Uneven
rates of sea level rise tied to climate change
The
pattern of uneven sea level rise over the last quarter century has been driven
in part by human-caused climate change, not just natural variability, according
to a new study.
The findings
suggest that regions of the world where seas have risen at higher than average
rates -- including the Eastern Seaboard of the United States and the Gulf of
Mexico -- can expect the trend to continue as the climate warms.
The study,
published in the Proceedings
of the National Academy of Sciences, was authored by scientists John
Fasullo at the National Center for Atmospheric Research (NCAR) and Steve Nerem
at the University of Colorado Boulder.
"By knowing
that climate change is playing a role in creating these regional patterns, we
can be more confident that these same patterns may linger or even intensify in
the future if climate change continues unabated," Fasullo said.
"With sea levels projected to rise a couple of feet or more this century on average, information about expected regional differences could be critical for coastal communities as they prepare."
"With sea levels projected to rise a couple of feet or more this century on average, information about expected regional differences could be critical for coastal communities as they prepare."
The research was funded by the National Science Foundation, which is NCAR's sponsor, the NASA Sea Level Change Team, and the U.S. Department of Energy.
Finding the signal of climate change
For the study,
Fasullo and Nerem, both members of the NASA Sea Level Change Team, analyzed the
satellite altimetry sea level record, which includes measurements of sea
surface heights stretching back to 1993. They mapped global average sea level
rise as well as how particular regions deviated from the average.
For example, the
oceans surrounding Antarctica and the U.S. West Coast have had
lower-than-average sea level rise, while the U.S. East Coast and Southeast
Asia, including the Philippines and Indonesia, have experienced the opposite.
In some parts of the world, the rate of local sea level rise has been as much
as twice the average.
Regional
differences in sea level rise are influenced by where heat is stored in the
ocean (since warm water expands to fill more space than cold water) and how
that heat is transported around the globe by currents and wind. Uneven sea
level rise is also influenced by ice sheets, which lose mass as they melt and
shift the gravitational forces affecting regional sea surface height.
Natural shifts
in ocean cycles -- including the Pacific Decadal Oscillation, a pattern of sea
surface temperatures similar to El NiƱo but longer lasting -- are therefore
known to affect sea levels. So scientists were not surprised to find that as
the ocean rises, it rises unevenly.
But it's been difficult to say whether these natural cycles were the dominant influence on regional differences.
But it's been difficult to say whether these natural cycles were the dominant influence on regional differences.
To investigate
the role of climate change, the scientists turned to two sets of climate model
runs, known as "large ensembles": one created using the NCAR-based
Community Earth System Model and one created using the Earth System Model at
the National Oceanic and Atmospheric Administration.
These large ensembles -- many model simulations by the same model, describing the same time period -- allow researchers to disentangle natural variability from the impacts of climate change. With enough runs, these impacts can be isolated even when they are relatively small compared to the impacts from natural variability.
These large ensembles -- many model simulations by the same model, describing the same time period -- allow researchers to disentangle natural variability from the impacts of climate change. With enough runs, these impacts can be isolated even when they are relatively small compared to the impacts from natural variability.
The climate
models suggest that in regions that have seen more or less sea level rise than
average, as much as half of that variation may be attributed to climate change.
The scientists also found that the impacts from climate change on regional sea
level rise sometimes mimic the impacts from natural cycles.
"It turns
out the sea level rise response to climate change in the Pacific resembles what
happens during a particular phase of the Pacific Decadal Oscillation,"
Fasullo said. "This explains why it's been so difficult to determine how
much of the pattern was natural or not, until now."
Improving forecasts
The research
findings have implications for local officials, who are interested in improved
forecasts of sea level rise for the areas they oversee. In the past,
forecasters have had to rely on the global rate of change -- about 3
millimeters a year and accelerating -- and knowledge of the uneven regional
impacts associated with continued melting of the ice sheets covering Greenland
and Antarctica.
The findings add the possibility that the regional patterns of sea level rise tied to climate change can also be included, because the models predict that the regional patterns observed in the satellite measurements will continue into the future.
"We now
have a new tool -- long-term satellite altimeter measurements -- that we can
use to help stakeholders who need information for specific locations,"
said Nerem, a fellow of the Cooperative Institute for Research in Environmental
Sciences at the University of Colorado Boulder and a professor of aerospace
engineering.