7 May 2021
adapted from the story by NOAA Communications
A new modeling study led by two NOAA researchers highlights the vast challenges and potentially damaging consequences of solar geoengineering actions large enough to ward off extreme warming by the end of the 21st century.
The study, published in the journal Atmospheric Chemistry and Physics, explored a set of climate model simulations generated by National Center for Atmospheric Research (NCAR) researchers called the Geoengineering Large Ensemble (GLENS). 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.
Lead author Antara Banerjee, a CIRES research scientist working at NOAA CSL, 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," said Banerjee.
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.
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.
While the sulfate aerosol injections in the NCAR model runs were carefully designed to keep the annual global-mean surface temperature, the equator-to-pole surface temperature gradients, and the interhemispheric temperature gradients constant 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° 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 (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 CSL, 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. Other members of the research team included scientists from NCAR, Colorado State University, Rutgers University, and Columbia University.
"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," said Butler. "This emphasizes the need for further investigation of potential unintended consequences of climate intervention techniques."
Banerjee, A., A.H. Butler, L.M. Polvani, A. Robock, I.R. Simpson, and L. Sun, Robust winter warming over Eurasia under stratospheric sulfate geoengineering – the role of stratospheric dynamics , Atmospheric Chemistry and Physics, doi:10.5194/acp-21-6985-2021, 2021.
It has been suggested that increased stratospheric sulfate aerosol loadings following large, low latitude volcanic eruptions can lead to wintertime warming over Eurasia through dynamical stratosphere-troposphere coupling. We here investigate the proposed connection in the context of hypothetical future stratospheric sulfate geoengineering in the Geoengineering Large Ensemble simulations. In those geoengineering simulations, we find that stratospheric circulation anomalies that resemble the positive phase of the Northern Annular Mode in winter is a distinguishing climate response which is absent when increasing greenhouse gases alone are prescribed. This stratospheric dynamical response projects onto the positive phase of the North Atlantic Oscillation, leading to associated side-effects of this climate intervention strategy, such as continental Eurasian warming and precipitation changes. Seasonality is a key signature of the dynamically-driven surface response. We find an opposite response of the North Atlantic Oscillation in summer, when no dynamical role of the stratosphere is expected. The robustness of the wintertime forced response stands in contrast to previously proposed volcanic responses.