IPCC Expert Meeting on Geoengineering

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Annex 4: Poster Abstracts Albedo enhancement over land to counteract global warming: Impacts on hydrological cycle Govindasamy Bala* and Bappaditya Nag Divecha Center for Climate Change & Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore -560012, India Increasing the land surface albedo via whitening the roofs and pavements in the urban area (Akbari et al., 2009; Oleson et al., 2010), covering deserts with more reflective polyethylene-aluminium to increase albedo (Gaskill, 2004) and making the color of crops lighter (Doughty et al., 2011; Ridgwell et al., 2009) are a few examples for land surface based Solar Radiation Management (SRM) schemes to counter global warming. Assuming these schemes are capable of countering global mean warming, what are the hydrological implications of large land albedo modification? Recent modelling studies have shown that Solar Radiation Management (SRM) schemes that counteract global warming by increasing the planetary albedo uniformly (no differentiation of land versus oceans) will cause a reduction in the intensity of the global water cycle when they cancel the global mean warming exactly (Bala et al., 2009, 2008). However, precipitation and runoff over land have been shown to increase when reduction in solar insolation is applied only over the oceans by increasing the reflectivity of marine clouds (Bala et al., 2011). This most recent study implies that large scale albedo modification should lead to drying of the continental regions. In this study, we perform idealized simulations using NCAR CAM3.1 to quantify the effect of SRM schemes that increase the albedo over land. We find that an increase in reflectivity over land that mitigates the global mean warming from a doubling of CO 2 leads to significant unmitigated warming in the southern hemisphere and cooling in the northern hemisphere since most of the land is located in northern hemisphere. Precipitation and runoff over land decrease by 13.4 and 22.3%, respectively because of a large residual sinking motion over land (see Figure A.4.1). The magnitude of hydrological changes are much larger than in the marine cloud albedo enhancement case (Bala et al., 2011) since the radiative forcing over land needed to counter global mean radiative forcing from a doubling of CO 2 is approximately twice the forcing needed over the oceans. Our results imply that albedo enhancement over oceans is superior to land albedo changes when the consequences on land hydrology are considered. Our study also has important implications for any intentional or unintentional large scale changes in land surface albedo such as deforestation/ afforestation/ reforestation, and desert and urban albedo modification. References Akbari H., S. Menon, and A. Rosenfeld, 2009: Global cooling: increasing world-wide urban albedos to offset CO 2. Climatic Change 94, 275–286. (DOI: 10.1007/s10584-008-9515-9). Bala G., K. Caldeira, and R. Nemani, 2009: Fast versus slow response in climate change: implications for the global hydrological cycle. Climate Dynamics 35, 423–434. (DOI: 10.1007/s00382-009-0583-y). Bala G., K. Caldeira, R, Nemani, L. Cao, G. Ban-Weiss, and H.-J. Shin, 2011: Albedo enhancement of marine clouds to counteract global warming: impacts on the hydrological cycle. Climate Dynamics 37, 915–931. (DOI: 10.1007/s00382-010-0868-1). Bala G., P.B. Duffy, and K.E. Taylor, 2008: Impact of Geoengineering Schemes on the Global Hydrological Cycle. Proceedings of the National Academy of Sciences of the United States of America 105, 7664–7669. (DOI: 10.1073/pnas.0711648105). Doughty C., C. Field, and A. McMillan, 2011: Can crop albedo be increased through the modification of leaf trichomes, and could this cool regional climate? Climatic Change 104, 379–387. (DOI: 10.1007/s10584-010-9936-0). <strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 43
Annex 4: Poster Abstracts Gaskill A., 2004: Gloabal Albedo Enhancement Project. Available at: http://www.global-warming-geo-engineering.org/Albedo-Enhancement/Surface-Albedo- Enhancement/Calculation-of-Coverage-Areas-to-Achieve-Desired-Level-of-ForcingOffsets/Desert-Area- Coverage/ag28.html. Oleson K., G. Bonan, and J. Feddema, 2010: Effects of white roofs on urban temperature in a global climate model. Geophysical Research Letters 37, L03701. Ridgwell A., J.S. Singarayer, A.M. Hetherington, and P.J. Valdes, 2009: Tackling regional climate change by leaf albedo biogeoengineering. Current Biology 19, 146–150. Figure A.4.1: Schematic diagram illustrating the changes in vertical motion at 500 mb over land and oceans when CO2 is doubled (left panel, 2xCO2-1xCO2), albedo over land is enhanced (middle panel, enhanced albedo case: Geo-2xCO2) and when CO2 is doubled and land albedo is enhanced (right panel, the geoengineered case: Geo-1xCO2) cases. Radiative forcings over land and oceans in each case are shown at the top. Vertical motion in height coordinates (w, meter/day) is obtained from w = -/(g) where is the model simulated pressure velocity. Changes in surface temperature and precipitation minus evaporation are also shown. Strong downward motion and the consequent decline in runoff and drying over land are indicated for the enhanced land albedo and geoengineering cases. <strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 44
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