social cost of carbon

Hold the presses! A U.S. Government interagency working group has just released its updated Technical Support Document (TSD) on the social cost of carbon (SCC).

This is joyous news in some circles. “The ‘Social Cost of Carbon’ Is Almost Double What the Government Previously Thought,” Climate Progress enthuses. Why are they pleased? Because the higher the SCC, the stronger the (apparent) case for suppressing the production and export of hydrocarbon energy in general, and for blocking the Keystone XL pipeline in particular.

SCC is an estimate of how much damage an incremental ton of carbon dioxide (CO2) emissions does to humanity and the biosphere. SCC estimates are driven by assumptions about such issues as climate sensitivity (how much warming results from a given increase in CO2 concentrations), climate impacts (how warming will affect weather patterns and sea-level rise), economic impacts (how changes in global temperature, weather, and sea-level rise will affect agriculture and other climate-sensitive activities), and technological change (how adaptive capabilities will develop as climate changes).

Modelers feed the assumptions into computer programs called “integrated assessment models” (IAMs). By tweaking those values, the modeler can get pretty much any result he desires. Outcomes also vary based on the discount rate selected, i.e., how much people are assumed to value income in the future compared to income in the present.

Using three IAMs, three discount rates (2.5,% 3,% and 5%), and a fourth value representing low-probability catastrophic impacts, the interagency group calculates four SCC estimates for the year 2020. In the working group’s 2010 TSD, the SCC estimates were $7, $26, $42, and $81 (2007$). In the updated TSD, the corresponding estimates are $12, $38, $58, and $129 (2007$). Excuse me, but even for the high-impact projections, the updated estimate ($129) is 59% higher than the 2010 estimate ($81), which is more than a tad shy of “almost double.”

Let’s cut to the chase. Those who say the SCC is bigger than the government previously thought merely recycle the old saw that climate change is “worse than scientists previously thought.” They are mistaken. The climate change outlook is better than we have long been told.

One reason the updated estimates are higher is that the IAMs contain an “explicit representation” of sea-level rise “dynamics.” Are the modelers keeping up with the scientific literature? Consider two recent studies

• King et al. (2012): The rate of Antarctic ice loss is not accelerating and translates to less than one inch of sea-level rise per century.
• Faezeh et al. (2013): Greenland’s four main outlet glaciers are projected to contribute 19 to 30 millimeters (0.7 to 1.1 inches) to sea level rise by 2200 under a mid-range warming scenario (2.8°C by 2100) and 29 to 49 millimeters (1.1 to 1.9 inches) under a high-end warming scenario (4.5°C by 2100).

If 21st century sea-level rise is more likely to be measured in inches rather than feet or meters, shouldn’t SCC estimates decline?

And what about the 15-year period of no-net warming, which the climate science establishment did not predict and still struggles to explain? The warming pause is hard to square with the mantra of “worse than we thought.” It is evidence that the SCC is lower than they thought.

Let’s look at the disconnect between what they predicted and what happened.  The graph below comes from NASA scientist Roy Spencer:  [click to continue…]