Climate change as a result of anthropogenic activities calls for reduction of greenhouse gas emissions to avoid dangerous consequences on society. However, abatement of emission is a costly process and adversely affects the economic growth. Recent proposals, therefore, suggested a different approach i.e. Geoengineering. Instead of controlling emissions, Geoengineering modifies the climate by changing global energy fluxes either by increasing the amount of outgoing infrared radiation through reduction of greenhouse gases (GHGs) or by decreasing the amount of solar radiation falling upon the earth’s surface by increasing the albedo (reflectivity) of the atmosphere. Most popular geoengineering strategies are Air Capture (AC) and Solar Radiation Management (SRM) and many economic studies have shown large net monetary benefits with their application. But, these studies neglected the risks which can arise due to potential failure to sustain SRM after few decade of its deployment. There is a concern that application of SRM will lead to increase in concentration of carbon-dioxide in atmosphere and its abrupt turning off can lead to rise in temperature and thereby huge monetary losses. In this report, consequences of abruptly turning off of SRM have been analyzed. A modified version of DICE (Dynamic Integrated model of Climate and the Economy) model that incorporates negative SRM forcing and a two phase optimization procedure has been used for the study. Different outcomes such as net change in NPV of climate damage and abatement costs, maximum mean temperature of earth surface, increase in temperature, emissions control rate, carbon taxes, etc due to abrupt ending of SRM have been analyzed. Results show that application of SRM with a risk of abrupt turnoff is still more profitable compared to not using it at all. / text

The climatic effects of Solar Radiation Management (SRM) geoengineering have been often modeled by simply reducing the solar constant. This is most likely valid only for space sunshades and not for atmosphere and surface based SRM methods. In this thesis, a global climate model is used to test if the climate response to SRM by stratospheric aerosols and uniform solar constant reduction are similar. Our analysis shows that when global mean warming from a doubling of CO2 is nearly cancelled by both these methods, they are similar when important surface and tropospheric climate variables are considered. However, a difference of 1K in the global mean stratospheric (61-9.8 hPa) temperature is simulated between the two SRM methods. Further, while the global mean surface diffuse radiation increases by about 15- 20% and direct radiation decreases by about 8% in the case of sulphate aerosol SRM method, both direct and diffuse radiation decrease by similar fractional amounts (~ -1.5%) when solar constant is reduced. When CO2 fertilization effects from elevated CO2 concentration levels are removed, the contribution from shaded leaves to gross primary productivity (GPP) increases by 6 % in aerosol SRM because of increased diffuse light. However, this increase is almost offset by a 7% decline in sunlit contribution due to reduced direct light. Overall both the SRM simulations show similar decrease in GPP (~ 1%) and NPP (~ 0.7%). Based on our results we conclude that the climate states produced by a reduction in solar constant and addition of aerosols into the stratosphere can be considered almost similar except for two important aspects: stratospheric temperature change and the consequent implications for the dynamics and the chemistry of the stratosphere and the partitioning of direct versus diffuse radiation reaching the surface. Further, the likely dependence of global hydrological cycle response on aerosol particle size and the latitudinal and height distribution of aerosols is discussed.

We study timing effects of carbon mitigation and solar radiation management (SRM) policies for correlated pollutants, CO₂ and SO₂. We show that national levels of carbon and sulfur emissions quotas and SRM implementation are positively correlated with each other. First-mover advantages exist when deciding both carbon quotas and SRM levels. Moreover, we use an example to illustrate that if international equity is considered, governments would be willing to choose SRM levels before carbon quotas since it yields higher payoffs and less acid rain and droughts damages. This timing was neglected by all previous theoretical economic models on geoengineering.

Geoengineering is the intentional alteration of the Earth’s climate system. The international Geoengineering Model Intercomparison Project (GeoMIP) seeks to identify the potential benefits and side effects of geoengineering on the Earth's climate. This thesis examines the first two experiments from the contribution of the Canadian Centre for Climate Modelling and Analysis to GeoMIP. In the first experiment (G1), atmospheric carbon dioxide concentrations are quadrupled and the solar constant is reduced to offset the increased greenhouse gas forcing. In the second experiment (G2), atmospheric carbon dioxide concentrations are increased at the rate of 1% per year and the solar constant is incrementally reduced to offset the greenhouse gas forcing. In concert with these experiments, results from two other experiments were analyzed, one in which the atmospheric greenhouse gas concentrations are quadrupled one in which they are increased at the rate of 1% per. The results obtained are in broad agreement with earlier work, showing that solar radiation management geoengineering schemes can prevent an increase in mean global surface temperature as atmospheric carbon dioxide concentrations increase. Though the mean global temperature remains constant while geoengineering is employed, there are regional and zonal differences from the control climate, with high latitude warming and cooling in the tropical and subtropical regions. In particular, the meridional temperature gradient is reduced compared to that of the control climate. The G2 experiment was very similar to the G1 experiment in terms of the spatial surface temperature changes, though the changes seen in the G2 experiment were less pronounced and the regions of statistical significance were smaller. During the geoengineering period, seasonal changes and a statistically significant decrease in global precipitation, particularly over the ocean were apparent in the G1 run. As with temperature, the spatial pattern of precipitation changes during the geoengineering period for G2 are similar to the same period in G1, but reduced in magnitude. However, most of the spatial changes to precipitation in the G2 experiment during geoengineering deployment fail to be statistically significant. Following geoengineering termination, the G1 experiment responds rapidly, with surface and ocean temperatures, NH and SH summer sea ice volume, AMOC transport volume and global precipitation following the same time evolution and reaching those same values found in the 4 × CO2 experiment’s first 10 years. Following geoengineering failure, the G2 run also experiences rapid climate change in all of the variables studied, but does not approach the first 10 years of the 1%CO2yr-1 experiment, because the forcings are quite different in the two runs. Taken together, these results suggest that, while geoengineering to reduce incoming solar radiation could offset the global temperature increase due to increased atmospheric greenhouse gas concentrations, there would be regional warming and cooling, as well as both global and regional impacts on the hydrological cycle. These results also suggest that, should geoengineering suddenly stop, the Earth’s climate would react immediately, with rapid changes in nearly all of the climate variables examined. / Graduate