Stephen Schwartz on climate sensitivity

A recent paper Heat capacity, time constant and sensitivity of Earth's climate system by Stephen Schwartz determines a climate sensitivity of 1.1 ± 0.5°C. Eg - if you doubled atmospheric CO2, global temperatures would go up 1.1°C. This is about a third of the consensus view (around 3°C) and unsurprisingly has been getting some attention on skeptic websites. Consequently I've added a new skeptic argument climate sensitivity is low.

Schwartz calculates sensitivity as the quotient of the climate "time constant" and global heat capacity. The "time constant", or time for the climate system to return to equilibrium after a perturbation, is a key aspect of the paper and Schwartz estimates around 5 years.

However, as Schwartz points out in his study, climate recovers at different rates depending on the nature of the forcing causing the perturbation. Short term changes such as a volcanic eruption result in a short time constant of a few years. A long term increase in CO2 levels results in a recovery spanning decades. Schwartz rightly points out "as the duration of volcanic forcing is short, the response time may not be reflective of that which would characterize a sustained forcing such as that from increased greenhouse gases because of lack of penetration of the thermal signal into the deep ocean."

In spite of that, Schwartz filters out long term changes by detrending the time series data which has the effect of biasing the result towards a shorter time constant. The time constant for non-detrended data yields a time constant of 15 to 17 years. Consequently, the estimated time constant of 5 years is questionable - a value the final result hinges on.

More on Stephen Schwartz and climate sensitivity...

Posted by John Cook on Wednesday, 22 August, 2007

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