrate the need for pragmatic action to reduce carbon dioxide emissions.
Given our need to reduce dependence on foreign oil, reduce ecological damage from fossil fuel use, transport and procurement and maintaining a relatively stable world economy, action needs to be taken anyway. From a decision making vantage it is enough to know that the climate is warming due in some measure to man's activities and the warming could be very bad!
A more accurate estimate of climate sensitivity is important to determine mitigation and adaption strategies for a warming environment. Realistically, substantial reductions in carbon emissions are unlikely before 2030. This makes alternate energy choices important by region and country, not only for economic reasons, but for reasons of adaption and mitigation.
So, in my mind the discussion of global warming should split into two paths. One, pragmatic action to reduce potential damage and two, action to improve our knowledge of climate sensitivity to decrease uncertainty.
With the exception of the 1.6 C revision by Arrhenius, most of the information discussed above is available at Wikipedia under Climate Sensitivity. Dr. Annan's papers discussed can be found here and here.
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