Clouds: The Wild Card Bruno Tremblay McGill University

Effect of Clouds Night Time

Facts about Clouds Clear-sky day are warm Clear-sky day are warm Cloudy-sky day day are cold Cloudy-sky day day are cold Clear-sky night are cold Clear-sky night are cold Clear-sky day are warm Clear-sky day are warm

Greenhouse effect of Clouds Clouds absorb LW radiation from the surface and then re-emits upwards at their own temperature Clouds absorb LW radiation from the surface and then re-emits upwards at their own temperature Since clouds are colder than the surface – temperature decreases with heights – the earth- atmosphere system emits less radiation to space Since clouds are colder than the surface – temperature decreases with heights – the earth- atmosphere system emits less radiation to space

Low level clouds Reflects SW emits at high temp Cools the earth High Level Clouds Transparent to SW Emits at low temp Warms the earth Thick Clouds Reflect SW Emits at low temp Neutral effect

Radiative Forcing of Clouds Low Cloud Forcing High Cloud Forcing

Different clouds

Global Annual Mean Cooling Effect of Clouds = 48 W/m2 Cooling Effect of Clouds = 48 W/m2 Warming Effect of Clouds = 31 W/m2 Warming Effect of Clouds = 31 W/m2 Net Effect of Clouds = 17 W/m2 (Cooling) Net Effect of Clouds = 17 W/m2 (Cooling) Clouds – in today’s climate – act to cool the climate Clouds – in today’s climate – act to cool the climate

Comparison with CO2 Anthropogenic increase in CO2: 280  490ppm (40% increase) Anthropogenic increase in CO2: 280  490ppm (40% increase) Reduction in thermal outgoing radiation: 1.6 W/m2 Reduction in thermal outgoing radiation: 1.6 W/m2 Equvalent to placing a 60W light buld every 6m by 6m surface area over the entire earth surface. Equvalent to placing a 60W light buld every 6m by 6m surface area over the entire earth surface.

Conparison with CO2 CO2 increase in the atmosphere is equivalent to: CO2 increase in the atmosphere is equivalent to: 1.6/48 * 100 = 3% change in reflection of solar radiation by clouds 1.6/48 * 100 = 3% change in reflection of solar radiation by clouds 1.6/31 * 100 = 5% change in the reduction of thermal radiation by clouds 1.6/31 * 100 = 5% change in the reduction of thermal radiation by clouds 1.6/17 * 100 = 9% change in net effect of clouds on radiation 1.6/17 * 100 = 9% change in net effect of clouds on radiation Are clouds sensitive to changes in earth temperature? Are clouds sensitive to changes in earth temperature?

Cloud feedbacks on Climate Warming GH Effect Cloud reflection Weaker Stronger Weaker Stronger Exacerbate mitigate

Positive feedback More high level of clouds More high level of clouds Fewer low level clouds Fewer low level clouds Increase in height of cloud top Increase in height of cloud top Decrease in cloud thickness Decrease in cloud thickness

Positive feedback Fewer high level of clouds Fewer high level of clouds More low level clouds More low level clouds Decrease in height of cloud top Decrease in height of cloud top Increase in cloud thickness Increase in cloud thickness

What are the cloud net effect on climate? We don’t know – But we can try! We don’t know – But we can try! Use Climate Model Use Climate Model Use data from the last 50 years Use data from the last 50 years

GCM - Uncertainties

Satellite Data Two problems: Two problems: Changes in clouds may not be linked with temperature changes Changes in clouds may not be linked with temperature changes Satellite were made for weather not climate monitoring Satellite were made for weather not climate monitoring Satellite data show decrease of high level clouds over ocean and land in accord with land obs Satellite data show decrease of high level clouds over ocean and land in accord with land obs Satellite data show a decrease in low level clouds over ocean and land in disagreement with land obs Satellite data show a decrease in low level clouds over ocean and land in disagreement with land obs