U43A-0038
Geoengineering and the Risk of Rapid Climate Change
Many scientists have proposed that geoengineering could be used to artificially cool the planet as a means of reducing CO2-induced climate warming. However, several recent studies have shown some of the potential risks of geoengineering, including negative impacts on stratospheric ozone, the hydrologic cycle and the possibility of rapid climate change in the case of abrupt failure, or rapid decommissioning of geoengineering technology. In this study, we have emulated a geoengineering scenario in the MAGICC climate model, by counteracting the radiative forcing from greenhouse gases. We have used a hypothetical scenario of business-as-usual greenhouse gas emissions, in which geoengineering is implemented at the year 2020, and is removed abruptly after 40 years. By varying the climate sensitivity of the MAGICC model, and using previously published estimates of climate sensitivity likelihoods, we are able to derive a probabilistic prediction of the rate of temperature change following the removal of geoengineering. In a simulation without geoengineering (considering only the A1B AIM emissions scenario) the maximum annual rate of temperature change (in the highest climate sensitivity simulation) was 0.5° C per decade. In the geoengineering simulations the maximum annual rate of temperature change, occurring in the year after geoengineering was stopped, varied from 0.22° C per decade for a climate sensitivity of 0.5° C to nearly 8° C per decade for a climate sensitivity of 10° C. The most likely maximum rate of change (corresponding to a climate sensitivity of 2.5° C) was just over 5° C per decade. There is a 99.8 percent probability that the rate of temperature change following the stoppage of geoengineering in this scenario would exceed 3° C per decade. This risk of rapid climate change associated with the use of planetary-scale geoengineering is highly relevant to discussion of climate policies aimed at avoiding "dangerous anthropogenic interference" in the climate system. Many ecosystems would be significantly stressed by the high rates of temperature change shown in this study, which could compromise ecosystems' ability to adapt to climate change There are also possible implications of rapid temperature change for other aspects of the climate system, such as the strength of the meridional overturning circulation. Based on the results of this study, we argue that the risk of rapid climate change following the abrupt removal of geoengineering could constitute increased risk of dangerous anthropogenic interference in the climate system.
U43A-0039
Ozone Depletion Caused by Rocket Engine Emissions: A Fundamental Limit on the Scale and Viability of Space-Based Geoengineering Schemes
Emissions from solid and liquid propellant rocket engines reduce global stratospheric ozone levels. Currently ~ one kiloton of payloads are launched into earth orbit annually by the global space industry. Stratospheric ozone depletion from present day launches is a small fraction of the ~ 4% globally averaged ozone loss caused by halogen gases. Thus rocket engine emissions are currently considered a minor, if poorly understood, contributor to ozone depletion. Proposed space-based geoengineering projects designed to mitigate climate change would require order of magnitude increases in the amount of material launched into earth orbit. The increased launches would result in comparable increases in the global ozone depletion caused by rocket emissions. We estimate global ozone loss caused by three space-based geoengineering proposals to mitigate climate change: (1) mirrors, (2) sunshade, and (3) space-based solar power (SSP). The SSP concept does not directly engineer climate, but is touted as a mitigation strategy in that SSP would reduce CO2 emissions. We show that launching the mirrors or sunshade would cause global ozone loss between 2% and 20%. Ozone loss associated with an economically viable SSP system would be at least 0.4% and possibly as large as 3%. It is not clear which, if any, of these levels of ozone loss would be acceptable under the Montreal Protocol. The large uncertainties are mainly caused by a lack of data or validated models regarding liquid propellant rocket engine emissions. Our results offer four main conclusions. (1) The viability of space-based geoengineering schemes could well be undermined by the relatively large ozone depletion that would be caused by the required rocket launches. (2) Analysis of space- based geoengineering schemes should include the difficult tradeoff between the gain of long-term (~ decades) climate control and the loss of short-term (~ years) deep ozone loss. (3) The trade can be properly evaluated only if our understanding of the stratospheric impact of rocket emissions is significantly improved. (4) Such an improved understanding requires a concerted effort of research including new in situ measurements in a variety of rocket plumes and a multi-scale modeling program similar in scope to the effort required to address the climate and ozone impacts of aircraft emissions.
U43A-0040
Measurements of Unexpected Ozone Loss in a Nighttime Space Shuttle Exhaust Plume: Implications for Geo-Engineering Projects
Measurements of ozone, carbon dioxide and particulate water were made in the nighttime exhaust plume of the Space Shuttle (STS-116) on 9 December 2006 as part of the PUMA/WAVE campaign (Plume Ultrafast Measurements Acquisition/WB-57F Ascent Video Experiment). The launch took place from Kennedy Space Center at 8:47 pm (local time) on a moonless night and the WB-57F aircraft penetrated the shuttle plume approximately 25 minutes after launch in the lowermost stratosphere. Ozone loss is not predicted to occur in a nighttime Space Shuttle plume since it has long been assumed that the main ozone loss mechanism associated with rocket emissions requires solar photolysis to drive several chlorine-based catalytic cycles. However, the nighttime in situ observations show an unexpected loss of ozone of approximately 250 ppb in the evolving exhaust plume, inconsistent with model predictions. We will present the observations of the shuttle exhaust plume composition and the results of photochemical models of the Space Shuttle plume. We will show that models constrained by known rocket emission kinetics, including afterburning, and reasonable plume dispersion rates, based on the CO2 observations, cannot explain the observed ozone loss. We will propose potential explanations for the lack of agreement between models and the observations, and will discuss the implications of these explanations for our understanding of the composition of rocket emissions. We will describe the potential consequences of the observed ozone loss for long-term damage to the stratospheric ozone layer should geo-engineering projects based on rocket launches be employed.
U43A-0041
Acid Deposition From Stratospheric Geoengineering With Sulfate Aerosols
We used a general circulation model of the Earth's climate to conduct geoengineering experiments involving stratospheric injection of sulfur dioxide [Robock et al., 2008] and analyzed the resulting deposition of sulfate. When sulfur is injected into the tropical or Arctic stratosphere, the main additional surface deposition occurs in midlatitude bands, because of strong cross-tropopause flux in the jet stream regions, and there are some larger local increases, specifically in Northern Canada and the Western Pacific Ocean. We used critical load studies to determine the effects of this increase in acid deposition on terrestrial ecosystems. For annual injection of 5 Tg of SO2 into the tropical stratosphere or 3 Tg of SO2 into the Arctic stratosphere, the additional surface sulfate deposition is not enough to negatively impact most ecosystems. Robock, Alan, Luke Oman, and Georgiy Stenchikov (2008), Regional climate responses to geoengineering with tropical and Arctic SO2 injections. J. Geophys. Res., 113, D16101, doi:10.1029/2008JD010050.
U43A-0042
Impact of Geoengineering Schemes on the Global Hydrological Cycle
The rapidly rising CO2 level in the atmosphere has led to proposals of climate stabilization by "geoengineering" schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earth's surface. In this article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO2 forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4%/K for solar forcing, but only 1.5%/K for CO2 forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.
U43A-0043
Cooling Earth's temperature by seeding marine stratocumulus clouds for increasing cloud cover by closing open cells
The transition from open to closed cellular convection in marine stratocumulus is very sensitive to small concentrations of cloud condensation nuclei (CCN) aerosols. Addition of small amounts of CCN (about 100 cm-3) to the marine boundary layer (MBL) can close the open cells and by that increase the cloud cover from about 40% to nearly 100%, with negative radiative forcing exceeding 100 wm-2. We show satellite measurements that demonstrate this sensitivity by inadvertent experiments of old and diluted ship tracks. With the methodology suggested by Salter and Latham for spraying sub-micron sea water drops that serve as CCN, it is possible to close sufficiently large area of open cells for achieving the negative radiative forcing that is necessary to balance the greenhouse gases positive forcing. We show calculations of the feasibility of such an undertaking, and suggest that this is an economically feasible method with the least potential risks, when compared to seeding marine stratocumulus for enhancing their albedo or with seeding the stratosphere with bright or dark aerosols. Global Circulation models coupled with the ocean and the ice are necessary to calculate the impact and the possible side effects.
U43A-0044
Modification of Cirrus Clouds to Reduce Global Warming
As far as we know, no studies have addressed the possibility of modifying cirrus clouds to reduce global
warming. Here we explore this possibility and associated feasibility issues.
To introduce this concept, some background information is needed. The effect of cirrus on climate can be
quantified through their predicted impact on climate sensitivity, S (i.e. the equilibrium response of global-
mean surface temperature to CO2 doubling) in global climate model (GCM) simulations. A recent study using
an ensemble of thousands of "perturbed physics" GCM simulations found that S was most strongly
influenced by the entrainment coefficient and the ice fall speed, indicating that S depends more on changes
in cirrus clouds than on low-level boundary layer clouds. It may be possible to modify the ice fall speed in
cirrus clouds which controls ice removal rates and affects the cirrus ice content, life cycle and coverage, as
well as the upper troposphere relative humidity. The main impact of reducing the ice fall speed was an
increase in longwave cloud forcing. In a different recent GCM study, we have used the mean size of the ice
particle size distribution to change the representative ice fall speed, V. By decreasing V, the cirrus coverage
was increased 5.5%, strongly affecting annual zonal means of cloud forcing, heating rates and temperatures
in the upper troposphere. This led us to speculate that the introduction of aerosol particles into the upper
troposphere (T < -40 C) that efficiently form ice crystals through heterogeneous nucleation may result in
larger ice particles with higher fall speeds since the heterogeneous nuclei would outcompete the natural
homogeneous freezing ice nuclei for water vapor. This would reduce longwave cloud forcing and lower
surface temperatures, as described above. A third recent GCM study supports our speculation, showing that
heterogeneous ice nucleation for these conditions produces larger ice crystals with higher fall velocities
(relative to ice crystals formed by homogeneous nucleation).
These studies and others beg the question of whether the introduction of efficient heterogeneous ice nuclei
in regions of the upper troposphere normally dominated by homogeneous nucleation would reduce cirrus
cloud coverage through higher ice fall speeds or would increase cirrus coverage by allowing nucleation in
otherwise clear-sky regions supersaturated with respect to ice. The introduction of efficient ice nuclei might
initially increase cirrus coverage in these regions, but once a new equilibrium of cirrus coverage is
established, it is unclear whether cirrus coverage would be more or less than present day conditions. This
question could be explored in climate simulations using microphysically advanced GCMs.
Should the method appear promising, it could be applied by introducing efficient ice nuclei into the upper
troposphere using commercial airliners. Weather modification research has developed ice nucleating
substances that are extremely effective at these cold temperatures, are non-toxic and are relatively
inexpensive. The strategy is to build-up a background concentration of efficient ice nuclei in the -40 to -60 C
zone so that cirrus forming by natural processes will experience these nuclei and grow larger crystals. High
level winds would disperse the nucleant aerosol from the flight corridors. While there are risks of affecting
the climate system in unforeseen ways, time scales in the atmosphere are relatively short, and this
geoengineering experiment could be terminated at any time.
http://www.dri.edu/Projects/Mitchell/
U43A-0045
Preparing Climate Engineering Responses to Climate Emergencies II: Impact Detection/Attribution and Field Testing
Through a one-week intensive study, the authors of this abstract explored the question: What program of comprehensive technical research over the next decade would maximally reduce the uncertainties associated with climate engineering responses to climate emergencies? The motivations underlying this question, our group's focus on climate engineering concepts for manipulating incident short-wave solar radiation, and our in-depth consideration of stratospheric aerosol interventions as a case example are all described in a previous presentation (Keith et al. in this session). This second of two presentations on our study group's findings concentrates specifically on our technical evaluation of the issues associated with climate impact detection and attribution. Our analyses begin by examining the natural variability (noise) and equilibration timescales (temporal response) of a number of specific climate parameters (e.g. surface radiative flux, surface temperature, atmospheric ozone concentrations, etc.) at both the global and regional scales. First, using the assumption of immediate response for all climate parameters, order-of-magnitude signal-to-noise ratio calculations are used to estimate the minimum intervention durations and amplitudes needed for climate impacts of predicted magnitude to be attributably detected. Next, a number of relevant processes (physical, chemical and biological) within the climate system are evaluated to provide order-of-magnitude estimates for the actual temporal response of these climate parameters (e.g. delay in global temperature response due to ocean heat capacity). Cumulatively, these first-order quantitative estimates reveal a number of basic limits to the timescale over which equilibrium climatic parameter impacts of a climate engineering intervention could be detected. Building from these basic results, we examine current climate monitoring capabilities across four broad categories of climate parameters: (1) radiative; (2) geophysical; (3) geochemical; and (4) ecological. The utility of present monitoring capabilities (e.g. the AeroNet network and ARM) for field-tests are considered, including the proposal and quantitative evaluation of methods for achieving maximal understanding from minimal amplitude tests (e.g. intermittent interventions with phase-sensitive "lock-in" detection methods to maximize sensitivity). Finally, large gaps between current monitoring capabilities and the basic detection limits are identified in each of these categories, and new detection systems are proposed to fill those gaps.
U43A-0046
The effect of deliberate stratospheric aerosols on direct sunlight and implications for concentrating solar power
Both calculations and data show that stratospheric aerosols reduce direct sunlight by about four watts for every watt reflected to outer space. The balance becomes diffuse sunlight. Calculations show how the amount of diffuse sunlight depends on the size of the stratospheric aerosol particles. One consequence of deliberate enhancement of the stratospheric aerosol layer would be a significant reduction in the power output of solar generation systems using parabolic or other concentrating optics to collect direct sunlight. This is evident in the output of solar electricity generating plans after the eruption of Mt. Pinatubo.
U43A-0047
Impact Of Geo-engineered Aerosols On Stratospheric Chemistry And Dynamics
Geo-engineering schemes have been proposed to alleviate the consequences of global warming; one proposed scheme is to inject sulfur into the stratosphere so as to mimic the effects of large volcanic eruptions. Past volcanic eruptions have shown that strongly enhanced sulfate aerosols in the stratosphere result in a higher planetary albedo, leading to surface cooling. However, the increase of sulfate aerosol surface area enhances heterogeneous reactions in the stratosphere that lead to ozone loss. The potential for high Arctic ozone depletion in the context of geo-engineering is known. On the other hand, halogen compounds are now decreasing in the atmosphere as a result of the enforcement of the Montreal Protocol and its amendments, and this is expected to bring about the recovery of the ozone layer and to lessen the potential impact of aerosols. In this study we present results of calculations made with NCAR's Whole Atmosphere Community Climate Model (WACCM), focusing on the impact of Geo-engineering on stratospheric chemistry and dynamics. Aside from changes in heterogeneous reactions, changes in stratospheric dynamics have a significant impact on ozone. On average, changes of both chemistry and dynamics result in a slowdown of the recovery of ozone for mid- and high latitudes. An increase of ozone depletion as a result of geo-engineering was found in both polar regions for the period between 2040-2050.