Feature: Checking their homework
For decades now we have heard how global warming is going to get us, how a doubling of atmospheric carbon dioxide will lead to scary amounts of warming. How's that working out?
First, a couple of boring numbers:
The IPCC (alt: IPCC) and the European Environment Agency both provide the formula for calculating change in radiative forcing (ΔF) in W/m2. For carbon dioxide (CO2) this formula is given as ΔF = αln(C/Co) where C and Co are the current and pre-industrial concentrations of CO2, respectively and α = 5.35. Some would immediately argue this overstates average net forcing from increasing atmospheric CO2 and we'd tend to agree but the inflated value is not important, at least, not yet -- call it an inbuilt "safety margin," if you like.
Atmospheric CO2 is presented in parts per million by volume (ppmv). There is no universal standard for what we mean by a doubling of CO2 and various numbers are used, most commonly 560 (2x280 -- the common pre-Industrial revolution reference) and 600 (2x300 -- presumably benchmarked from early in the Twentieth Century).
Since most people seem to conceive the situation as two times "natural," which we take to mean immediately pre-Industrial Revolution, we'll be
using the former. From the above formula then, the change in forcing from a doubling of pre-Industrial Revolution atmospheric CO2 =
According to the National Academies' Climate Change Science: An Analysis of Some Key Questions (2001), doubling CO2 (to 600 ppmv) would lead to a forcing of about 4 W/m2, so we guess these figures are close enough for our purposes here.
So, how's that working out in the real world? How could we tell?
The Carbon Dioxide Information Analysis Center provides Recent Greenhouse Gas Concentrations, complete with increased radiative forcing. Going by the July 2009 update Earth is already experiencing three-fourths of the increased forcing estimated from 2xCO2 (2.99 W/m2) from greenhouse gas changes alone.
Now, observations tell us that Earth responds quite rapidly to forcing changes, for example the range from 12 °C-15.8 °C and back each year as the greater land mass of the northern hemisphere receives more and less solar forcing with the changing seasons:
According to the IPCC's AR4 assessment for policymakers, "The total temperature increase from 1850 – 1899 to 2001 – 2005 is 0.76 [0.57 to 0.95] °C", so three-fourths of the expected forcing has delivered three-fourths of a degree warming...
Given that black carbon has been "blamed" for a percentage of estimated warming, as has solar activity and lets not forget land use change with only about one-third of estimated warming due to changes in atmospheric carbon dioxide, the chances of 2xCO2 delivering even 1 degree warming would appear, um... limited, much less the rather imaginative 3 °C proposed above.
And since carbon dioxide increases have so little effect on global mean temperature it should be obvious that spending fortunes restricting carbon dioxide emissions will also have irrelevantly small effect.
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