Most of the Observed Warming since the Mid-20th Century Likely Not from Human GHG Emissions?
A few weeks ago, over at the blog MasterResource.org, WCR’s Chip Knappenberger took a look at just how confident one should be regarding the amount of warming that anthropogenic greenhouse gas (GHG) emissions have caused since the mid-20th century.
The IPCC claims that it is “very likely” that “most” of the warming since then has been the result of human GHG emissions. In IPCC parlance, “very likely” means with a greater than 90% likelihood. The EPA parrots the IPCC’s claim in the Technical Support Document for their Endangerment Finding (TSD, p. 2):
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG [greenhouse gas] concentrations.
But, in his MasterResource.org article, Knappenberger shows that this statement is not supported by recent findings in the scientific literature—findings that have appeared in the literature subsequent to the publication of the IPCC’s statement. He concluded that the IPCC’s statement—especially the likelihood designation—should be re-evaluated in light of what we know now.
In some sense, however, Knappenberger’s analysis did not go far enough. While he used middle-of-the-road estimates for the warming influence of some non-GHG factors, in some cases he was being too conservative—like when it comes to the non-climatic influences on local thermometers—and, further, he failed to include a potential impact from solar changes.
So here, we take Knappenberger’s analysis a bit further, and show that it is easy to demonstrate, using the contents of the peer-reviewed scientific literature, that anthropogenic GHG emissions could be responsible for less than one-third of the warming in the extant global temperature records.