(Part of the How to Talk to a Global Warming Skeptic guide)

Objection: Taking into account the logarithmic effect of CO2 on temperature, the 35 percent increase we have already seen in CO2 concentrations represents about three-quarters of the total forcing to be expected from a CO2 doubling. Since we have warmed about 0.7 degrees Celsius so far, we should only expect about 0.3 degrees more for a doubling from pre-industrial levels, so about 1 degree total, not 3 degrees as the scientists predict. Clearly the climate model sensitivity to CO2 is much too high.

Answer: Even without addressing the numbers in this argument, there is a fundamental flaw in its reasoning.

We don't yet know exactly how much the climate will warm from the CO2 already in the air. There is a delay of several decades between forcing and final response. Until an equilibrium temperature is reached, present day observations will not tell us the exact value of the climate's sensitivity to CO2.

The reason for this is primarily the large heat capacity of the oceans. The enhanced greenhouse effect from higher CO2 levels is indeed trapping energy in the climate system according to expectations, but the enormous quantity of water on earth is absorbing most of the resulting heat. Due to water's high heat capacity, this absorbed energy shows up as only a modest ocean warming, which in turn dampens the temperature change on land and lowers the global average trend.

This is commonly referred to as the climate system's thermal inertia. According to model experiments and consistent with data from past climate changes, this inertia results in a lag of several decades between the imposition of a radiative forcing and a final equilibrium temperature.

Now let's look at a couple of further details. CO2 is not the only factor affecting global temperature. There is a phenomenon called "global dimming" counteracting greenhouse gas warming. Global dimming refers to the blocking of incoming sunlight by particulate pollution in the troposphere and airplane contrails in the stratosphere. It is not a well quantified effect, but it may well be masking a great deal more warming; it is definitely masking some.

This is just one example of why we cannot attribute global temperature trends entirely to CO2 -- the same mistaken premise that fuels arguments about the mid-century cooling trend.

I believe it was Richard Lindzen who first made this argument [PDF] about climate sensitivity. The numbers he uses don't add up. A 35 percent increase in CO2 should correspond to 43 percent of the forcing from two times CO2 (ln(1.35)/ln(2)= 43%), which is not three-fourths.

The original article for this Skeptic Guide entry had an extremely interesting discussion under it, for anyone interested.

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