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Counteracting climate change via solar radiation management

Author(s): Bala G

Published: December, 2011

Publisher: Current Science (Bangalore)

Tags: Overview

URL: http://cs-test.ias.ac.in/cs/Volumes/101/11/1418.pdf

Abstract: Since the pre-industrial period, increase in atmospheric carbon dioxide (CO2) and other greenhouse gases (GHGs) has exerted a positive radiative forcing on the climate system by trapping longwave radiation. Solar radiation management (SRM) schemes aim to offset the warming influence from GHGs by reducing the amount of solar radiation absorbed by the Earth. This can be achieved in two ways: (1) reducing the amount of solar radiation reaching the Earth and (2) increasing the reflectivity of the planet. Radiation reaching the earth can be reduced by space-based sunshades and the reflectivity of the planet can be increased by increasing the albedo of clouds in the atmosphere or the reflectivity of the land surface or ocean surface. Reflectivity could be also increased by artificially injecting sulphate aerosols into the stratosphere.


Albedo enhancement over land to counteract global warming: impacts on hydrological cycle

Author(s): Bala G, Nag B

Published: November, 2011

Publisher: Climate Dynamics

DOI: 10.1007/s00382-011-1256-1

Tags: Surface Albedo Modification, Afforestation, Land Use Management, Precipitation, Climate Modelling

URL: https://www.see.ed.ac.uk/~shs/Climate%20change/Climate%20model%20results/Bala_and_Nag_CD2011.pdf

Abstract: A recent modelling study has shown that precipitation and runoff over land would increase when the reflectivity of marine clouds is increased to counter global warming. This implies that large scale albedo enhancement over land could lead to a decrease in runoff over land. In this study, we perform simulations using NCAR CAM3.1 that have implications for Solar Radiation Management geoengineering 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 CO2 leads to a large residual 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 triggered by albedo enhancement over land. Soil water content also declines when albedo over land is enhanced. The simulated magnitude of hydrological changes over land are much larger when compared to changes over oceans in the recent marine cloud albedo enhancement study since the radiative forcing over land needed (-8.2 W m-2) to counter global mean radiative forcing from a doubling of CO2 (3.3 W m-2) is approximately twice the forcing needed over the oceans (-4.2 W m-2). Our results imply that albedo enhancement over oceans produce climates closer to the unperturbed climate state than do albedo changes on land 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, air pollution, and desert and urban albedo modification.


Dependence of climate forcing and response on the altitude of black carbon aerosols

Author(s): Ban-Weiss GA, Cao L, Bala G, Caldeira K

Published: April, 2011

Publisher: Climate Dynamics

DOI: 10.1007/s00382-011-1052-y

Tags: Stratospheric Aerosols, Climate Modelling, Precipitation, Short-Lived Climate Forcers, Tropospheric Aerosols

URL: http://dge.stanford.edu/labs/caldeiralab/Caldeira_research/BanWeiss_Cao.html

Abstract: Black carbon aerosols absorb solar radiation and decrease planetary albedo, and thus can contribute to climate warming. In this paper, the dependence of equilibrium climate response on the altitude of black carbon is explored using an atmospheric general circulation model coupled to a mixed layer ocean model. The simulations model aerosol direct and semi-direct effects, but not indirect effects. Aerosol concentrations are prescribed and not interactive. It is shown that climate response of black carbon is highly dependent on the altitude of the aerosol. As the altitude of black carbon increases, surface temperatures decrease; black carbon near the surface causes surface warming, whereas black carbon near the tropopause and in the stratosphere causes surface cooling. This cooling occurs despite increasing planetary absorption of sunlight (i.e. decreasing planetary albedo). We find that the trend in surface air temperature response versus the altitude of black carbon is consistent with our calculations of radiative forcing after the troposphere, stratosphere, and land surface have undergone rapid adjustment, calculated as “regressed” radiative forcing. The variation in climate response from black carbon at different altitudes occurs largely from different fast climate responses; temperature dependent feedbacks are not statistically distinguishable. Impacts of black carbon at various altitudes on the hydrological cycle are also discussed; black carbon in the lowest atmospheric layer increases precipitation despite reductions in solar radiation reaching the surface, whereas black carbon at higher altitudes decreases precipitation.


Albedo enhancement of marine clouds to counteract global warming: Impacts on the hydrological cycle

Author(s): Bala G, Caldeira K, Nemani R, Cao L, Ban-Weiss GA, Shin HJ

Published: June, 2010

Publisher: Climate Dynamics

DOI: 10.1007/s00382-010-0868-1

Tags: Cloud Brightening, Environmental Side-Effects, Precipitation, Climate Modelling, Cloud Physics

URL: http://www.springerlink.com/content/9569172415150486/

Abstract: Recent studies have shown that changes in solar radiation affect the hydrological cycle more strongly than equivalent CO2 changes for the same change in global mean surface temperature. Thus, solar radiation management "geoengineering" proposals to completely offset global mean temperature increases by reducing the amount of absorbed sunlight might be expected to slow the global water cycle and reduce runoff over land. However, proposed countering of global warming by increasing the albedo of marine clouds would reduce surface solar radiation only over the oceans. Here, for an idealized scenario, we analyze the response of temperature and the hydrological cycle to increased reflection by clouds over the ocean using an atmospheric general circulation model coupled to a mixed layer ocean model. When cloud droplets are reduced in size over all oceans uniformly to offset the temperature increase from a doubling of atmospheric CO2, the global-mean precipitation and evaporation decreases by about 1.3% but runoff over land increases by 7.5% primarily due to increases over tropical land. In the model, more reflective marine clouds cool the atmospheric column over ocean. The result is a sinking motion over oceans and upward motion over land. We attribute the increased runoff over land to this increased upward motion over land when marine clouds are made more reflective. Our results suggest that, in contrast to other proposals to increase planetary albedo, offsetting mean global warming by reducing marine cloud droplet size does not necessarily lead to a drying, on average, of the continents. However, we note that the changes in precipitation, evaporation and P-E are dominated by small but significant areas, and given the highly idealized nature of this study, a more thorough and broader assessment would be required for proposals of altering marine cloud properties on a large scale.


Problems with geoengineering schemes to combat climate change

Author(s): Bala G

Published: November, 2009

Publisher: Current Science

Tags: Environmental Side-Effects, Overview

URL: http://eprints.iisc.ernet.in/19494/

Abstract: The accelerated rate of increase in atmospheric CO2 concentration in recent years has revived the idea of stabilizing the global climate through geoengineering schemes. Majority of the proposed geoengineering schemes will attempt to reduce the amount of solar radiation absorbed by our planet. Climate modelling studies of these so called 'sunshade geoengineering schemes' show that global warming from increasing concentrations of CO2 can be mitigated by intentionally manipulating the amount of sunlight absorbed by the climate system. These studies also suggest that the residual changes could be large on regional scales, so that climate change may not be mitigated on a local basis. More recent modelling studies have shown that these schemes could lead to a slow-down in the global hydrological cycle. Other problems such as changes in the terrestrial carbon cycle and ocean acidification remain unsolved by sunshade geoengineering schemes. In this article, I review the proposed geoengineering schemes, results from climate models and discuss why geoengineering is not the best option to deal with climate change.


Combined climate and carbon-cycle effects of large-scale deforestation

Author(s): Bala G, Caldeira K, Wickett M, Phillips TJ, Lobell DB, Delire C, Mirin A

Published: April, 2007

Publisher: Proceedings of the National Academy of Sciences

DOI: 10.1073/pnas.0608998104

Tags: Afforestation, Climate Modelling, Climate Science, Carbon Cycle

URL: http://www.pnas.org/content/104/16/6550.abstract

Abstract: The prevention of deforestation and promotion of afforestation have often been cited as strategies to slow global warming. Deforestation releases CO2 to the atmosphere, which exerts a warming influence on Earth's climate. However, biophysical effects of deforestation, which include changes in land surface albedo, evapotranspiration, and cloud cover also affect climate. Here we present results from several large-scale deforestation experiments performed with a three-dimensional coupled global carbon-cycle and climate model. These simulations were performed by using a fully three-dimensional model representing physical and biogeochemical interactions among land, atmosphere, and ocean. We find that global-scale deforestation has a net cooling influence on Earth's climate, because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.


Biogeophysical effects of CO2 fertilization on global climate

Author(s): Bala G, Caldeira K, Mirin A, Wickett M, Delire C, Phillips TJ

Published: June, 2006

Publisher: Tellus B

DOI: 10.1111/j.1600-0889.2006.00210.x

Tags: Terrestrial Carbon Storage, Afforestation, Climate Modelling, Climate Science

URL: http://adsabs.harvard.edu/full/2006TellB..58..620B

Abstract: CO2 fertilization affects plant growth, which modifies surface physical properties, altering the surface albedo, and fluxes of sensible and latent heat. We investigate how such CO2-fertilization effects on vegetation and surface properties would affect the climate system. Using a global three-dimensional climate-carbon model that simulates vegetation dynamics, we compare two multicentury simulations: a ‘Control’ simulation with no emissions and a ‘Physiol-noGHG’ simulation where physiological changes occur as a result of prescribed CO2 emissions, but where CO2-induced greenhouse warming is not included. In our simulations, CO2 fertilization produces warming; we obtain an annual- and global-mean warming of about 0.65 K (and land-only warming of 1.4 K) after 430 yr. This century-scale warming is mostly due to a decreased surface albedo associated with the expansion of the Northern Hemisphere boreal forests. On decadal timescales, the CO2 uptake by afforestation should produce a cooling effect that exceeds this albedo-based warming; but if the forests remain in place, the CO2-enhanced-greenhouse effect would diminish as the ocean equilibrates with the atmosphere, whereas the albedo effect would persist. Thus, on century timescales, there is the prospect for net warming from CO2 fertilization of the land biosphere. Further study is needed to confirm and better quantify our results.


Geoengineering Earth's radiation balance to mitigate climate change from a quadrupling of CO2

Author(s): Bala G, Caldeira K, Duffy PB

Published: December, 2002

Publisher: Global and Planetary Change

DOI: 10.1016/S0921-8181(02)00195-9

Tags: Stratospheric Aerosols, Climate Modelling

URL: http://www.sciencedirect.com/science/article/pii/S0921818102001959

Abstract: It has been suggested that climate change induced by anthropogenic CO2 could be counteracted with geoengineering schemes designed to diminish the solar radiation incident on Earth's surface. Though the spatial and temporal pattern of radiative forcing from greenhouse gases differs from that of sunlight, it was shown in a recent study that these schemes would largely mitigate regional or seasonal climate change for a doubling of the atmospheric CO2 content. Here, we examine the ability of reduced solar luminosity to cancel the effects of quadrupling of CO2 content. In agreement with our previous study, geoengineering schemes could markedly diminish regional and seasonal climate change. However, there are some residual climate changes: in the geoengineered 4×CO2 climate, a significant decrease in surface temperature and net water flux occurs in the tropics; warming in the high latitudes is not completely compensated; the cooling effect of greenhouse gases in the stratosphere persists and sea ice is not fully restored. However, these residual climate changes are much smaller than the change from quadrupling of CO2 without reducing solar input. Caution should be exercised in interpretation because these results are from a single model with a number of simplifying assumptions. There are also many technical, environmental and political reasons not to implement geoengineering schemes.


Impact of geoengineering schemes on the terrestrial biosphere

Author(s): Bala G, Thompson S, Duffy PB, Caldeira K, Delire C

Published: November, 2002

Publisher: Geophysical Research Letters

DOI: 10.1029/2002GL015911

Tags: Stratospheric Aerosols, Environmental Side-Effects, Climate Modelling

URL: http://www.agu.org/journals/ABS/2002/2002GL015911.shtml

Abstract: Climate stabilization via “Geoengineering” schemes seek to mitigate climate change due to increased greenhouse gases by compensating reduction in solar radiation incident on earth's surface. In this paper, we address the impact of these climate stabilization schemes on terrestrial biosphere using equilibrium simulations from a coupled atmosphere-terrestrial biosphere model. Climate stabilization would tend to limit changes in vegetation distribution brought on by climate change, but would not prevent CO2-induced changes in Net Primary Productivity (NPP) or biomass; indeed, if CO2 fertilization is significant, then a climate-stabilized world could have higher NPP than our current world. Nevertheless, there are many reasons why geoengineering is not a preferred option for climate stabilization.


Geoengineering Earth's radiation balance to mitigate CO2-induced climate change

Author(s): Bala G, Caldeira K

Published: July, 2000

Publisher: Geophysical Research Letters

DOI: 10.1029/1999GL006086

Tags: Stratospheric Aerosols, Climate Modelling

URL: http://www.agu.org/pubs/crossref/2000/1999GL006086.shtml

Abstract: To counteract anthropogenic climate change, several schemes have been proposed to diminish solar radiation incident on Earth's surface. These geo-engineering schemes could reverse global annual mean warming; however, it is unclear to what extent they would mitigate regional and seasonal climate change, because radiative forcing from greenhouse gases such as CO2 differs from that of sunlight. No previous study has directly addressed this issue. In the NCAR CCM3 atmospheric general circulation model, we reduced the solar luminosity to balance the increased radiative forcing from doubling atmospheric CO2. Our results indicate that geo-engineering schemes could markedly diminish regional and seasonal climate change from increased atmospheric CO2, despite differences in radiative forcing patterns. Nevertheless, geo-engineering schemes could prove environmentally risky.


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