The Unintended Consequences of Sulfate Aerosols in the Troposphere and Lower Stratosphere Massachusetts Institute of Technology Department of Civil and Environmental Engineering 11/29/2009 1Executive Summary Sulfate aerosol injection is a geoengineering scheme that has been put forward in order to combat the rise in surface temperature of the Earth due to the build up of carbon dioxide in the atmosphere. The build up of aerosols in the atmosphere reflects sunlight away from the Earth, as shown by large volcanic eruptions such as those of Mount Pinatubo and El Chichón. Both these eruptions caused a significant decrease in regional surface temperatures for several years following the eruptions; an injection of sulfate aerosols is expected to do the same. There are potential side effects of the scheme, however, such as regional precipitation changes, ozone depletion and acid rain. In addition, sulfate aerosol injection does not address continued build-up of carbon dioxide. In order to quantify the severity of the potential consequences and the effectiveness of the scheme at decreasing global surface temperatures, I have proposed an experiment involving a small-scale aerosol injection scheme. The experimental design is based on computer simulation models done by Robock et al. (2008). The goal of the experiment is to ascertain the side effects of a scheme meant to increase the amount of summer sea ice in the Arctic. Although the experiment will provide us with valuable knowledge regarding the consequences of a sulfate aerosol injection scheme, further experimentation will be necessary before any type of geoengineering scheme is undertaken. 2An Introduction to Global Climate Change Since the advent of the Industrial Revolution, societies across the globe have endeavored to increase their production capacity and, in turn, have increased their contribution to global emissions. As a result, the amount of carbon dioxide in the atmosphere has increased from 280 parts per million (ppm) to 380 ppm, and the average global surface temperature has increased by 0.8°C (Bala, 2009). These increases are the result of human perturbation, and are likely to continue unchecked, leading to significant climate change that would vastly change ecosystems all over the world. Global warming and climate change are the result of increases in the concentrations of anthropogenic greenhouse gases in the atmosphere. Some of the most prevalent greenhouse gases are carbon dioxide, methane, water vapor, ozone and nitrous oxide. Although all of these gases play a significant role in global climate science, carbon dioxide is the one on which many models and estimates are based, and which humans play the greatest role in emitting into the atmosphere. Natural greenhouse gases play an important role in our global climate. They act as a blanket keeping the Earth's surface at a habitable temperature. This is called the greenhouse effect. Without the presence of greenhouse gases such as water vapor and carbon dioxide, the temperature of Earth's surface would be about -19°C instead of the global mean surface temperature of 14°C (Le Treut et al., 2007). This temperature has allowed for the survival and productivity of the human race and has thus permitted the industrialization of the Earth which is now contributing to global climate change and ecosystem destruction. The increase of carbon dioxide in the atmosphere has two major components: the emissions from the burning of fossil fuels and cement production, and changes in land-use, such as deforestation and wood-harvesting. In a typical cycle, natural sinks on land and in the ocean 3regulate and remove carbon dioxide from the atmosphere. However, carbon dioxide is less soluble in warmer waters, which decreases the “downward transport of anthropogenic carbon” (Bala et al., 2005). In the past 20 years, the growth rate of fossil fuel emissions has increased from 1.3% per year in 1990 to 3.3% per year from 200-2006 (Canadell et al., 2007). As the amount of carbon dioxide emitted into the atmosphere increases, the efficiency of natural carbon sinks decreases. Potential consequences If global emissions continue to grow at such rates, the consequences could be dire. Schneider (2009) presents the scenario where our atmosphere has 1,000 ppm of CO2. This scenario would lead to a world without the ecosystems unique to Earth such as mountain glaciers and coral-reef communities. Although the scenario Schneider presents is extreme, the ideas he presents are reasonable. Even if only a few of the consequences he lays out come true, we would face a world significantly different than it is today. For instance, an increase of 1°C above current global temperatures could lead to coral bleaching. Coral reef systems are incredibly sensitive to environment changes, and severe coral bleaching has already occurred, such as in 1998 when nearly 16% of reef coral died (Walther et al., 2002). An increase of 2.7°C may cause the Greenland ice-cap to melt and at a change of 3°C, catastrophic changes such as “reversal of the land carbon sink and possible destabilization of the Antarctic ice sheets” (Bala, 2009). Temperatures would vary between regions, because maximum temperatures are increasing more slowly than minimum temperatures, meaning that there are longer freeze-free periods at high latitudes whereas the temperature change in mid-latitude regions is less noticeable (Walther et al., 2002). Climate change has also caused different reactions in many species. Species whose breeding or migration depends on seasons have been arriving earlier, 4such as the birds that migrate in the spring. (Walther et al., 2002). In fact, ecosystems may already be committed to long-term changes despite efforts to curb climate change. Jones et al. (2009) details a model involving the Amazon rainforest, which suggests that an increase in the global mean temperature increases the decomposition of soil and, coupled with the decrease in precipitation, causes plant productivity to be significantly decreased. Because some effects of climate change, such as “vegetation cover and carbon storage” (Jones et al., 2009) differ in the rate of change from precipitation and temperature, it is likely that both terrestrial and marine ecosystems would continue to change even if the climate remained the same or returned to its previous state. This may mean that by 2050, the tree cover in the