Geo-engineering attempts to mitigate global warming by blocking sunlight could succeed and drastically cool the planet, bringing global temperatures back to around year 1900 levels—but could just as easily worsen the situation if these projects fail or are suddenly halted, according to a new computer modeling study.
The study, described in an open access paper in the 4 June early online edition of The Proceedings of the National Academy of Sciences, modeled possible outcomes if society tries to slow global warming by geo-engineering a solar filter while greenhouse gas emissions continue on a business-as-usual basis rather than reducing carbon dioxide emissions.
Given current political and economic trends, it is easy to become pessimistic about the prospect that needed cuts in carbon dioxide emissions will come soon enough or be deep enough to avoid irreversibly damaging our climate. If we want to consider more dramatic options, such as deliberately altering the Earth’s climate, it’s important to understand how these strategies might play out.—co-author Ken Caldeira, Carnegie Institution’s Department of Global Ecology
Although the term “geo-engineering” describes any measure intended to modify the Earth at the planetary scale, the current study focuses on changes that reduce the amount of solar radiation that reaches the planet’s surface. Several methods to accomplish this have been suggested, from filling the upper atmosphere with light-reflecting sulfate particles to installing mirrors in orbit around the planet.
According to the model, the global climate system responds quickly to artificially reduced insolation; hence, the researchers conclude, there may be little cost to delaying the deployment of geo-engineering strategies until such a time as “dangerous” climate change is imminent.
However, the study also finds that if any hypothetical geo-engineering program were to fail or be cancelled for any reason, a catastrophic, decade-long spike in global temperatures could result, along with rates of warming 20 times greater than we are experiencing today.
If we become addicted to a planetary sunshade, we could experience a painful withdrawal if our fix was suddenly cut off. This needs to be taken into consideration if we ever think seriously about implementing a geo-engineering strategy.—Ken Caldeira
The authors concluded that lower temperatures in a geo-engineered world would result in more efficient storage of carbon in plants and soils. However, if the geo-engineering system failed and temperatures suddenly increased, much of that stored carbon would be released back into the atmosphere. This, in turn, could lead to accelerated greenhouse warming.
Reduced solar radiation not only affects temperatures in the simulations, but also global rainfall patterns. In a model run with no simulated geo-engineering, warmer temperatures resulted in more rainfall over the oceans, while increased carbon dioxide levels caused a decrease in evaporation from plants’ leaves, and consequently a decrease in rainfall over tropical forests. In contrast, the geo-engineering scenario—which had lower temperatures but the same high levels of carbon dioxide—resulted only in a decrease in tropical forest rainfall.
Many people argue that we need to prevent climate change. Others argue that we need to keep emitting greenhouse gases. Geoengineering schemes have been proposed as a cheap fix that could let us have our cake and eat it, too. But geo-engineering schemes are not well understood. Our study shows that planet-sized geo-engineering means planet-sized risks.—Ken Caldeira
“Transient climate-carbon simulations of planetary geo-engineering” H. Damon Matthews and Ken Caldeira; Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0700419104