Tuesday, January 5, 2016

New Light On The Role Of Low Altitude Clouds In a Changing Climate

That the properties of clouds play a significant role in climate change models should not come as a huge surprise to anyone who's been paying attention. This is given the fact that Earth's climate is the product of complex interactions between the Sun, the ocean and the atmosphere. For reference, climate scientists explore these exchanges - especially between ocean and atmosphere - using general circulation models (GCM) which compute the respective circulations in ocean and atmosphere.

The GCMs take into account Earth's rotation as well as thermodynamic interactions such as radiation and latent heat. This is why  - for example - whenever I enter a public forum where climate change is being discussed (as I did over the weekend via an article in the Financial Times) I always publish a thermal physics test for the plethora of deniers popping off about "hoaxes". Because if they don't know the basics of thermal processes - including about evaporation, condensation, convection, latent heat, radiation, sublimation etc.-  they really are just parroting swill from the denier think tanks.

Anyway, a major headache has been how exactly clouds influence climate feedback.  Now, Hu et al (Geophysical Research Letters, 2015) have developed a methodology to identify how low lying tropical marine cloud cover responds to temperature changes in GCMs, thereby more precisely assessing the role of clouds in a changing climate.

The team first identified two major variables that control cloud coverage: latent heat flux and moisture gradient. The first describes the energy released as water evaporates from Earth's surface and collects in the troposphere. The second represents how moisture collects in the air with respect to altitude.  The important fact here is that the boundary layer closest to the Earth's surface gets drier with increasing altitude. (In the Hu et al study, sea surface temperature was used as a proxy for the two variables)

The investigators found that the cloud feedbacks created in GCMs could be attributed to three key changes. The first of these is stronger tropical inversions - or meteorological phenomena in which the normal stratification of the atmosphere is reversed so cool, moist air is trapped under warm, dry air. The stronger the inversion, the less mixing occurs between the boundary layer and the dry, upper part of the troposphere. . Less mixing produces a shallower, wetter, cloudier boundary layer.

The other changes are an increase in surface latent heat flux and an increase in the difference in moisture content at different heights - arising from a greater increase in moisture near the surface compared with higher altitudes. By contrast to the tropical inversions, these last two hinder the formation of cloud cover.

In general, the Hu team found that when the models accurately simulated the response of low cloud coverage to inversion strength and sea surface temperature variations, low cloud cover decreased. Note here that without the reflective layer provided by clouds, there is lower albedo so the Earth's surface absorbs more solar radiation. Thus, a decrease in cloud coverage can drive an overall increase in warming.

This recent work must therefore be seen in perspective with earlier research (Eos Transactions paper: 'Can Earth’s Albedo and Surface Temperature Increase Together’ in Vol. 87, No. 4, Jan. 24, 2006, p. 37) wherein the authors pointedly noted evidence that Earth’s albedo increased from 2000 to 2004 but that this had NOT led to a reversal in global warming. They also remarked on apparent temperature anomalies and divergence between differing altitudes but pointed to the differences between clouds at those altitudes.

The authors cited (to then) the most up to date cloud data from the International Satellite Cloud Climatology Project (ISCCP). The data – from a range of meteorological satellites covering the entire Earth- disclosed the most likely reason for the anomaly was primarily in the redistribution of the clouds. As the authors observed:

"whereas low clouds have decreased during the most recent years, high clouds have increased to a larger extent leading to both an increase in cloud amount AND an increased trapping of infrared radiation.”

Clearly, further studies of cloud sensitivity are warranted and one hopes the Hu et al paper will provide a basis to improve on the GCM model forecast potential.

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