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Understanding the Disagreement in Climate Models on 20th Century Sea Level Rise

This article explores why climate models provide different estimates of sea level rise in the 20th century. It discusses the role of thermal expansion and glaciers in sea level rise and highlights the factors that contribute to the discrepancies in climate model projections. The article concludes with suggestions for improving sea level estimates and reducing uncertainty in future projections.

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Understanding the Disagreement in Climate Models on 20th Century Sea Level Rise

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  1. Whyclimatemodelsdisagree on the 20thcenturysealevelrise? Can sealevel observations from satellite altimetry and tide gauge records provide a constraint on climate model projections of futur sealevelrise? Benoit Meyssignac (LEGOS/CNES) in collaboration withA.Melet and others

  2. Context Sea level rise 1993-2015 Altimetry : 3.3 mm/yr (source : CCI sea level) Sea level rise 2081-2100 Climate models RCP8.5 : 11.2 mm/yr (source IPCC AR5) 120 IPCC AR5 100 80 GIEC 2013 Moyenne globale 60 40 20 Sea level rise (mm) 0 1700 1800 1900 2000 2100

  3. 20thcenturyobservedsealevelrise and climatemodelsestimates IPCC AR5

  4. Global meanthermostericsealevel (GMSLT) in CMIP5 climatemodels Ensemble mean consistent with observations but large spread over the 20th century Observations Models ensemble mean Trend (mm/yr) 1900-2005 GMTSL (mm) 1961-2005

  5. Earthenergyimbalance Climate change: radiative imbalance

  6. FromEarthenergyimbalance to thermal expansion How ocean warming yield to sea level rise? Where is global warming going? ε : expansion efficiency of heat in m J-1 [Russell et al. 2000, Kuhlbrodt et Gregory 2012] β : fraction of energy stored in the ocean S : Earth surface Under transient climate change: α : climate sensitivity κ : ocean heat uptake efficiency

  7. FromEarthenergyimbalance to thermal expansion in climatemodels Over 1900-1999 : avec

  8. Whyclimatemodelsdisagree on 20thcentury GMSLT ? thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  9. Whyclimatemodelsdisagree on 20thcentury GMSLT ? Expansion efficiency of heat ε = 0.12 ± 0.02 m YJ-1 Levitus et al. 2005 : ε = 0.12± 0.01 m YJ-1 Church et al. 2011 : ε = 0.15± 0.03 m YJ-1 IPCC 2013 : ε = 0.15± 0.02 m YJ-1 thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  10. Whyclimatemodelsdisagree on 20thcentury GMSLT ? Fraction of energy in the ocean β = 0.93 ± 0.2 3 out of 24 models with β ≠ 0.93 ± 0.2 disregarded Church et al. 2011 : β = 0.93 IPCC 2013 : β = 0.94 thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  11. Whyclimatemodelsdisagree on 20thcentury GMSLT ? thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  12. Whyclimatemodelsdisagree on 20thcentury GMSLT ? spread in κ only differences in heat transport in ocean~25% of the spread thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  13. Whyclimatemodelsdisagree on 20thcentury GMSLT ? spread in κ only spread in α only differences in heat transport in ocean~25% of the spread differences in climate feedbacks (clouds)~25% of the spread thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  14. Whyclimatemodelsdisagree on 20thcentury GMSLT ? spread in κ only spread in F only spread in α only differences in radiative forcing (aerosols) ~20% of the spread differences in heat transport in ocean~25% of the spread differences in climate feedbacks (clouds)~25% of the spread thermal expansion (m) CMIP5 ΔGMTSL from 3D T, S

  15. Whyclimatemodelsdisagree on 20thcentury GMSLT ? • Observational constraint on μ would help reducing uncertainties in ΔGMTSL in climate models and yield to better projections • Under transient climate change, the global mean thermosteric sea level rise verifies: GMTSL (mm) 1900-2005 More in Melet and Meyssignac, 2015, Journal of Climate

  16. 20thcenturyobservedsealevelrise and climatemodelsestimates From Church et al. 2013

  17. Glaciers mass loss in CMIP5 climatemodels

  18. Earthenergyimbalance Earth radiative balance Climate change: radiative imbalance

  19. FromEarthenergyimbalance to glaciers icemelt How glaciers ice melt yield to sea level rise? Where is global warming going? ρfw : fresh water density S : Earth surface βG : fraction of energy which melts glaciers Lfusion : Latent heat of fusion Under transient climate change: α : climate sensitivity κ : ocean heat uptake efficiency

  20. FromEarthenergyimbalance to glaciers icemelt in climatemodels avec The linear relationship does not hold because of the glaciers response to the LIA!

  21. FromEarthenergyimbalance to glaciers icemelt in climatemodels avec WORK IN PROGRESS The linear relationship does not hold because of the glaciers response to the LIA!

  22. CONCLUSION • 20th century sea level rise is essentially due to the thermal expansion of the ocean and glaciers ice melt • Climate models give different estimates of the 20th century GMSL because they give different estimates of 1) the 20th century thermal expansion and 2) to a lesser extent glaciers mass loss • As for the thermal expansion, it depends the climate sensitivity the ocean heat uptake efficiency and the radiative forcing and climate models disagree on these parmeters because they disagree on the clouds radiative feedback, the verticl transport of heat in the ocean and the radiative forcing of aerosols • These are the processes that need to be improved to improve the 20th century sea level estimates and reduce the uncertainty in sea level projections • The linear relationship between GMSLT and the radiative forcing suggest that observations of the ocean thermal expansion through Argo measurement or satellite altimetry corrected for mass signal with GRACE can help in constraining these processes and imrpove climate model projections • The relationship between GMSLG and the radiative forcing does not follow a similar linear scheme because of the low response to the LIA and needs more investigation

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