Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

Simulated Geoengineering Evaluation: Cooler Planet, With Side Effects

A new modeling study led by CIRES and NOAA researchers highlights the vast challenges and potentially damaging consequences of a solar geoengineering program large enough to ward off extreme warming by the end of the 21st century. 

The study, published today in the journal Atmospheric Chemistry and Physics, explored a set of climate model simulations generated by NCAR researchers called the Geoengineering Large Ensemble. This group of 20 simulations projected the climate-forcing influence of hypothetical sulfate aerosol injections in the stratosphere sufficient to reflect enough sunlight to counter global warming from rising carbon dioxide levels throughout the end of the 21st century. 

A massive intervention would be needed

Lead author Antara Banerjee, a CIRES research scientist working at NOAA’s Chemical Sciences Laboratory, said that the model required enormous inputs of sulfur dioxide to mitigate the expected warming: as much as 50 million metric tons would need to be continuously injected into the stratosphere every year by the end of the century to obtain zero global-mean temperature change even as CO2 continues to increase.

“While these sulfate aerosols would largely mitigate the impacts of greenhouse gas-induced climate change, there are unintended side effects in these simulations that we need to understand,” Banerjee said. 

Some scientists and policy makers view climate intervention scenarios, such as reflecting sunlight into space to cool the planet, as a temporary “Plan B” in case humans do not act aggressively enough to tackle the root cause of climate change: fossil fuel pollution. 

Solar radiation management, as it is called, is widely considered to be the climate intervention method most likely to work. Although the technology needed to place reflective particles in the stratosphere does not yet exist, scientists are confident that a sufficient amount of aerosols would cool the planet, based on the observed cooling effect that large volcanic eruptions have had on the global climate in the past. 

This figure illustrates potential methods for climate intervention that would either intercept incoming or make it easier for outgoing solar radiation to escape the atmosphere. Credit: Chelsea Thompson, CIRES/NOAA

At the direction of Congress, NOAA initiated a research program in 2020 to establish the scientific foundation needed to inform decision makers who may one day evaluate climate intervention proposals. NOAA scientists and partners are investigating the climate effects of aerosols potentially added to the stratosphere and troposphere, and evaluating modeling systems that realistically assess aerosol impacts on the Earth system and on society. Research into atmospheric aerosols will also improve weather and climate models.

Unexpected side effects could appear 

While the sulfate aerosol injections in the NCAR model runs were carefully designed to keep constant the annual global-mean surface temperature, the equator-to-pole surface temperature gradients, and the interhemispheric temperature gradients as carbon dioxide rose throughout this century, the analysis indicated that potentially unexpected side effects were still possible in different seasons. 

For example, while the simulations mitigated around two-thirds of expected winter warming trends due to climate change in Eurasia, a robust surface warming of up to 1.5 Celsius, or almost 3 degrees Fahrenheit, every 30 years still occurred.

Another side effect identified in the simulations is reduced precipitation in the Mediterranean during winter, when the arid region normally receives most of its annual moisture. This begins mid-century, when the simulated geoengineering effort scales up. However, the loss of winter precipitation is balanced by an increase in summer moisture. The opposite would occur in Scandinavia: wetter winters and drier summers. 

These side effects, while considerably weaker in magnitude than the changes in temperature and precipitation expected from high-end CO2 emission scenarios by the end of the century, occur because the additional sulfate aerosols cause a strengthening of the Northern Hemisphere stratospheric polar vortex, a band of strong westerly winds that forms between about 10 and 30 miles above the North Pole every winter. A stronger polar vortex in turn shifts the North Atlantic Oscillation, or NAO, which influences the location of storm tracks across the North Atlantic, to a more positive phase, resulting in a stronger Atlantic jet stream and a northward shift of the storm track. 

During a positive NAO, northern Europe sees warmer-than-average temperatures that are associated with the air masses that arrive from lower latitudes, along with increased precipitation. At the same time, southern Europe sees less precipitation. 

The model simulations show that these trends reverse during summer when the stratospheric polar vortex is not present. 

Co-author Amy Butler, of NOAA’s Chemical Sciences Lab, who studies how the stratosphere influences weather at Earth’s surface, said that the model runs demonstrate how large amounts of aerosols in the upper atmosphere can change hemispheric circulation patterns.

“Our results suggest that under the sulfate climate intervention approach adopted here, the Eurasian continent would still need to adapt to climate changes—specifically, warmer winters, a drier Mediterranean and wetter Scandinavia,” Butler said. “This emphasizes the need for further investigation of potential unintended consequences of climate intervention techniques."

Other members of the research team included scientists from NCAR, Colorado State University, Rutgers University, and Columbia University

This story was written by NOAA Communications

CIRES is a partnership of NOAA and CU Boulder.


Antara Banerjee
CIRES and NOAA scientist
Theo Stein
NOAA Communications
Karin Vergoth
CIRES communications

Recent Stories