Understanding Global Climate Forcing from Land-Cover Changes

1. Impact ofAnthropogenic Land-Cover Changes on1) the global radiative forcing of planet EarthEdouard Davin, Nathalie de Noblet, Pierre Friedlingstein2) the characteristics of the El-Niño Southern OscillationEdouard Davin, Nathalie de Noblet, Christian Laguerre,Pascal Terray, Eric GuilyardiLaboratoire des Sciences du Climat et de l’EnvironnementUnité mixte CEA-CNRS-UVSQ / Gif-sur-Yvette / France

2. 3 snap-Shot experimentsfully coupled Atmosphere-Ocean (IPSL-CM4)+ interactive seasonally varying foliage density (LAI)200-year long simulations, analyses on last 50 years,all with pre-industrial aerosols and GHGs PAST- pre-industrial land-cover (year 1870) PRES – present-day land-cover FUTU – Future land-cover (year 2100, SRES A2) Change in anthropogenic land fraction (crops + pastures) PRES - PAST FUTU - PRES IMAGE2 SRES A2 (Alcamo et al. 1998) Ramankutty & Foley, 1999 ; Goldewijk, 2001

3. Part I:Impact of land cover change on surface climate:relevance of the radiative forcing concept Edouard Davin, Nathalie de Noblet-Ducoudré, Pierre Friedlingstein Submitted to GRL

4. Question addressed in this paper :is the radiative forcing concept applicable to the climatic impacts of the land-use induced land-cover changes ?

5. Instantaneous forcing Adjusted forcing Climatic Change Reminder: concept of radiative forcing Quantity (e.g. CO2)  ΔQi (change in radiative forcing, W/m-2) ΔTs = λ * ΔQ

6. 1750 8-9 millions km2 of anthropogenic land-cover (6-7% of land areas) 1990 46-51 millions km2 of anthropogenic land-cover (35-39% of land areas) due to the sole changes in land-surface albedo Radiative forcings from 1750 till 2005

7. Land cover change land surface model ORCHIDEE Simulated climatology Change in land surface characteristics prior to any atmospheric feedbacks Δ evapotranspiration Δ albedo Δ water vapor content radiative transfer scheme of our AGCM (LMDz) Conversion into radiative forcings ΔF water vapor ΔF albedo We have computed the radiative forcingdue to land-cover changes from 1870 to present-dayand from present-day to 2100

8. Change in anthropogenic land fraction (crops + pastures) PRES - PAST FUTU - PRES Annual mean radiative forcing (albedo + water vapor) PRES – PAST FUTU - PRES

9. Changes in the global annual radiative forcing (W/m-2) - 0.29 W/m-2 • 0.70 W/m-2 due to albedo changes alone due to evapotranspiration changes alone

10. Changes in the global annual mean surface temperature (°C) - 0.05°C - 0.14°C due to albedo changes alone due to evapotranspiration changes alone

11. PRES - PAST FUTU – PRES Spatial patterns of the changes in the global annual mean surface temperature (°C) ΔT = - 0.14 K ΔT = - 0.05 K

12. Derived climate sensitivityΔ Ts / (Δ F - Δ R) • Climate exhibits the same sensitivity to both historical and future land cover change. Climate sensitivity inferred from a 2xCO2 experiment : 1 K/(W.m-2) Climate sensitivity to land cover change is 70 % lower than the sensitivity to CO2 forcing.

13. 2 hypothesized causes: Spatial inhomogeneity  limited poleward extent of land-cover changes  limited sea-ice feedback. but this only explains about 20% of the reduced sensitivity Non-radiative effects Land-cover changes applied  decreased net radiation absorbed by the land-surface  decreased evapotranspiration BUT increased sensible heat (due to increased Bowen Ratio) Why is the climate sensitivityderived from land-cover changes different from the one derived from CO2?

14. DT - + Decreased evapotranspiration  decreased water vapor greenhouse effect  cooling of the troposphere Value used to quantify the climate sensitivity Increased sensible heat  warming of the boundary layer over deforested areas (figures taken from Kabat et al.: Vegetation, Water, Humans, and the Climate, IGBP BAHC)

15. In conclusion Deforestation, wherever it occurs, leads to global cooling through its biophysical effects on climate The climate sensitivity derived is the same wherever deforestation occurs  the radiative concept may be applicable(similar work needs to be carried out with other models, other deforestation locations and intensities) But the derived climate sensitivity is 70% lower than the one derived from changes in CO2.

16. Part II: Influence of future land-use inducedland-cover changeon the characteristics ofthe El-Niño Southern Oscillation Edouard Davin, Nathalie de Noblet-Ducoudré, Christian Laguerre, Pascal Terray, Eric Guilyardi (Prelimirary results)

17. Large changes in the tropical regions + Global oceanic cooling  Potential effects on ENSO Land-use induced land cover changes between present-day and year 2100

18. First EOF of sea-surface temperature % of tropical variability explained by this first EOF PRES 30 % FUTU 34 % Observed (HADISST) 30 % futur

19. Question addressed :what is the impact of this increased variance on the frequency and/or characteristics of the El-Niño events ?

20. 1) No significant change in frequency PRES El Niños are more frequent in the model than in reality Power spectrum FUTU Year-1 Observations Year-1

21. 2) Selection of modelled El-Niños over a 100 year-long time period • Based on the following criterias: • examine anomalous SSTs (with respect to modelled climatology) in the Niño 3 box: 5°S-5°N / 150°W-90°W • Select the years exhibiting anomalous SSTs larger than 1.5*s during 3 consecutive months between October and February

22. Selected events anomalous SSTs over a 3-year long time period El Niño events PRES FUTU

23. Selected events anomalous SSTs over a 3-year long time period SSTs are warmer in FUTU at the peak of the event SSTs are colder at present, prior to the event PRES FUTU

24. El Niño composites anomalous SSTs over a 3-year long time period PRES FUTU

25. time versus longitude diagram of anomalous SSTs composite El Niño for each set of simulations PRES FUTU similar timing of termination TIME earlier onset of warming WARMER COLDER

26. Statistically significant spatial patterns of anomalous SSTs through time Earlier warming of the Indian ocean PRES FUTU March January April June September July December October

27. Comparing SST changes in the Niño 3 region to those in the Indian Ocean Amplitude of warming in Indian ocean almost as large as in Pacific PRES ~3.5 months FUTU Niño 3 Indian Ocean ~3 months

28. What can explain the earlier onset of El Niño ?

29. What can explain the earlier onset of El Niño ? hypothesis With a deforested Amazon Ascending motion reduced over the Amazon Shift of the main convective cell towards the East Pacific Reduced Walker cirulation

30. What can explain the earlier onset of El Niño ? The model shows changes in upper-level divergence PRES Divergence at 200hPa Annual means Reduced upper-level divergence Change in divergence at 200hPa FUTU - PRES decreased upper-level convergence

31. Change in divergence at 200hPa DJF MAM FUTU - PRES JJA SON

32. Weakening of the Walker circulation differences in zonal mean wind between 200hPa and 850hPa. Winter values (DJF) PRES FUTU - PRES

33. FUTU – PRES + - Changes in surface temperature in global annual mean already exhibits an El Niño like pattern

34. In conclusion • Tropical deforestation reduces ascending motions above the Amazon basin  reduced subsidence in the eastern side of equatorial Pacific  slowing down of the Walker circulation. • The frequency of ENSO events is not modified • But the warm phases of ENSO start earlier, and the maximum temperature anomaly reached is larger. • + Temperature changes in the Indian ocean are as large as in the eastern Pacific.