The G4Foam Experiment: global climate impacts of regional ocean albedo modification

Abstract. Reducing insolation has been proposed as a geoengineering response to global warming. Here we present the results of climate model simulations of a unique Geoengineering Model Intercomparison Project Testbed experiment to investigate the benefits and risks of a scheme that would brighten certain oceanic regions. The National Center for Atmospheric Research CESM CAM4-Chem global climate model was modified to simulate a scheme in which the albedo of the ocean surface is increased over the subtropical ocean gyres in the Southern Hemisphere. In theory, this could be accomplished using a stable, nondispersive foam, comprised of tiny, highly reflective microbubbles. Such a foam has been developed under idealized conditions, although deployment at a large scale is presently infeasible. We conducted three ensemble members of a simulation (G4Foam) from 2020 through to 2069 in which the albedo of the ocean surface is set to 0.15 (an increase of 150 %) over the three subtropical ocean gyres in the Southern Hemisphere, against a background of the RCP6.0 (representative concentration pathway resulting in +6 W m−2 radiative forcing by 2100) scenario. After 2069, geoengineering is ceased, and the simulation is run for an additional 20 years. Global mean surface temperature in G4Foam is 0.6 K lower than RCP6.0, with statistically significant cooling relative to RCP6.0 south of 30° N. There is an increase in rainfall over land, most pronouncedly in the tropics during the June–July–August season, relative to both G4SSA (specified stratospheric aerosols) and RCP6.0. Heavily populated and highly cultivated regions throughout the tropics, including the Sahel, southern Asia, the Maritime Continent, Central America, and much of the Amazon experience a statistically significant increase in precipitation minus evaporation. The temperature response to the relatively modest global average forcing of −1.5 W m−2 is amplified through a series of positive cloud feedbacks, in which more shortwave radiation is reflected. The precipitation response is primarily the result of the intensification of the southern Hadley cell, as its mean position migrates northward and away from the Equator in response to the asymmetric cooling.

[1]  Manfred Mudelsee,et al.  Holocene Forcing of the Indian Monsoon Recorded in a Stalagmite from Southern Oman , 2003, Science.

[2]  R. Neely,et al.  Representation of the Community Earth System Model (CESM1) CAM4-chem within the Chemistry-Climate Model Initiative (CCMI) , 2016 .

[3]  G. Meehl,et al.  Monsoon Regimes and Processes in CCSM4. Part I: The Asian–Australian Monsoon , 2012 .

[4]  H. Hailes,et al.  Long-term stabilization of reflective foams in sea water , 2014 .

[5]  Shingo Watanabe,et al.  The hydrological impact of geoengineering in the Geoengineering Model Intercomparison Project (GeoMIP) , 2013 .

[6]  Shingo Watanabe,et al.  The Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6): simulation design and preliminary results , 2015 .

[7]  R. Poore,et al.  Century‐scale movement of the Atlantic Intertropical Convergence Zone linked to solar variability , 2004 .

[8]  Alan Robock.  Bubble, bubble, toil and trouble , 2011 .

[9]  A. Jones,et al.  Climate impacts of geoengineering marine stratocumulus clouds , 2009 .

[10]  Karl E. Taylor,et al.  An overview of CMIP5 and the experiment design , 2012 .

[11]  Sarah M. Kang,et al.  The Response of the ITCZ to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with a GCM , 2008 .

[12]  Alan Gadian,et al.  Marine cloud brightening , 2012, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[13]  Sarah M. Kang,et al.  Anthropogenic sulfate aerosol and the southward shift of tropical precipitation in the late 20th century , 2013 .

[14]  Swadhin K. Behera,et al.  Subtropical SST dipole events in the southern Indian Ocean , 2001 .

[15]  P. Rasch,et al.  Geoengineering by cloud seeding: influence on sea ice and climate system , 2009 .

[16]  R. Stouffer,et al.  Response of the ITCZ to Northern Hemisphere cooling , 2006 .

[17]  Alan Robock,et al.  20 reasons why geoengineering may be a bad idea , 2008 .

[18]  J. Kay,et al.  Global Climate Impacts of Fixing the Southern Ocean Shortwave Radiation Bias in the Community Earth System Model (CESM) , 2015 .

[19]  T. Stocker,et al.  Stable Carbon Cycle–Climate Relationship During the Late Pleistocene , 2005, Science.

[20]  Syukuro Manabe,et al.  Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere , 1980 .

[21]  S. Emori,et al.  Dynamic and thermodynamic changes in mean and extreme precipitation under changed climate , 2005 .

[22]  C. Reason.  Subtropical Indian Ocean SST dipole events and southern African rainfall , 2001 .

[23]  A. Robock.  Albedo enhancement by stratospheric sulfur injections: More research needed , 2016 .

[24]  C. Bretherton,et al.  Boundary Layer Depth, Entrainment, and Decoupling in the Cloud-Capped Subtropical and Tropical Marine Boundary Layer , 2004 .

[25]  C. Bretherton,et al.  Ocean–Cloud–Atmosphere–Land Interactions in the Southeastern Pacific: The VOCALS Program , 2014 .

[26]  Ryan R. Neely,et al.  Stratospheric sulfate geoengineering could enhance the terrestrial photosynthesis rate , 2016 .

[27]  Baode Chen,et al.  The Origin of Monsoons , 2001 .

[28]  S. Xie,et al.  Interdecadal variations in ENSO teleconnection to the Indo-Western Pacific for 1870-2007 , 2012 .

[29]  C. Bretherton,et al.  On the Relationship between Stratiform Low Cloud Cover and Lower-Tropospheric Stability , 2006 .

[30]  K. Taylor,et al.  The Geoengineering Model Intercomparison Project (GeoMIP) , 2011 .

[31]  A. Oschlies,et al.  Assessing climate impacts and risks of ocean albedo modification in the Arctic , 2016 .

[32]  T. Yamagata,et al.  Indian Ocean subtropical dipole simulated using a coupled general circulation model , 2004 .

[33]  R. Lindzen,et al.  Hadley Circulations for Zonally Averaged Heating Centered off the Equator , 1988 .

[34]  J. Lamarque,et al.  CAM-chem: description and evaluation of interactive atmospheric chemistry in the Community Earth System Model , 2012 .

[35]  R. Seitz,et al.  Bright water: hydrosols, water conservation and climate change , 2011 .

[36]  David W Keith,et al.  Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar geoengineering , 2014, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[37]  P. Crutzen.  Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma? , 2006 .

[38]  Shaun A Marcott,et al.  A Reconstruction of Regional and Global Temperature for the Past 11,300 Years , 2013, Science.

[39]  Richard J. Rosen,et al.  BUBBLE, BUBBLE, TOIL AND TROUBLE! , 1977 .

[40]  A. Robock.  CHAPTER 7:Stratospheric Aerosol Geoengineering , 2014 .

[41]  Johannes Quaas,et al.  Climate extremes in multi-model simulations of stratospheric aerosol and marine cloud brightening climate engineering , 2014 .

[42]  Andy Ridgwell,et al.  Climatic effects of surface albedo geoengineering , 2011 .

[43]  Jean-Francois Lamarque,et al.  A new Geoengineering Model Intercomparison Project (GeoMIP) experiment designed for climate and chemistry models , 2014 .

[44]  R. Wood,et al.  Subseasonal variability of low cloud radiative properties over the southeast Pacific Ocean , 2009 .

[45]  D. Frierson,et al.  Extratropical Influence on ITCZ Shifts in Slab Ocean Simulations of Global Warming , 2012 .

[46]  G. Hegerl,et al.  Changes in temperature and precipitation extremes in the IPCC ensemble of global coupled model simulations , 2007 .

[47]  Alan Robock,et al.  Stratospheric aerosol geoengineering , 2015 .

[48]  David W Keith,et al.  Field experiments on solar geoengineering: report of a workshop exploring a representative research portfolio , 2014, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[49]  Markus Reichstein,et al.  Improving canopy processes in the Community Land Model version 4 (CLM4) using global flux fields empirically inferred from FLUXNET data , 2011 .

[50]  R. Neale,et al.  The Mean Climate of the Community Atmosphere Model (CAM4) in Forced SST and Fully Coupled Experiments , 2013 .

[51]  Andrew S. Jones,et al.  Asymmetric forcing from stratospheric aerosols impacts Sahelian rainfall , 2013 .

[52]  S. Xie,et al.  A coupled ocean‐atmosphere model of relevance to the ITCZ in the eastern Pacific , 1994 .

[53]  R. Neely,et al.  A Consistent Prescription of Stratospheric Aerosol for Both Radiation and Chemistry in the Community Earth System Model (CESM1) , 2015 .

[54]  KenCaldeira,et al.  Impacts of ocean albedo alteration on Arctic sea ice restoration and Northern Hemisphere climate , 2015 .

[55]  W. Boos,et al.  Interannual Variability of Monsoon Precipitation and Local Subcloud Equivalent Potential Temperature , 2013 .

[56]  David B. Mechem,et al.  The Time Scales of Variability of Marine Low Clouds , 2016 .

[57]  Kevin E. Trenberth,et al.  Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering , 2007 .

[58]  W. Cai,et al.  Teleconnection Pathways of ENSO and the IOD and the Mechanisms for Impacts on Australian Rainfall , 2011 .

[59]  Alan Robock,et al.  Stratospheric geoengineering impacts on El Niño/Southern Oscillation , 2015 .

[60]  B. Soden,et al.  Robust Responses of the Hydrological Cycle to Global Warming , 2006 .

[61]  T. Palmer,et al.  Sahel rainfall and worldwide sea temperatures, 1901–85 , 1986, Nature.

[62]  C. Bitz,et al.  Influence of high latitude ice cover on the marine Intertropical Convergence Zone , 2005 .

[63]  K. Calvin,et al.  The RCP greenhouse gas concentrations and their extensions from 1765 to 2300 , 2011 .