Combining geoengineering approaches would keep constant temperatures and overall rainfall
New research from an international team of atmospheric scientists published by Geophysical Research Letters investigates for the first time the possibility of using a "cocktail" of geoengineering tools to reduce changes in both temperature and precipitation caused by atmospheric greenhouse gases.
Carbon dioxide emissions from the burning of coal, oil, and gas not only cause the Earth to get hotter, they also affect weather patterns around the world. Management approaches need to address both warming and changes in the amount of rainfall and other forms of precipitation.
So-called solar geoengineering aims to cool the planet by deflecting some of the Sun's incoming rays. Ideas for accomplishing this include the dispersion of light-scattering particles in the upper atmosphere, which would mimic the cooling effect of major volcanic eruptions.
However, climate-modeling studies have shown that while this scattering of sunlight should reduce the warming caused by greenhouse gases in the atmosphere, it would tend to reduce rainfall and other types of precipitation less than would be optimal.
Another approach involves thinning of high cirrus clouds, which are involved in regulating the amount of heat that escapes from the planet to outer space. This would also reduce warming, but would not correct the increase in precipitation caused by global warming.
One method reduces rain too much. Another method reduces rain too little.
The team-which includes Carnegie's Ken Caldeira, Long Cao and Lei Duan of Zhejiang University, and Govindasamy Bala of the Indian Institute of Science-used models to simulate what would happen if sunlight were scattered by particles at the same time as the cirrus clouds were thinned. They wanted to understand how effective this combined set of tools would be at reversing climate change, both globally and regionally.
Caldeira's key contributions to science are his relatively early recognition of the threats posed by ocean acidification, his pioneering investigations into the environmental consequences of intentional intervention in the climate system ("geoengineering"), and the first peer-reviewed study to estimate near-zero-emission energy needs consistent with a 2C climate stabilization target. Kenneth Caldeira is an atmospheric scientist who works at the Carnegie Institution for Science's Department of Global Ecology. He researches ocean acidification, climate effects of trees, intentional climate modification, and interactions in the global carbon cycle/climate system. He also acted as an inventor for Intellectual Ventures, a Seattle-based invention and patent company headed up by Nathan Myhrvold.
"As far as I know, this is the first study to try to model using two different geoengineering approaches simultaneously to try to improve the overall fit of the technology," Caldeira explained.
The good news is that their simulations showed that if both methods are deployed in concert, it would decrease warming to pre-industrial levels, as desired, and on a global level rainfall would also stay at pre-industrial levels. But the bad news is that while global average climate was largely restored, substantial differences remained locally, with some areas getting much wetter and other areas getting much drier.
"The same amount of rain fell around the globe in our models, but it fell in different places, which could create a big mismatch between what our economic infrastructure expects and what it will get," Caldeira added. "More complicated geoengineering solutions would likely do a bit better, but the best solution is simply to stop adding greenhouse gases to the atmosphere."
Caldeira said that the international collaboration of scientists (including scientists from China and India) undertook this research as part of a broader effort aimed at understanding the effectiveness and unintended consequences of proposed strategies for reducing climate change and its impacts.
Abstract
Solar geoengineering has been proposed as a backup plan to offset some aspects of anthropogenic climate change if timely CO2 emission reductions fail to materialize. Modeling studies have shown that there are trade-offs between changes in temperature and hydrological cycle in response to solar geoengineering. Here we investigate the possibility of stabilizing both global mean temperature and precipitation simultaneously by combining two geoengineering approaches: stratospheric sulfate aerosol increase (SAI) that deflects sunlight to space and cirrus cloud thinning (CCT) that enables more longwave radiation to escape to space. Using the slab ocean configuration of National Center for Atmospheric Research Community Earth System Model, we simulate SAI by uniformly adding sulfate aerosol in the upper stratosphere and CCT by uniformly increasing cirrus cloud ice particle falling speed. Under an idealized warming scenario of abrupt quadrupling of atmospheric CO2, we show that by combining appropriate amounts of SAI and CCT geoengineering, global mean (or land mean) temperature and precipitation can be restored simultaneously to preindustrial levels. However, compared to SAI, cocktail geoengineering by mixing SAI and CCT does not markedly improve the overall similarity between geoengineered climate and preindustrial climate on regional scales. Some optimal spatially nonuniform mixture of SAI with CCT might have the potential to better mitigate climate change at both the global and regional scales.
Plain Language Summary
Increases in atmospheric carbon dioxide cause increase in both global temperatures and precipitation. Solar geoengineering has been proposed as a means to counteract this climate change by deliberately deflecting more sunlight from the Earth's climate system. Numerous climate modeling studies have shown that proposed solar geoengineering schemes, such as injection of sulfate aerosols into the stratosphere, can cool climate, but the amount of precipitation change per degree of temperature change is greater than that for CO2, meaning that such proposals cannot simultaneously globally restore both average temperatures and average precipitation. It has also been suggested that the Earth could be cooled by thinning cirrus clouds, but the amount of precipitation change per degree of temperature change for this method is less than that for CO2. Our climate modeling study shows, for the first time, that a cocktail of these two approaches would decrease precipitation and temperature in the same ratios as they are increased by CO2, which would allow simultaneous recovery of preindustrial temperature and precipitation in a high CO2 world at global scale. We show that although the average temperatures and precipitation can be recovered at global scale, substantial differences between the geoengineered and natural climates persist at regional scale.
