Cloud feedback adds 50-100% to climate sensitivity

According to Atmospheric Sciences Professor Andrew Dessler, of Texas A&M University, in an interview with BBC’s Science in Action.

Dessler is author to a new study in Science, ‘A Determination of the Cloud Feedback from Climate Variations over the Past Decade,’ published in Friday’s edition. He has investigated cloud responses over the short-lived but relatively strong temperature rises and declines that are associated with the ENSO oscillation, better known for its extreme states, El Niño and La Niña.
The [long] question is: do increased water evaporation and condensation over the world’s oceans, leading to a net increase in clouds with warming, lead to amplification of that warming (a positive climate feedback) due to further infrared absorption – or to a compensating cooling (a negative feedback), due to increased solar reflection with increased cloud cover and Earth surface albedo?

His findings support what climate models suggest and what the majority of climate scientists suspected: increased cloud formation acts as a positive feedback.

On October 19 we wrote about a new paper by NASA, called ‘CO2: The Thermostat that controls Earth’s Temperature’, and that argues, based on the GISS climate models, that water feedbacks are the main engine to CO2-induced climate change. ‘Real climate sensitivity’ can be attributed for 25 percent to direct warming effects of CO2 itself. Clouds are responsible for another 25 percent and water vapour increases contribute 50 percent.

The difficulty with assessing the warming impacts of clouds is they differ with cloud type. Stratus and cirrus types offer good reflection for relatively little mass and therefore infrared absorption. Cumulus clouds may look nice and white when we see the Sun shining on their sides, but they also pack a lot of excess heat.

It’s yet unclear what the ramifications of this news are for geoengineering research. One of the best favoured ideas so far – sometimes referred to as ‘cloud seeding’ – suggests deliberate attempts to increase cloud cover could have a cooling effect. With the Science and NASA studies in mind, the enthusiasts better choose the right air pressure circumstances for their experiments. On the one hand you need a good temperature inverse up high to get your desired stratus layer. On the other you require the nice thermodynamic instability to ever reach that level with your aerosols. These don’t match. You’d either end up with your aerosols on your feet or with more cumulus clouds, only exacerbating the problem – we now know.

© Rolf Schuttenhelm |

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