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The Ocean Heat Trap
By NASA
By comparing computer simulations of Earth’s climate with
millions of measurements of ocean heat content collected by satellites
and in-the-water sensors, a team of climatologists and oceanographers
has provided what leading NASA climate scientist James Hansen calls the
“smoking gun” of human- caused global climate change: a prediction of
Earth’s energy imbalance that closely matches real-world observations.
Where Greenhouse Heat Hides Like any planet, the Earth absorbs
some radiation and emits some radiation back into space. If the amount
of energy Earth emits matches the amount it absorbs, the planet’s
energy budget is in balance, and its
temperature remains steady. If the incoming and outgoing energy don’t
match, the planet is either warming or cooling over time, even if the
change isn’t immediately obvious. If greenhouse gases are forcing Earth
to absorb more energy than it emits, why wouldn’t global surface
temperatures increase right away?
It sounds reasonable that if excess greenhouse gases in the
atmosphere are causing Earth to absorb more energy than it reflects
back into space, that excess energy should heat up the atmosphere
first. Something that many people find odd—but climate scientists have
long known—is that most excess energy would really hide elsewhere.
“It turns out that the atmosphere, the air, really can’t hold
that much heat,” explains Josh Willis, an oceanographer with the
California Institute of Technology working at NASA’s Jet Propulsion
Laboratory. Heat capacity is the amount of energy that must be put into
something to change its temperature, and air has a very low heat
capacity. “If you put energy into the ocean, on the other hand, its
temperature changes only very slightly.”
One reason the ocean heats more slowly than the atmosphere is
the difference in their total mass. “The atmosphere only weighs a tiny
fraction of what the ocean weighs,” Willis explains. “But there’s also
a sort of intrinsic property of the air that makes it not quite as good
at holding heat as the ocean. That property is called the specific
heat. You probably have a feel for this if you’ve ever tried to boil a
pot of water. You have to burn a lot of gas or wood to heat up the
water. But if you had a similar quantity of air, it would take a lot
less energy to heat it up to the same temperature. The water’s heavier,
and it has a higher specific heat, and both of those things give
it a
much bigger heat capacity.”
What this means for planet Earth is that excess energy might not
make itself immediately obvious by strongly warming the atmosphere.
Instead, that energy might hide in the ocean, in the form of warmer
ocean temperatures.
“The ocean’s a big place, and it’s not doing the same thing everywhere,” says Willis. “In some places, it’s
warming. In some places, it’s cooling, and the quantity we need is the
average over the whole thing.” Getting that average has entailed
overcoming several challenges, the biggest of which is probably
learning how to take the ocean’s temperature everywhere.
“There’s a long history of shipping back and forth between the U.S. and
Europe,” Willis explains. As a result, temperatures in the North
Atlantic have been studied carefully over the last century. Ships often
dropped thermistors—devices that exhibit rapid changes in electrical
resistance based on small temperature changes—along their routes.
Although this approach yielded excellent temperature records for some
areas, it offered far from universal coverage.
“The Pacific is a huge ocean, and even if you count all the
shipping lanes—the places where people go all the time—there are still
big gaps.” Sparsely sampled as it historically has been, however, the
Pacific ranks above what Willis calls the “worst place for sampling”:
the Southern Ocean.
Global measurements of changes in ocean temperature could only
come from satellite. In 1992, NASA and the French Centre National
d’Etudes Spatiales launched the TOPEX/Poseidon oceanography satellite;
in 2001, they launched the successor, Jason 1. These satellites have
provided researchers with global records of sea surface height.
“As the water column warms up, the water expands, and that causes a
change in the height of the sea surface. A satellite can measure
sea-surface height very accurately, and it can cover the globe,” Willis
says.
The Jason-1 satellite doesn’t work alone. In the ancient Greek
myth, the explorer Jason was accompanied by his trusty shipmates, the
Argonauts. Reminiscent of that legend, the international Argo float
program complements the satellite’s observations. By the end of 2006,
the Argo program is expected to operate 3,000 floats, mechanical
devices that measure temperature and salinity, drifting at specified
depths and spaced roughly every 3 degrees (at the Equator, this equals
360 kilometers).
“We haven’t sampled the ocean with direct temperature
measurements nearly as much as we probably should, but the Argo floats
are starting to provide really good coverage,” Willis remarks.
A growing collection of satellite observations and direct
temperature measurements of the ocean enabled Willis to try to assess
the ocean’s averagetemperature. “Many, many researchers worked over
many, many years” to collect all the data, says Willis. In 2004, he and
two colleagues assembled the pieces for a paper published in the
Journal of Geophysical Research. Along with satellite data, he
incorporated roughly 1 million temperature profiles. “Each profile
really includes measurements of temperature with depth between the sea
surface and several hundred meters, so there are actually lots of
measurements within each of those million profiles,” he explains.
After combining all the data, Willis found that between mid-1993
and mid-2003, the heat content of the upper 750 meters of Earth’s
global ocean increased at an average rate of 0.86 watts (plus or minus
0.12 watts) per square meter. Just 0.86 watts per square meter may not
sound like much until you consider that we are talking about an area of
about 337 trillion square meters (the 93 percent of the world ocean
that Willis studied).
He also points out that ocean temperatures actually fall into
two categories. “It’s important to make a distinction between warming
at different depths. People are most familiar with sea surface
temperature because it’s where we live. We live on the Earth’s surface.
But a lot of this action and this heat-content signal are really
beneath the surface of the ocean, and you have to go to hundreds of
meters in depth to measure it. And what’s going on right at the surface
and what’s going on at depth can sometimes be different.” 
Sea-surface temperature is a bigger driver of weather, but ocean
temperature at depth tells researchers more about the planet’s energy
imbalance. To a climate modeler, a real-world estimate of the ocean’s
energy imbalance provides a rare opportunity to test model predictions
of how increasing amounts of greenhouse gases are affecting Earth’s
climate. Soon after he published his paper, Willis learned he had
caught someone’s attention.
Watching a Changing Planet
James Hansen heads NASA’s Goddard Institute for Space Studies
(GISS) at Columbia University in New York. In the mid-1970s, he was
studying clouds on Venus when he fielded a request for help in studying
the greenhouse effects of trace gases—gases that exist in miniscule
amounts but that may still have significant climatic effects. About
that time, Hansen’s research interests shifted away from Venus and
closer to home. “A planet that is changing on timescales that
people are going to notice is a more interesting planet, and it obviously has more social relevance,” he says.
After shifting his research focus in the mid-1970s, Hansen spent
more than a decade studying the effects of greenhouse gases on Earth’s
climate. His research covered the effects of volcanic eruptions and
aerosols (tiny liquid or solid particles suspended in the air), solar
variability, increasing carbon dioxide, and increasing trace gases. He
also studied atmospheric circulation, the sinking of surface water down
into the deep ocean in the North Atlantic, and global trends in both
sea level and surface air temperatures. He developed three-dimensional
models of global climate. In 1988 Hansen testified before Congress,
describing how different levels of greenhouse gases might affect future
temperatures. Over the next 17 years, observed temperatures closely
agreed with Hansen’s 1988 predictions. But are recent temperature
changes really from human-triggered global warming, or could they just
be coincidence? The way to find out, Hansen explains, is to look at the
planet’s overall energy budget. If increasing greenhouse gases are
exerting pressure on the planet’s climate, the gases should create an
energy imbalance in which the Earth absorbs more energy than it
radiates back to space. Because most of the planet is ocean-covered and
because those oceans have a high heat capacity, excess energy should
show up in the ocean. So when Hansen saw Willis’s paper on ocean
warming, he sent an e-mail suggesting that they collaborate on a new
study.
“Josh Willis’ paper spurred my colleagues and me to compare our
climate model results with observations,” Hansen says. Hansen, Willis,
and several colleagues used the global climate model of the NASA
Goddard Institute for Space Studies
(GISS), which predicts the evolution of climate based on various
forcings—conditions or events that can cause climate change, such as
water vapor and greenhouse gases, changes in solar radiation, or
volcanic eruptions.
Hansen and his collaborators ran five climate simulations
covering the years 1880 to 2003 to estimate change in Earth’s energy
budget. Taking the average of the five model runs, the team found that
over the last decade, heat content in the top 750 meters of the ocean
increased by 6.0 plus or minus 0.6 watt-years per square meter. (A watt
year is the amount of energy delivered by one watt of power over one
year.) What kind of energy imbalance would it take to generate that
much heat? The models predicted that as of 2003, the Earth would have
to be absorbing about 0.85 watts per square meter more energy than it
was radiating back into space—an amount that closely matched the
measurements of ocean warming that Willis had compiled in his previous
work. The Earth, they conclude, has an energy imbalance.
“I describe this imbalance as the smoking gun or the innate
greenhouse effect,” Hansen says. “It’s the most fundamental result that
you expect from the added greenhouse gases. The (greenhouse) mechanism
works by reducing heat radiation to space and causing this imbalance.
So if we can quantify that imbalance (through our predictions), and
verify that it not only is there, but it’s of the magnitude that we
expected, then that’s a very big, fundamental confirmation of the whole
global warming problem.”
If Earth’s oceans are soaking up the excess
heat energy caused by greenhouse gases, then exactly what is the
problem? The problem—and perhaps part of the solution as well—is
thermal inertia. Inertia is the tendency of an object to resist a
change in its current state. The huge heat capacity of the oceans
creates thermal inertia in the climate system. Just as a speeding car
can take some time to stop after the driver hits the brakes, the
Earth’s climate system may take awhile to reflect the change in its
energy balance. In other words, there’s a time lag between when the
Earth begins to experience an energy imbalance and when the climate
fully responds to it.Willis explains, “If we stopped burning all fossil
fuels today, the Earth would still warm up a little bit. It would
continue absorbing excess energy until it reached [a new] equilibrium
point. But if we keep doing what we’re doing, the equilibrium point is
going to move further away.” Hansen remarks,
“We’re putting in the pipeline additional change that will occur
over the next several decades, and which will be difficult if not
impossible to avoid.” By his estimates, the current energy imbalance is
likely to produce an additional 0.5 to 0.6 degrees Celsius of warming
in global average surface Yet the same inertia that could worsen
future climate change also gives us time to mitigate the unwanted
effects. Many people associate mitigation of global warming with
reducing carbondioxide emissions, but Hansen suggests that this isn’t
the only—or even the best—approach.
“We’ve reached a point where large climate changes are
likely—almost inevitable—this century, and if you want to minimize
those, you have to slow the emissions of carbon dioxide. But it will
take at least several decades to stabilize carbon dioxide, so, to avoid
large climate change, we must reduce other positive [warming] climate
forcings, such as soot and methane,” he explains. Dealing with them
will also benefit human health. Soot, for instance, can be reduced
through more efficient use of diesel fuel and coal, and reducing it
would improve air quality. Methane emissions can be reduced by
capturing methane at coal mines, landfills, and waste management
facilities. Because bacterial activity in flooded soil releases
methane, changing fertilizers and irrigation techniques to reduce
standing water could reduce methane and lower the risk of
mosquito-borne diseases at the same time.
Despite Earth’s inertia, some results of
climate change have already become apparent. Hansen and colleagues
identified 2005 as the warmest year on record, although other global
monitoring teams’ assessments differ slightly, placing 1998 just ahead
of 2005. “Whether 2005 or 1998 is the warmest is not scientificallythat
important,” says Hansen. “Nineteen ninety-eight leaped up far above any
previous temperature because it coincided with the El Niño of the
century. Now we’ve got back to approximately the same temperature
without the help of an El Niño, so it just confirms the strong
underlying global warming trend.”
Besides changes to ecosystems resulting in phenomena like coral
bleaching, warmer ocean temperatures can affect those of us on land by
bringing different weather patterns. No one can conclusively pin the
unusually destructive hurricane
season of 2005 squarely on global warming, but storms like Katrina are
more likely to occur in an environment of warmer sea-surface
temperatures. “Global warming probably does play a role,” says Hansen.
“That doesn’t mean there aren’t natural fluctuations, and it doesn’t
mean storms like Katrina and Rita will happen every year. It depends on
the weather, not just the average sea-surface temperature.
Nevertheless, I think that more severe storms are one
impact of a warmer ocean.”
Another consequence of warmer ocean temperatures that concerns
Hansen is a potential change to sea level from melting ice shelves. Ice
shelves are thick slabs of ice attached to coastlines. Fed by glaciers,
these shelves extend out over the ocean’s surface. “Something that
needs to be understood better is the effect of this ocean heat on ice
shelves. There are some ice shelves, both in West Antarctica and
Greenland, that have thinned substantially over the past several
years,” he says.
Ice shelves have shown other changes besides thinning. Between
1995 and 2002, parts of the Larsen Ice Shelf on the Antarctic Peninsula
disintegrated. In the wake of the 2002 disintegration event, glaciers
feeding that part of the Larsen Ice Shelf accelerated. Ted Scambos,
lead scientist at the National Snow and Ice Data Center at the
University of Colorado, Boulder, agrees with Hansen’s assessment.
“Warmer oceans are definitely having an effect on some of the world’s
largest glaciers. While in some areas, like the Larsen, it’s warmer air
temperatures that have done Larsen, it’s warmer air temperatures that
have done the most damage, for the large, floating ice tongues— a kind
of shelf—off Greenland and Antarctica, the warmth in the ocean is
leading the way.” Neither Willis’ paper on ocean temperatures, nor
Hansen’s paper on the energy budget is the last word on this issue.
Willis stresses that taking the ocean’s temperature is an ongoing
process. Meanwhile, climate models continue to be refined. What the
researchers do know, however, is that the Earth now
absorbs more energy than it emits back to space. While that energy
hides in the ocean, its effects aren’t immediately felt in the average
global surface temperature. The future may bring additional warming,
however, as the Earth’s entire climate system responds to the current
energy imbalance.
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