Does Cloud Cover Explain Recent Temperature Trends?

Climate science is in its infancy, and much of what drives the Earth’s climate is understood poorly if at all. It is clear, however, that clouds have a significant impact on temperatures. A recent paper by Australian physicist John McLean suggests that cloud cover, combined with the El Nino-Southern Oscillation, explains temperature trends from 1950 to the present. Anthony Watts summarizes the paper’s findings:

Key points of the paper:

* Indicates that the temperature pattern can be attributed to a sequence of events, namely a shift in the prevailing ENSO conditions, then a reduction in total cloud cover and then a shift on cloud [cover] (decrease in low level cloud that was largely offset by an increase in mid and upper level cloud)

* Uses the Trenberth, Fasulo & Kiehl energy balance diagram to show that the loss in total cloud cover caused an increase in heat energy being absorbed at the Earth’s surface that was greater than the increase that IPCC 5AR claims was due to greenhouse gases

* Indicates that greenhouse gases played little if any part in the warming, which not only refutes the IPCC’s belief or opinion but also means that there is negligible, or even no, 16 or more years’ of “missing heat” to be found.

* Shows the changes in cloud cover and temperature both as global averages and then for the six latitude bands each of 30 degrees, the latter indicating the changes in cloud cover applied to most latitude bands except the Antarctic and to a less extent 30S-60S.

* Doesn’t attempt to identify the reason for the reduction in total cloud cover or the shift from low level cloud.

You can read the paper for yourself here. These are some highlights:

The pattern of the residual temperature anomaly does not correspond to changes in atmospheric carbon dioxide, which has been increasing almost linearly from 1958, when monitoring began and certainly has not stabilised since 2000. The pattern is also inconsistent with other greenhouse gases, including methane, whose concentration rose from 1984 to 1999, and CFC-12, which increased from 1979 to year 2000.

The residual temperature anomaly is however consistent with two variations in cloud cover. The first is a reduction in total cloud cover, which would allow more solar insolation to strike the Earth’s surface and in particular more radiation in the UV-B range; the second is a decrease in low-level cloud and increase in mid- and upper-level cloud.

McLean notes that the effect of documented changes in cloud cover is significantly greater than the impact that the IPCC attributes (incorrectly, in all likelihood) to greenhouse gases:

The reduction in total cloud cover is significant in the context of the energy budget described by Trenberth et al. [34] , which indicates that clouds reflect 23% of the 341 Wm−2 (i.e. 79 Wm−2) of incoming solar radiation. The reduction in total cloud cover of 6.8% means that 5.4 Wm−2 (6.8% of 79) is no longer being reflected but acts instead as an extra forcing into the atmosphere, some of which will be lost when it adds to the longwave radiation to space. Of course clouds have many other effects on the earth’s radiation budget many of which are not fully understood, but a change of 5.4 Wm−2 is potentially of considerable significance.

To put this into context, the IPCC Fifth Assessment Report [1], section 8.5.2, states that the total anthropogenic radiative forcing for 2011 relative to 1750 is 2.29 [1.13 to 3.33] Wm−2 for all greenhouse gases and for carbon dioxide alone is 1.68 [1.33 to 2.03] Wm−2.

The increase in radiative forcing caused by the reduction in total cloud cover over 10 years is therefore more than double the IPCC’s estimated radiative forcing for all greenhouse gases and more than three times greater than the forcing by carbon dioxide alone. Even the upper limits of the IPCC’s estimates fall well short of the increase in radiative forcing caused by the reduction in total cloud cover.

McLean states his conclusions as follows:

Since 1950, global average temperature anomalies have been driven firstly, from 1950 to 1987, by a sustained shift in ENSO conditions, by reductions in total cloud cover (1987 to late 1990s) and then a shift from low cloud to mid and high-level cloud, with both changes in cloud cover being very widespread.

According to the energy balance described by Trenberth et al. (2009) [34], the reduction in total cloud cover accounts for the increase in temperature since 1987, leaving little, if any, of the temperature change to be attributed to other forcings.

With ISCCP cloud cover data available only for the period from 1984 to 2009 this hypothesis should be regarded as tentative.

Pretty much all hypotheses in the filed of climate science should be regarded as tentative, given the complexity of the Earth’s climate system and our rudimentary understanding of it. Alleged certainties about climate are the product of politics, not science.