The research team found that reductions in four pollutants
that cycle comparatively quickly through the atmosphere could temporarily
forestall the rate of sea level rise by roughly 25 to 50%.
"To avoid potentially dangerous sea level rise, we
could cut emissions of short-lived pollutants even if we cannot immediately cut
carbon dioxide emissions," says Aixue Hu of the National Center for
Atmospheric Research (NCAR), the first author of the study. "This new
research shows that society can significantly reduce the threat to coastal
cities if it moves quickly on a handful of pollutants."
How the study was
The study, a collaboration of the Scripps Institution for
Oceanography, NCAR, and Climate Central, is being published this week in the
journal Nature Climate Change. It was
funded by the National Science Foundation and the Department of Energy.
"It is still not too late, by stabilizing carbon
dioxide concentrations in the atmosphere and reducing emissions of
shorter-lived pollutants, to lower the rate of warming and reduce sea level
rise," says Veerabhadran Ramanathan of Scripps, who led the study.
"The large role of the shorter-lived pollutants is encouraging since
technologies are available to drastically cut their emissions."
Protecting the coasts
The potential impact of rising oceans on populated areas is
one of the most concerning effects of climate change. Many of the world's major
cities, such as New York, Miami, Amsterdam, Mumbai, and Tokyo, are located in
low-lying areas by the water.
As glaciers and ice sheets melt and warming oceans expand,
sea levels have been rising by an average of about 3 millimetres annually in
recent years (just more than one-tenth of an inch). If temperatures continue to
warm, sea levels are projected to rise between 18 and 59 centimeters (7 to 23
inches) this century, according to a 2007 assessment by the Intergovernmental
Panel on Climate Change. Some scientists, however, feel those estimates are too
Such an increase could submerge densely populated coastal
communities, especially when storm surges hit.
Despite the risks, policy makers have been unable to agree
on procedures for reducing emissions of carbon dioxide. With this in mind, the
research team focused on emissions of four other heat-trapping pollutants:
methane, tropospheric ozone, hydrofluorocarbons, and black carbon. These gases
and particles last anywhere from a week to a decade in the atmosphere, and they
can influence climate more quickly than carbon dioxide, which persists in the
atmosphere for centuries.
Previous research by Ramanathan and Yangyang Xu of Scripps,
a co-author of the new paper, has shown that a sharp reduction in emissions of
these shorter-lived pollutants beginning in 2015 could offset warming
temperatures by up to 50 percent by 2050.
Applying those emission reductions to sea level rise, the
new research found that the cuts could dramatically slow rising sea levels.
Their results showed that total sea level rise would be reduced by an estimated
22 to 42% by 2100, depending on the extent to which emissions were
However, the new study also found that delaying emissions
cuts until 2040 would reduce the beneficial impact on year-2100 sea level rise
by about a third.
30% less sea level
If society were able to substantially reduce both emissions
of carbon dioxide as well as the four other pollutants, total sea level rise
would be lessened by at least 30 % by 2100, the researchers concluded.
The researchers used mostly percentage changes for sea level
rise, rather than actual estimates in centimetres, because of uncertainties
over future temperature increases and their impacts on rising sea levels.
"We still have some control over the amount of sea
level rise that we are facing," Hu says.
Another co-author, Claudia Tebaldi of Climate Central, adds:
"Without diminishing the importance of reducing carbon
dioxide emissions in the long term, this study shows that more immediate gains
from shorter-lived pollutants are substantial. Cutting emissions of those gases
could give coastal communities more time to prepare for rising sea levels. As
we have seen recently, storm surges in very highly populated regions of the
East Coast show the importance of both making such preparations and cutting
To conduct the study, Hu and his colleagues turned to the
NCAR-based Community Climate System Model, as well as a second computer model
that simulates climate, carbon, and geochemistry. They also drew on estimates
of future emissions of heat-trapping gases under various social and economic
scenarios and on computer models of melting ice and sea level rise.
The study assumes that society could reduce emissions of the
four gases and particles by 30 to 60 percent over the next several decades.
That is the steepest reduction believed achievable by economists who have
studied the issue at Austria's International Institute for Applied Systems
Analysis, one of the world's leading research centres into the impact of
economic activity on climate change.
"It must be remembered that carbon dioxide is still the
most important factor in sea level rise over the long term," says NCAR
scientist Warren Washington, a co-author. "But we can make a real
difference in the next several decades by reducing other emissions."