Global Environmental Change [GC]

GC52A  MW:3002   Friday
Geoengineering
Presiding: A Robock, Rutgers University; K G Caldeira, Carnegie Institution of Washington; R P Turco, University of California, Los Angeles

GC52A-01 

"On the Possibilities of Climate Control" in 1962: Harry Wexler on Geoengineering and Ozone Destruction

* Fleming, J R (jfleming@colby.edu), Colby College, Science, Technology and Society Prog. 5881 Mayflower Hill, Waterville, ME 04901, United States

In 1962, in the early days of GCMs and satellites, Harry Wexler, Chief of the Scientific Services Division of the U.S. Weather Bureau and one of the most influential meteorologists of the 20th century, turned his attention to techniques that could raise or lower the overall temperature of the planet or rearrange its thermal structure. He also investigated possible inadvertent and purposeful damage to the ozone layer involving catalytic reactions of chorine and bromine. This work pre-dated the Nobel Prize-winning work on ozone depletion of P. Crutzen, M. Molina, and S. Rowland by about a decade. Wexler revealed his concerns about geoengineering and ozone destruction in a series of lectures "On the Possibilities of Climate Control" presented to technical audiences in Boston, Hartford, and Los Angeles in 1962. Using newly available results from GCMs and satellite heat budget experiments, Wexler pointed out that strategic manipulations of the Earth's shortwave and longwave radiation budgets could result in rather large-scale effects on general circulation patterns in short or longer periods, even approaching that of climatic change. These techniques, included increasing world temperature by several degrees by detonating up to ten H-bombs in the Arctic Ocean; decreasing world temperature by launching powder into an equatorial orbit to shade the Earth and make it look somewhat like Saturn and its rings; warming the lower atmosphere and cooling the stratosphere by artificial injections of water vapor or other substances; and notably, destroying all stratospheric ozone above the Arctic circle or near the equator using a relatively small amount of a catalytic agent such as chlorine or bromine. Wexler was preparing a new lecture in the summer of 1962 on "The Climate of Earth and Its Modifications," and might, under normal circumstances, have prepared his ideas for publication, as he had done earlier. However, he was cut down in his prime by a sudden heart attack on August 11, 1962. His previously unexamined notes and papers on climate control and ozone destruction are located in the Library of Congress. http://www.colby.edu/sts/wexlerozone.pdf

GC52A-02 INVITED 

Effects of Mount Pinatubo Volcanic Eruption on the Hydrological Cycle as an Analog of Geoengineering

Trenberth, K E (trenbert@ucar.edu), National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, United States * Dai, A (adai@ucar.edu), National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, United States

The problem of global warming arises from the buildup of greenhouse gases (GHGs) such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine through emulating volcanic eruptions by injecting large amounts of aerosols into the stratosphere. Different characteristics of the forcing induced by the increased GHGs and stratospheric aerosols have the potential to cause unintended responses in the climate system, and thus they require careful studies. Here we use the large volcanic eruption of Mount Pinatubo as an analog of potential effects on the terrestrial hydrological cycle from similar geoengineering solutions to global warming. Precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from Agung in May 1963, El Chichón in March 1982, and Pinatubo in June 1991, taking into account effects from El Niño-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991-September 1992 caused by wide-spreading drying over land. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions emulating large volcanic eruptions. http://www.cgd.ucar.edu/cas/adai/papers/TrenberthDai_GRL07.pdf

GC52A-03 INVITED 

Transient climate-carbon simulations of planetary geoengineering

* Matthews, D (dmatthew@alcor.concordia.ca), Concordia University, 1455 de Maisonneuve Blvd W., Montreal, QC H3G 1M8, Canada Caldeira, K (kcaldeira@globalecology.stanford.edu), Carnegie Institution, 260 Panama Street, Stanford, CA 94305, United States

Geoengineering (the intentional modification of Earth's climate) has been proposed as a means of reducing CO2-induced climate warming while greenhouse gas emissions continue. Most proposals involve managing incoming solar radiation such that future greenhouse gas forcing is counteracted by reduced solar forcing. In this study, we assess the transient climate response to geoengineering under a business-as-usual CO2 emissions scenario, using an intermediate complexity global climate model which includes an interactive carbon cycle. We find that the climate system responds quickly to artificially reduced solar insolation; hence, there may be little cost to delaying the deployment of geoengineering strategies until such a time as "dangerous" climate change is imminent. Spatial temperature patterns in the geoengineered simulation are comparable to pre-industrial, though this is not true for precipitation. Carbon sinks in the model increase in response to geoengineering: since geoengineering acts to mask climate warming, there is a direct CO2- driven increase in carbon uptake without an offsetting temperature-driven suppression of carbon sinks. However, this strengthening of carbon sinks, combined with the potential for rapid climate adjustment to changes in solar forcing, leads to serious consequences should geoengineering fail or be stopped abruptly. Such a scenario could lead to very rapid climate change, with warming rates up to 20 times greater than present day. This warming rebound would be larger and more sustained should climate sensitivity prove to be higher than expected. Thus, employing geoengineering schemes with continued carbon emissions could lead to severe risks for the global climate system.

GC52A-04 

Climate Model Simulations of Tropical and Polar Stratospheric Aerosol Injection: Cooling but Drought

* Robock, A (robock@envsci.rutgers.edu), Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, United States Oman, L (oman@jhu.edu), Dept. of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, United States Stenchikov, G (gera@envsci.rutgers.edu), Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, United States

In response to the global warming problem, there has been a recent renewed call for geoengineering "solutions" involving injecting particles into the stratosphere or blocking sunlight with satellites between the Sun and Earth. Here we describe different proposed geoengineering designs, and then show climate model calculations with the coupled atmosphere-ocean NASA GISS ModelE GCM that evaluate both their efficacy and their possible adverse consequences. We conduct experiments by simulating global warming with and without continuous emissions of sulfate aerosol precursors both into the tropical lower stratosphere and into the high latitude Northern Hemisphere lower stratosphere. We find that while stratospheric aerosols can cool the planet on a global average basis with tropical emissions or cool the Northern Hemisphere with high latitude emissions, there are also large regional climate changes in temperature and precipitation, with large areas of drought. At the current level of understanding, there are too many potential problems with geoengineering, and it would be much cheaper and easier to solve the global warming problem by reducing greenhouse gas emissions. These problems include cost, continued ocean acidification, obtaining global agreement on the optimum climate, regional climate changes, ozone depletion, reduction of solar energy for power generation, and unexpected consequences.

GC52A-05 

Will sunshade geoengineering schemes lead to reduced rainfall?

* Bala, G (bala1@LLNL.GOV), Lawrence Livermore National Laboratory, L-103 Atmospheric Science Division Energy and Environment Direcotrate, Livermore, CA 94550,

The rapidly rising CO2 level in the atmosphere has led to proposals of climate stabilization via "Geoengineering" schemes that would mitigate climate change by intentionally manipulating the solar radiation incident on earth's surface. In this paper, we address the impact of these climate stabilization schemes on the global hydrological cycle using equilibrium simulations from an atmospheric general circulation model coupled to a slab ocean model. We show that climate stabilization would tend to mitigate the surface temperature changes, but it could lead to a decline in the intensity of the hydrologic cycle. Shortwave forcing is more effective in driving changes in global precipitation than is CO2 forcing of a similar magnitude; this results in a less intense hydrologic cycle in the geoengineered climate. Further research is recommended to explore the response of different components of the climate system to forcing mechanisms.

GC52A-06 

A Fully Coupled GCM Study of a "Geoengineered World"

* Lunt, D J (d.j.lunt@bristol.ac.uk), University of Bristol, School of Geographical Sciences University of Bristol University Road, Bristol, BS8 1SS, United Kingdom Ridgwell, A (andy@seao2.org), University of Bristol, School of Geographical Sciences University of Bristol University Road, Bristol, BS8 1SS, United Kingdom Valdes, P J (p.j.valdes@bristol.ac.uk), University of Bristol, School of Geographical Sciences University of Bristol University Road, Bristol, BS8 1SS, United Kingdom

Several schemes have been proposed with the explicit aim of modifying the future climate of the planet as a mitigation strategy in a response to anthropogenic global warming. A selection of these, including the placing of mirrors at the Lagrange point between the Earth and the Sun, and the injection of aerosols into the stratosphere, have at their heart the goal of effectively reducing the incoming solar radiation near the top of the atmosphere, to "balance" increased surface warming due to increased greenhouse gas concentrations. However, it is likely that an exact balance of the radiative forcing would be very difficult to obtain, due to differing spatial characteristics of the solar forcing applied (greatest at the equator and least at the poles) and that of long wave absorption (more equal over all latitudes), as well as differing temporal characteristics of the radiative forcings. In this study, we model the different climate expected in a "Geoengineered World", compared to the "Preindustrial World", if both have the same global annual mean surface temperature. We use the UK Met Office GCM, HadCM3L, and carry out 5 simulations: Pre-industrial, Doubled CO2, Quadrupled CO2, and 2 simulations in which the increased CO2 is balanced in the global annual mean by a reduction in incoming solar radiation. The "strength" of mirror/aerosol required is calculated using an iterative procedure, until balance is obtained. Our results indicate significant differences between the Geoengineered World and the Preindustrial World, despite near identical global annual mean surface temperatures. In particular, we obtain relatively large differences in surface temperature over mid-latitude continental regions, in particular North America, and significant changes in upwelling on the West African tropical coast. The drying of the American Mid-West, and impacts on Africa fisheries, are likely to have significant consequences for global and local food production.

GC52A-07 

Artificial Enhancement of Natural Forcings to Cool The Earth and Their Effects on the Climate System

* Ammann, C M (ammann@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States Washington, W M (wmw@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States Buja, L (southern@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States Teng, H (hteng@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States Strand, W G (strandwg@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States Middleton, A (adrianne@ucar.edu), Climate and Global Dynamics Division National Center for Atmospheric Research, 1850 Table Mesa Dr, Boulder, CO 80307-3000, United States

Natural climate variability has always existed and will continue to operate in the future. Current climate changes, however, are quickly exceeding the range that can be explained with natural factors both internal and external to the climate system. Anthropogenic modifications to the atmosphere and essentially all other components of the climate system are causing a rapid change in climate. Because of the speed and expected magnitude of changes, and because of the longevity of the already committed as well as future forcing (the greenhouse gases), various alternatives for "corrections" are being discussed in the climate community and their potential impacts have to be carefully assessed. Using the NCAR Community Climate System Model, we study the effects of two different approaches of how the Earth's energy balance could be altered to keep global climate from continued warming. One approach deals with a possible reduction of solar irradiance (e.g., achieved through a large number of reflectors in space that would divert sunlight away from Earth), the other considers maintaining an artificial sulfate "blanket" in the stratosphere (achieved through continuous injection of sulfur into the tropical stratosphere). These strategies mimic natural forcings of solar irradiance changes and the effects from explosive volcanism, both factors that are included in their natural setting in standard simulations of the recent past. Branching off the IPCC-AR4 assessment simulations we determine the necessary magnitude of artificial enhancement of these natural forcings that would required to counter a high-emission scenario (A2). These ensemble simulations done for both cases demonstrate that the required enhancement of natural forcing factors - and thus the anthropogenic changes expected under a high-emission scenario themselves - are far larger than their natural range. Moreover, our simulations with the fully coupled model allow us to document the significant spatial impacts of such artificially "stabilized" climate that could be expected under the two geoengineering scenarios. While global mean temperature can be "restored" to a specified level, some regional climates will be very different.

GC52A-08 INVITED 

The Impact of Cloud Seeding of Marine Stratocumulus on the Ocean Atmosphere System

* Rasch, P J (pjr@ucar.edu), National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307, United States Chen, C (cchen@ucar.edu), National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307, United States Latham, J (latham@ucar.edu), National Center for Atmospheric Research, P. O. Box 3000, Boulder, CO 80307, United States

Clouds play a profound role in influencing the energy balance of the Earth system. It is also well known that relatively small variations in cloud properties (drop size, precipitation efficiency, areal extent, cloud lifetime) can produce large changes in the radiative properties of clouds. In 1990 John Latham suggested that a deliberate increase in cloud brightness and longevity (through the introduction of extra Cloud Condensation Nuclei (CCN)) over selected regions of the globe might be used to counteract global warming resulting from increases in greenhouse gases as a geo-engineering strategy. Since that time Latham and colleagues have explored this concept in a variety of studies, exploring the consequences of increases in CCN theoretically, in simple models of particular cloud types, and in an atmospheric general circulation model study employing fixed sea surface temperatures. These latter studies have explored only the change in forcing resulting from a change in cloud albedo, and did not explore the response of the climate system to such a change. They indicated that low (warm) clouds, in regions of lots of sunlight, and susceptible to the introduction of additional CCN might be optimal for this kind of geo-engineering (i.e. the subtropical marine stratocumulus regions). The studies suggested that a significant change in cloud albedo over only a few (approximately 10) percent of the planet would be sufficient to counteract the approximately 4 W/m2 forcing resulting from a doubling of CO2. However, implicit in that result is also the fact that in those relatively small regions where the geoengineering is done, the cloud forcing will be changed very substantially. (As a consequence, it may be preferable to produce smaller albedo changes over a correspondingly larger area). The marine stratocumulus regions are important to the climate system in many ways. They are areas of subsidence above the boundary layer, and play a large role in the Walker and Hadley atmospheric circulations, and they are also areas of oceanic upwelling, and participate in the many aspects of upper ocean dynamical circulations, influencing for example, the source regions of the cold tongue of SSTs that extends into the central pacific, an area important for features like ENSO. In this study we will describe our preliminary findings about the consequences of geoengineering of marine stratus and stratocumulus to the ocean atmosphere system through simulations with slab ocean and fully coupled version of the NCAR Community Climate Sytem Model.

GC52A-09 INVITED 

Lowering Global Temperature by Enhancing the Natural Sulfur Cycle

* Wingenter, O W (oliver@nmt.edu), Department of Chemistry, New Mexico Tech, Socorro, NM 87801, United States * Wingenter, O W (oliver@nmt.edu), Geophysical Research Center, New Mexico Tech, Socorro, NM 87801, Elliot, S M (sme@lanl.gov), Climate Ocean Sea Ice Modeling Project, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Blake, D R (DRBlake@uci.edu), Department of Chemistry, University of California, Irvine, Irvine, CA 92697, United States

We describe a well leveraged approach to partially regulate climate using limited iron enhancement to stimulate the natural sulfur cycle resulting in increased cloud reflectivity that could cool large regions of our planet. Our plan differs greatly in size and intended outcome from full scale ocean iron fertilization of the Southern Ocean (SO) as proposed previously to help mitigate rising CO2 in the atmosphere. Some regions of the Earth's oceans are high in nutrients but low in primary productivity. The largest such region is the SO followed by the equatorial Pacific. Several mesoscale (100 km2) experiments have shown that the limiting nutrient to productivity is iron. Yet, the effectiveness of iron fertilization for sequestering significant amounts of atmospheric CO2 is still in question. However, marine microorganisms not only consume inorganic carbon but also produce and consume many climate relevant organic gases. The greatest climate effect of iron fertilization may be in enhancing dimethyl sulfide (DMS) production, leading to changes in the optical properties of the atmosphere and cooling of the region. It appears that that full scale fertilization of the SO is not a viable solution because it would lead to over cooling of the region. Furthermore, our initial proposal differs from other solar shading plans as primary productivity may actually increase somewhat despite the slight loss in sunlight.

GC52A-10 

Climate management through agricultural albedo manipulation

* Doughty, C E (cdoughty@uci.edu), Department of Earth System Science University of California, Irvine, Department of Earth System Science University of California, Irvine Croul Hall, Irvine, Ca 92697-3100, United States McMillan, A (mcmillaa@uci.edu), Department of Earth System Science University of California, Irvine, Department of Earth System Science University of California, Irvine Croul Hall, Irvine, Ca 92697-3100, United States Goulden, M (mgoulden@uci.edu), Department of Earth System Science University of California, Irvine, Department of Earth System Science University of California, Irvine Croul Hall, Irvine, Ca 92697-3100, United States

Agriculture is the largest land surface that people actively manipulate, and it could be adversely affected by increased temperatures due to global warming. Crop species, already efficient reflectors of near infrared (NIR) radiation, could be modified to be even more reflective with the aim of causing albedo driven cooling. Agricultural scientists have bred soybean varieties with a fourfold increase in pubescence (leaf hairs) that increases shortwave reflectivity by 3-5%. We used NCAR's community atmosphere model (CAM) coupled to the community land model (CLM) to understand the effect on climate of a worldwide replacement of agriculture with crop varieties 3-5% more reflective in the shortwave. Increasing reflectance significantly decreases daily maximum temperature by an average of 1.85 ° C in agricultural regions above 30 ° latitude where lower vapor pressure deficits (VPD) cause the energy lost from increased reflectivity to be lost as sensible heat instead of latent heat. However, in latitudes between 10-30 ° N and S there are few significant temperature changes because higher vapor pressure deficits decrease latent heat, which decreases cloud formation and increased solar radiation absorbed by agricultural systems.

GC52A-11 

Land surface albedo in forestry management and land use change - possible offsets or augmentation to the radiative impact of carbon sequestration

* McMillan, A M (mcmillaa@uci.edu), University of California, Irvine, Department of Earth System Science, Irvine, CA 92697-3100, United States Doughty, C E (cdoughty@uci.edu), University of California, Irvine, Department of Earth System Science, Irvine, CA 92697-3100, United States Goulden, M L (mgoulden@uci.edu), University of California, Irvine, Department of Earth System Science, Irvine, CA 92697-3100, United States

Slowing or reversal of regional warming trends may be achieved by manipulation of land surface albedo. This approach is most feasible in agricultural and forestry areas, where the land surface is already under significant human influence. Afforestation, reforestation and deforestation are activities that not only affect carbon exchange but also affect energy exchange. In areas undergoing land use change to forestry, the "cooling" associated with increased carbon sequestration may be offset, or even canceled, by a "warming" associated with a darker land surface. In existing managed forests, the decision to replace darker conifer tree species with brighter broadleaf species can also potentially reduce surface albedo. We combined satellite and in situ observations of albedo with eddy covariance and inventory measurements of carbon exchange to assess the trade-off between a net warming due the darkening associated with dark forests and the net cooling associated with increased carbon sequestration. We also examined the net radiative effect of converting existing conifer plantations to broadleaf plantations. Initial results from a coupled climate-land surface model indicated that significant surface cooling occurred when darker evergreen forests were replaced by brighter broadleaf forests. Albedo manipulation may cause decreases in latent heat flux rather than decreases in sensible heat fluxes, with possible consequences for cloud formation.

GC52A-12 

Geoengineering and the Problem of Comparative Judgments of Harm.

* Bunzl, M (bunzl@rutgers.edu), Rutgers University, Initiative on Climate Change, Social Policy and Politics, New Brunswick, NJ 08903, United States

Suppose geoengineering "works" and that there are no competing antagonistic interventions. Even on this rosy scenario, it is extremely unlikely that it will "work" for everyone since (as Schneider pointed out many years ago) there is no reason to think the effects of geoengineering will offset the effects of global warming locally. It may seem obvious that at best then, the benefits of geoengineering will be unequal and at worst, some will benefit while some will be harmed. I argue that this is a much harder claim to formulate operationally than one might think since it is by no means clear what the appropriate base line for making such judgments is, nor whether such base lines are commensurable. http://www.ccspp.rutgers.edu