Clouds:Clouds:The Wild CardThe Wild Card

Bruno TremblayBruno Tremblay

McGill UniversityMcGill University

bruno.tremblay@mcgill.ca

Effect of CloudsEffect of CloudsNight TimeNight Time

Facts about CloudsFacts about Clouds

Clear-sky day are warmClear-sky day are warm Cloudy-sky day day are coldCloudy-sky day day are cold

Clear-sky night are coldClear-sky night are cold Clear-sky day are warmClear-sky day are warm

Greenhouse effect of Greenhouse effect of CloudsClouds

Clouds absorb LW radiation from Clouds absorb LW radiation from the surface and then re-emits the surface and then re-emits upwards at their own upwards at their own temperaturetemperature

Since clouds are colder than Since clouds are colder than the surface – temperature the surface – temperature decreases with heights – the decreases with heights – the earth-atmosphere system emits earth-atmosphere system emits less radiation to spaceless radiation to space

Low level cloudsReflects SWemits at high tempCools the earth

High Level CloudsTransparent to SWEmits at low tempWarms the earth

Thick CloudsReflect SWEmits at low tempNeutral effect

Radiative Forcing of Radiative Forcing of CloudsClouds

Low Cloud Forcing High Cloud Forcing

Different cloudsDifferent clouds

Global Annual MeanGlobal Annual Mean

Cooling Effect of Clouds = 48 Cooling Effect of Clouds = 48 W/m2W/m2

Warming Effect of Clouds = 31 Warming Effect of Clouds = 31 W/m2W/m2

Net Effect of Clouds = 17 W/m2 Net Effect of Clouds = 17 W/m2 (Cooling)(Cooling)

Clouds – in today’s climate – Clouds – in today’s climate – act to cool the climateact to cool the climate

Comparison with CO2Comparison with CO2

Anthropogenic increase in CO2: Anthropogenic increase in CO2: 280 280 490ppm (40% increase) 490ppm (40% increase)

Reduction in thermal outgoing Reduction in thermal outgoing radiation: 1.6 W/m2radiation: 1.6 W/m2

Equvalent to placing a 60W light Equvalent to placing a 60W light buld every 6m by 6m surface area buld every 6m by 6m surface area over the entire earth surface. over the entire earth surface.

Conparison with CO2Conparison with CO2

CO2 increase in the atmosphere is CO2 increase in the atmosphere is equivalent to:equivalent to: 1.6/48 * 100 = 3% change in reflection 1.6/48 * 100 = 3% change in reflection of solar radiation by cloudsof solar radiation by clouds

1.6/31 * 100 = 5% change in the 1.6/31 * 100 = 5% change in the reduction of thermal radiation by cloudsreduction of thermal radiation by clouds

1.6/17 * 100 = 9% change in net effect 1.6/17 * 100 = 9% change in net effect of clouds on radiationof clouds on radiation

Are clouds sensitive to changes in Are clouds sensitive to changes in earth temperature?earth temperature?

Cloud feedbacks on Cloud feedbacks on ClimateClimate

Warming

GH Effect

Cloud reflection

Weaker

Stronger

Weaker

Stronger

Exacerbate

mitigate

Positive feedbackPositive feedback

More high level of cloudsMore high level of clouds Fewer low level cloudsFewer low level clouds Increase in height of cloud topIncrease in height of cloud top Decrease in cloud thicknessDecrease in cloud thickness

Positive feedbackPositive feedback

Fewer high level of cloudsFewer high level of clouds More low level cloudsMore low level clouds Decrease in height of cloud topDecrease in height of cloud top Increase in cloud thicknessIncrease in cloud thickness

What are the cloud net What are the cloud net effect on climate?effect on climate?

We don’t know – But we can try!We don’t know – But we can try!

Use Climate ModelUse Climate Model Use data from the last 50 yearsUse data from the last 50 years

GCM - UncertaintiesGCM - Uncertainties

Satellite DataSatellite Data

Two problems:Two problems: Changes in clouds may not be linked with Changes in clouds may not be linked with temperature changestemperature changes

Satellite were made for weather not Satellite were made for weather not climate monitoringclimate monitoring

Satellite data show decrease of high Satellite data show decrease of high level clouds over ocean and land in level clouds over ocean and land in accord with land obsaccord with land obs

Satellite data show a decrease in low Satellite data show a decrease in low level clouds over ocean and land in level clouds over ocean and land in disagreement with land obsdisagreement with land obs