Key Feedbacks in the Climate System

Transcription

1 Key Feedbacks in the Climate System With a Focus on Climate Sensitivity SOLAS Summer School 12 th of August 2009 Thomas Schneider von Deimling, Potsdam Institute for Climate Impact Research

2 Why do Climate Projections span a broad range of temperature increases? Emission Scenarios Uncertainty of Emissions Model Uncertainty IPCC 2007, Summary Report for Policy Makers

3 Glacial Cycles Strong Amplification of a Small Perturbation Vostoc Ice-Core (Petit et al., Barnola et al., 1999) CO 2 ature Tempera Kyrs B.P.

4 Incoming Solar - Reflected = Outgoing Longwave σ T 4 = S (1 α ) T = 255K = -18 o C T obs = 287K=15 o C T=33 o C greenhouse effect Earth/Atmosphere is in a delicate radiative balance Slight changes in this balance can cause large changes in global climate These changes can be amplified or dampened by positive or negative feedbacks

5 The Greenhouse Effect

6 Climate Stability The Earth s climate changes as a result of internal/external forcing: Changes in incoming solar radiation Volcanoes short time scale Anthropogenic emission of GHGs and aerosols Changes in the earth s orbit Plate tectonics Weathering long time scale These forcings can be thought of as a perturbation (or push) to climate stability. These changes can be amplified by positive or damped by negative feedbacks.

7 Climate Feedbacks 1. No Feedback Case Radiative Forcing RF λ 0!T 0 E.g.: Perturbing the System by Doubling CO 2 :!T 0 =λ 0 * RF => The climate system perturbation (Radiative Forcing RF) is about 3.7 W/m 2 Without Feedbacks acting the global temperature would adjust to this perturbation by an increase of about 1.2 C (models strongly agree! physical basis: Stefan Boltzmann Law of Radiation).

8 However: the climate system undergoes changes while warming up (e.g. cloud changes). If these changes impact global mean temperature, climate feedbacks come into play. 2. Feedback Case Radiative Forcing RF λ 0!T!T=λ 0 *(RF+F!T) F F!T T0 T0 T = = 1 λ F 1 f 0

9 A Question of Timescale The standard definition of Climate Sensitivity (i.e. the equilibrium change in global mean surface temperature following a doubling of CO 2 ) refers to fast feedbacks: Clouds ~ hours Water vapour ~10 days Measure of Feedback Strength Key Characteristic for Comparing Models Other components can undergo huge changes, but on slower timescales: Vegetation ~ decades to centuries Ice sheets ~ Centuries to millennia, but current estimates suggest that Ice sheets might respond on much shorter timescales! What measure of sensitivity is describing the feedbacks acting on What measure of sensitivity is describing the feedbacks acting on timescale of human interference with the climate system? (see discussing of Earth System Sensitivity by James Hansen and colleagues)

10 Ice-Albedo Feedback (+) E.g. by Changes in Solar Radiation Anthropogenic Emission of GHGs Initial Perturbation T Snow and Ice Area => Positive Feedback Albedo Absorption of Sunlight

11 National Snow and Ice National Snow and Ice Data Center, Boulder, CO 1979

12 Water Vapor Feedback As surface warms, equilibrium vapor pressure will increase (Clausius-Clapeyron) 7% increase per 1K warming 20% increase for 3K warming ~ 20 for general terrestrial conditions

13 Water Vapor Feedback (+) Initial Perturbation Earth Warms Water Holding Capacity of the Atmosphere (Clausius Clapeyron) IR Opacity of the Atmosphere Outgoing longwave radiation

14 Lapse Rate Feedback (-) Moist convective processes control the vertical temperature distribution over much of the earth (i.e., tropics and much of summer hemisphere). The moist adiabatic lapse rate is smaller in a warmer climate, thus temperature changes in the upper troposphere are greater than those at the surface. Greater warming aloft increases the outgoing longwave radiation, thus cooling the atmosphere. -> Negative Feedback

15 Effect of Clouds on Earth s Climate Clouds double Earth albedo (15% => 30%) surface cooling Clouds reduce longwave emission by about 30 W/m 2 surface warming Net Effect: Reduction of net radiative flux into the Earth System of about 20 W/m 2 Cover about 50% of Earth Figs from

16 Cloud Feedback How will clouds change if climate changes? Clouds can either cool or warm the climate system (mainly depending on their height). All cloud types reflect solar radiation, but only high clouds decrease infrared emission. The net effect of low clouds is to cool the climate (reflect solar, but little effect on infrared). The net effect of high clouds is to warm the climate (reflect some solar, strongly decrease infrared emission). => Negative or Positive Feedback (Current generation of GCMs show neutral to positive cloud feedback)

17 Climate feedbacks analysed from GCMs Negative Lapse Rate FB partially compensates positive water vapor feedback => Reduction of spread in WV+LR

18 Conclusions Temperature increase for 2xCO 2 The impact of a given feedback on T is strongly dependent on the presence of other feedbacks: If once a strong positive feedback exists, the addition of a rather T0 T0 T = = moderate positive feedback may produce a 1 λ F 1 f large additional temperature change K without feedbacks Assuming a feedback factor f 1 of 0.5 for water vapour, then T will be twice the magnitude of T 0. Adding a further feedback factor f 2 of only half the magnitude of f 1 (f 2 =0.25), then the T will be four times the value of T 0,S If f 2 was of the same magnitude as for water vapour (f 2 =0.5) would yield an infinite sensitivity!

19 What does this mean for uncertainty in CS? => Uncertainty in cloud feedback is largely responsible for the wide range of sensitivities displayed by models - despite the fact that their contribution to the feedback factor is smaller than that of water vapor! Roe 2008

20 What does this mean for uncertainty in CS? Gaussian distribution of feedback factor => skewed distribution => skewed distribution of temperature change

21 Take Home Messages Feedbacks play a crucial role in amplifying or dampening a forced temperature change (e.g. initiated by changes in Earth s Orbit). Observed changes in past climate and climate model diagnostics suggest an important contribution of positive feedbacks. Water Vapor Feedback is the largest contributor to Climate Sensitivity, while the spread in cloud feedback crucially determines the uncertainty range of Climate Sensitivity. Climate Sensitivity refers to fast feedbacks (water vapor, clouds, lapse rate, albedo), but for climate changes relevant on the time scale of anthropogenic inference with the climate system additional feedbacks have to be considered (such as changes in vegetation, ice sheets, aerosol content,...) and these feedbacks tend to increase the overall sensitivity.

22 Contributions to the uncertainty in climate feedbacks Cloud feedback Surface albedo feedback Water vapor feedback Radiative effects only Source: Dufresne & Bony, Journal of Climate 2008

23 All Feedbacks: Temp, W. Vapor, and Sfc Albedo are zonal land/ocean scale Cloud Feedback has much more zonal structure: ~ 1000 km. Soden et al. J. Climate July 2008