Warming Oceans

Les Schaffer schaffer at optonline.net
Fri Jun 22 10:33:02 MDT 2001

[ from current issue of Science]

Warming Oceans Appear Linked to Increasing Atmospheric Greenhouse Gases 
Barbara Goss Levi 

An expanded data set for ocean temperatures has enabled climate
modelers to compare predicted heating of the ocean to what has been
observed over the past 40 years.

Global surface temperatures rose by about 0.6°C in the last century,
according to the most recent report from the Intergovernmental Panel
on Climate Change.[1] That estimate was based on observations of
surface air temperatures and sea surface temperatures--the most
commonly used measures of climate change. The IPCC also concluded that
the observed warming is caused at least in part by increased
greenhouse gases such as carbon dioxide, which absorb the infrared
radiation that otherwise would escape from Earth. If the trapped
infrared radiation is heating the atmosphere, we might expect it to be
warming the world's oceans as well.[2] Covering nearly three-fourths
of Earth's surface and having a high specific heat, the oceans have
the greatest capacity to store heat of any component of the climate
system. Unfortunately, detailed studies of ocean heat storage have
been hampered by the paucity of easily accessible data on subsurface

Sydney Levitus and his colleagues have been working for over ten years
to correct this problem. Levitus, who works at the National
Oceanographic and Atmospheric Administration's National Oceanographic
Data Center in Silver Spring, Maryland, now heads an international
project under the auspices of the United Nations-sponsored
Intergovernmental Oceanographic Commission. Scouring the world,
Levitus and his team have added 2.3 million historical temperature
profiles (temperatures as a function of ocean depth) to the 3 million
profiles that were previously available in electronic form. Many of
the newly added data had existed only in manuscript form. The team put
the data into a comprehensive, integrated, publicly available
scientific database.[3]

In a report last year, Levitus and three NODC colleagues used[4] the
database to estimate that from 1948 to 1998, the volume-averaged (down
to 3000 meters) temperature of the oceans rose by 0.06°C,
corresponding to a heat input of 18.2 x 10^22 J. Some regions, such as
the deep subarctic region of the North Atlantic, had cooled, but each
ocean as a whole showed a net warming. (The heat content is calculated
on a three-dimensional grid based on the local temperature, seawater
density, specific heat, and the volume represented.)

Recently, three independent groups have compared the changes in ocean
heat content seen in the past 40-45 years with the predictions of
global climate models that coupled the oceans and atmosphere.[5-7] In
each of the three studies, the modelers found that they could get
reasonably good agreement with the observations only when they
included the effects of manmade greenhouse gases. Until these recent
studies, climate modelers were allowing for heat flux into the oceans
but had no way to assess how realistic those heat fluxes were. The
newly available ocean data will now act as an important additional
constraint on the models.


Oceans dominate 

Levitus and his group compared the heat stored in the oceans to the
heat that has gone into other parts of the climate system over the
last 40-odd years. Specifically, they calculated the heat stored in
the warmer atmosphere (open circles in figure 1), the heat absorbed in
the melting of glaciers, and the heat taken to reduce the extent and
thickness of sea ice. All of these terms were more than an order of
magnitude smaller than the heat flux into the oceans in the same
period. Barnett notes that neither changes in solar irradiance nor
geothermal heating can come close to supplying the heat required to
make the changes seen over the last 40 years. "That pretty much leaves
anthropogenic sources as our only explanation," he says.

Because of their large thermal inertia, the oceans are clearly the
memory of the world's climate system. That result was not unexpected
but needed to be confirmed with data. As estimated by a recent paper,
even if manmade greenhouse gas emissions were to cease immediately,
heat coming out of the oceans would continue to raise atmospheric
temperatures by an additional 1°C before the system equilibrated.[9]

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