Another Explanation of the Global Warming Pause in Doubt

One of the leading explanations of the decade-and-a-half “pause” in global warming is that aerosols—fine particles of various kinds, mostly man-made, but also volcanic in origin—have increased the albedo (reflectiveness) of clouds.  The cooling effect of high pollution in the early period of the industrial revolution in the 19^th and early 20^th century is well known and documented.

Geophysical Research Letters, one of the leading scientific journals in the climate world, has posted a pre-publication copy of a forthcoming paper that has gone through complete peer review casting significant doubt on this hypothesis. “Uncertainty in the magnitude of aerosol-cloud radiative forcing over recent decades” was conducted by ten scientists from the leading climate science establishments mostly in Britain.

The complete article is typically dense and nearly unreadable for the layperson, but the abstract, read closely, contains a significant finding:

Aerosols and their effect on the radiative properties of clouds are one of the largest sources of uncertainty in calculations of the Earth’s energy budget. Here the sensitivity of aerosol cloud-albedo effect forcing to 31 aerosol parameters is quantified. Sensitivities are compared over three periods; 1850-2008, 1978-2008 and 1998-2008. Despite declining global anthropogenic SO2 emissions during 1978-2008, a cancellation of regional positive and negative forcings leads to a near-zero global mean cloud-albedo effect forcing. In contrast to existing negative estimates, our results suggest that the aerosol cloud-albedo effect was likely positive (0.006 to 0.028 Wm^-2) in the recent decade, making it harder to explain the temperature hiatus as a forced response. Proportional contributions to forcing variance from aerosol processes and natural and anthropogenic emissions are found to be period dependent. To better constrain forcing estimates, the processes that dominate uncertainty on the timescale of interest must be better understood. (Emphasis added.)

Translation: the findings of this study indicate that the aerosol explanation of the warming pause is not supported by this evidence. And, as the last highlighted sentence indicates, at least these ten scientists are saying we don’t know enough about how this works.

The conclusion of the main text of the study is slightly more emphatic:

Here we show that the credible range of CAE forcing during the 1998-2008 period is (0.006 W m−2, 0.028 W m−2), indicating that a positive CAE [Cloud Aerosol Effect] forcing is likely during the 1998-2008 period. In contrast to existing negative estimates of aerosol indirect forcing our results suggest that the aerosol cloud-albedo effect was likely positive during the last decade, indicating that the hiatus in surface warming cannot be attributed to CAE forcing. A likely positive CAE forcing during this period re-frames the role of CAE forcing in explaining model overestimation of recent warming using external forcings. The attribution of the present pause in surface warming as a forced response is more difficult given the present results. The existing role of aerosols in explaining the hiatus therefore needs to be re-evaluated.  (Emphasis added.)

Understand that the climateers claim that current albedo cloud effects are negative, and thus holding down warming, while this study suggests the current cloud albedo effect is slightly positive (that is, warming, though very slightly). One argument of the climatistas is that if China, India, and other countries follow the United States and Europe in reducing particulate pollution, the reflective effect of clouds will diminish and warming will resume.  But with this study that hypothesis needs to be rethought. And as John noted yesterday, the other leading explanation—ocean warming—isn’t looking to hot (pun intended) right now either.

Let’s see if this study gets any ink in the New York Times or other mainstream media publications. I predict you won’t need to turn your cricket detector up to eleven.