Reducing the Solar Constant as a Mitigation Technique to the Increasing CO2 Levels

Due to the detrimental effects of global warming, in particular the drastically increasing levels of CO2, the Danish Climate Centre (DKC) focused on methods of mitigation. In particular, DKC focused on solar radiation management experiments. The DKC uses the EC-Earth climate model in their experiments because it includes a good representation of the stratosphere. Their experiment includes three simulations: a pre-industrial control simulation (Control), a quadrupled CO2 simulation (4CO2), and a quadrupled CO2 simulation balanced by a reduction of the solar constant (Balanced). The authors also considered the situation in the Northern Hemisphere extra-tropical winter. The mean temperature and precipitation responses in each simulation were analyzed. The authors conclude that reducing the solar constant can significantly mitigate the detrimental affects of increasing CO2 levels. However, they acknowledge that a better understanding about the effects of cooling on the stratospheric ozone is needed. —Michela Isono
Christiansen, B., Yang, S., 2011. Mitigating a Quadrupling of CO2 by a Reduction of the Solar Constant: A Geoengineering Experiment with the EC-Earth Climate Model. Danish Meteorological Institute, 1399–1973.

The solar radiation constant is a measure of the amount of incoming solar electromagnetic radiation per unit area. The experiment lasted for 50 years and the second 25 years are used for analysis purposes. A reduction of the solar constant of 56 W/m2 was used. This value was derived from the equation ∆F = (1-a) / 4∆S, where ∆F is the radiative forcing from the quadrupled CO2(~8.5 W/m2) and where a is the planetary albedo (0.33). The authors investigated the annual mean temperature responses and the annual mean precipitation responses, and acknowledged that their experiments have been performed in previous scientific studies. The authors determine the statistical significance of the differences among the simulations by using a t-test.
The authors chose to study the Northern Hemisphere extra-tropical winter region. In this area, the impact of the reduced solar radiation is minimal; however the indirect effects associated with dynamical changes were expected. The authors stated that changes in the stratospheric temperature cause changes in the stratospheric vortex, which causes changes in the North Atlantic Oscillation (NAO) and then affects the troposphere.
Annual Mean Temperature Responses
            The annual mean surface temperature responses were calculated over a 25-year time period. The 4CO2simulation showed significant warming across all regions (values ranged from 2 to over 16 K). The largest response was in the Arctic, and the smallest response was in the tropics and over the Southern Ocean. The Balance simulation showed a much lower response across all regions (values infrequently went above 1 K). The warming that occurred was located in the polar areas contrasted by the slight cooling in the tropics. The responses in the tropics and Arctic were statistically significant, however responses in many parts of the extra-tropics proved statistically insignificant.
            The zonal and annual mean temperature responses as a function of latitude and altitude were also performed. The 4CO2 simulation showed that the troposphere warmed ubiquitously. The most warming was observed in the mid-troposphere over the tropics. In contrast, the stratosphere cooled ubiquitously (values reached –15 K). The Balance simulation showed much smaller responses in the troposphere, although cooling was seen overall in the tropical and extra-tropical troposphere (values were statistically significant decreasing by –1 or –2 K). Comparatively, there was more cooling in the stratosphere in the Balance simulation than in the 4CO2 simulation.
Annual Precipitation Responses
            The monthly global mean precipitation as a function of time for the three simulations over 25 years was analyzed. The Control simulation showed a consistent value of about 2.85 mm/day/m2. The value in the 4CO2 simulation increased about 0.2 mm/day. The authors noted that this result could be caused by the fact that saturation water vapor pressure increases with temperature. The value in the Balance simulation decreased about 0.1 mm/day. The authors stated that this finding has been found in previous studies and is the result of the reduced solar radiation at the Earth’s surface, which ultimately slows the continuous movement of water on, above, and below the surface of the Earth (hydrologic cycle). 
            The geographical distributions of the annual mean precipitation responses were also shown. In the 4CO2simulation, the polar-regions and tropics showed the greatest increase in precipitation and were statistically significant. The extra-tropics showed a weaker change, in which there was a negative response in some locations. In the Balance simulation, a small decrease in precipitation response was generally shown. The response of only a small number of regions was statistically significant— over seas or the Western part of tropical Africa. 
NH Winter Responses
            The annual zonal mean temperature response showed that the troposphere warmed and the stratosphere cooled in the 4CO2 simulation. This occurrence is statistically significant in all regions except in the polar stratosphere. However, a greater response in the stratosphere in the winter towards the poles was found as well. This is the result of the induced temperature response that reduces the negative temperature gradient in the stratosphere, which reduces the stratospheric vortex. The results of the zonal mean wind responses support this idea.
Additionally, the Balance simulation for winter (December, January, and February) zonal mean temperature response over the last 25 years showed that the gradient in the stratosphere temperature response was almost gone. This is due to cooling in the polar stratosphere and causes smaller changes in the stratospheric vortex and no changes in the troposphere. The mean surface temperature and precipitation responses during the winter were determined as well. The results showed a significant response in the 4CO2 simulationbut not in the Balance simulation.
            These experiments showed that reducing the solar constant can mitigate the increase of the annual mean surface temperature that is caused by the drastically increasing levels of CO2. This mitigation method proved successful near the surface, and cooled both the troposphere and the stratosphere. However, the effects of significant cooling in the upper stratosphere could affect the stratospheric ozone.
The authors also took into account the NH winter response. In this case, the effect of reducing the solar constant was small. However, the surface temperature and precipitation was combated well.

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