Atmospheric Processes

Transcription

1 Atmospheric Processes Steven Sherwood Climate Change Research Centre, UNSW Yann Arthus-Bertrand / Altitude

2 Where do atmospheric processes come into AR5 WGI? 1. The main feedbacks that control equilibrium climate sensitivity - hence long-term warming - are governed by atmospheric processes. 2. Rainfall changes that occur in a warmer climate depend on processes related to those above. A new chapter (7) was added to AR5 to cover key areas of uncertainty, aerosols and clouds. This chapter also assessed process studies relevant to precipitation change.

3 Feedbacks: What was known going into AR5

4 Feedbacks: What was known going into AR5 The largest feedback on climate is due to water vapour

5 Feedbacks: What was known going into AR5 The largest feedback on climate is due to water vapour It is offset by a smaller, and highly correlated lapse rate feedback due to the atmosphere warming faster than the surface.

6 Feedbacks: What was known going into AR5 The largest feedback on climate is due to water vapour It is offset by a smaller, and highly correlated lapse rate feedback due to the atmosphere warming faster than the surface. Most of the spread in model climate sensitivity is due to spread in the net feedback from clouds. In many models, the cloud feedback is positive and stronger than the sum of the two above. In others it is near zero.

7 Feedbacks: What was known going into AR5 The largest feedback on climate is due to water vapour It is offset by a smaller, and highly correlated lapse rate feedback due to the atmosphere warming faster than the surface. Most of the spread in model climate sensitivity is due to spread in the net feedback from clouds. In many models, the cloud feedback is positive and stronger than the sum of the two above. In others it is near zero. Climate models represent clouds very crudely and could have the feedbacks wrong.

8 Feedbacks: What was known going into AR5 The largest feedback on climate is due to water vapour It is offset by a smaller, and highly correlated lapse rate feedback due to the atmosphere warming faster than the surface. Most of the spread in model climate sensitivity is due to spread in the net feedback from clouds. In many models, the cloud feedback is positive and stronger than the sum of the two above. In others it is near zero. Climate models represent clouds very crudely and could have the feedbacks wrong. Independent insights into cloud feedback are potentially available from more detailed modeling efforts and from observations.

9 timescale (day) Cloud Processes Climate System General Circulation Model (GCM) Super Parameterization (MMF) & Global Cloud Resolving Model (GCRM) Cloud Resolving Model (CRM) & Large Eddy Simulation (LES) spatial scale (m)

10 Fig. 7.5

11 Fig. 7.7

12 Feedbacks: What is in the AR5 SPM

13 Feedbacks: What is in the AR5 SPM The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. (Not new, but much more confidence).

14 Feedbacks: What is in the AR5 SPM The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. (Not new, but much more confidence). The net radiative feedback due to all cloud types combined is likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds. {7.2} (First time explicitly claimed in IPCC)

15 Feedbacks: What is in the AR5 SPM The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. (Not new, but much more confidence). The net radiative feedback due to all cloud types combined is likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds. {7.2} (First time explicitly claimed in IPCC) The equilibrium climate sensitivity...is likely in the range 1.5 C to 4.5 C (high confidence), extremely unlikely less than 1 C (high confidence), and very unlikely greater than 6 C (medium confidence)16. {TFE6.1, Figure 1; Box 12.2} Text

16 Feedbacks: What is in the AR5 SPM The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. (Not new, but much more confidence). The net radiative feedback due to all cloud types combined is likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds. {7.2} (First time explicitly claimed in IPCC) The equilibrium climate sensitivity...is likely in the range 1.5 C to 4.5 C (high confidence), extremely unlikely less than 1 C (high confidence), and very unlikely greater than 6 C (medium confidence)16. {TFE6.1, Figure 1; Box 12.2} Text Note: a positive cloud feedback --> sensitivity > 2 C

17 = positive feedback contribution = ambiguous feedback contribution Rising High Clouds Broadening of the Hadley Cell Narrowing of Tropical Ocean Rainfall Zones Rising High Clouds Rising of the Melting Level Poleward Shift of Storms Less Low Clouds More Polar Clouds Equator 30º 60º Pole Fig. 7.11

18 = positive feedback contribution = ambiguous feedback contribution Rising High Clouds Broadening of the Hadley Cell Narrowing of Tropical Ocean Rainfall Zones Rising High Clouds Rising of the Melting Level Poleward Shift of Storms Less Low Clouds More Polar Clouds Equator 30º 60º Pole What could be missing? - low cloud feedback remains highly variable in models - cirrus cloud amount/thickness feedback might also be possible Fig. 7.11

19 Tropics Midlatitudes Greenhouse Warming Cloud Response Feedback Mechanism High clouds rise as troposphere deepens, increasing difference between cloud top and surface temperature. High clouds more effectively trap infrared radiation, increasing surface warming. Reduction in mid- and low-level cloudiness (left). Shift of cloudy storm tracks poleward into regions with less sunlight (right). Less sunlight reflected by clouds back to space, increasing surface warming. DRAFT Fig. FAQ 7.1

20 Tropics Midlatitudes Greenhouse Warming Cloud Response Feedback Mechanism High clouds rise as troposphere deepens, increasing difference between cloud top and surface temperature. High clouds more effectively trap infrared radiation, increasing surface warming. Reduction in mid- and low-level cloudiness (left). Shift of cloudy storm tracks poleward into regions with less sunlight (right). Less sunlight reflected by clouds back to space, increasing surface warming. These mechanisms are tied to large-scale circulation changes that are robust and relatively well understood. DRAFT Fig. FAQ 7.1

21 Rainfall: What is in the AR5 SPM

22 Rainfall: What is in the AR5 SPM Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases (see Table SPM.1). {7.6, 12.4}

23 Rainfall: What is in the AR5 SPM Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases (see Table SPM.1). {7.6, 12.4} Anthropogenic influences have contributed...to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence)...

24 Change in Precipitation (% ºC -1 ) A. GCM B. Observed Trends C. GCM Constrained by Obs D. CRM RCE E. LES RCE A observed trends global models temperature only A A B extratropics only tropics only C D global models combined with observations detailed storm models D E 0 Increasing CO 2 Time average 24 hr Extreme Fig. 7.11

25 Further Information Yann Arthus-Bertrand / Altitude