Simulated geoengineering evaluation: cooler planet, but with side effects
NOAA
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| This figure illustrates proposed methods for climate intervention that would affect climate by modifying either incoming or outgoing solar radiation. Credit: Chelsea Thompson, NOAA/CIRES |
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 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 counter 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 carbon dioxide 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.
FUTURE CLIMATE SIMULATIONS: These illustrations depict the simulated change in wintertime trends for temperature and precipitation between 2020 and 2095 under a geoengineering scenario (first column) and without geoengineering (second column). Units are degrees Celsius per 30 years and millimeters of precipitation day per 30 years. Credit: Antara Banerjee, NOAA/CIRES.
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.
Unexpected side effects could
appear
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 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. 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."
For more information, contact
Theo Stein at NOAA Communications: theo.stein@noaa.gov