Wednesday, December 19, 2012

Matt Ridley vs. Marty Weitzman

In today's WSJ,  Matt Ridley has an optimistic climate change piece.  Unlike my work on climate change adaptation, he ignores how capitalist cities, individuals and firms respond to an anticipated challenge.  Instead, Ripley focuses on what we know about the function mapping aggregate global concentrations of CO2 increases into average temperature increases.  Here is a quote:

"Given what we know now, there is almost no way that the feared large temperature rise is going to happen. Mr. Lewis comments: "Taking the IPCC scenario that assumes a doubling of CO2, plus the equivalent of another 30% rise from other greenhouse gases by 2100, we are likely to experience a further rise of no more than 1°C."
A cumulative change of less than 2°C by the end of this century will do no net harm. It will actually do net good—that much the IPCC scientists have already agreed upon in the last IPCC report. Rainfall will increase slightly, growing seasons will lengthen, Greenland's ice cap will melt only very slowly, and so on.
Some of the best recent observationally based research also points to climate sensitivity being about 1.6°C for a doubling of CO2." 
Ripley does not bother to engage with Marty Weitzman's work on fat tails (see any of his post 2008 papers posted here).  Weitzman's starting point is that we don't know the mapping from CO2 to temperature increases and that there are "fat probabilities" of truly horrible scenarios.  A risk averse individual or nation might seek the insurance of carbon mitigation.  
Ripley's certainty about a random variable strikes me to be quite strange and unlikely to encourage robustness and resilience.   How does he "know that he knows" that function?  I know that I don't know that function and I'm planning my life and investments accordingly.  

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