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| Worst case scenario for New England - ocean front in Providence |
New research by a team of Lawrence
Livermore National Laboratory scientists and international collaborators shows
that the observed ocean warming over the last 50 years is consistent with
climate models only if the models include the impacts of observed increases in
greenhouse gas during the 20th century.
Though the new research is not the
first study to identify a human influence on observed ocean warming, it is the
first to provide an in-depth examination of how observational and modeling
uncertainties impact the conclusion that humans are primarily responsible.
"We have taken a closer look at factors that influence these results," said Peter Gleckler, an LLNL climate scientist and lead author of the new study that appears in the June 10 edition of the journal Nature Climate Change. "The bottom line is that this study substantially strengthens the conclusion that most of the observed global ocean warming over the past 50 years is attributable to human activities."
The group looked at the average
temperature (or heat content) in the upper layers of the ocean. The observed
global average ocean warming (from the surface to 700 meters) is approximately
0.025 degrees Celsius per decade, or slightly more than 1/10th of a degree
Celsius over 50 years.
The sub-surface ocean warming is
noticeably less than the observed Earth surface warming, primarily because of
the relatively slow transfer of ocean surface warming to lower depths.
Nevertheless, because of the
ocean's enormous heat capacity, the oceans likely account for more than 90
percent of the heat accumulated over the past 50 years as Earth has warmed.
In this study the team, including
observational experts from the United States ,
Japan and Australia , examined the causes of
ocean warming using improved observational estimates.
They also used results from a large
multi-model archive of control simulations (that don't include the effects of
humans, but do include natural variability), which were compared to simulations
that included the effects of the observed increase in greenhouse gases over the
20th century.
"By using a "multi-model
ensemble," we were better able to characterize decadal-scale natural
climate variability, which is a critical aspect of the detection and
attribution of a human-caused climate change signal.
What we are trying to do is
determine if the observed warming pattern can be explained by natural
variability alone," Gleckler said. "Although we performed a series of
tests to account for the impact of various uncertainties, we found no evidence
that simultaneous warming of the upper layers of all seven seas can be
explained by natural climate variability alone. Humans have played a dominant
role."
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Journal Reference:
1.
P. J. Gleckler, B. D.
Santer, C. M. Domingues, D. W. Pierce, T. P. Barnett, J. A. Church, K. E.
Taylor, K. M. AchutaRao, T. P. Boyer, M. Ishii, P. M. Caldwell. Human-induced global ocean
warming on multidecadal timescales. Nature Climate Change, 2012;
DOI:10.1038/nclimate1553
