AGU Journal Highlights — June 7, 2007
Public release date: 8-Jun-2007

Contact: Peter Weiss
American Geophysical Union
Climate change could transform Venice Lagoon

A new model that couples biotic and abiotic
processes in intertidal zones predicts that salt
marshes in Italy’s Venice Lagoon may not survive
future climate changes. These projections of the
model developed by study authors Marani et al.
stem from current forecasts of sea-level rise.
Tidal ecosystems and landforms in lagoons and
estuaries manifest extremely high biodiversity
and rates of primary productivity. Because these
zones are associated with important
socio-economic activities sustaining a large
population worldwide, their evolution through
different climate-change scenarios could bear
important consequences for regional economies of
several areas. Applying their new model, the
authors find that vegetation type, disturbances
to microbial biofilm, sediment availability, and
marine transgressions or regressions drive
evolution of such systems. Besides making
specific predictions for the Venice Lagoon’s
future, the model also reproduces empirical
observations from the lagoon’s past, spanning the
last five centuries.

Title: Biologically-controlled multiple
equilibria of tidal landforms and the fate of the
Venice Lagoon

Authors: Marco Marani, Andrea D’Alpaos,
Stefano Lanzoni, Luca Carniello and Andrea
Rinaldo: Dipartimento di Ingegneria Idraulica,
Marittima, Ambientale e Geotecnica, and
International Center for Hydrology, Universití  di
Padova, Padova, Italy.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL030178, 2007 (in press, as of
7 June 2007)
Eastern Amazon copiously emits greenhouse gas methane

Recent satellite and laboratory
observations suggested that the Amazon rainforest
emits large amounts of methane, a potent
greenhouse gas. Yet, direct measurements of
methane in the air high above the Amazon basin
were lacking. Using a light aircraft above two
sites in the central and eastern Amazon, Miller
et al. collected air samples for four years along
vertical profiles spanning four kilometers.
Comparing those profiles with data collected at
islands in the Atlantic Ocean, which represent
background air entering the Amazon basin, the
authors find large enhancements of methane over
the eastern Amazon, averaging about 34 parts per
billion each year. For comparison, the entire
north-south, pole-to-pole difference in methane
concentrations is about 150 parts per billion.
Previous estimates of methane emissions from
sources such as wetlands, fires, and aerobic
processes in plants are not large enough to
explain the observed concentrations above the
Amazon. However, the authors suspect that
wetlands may release the bulk of this added

Title: Airborne measurements indicate large
methane emissions from the eastern Amazon basin

Authors: John B. Miller, Andrew M.
Crotwell, Edward. J. Dlugokencky, Peter Bakwin
and Pieter P. Tans: National Oceanic and
Atmospheric Administration Earth System Research
Laboratory, Boulder, Colorado, U.S.A.;
Luciana V. Gatti and Monica T.S. d’Amelio:
Divisao de Quimica Ambiental, Laboratorio de
Quimica Atmosferica, Insituto de Pesquisas
Energéticas e Nucleares, Sí£o Paulo, Brazil;
Paulo Artaxo: Instituto de Fí­sica,
Universidade de Sí£o Paulo, Sí£o Paulo, Brazil.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2006GL029213, 2007
Winds flood an Arctic shelf with warm waters

Many scientists expect that Arctic regions
will experience amplified warming as the global
climate changes, a prediction supported by models
and recent observations. Cottier et al. studied
interactions between warm Atlantic water carried
by the West Spitsbergen Current (situated between
Greenland and Norway’s Svalbard Islands) and cold
waters of the Arctic Front. Their examination
reveals that, during the Arctic winter of
2005-2006, periods of sustained winds over
Svalbard’s West Spitsbergen Shelf caused
extensive flooding of the typically cold coastal
waters with warm Atlantic water from the West
Spitsbergen Current. The winter temperature of
water on the West Spitsbergen Shelf reverted to
readings typical of Fall, interrupting the
region’s normal cycle of sea-ice formation. Winds
may also play an important role in distributing
heat to other Arctic areas, the authors say.

Title: Wintertime warming of an Arctic
shelf in response to large-scale atmospheric

Authors: F. R. Cottier and M. E. Inall:
Scottish Association for Marine Science, Oban,
Scotland, United Kingdom;

F. Nilsen: University Centre in Svalbard,
Longyearbyen, Norway; and Geophysical Institute,
University of Bergen, Bergen, Norway;
S. Gerland and V. Tverberg: Norwegian Polar Institute, Tromsí¸, Norway;
H. Svendsen: Geophysical Institute, University of Bergen, Bergen, Norway.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029948, 2007
Human influence could rev up oceanic planetary waves by 35 percent

By the end of the 21st century,
human-induced ocean warming is likely to
accelerate oceanic planetary wave speeds by 35
percent relative to pre-industrial times, a new
study suggests. Oceanic planetary waves are
long-wavelength, westward-traveling waves linked
to Earth’s rotation and shape. Critical to
transmitting energy and nutrients throughout the
oceans, they can intensify western boundary
currents such as the Gulf Stream and the Kuroshio
Current. Theory predicts that the waves’
propagation speeds increase as density
differences in the ocean’s water column become
more sharply delineated. Fyfe and Saenko note
that human-induced ocean warming over the last 40
years has likely increased density contrast
between surface and deep-ocean zones. Through
analysis of climate models, they forsee the
potentially large speed-up of oceanic planetary
waves by century’s end as a result. Morever, they
find that such warming may have already produced
a detectable speed-up of low-latitude North
Pacific oceanic planetary waves. These wave
speed-ups, based on emissions scenarios from the
Intergovernmental Panel on Climate Change, will
have important effects on year-to-year climate
variability, the authors say.

Title: Anthropogenic speed-up of oceanic planetary waves

Authors: John C. Fyfe and Oleg A. Saenko:
Canadian Centre for Climate Modelling and
Analysis, Environment Canada, Victoria, British
Columbia, Canada.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029859, 2007
Evaluating river discharge from space

A proposed joint U.S.-European satellite
mission could provide global estimates of
surface-water elevation within river channels.
Although such a satellite would not directly
observe river discharge, water-level observations
could be assimilated into a
hydrology/hydrodynamics model to provide
spatially and temporally continuous fields of
river discharge. Andreadis et al. evaluate such a
system using synthetically generated satellite
observations over a reach of the Ohio River.
Using assumed satellite overpass frequencies of
8, 16, and 32 days, the study demonstrates
significant error reductions in estimates of
water-surface elevation and discharge relative to
simulations without assimilation-especially
during the period between satellite overpasses.
While humans use 54 percent of accessible global
stream runoff for agriculture, industry, and
municipal consumption, estimates of river
discharge globally remain highly uncertain due to
limitations of direct observations, especially in
developing countries and remote regions. The
proposed WatER (Water Elevation Recovery)
satellite would include an altimeter using a pair
of synthetic aperture radar antennae.

Title: Prospects for river discharge and
depth estimation through assimilation of
swath-altimetry into a raster-based hydrodynamics

Authors: Konstantinos M. Andreadis and
Dennis P. Lettenmaier: Department of Civil and
Environmental Engineering, University of
Washington, Seattle, Washington, U.S.A.;
Elizabeth A. Clark: Department of
Geological and Environmental Sciences, Stanford
University, Stanford, California, U.S.A.;
Douglas E. Alsdorf: School of Earth
Sciences, Ohio State University, Columbus, Ohio,

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029721, 2007
Black carbon pollutants scrutinized above Japan

To better understand how an important
atmospheric pollutant disperses and transforms,
Moteki et al. used an airborne, single-particle,
soot photometer to study urban plumes in the
lower troposphere around Nagoya, Japan. The
researchers observed particles of black carbon-an
aerosol that is emitted from fossil-fuel
combustion and biomass burning and known to
efficiently absorb visible sunlight and heat the
atmosphere. Many climate models include the
effect of these particles on the atmosphere, but
approach their dispersion in an idealized way.
From the new observations of freshly emitted
black-carbon particles, the authors conclude that
those particles become well-mixed within the
atmosphere 12 hours after emission. The
observations also indicated changes in particle
size. In the Nagoya plumes, the authors found
that black carbon particles with a core diameter
of 180 nanometers increased in size at a rate of
2.3 percent per hour. Larger particles increased
at a slower rate of one percent per hour.
Particles grew as sulfates and organic carbons
coated them. Light absorption by black-carbon
particles can almost double if the particles are
coated by other pollutants.

Title: Evolution of mixing state of black
carbon particles: Aircraft measurements over the
western Pacific in March 2004

Authors: N. Moteki, Y. Kondo, Y. Miyazaki,
N. Takegawa, and T. Miyakawa: Research Center of
Advances Science and Technology, University of
Tokyo, Tokyo, Japan;
Y. Komazaki: Japan Agency for Marine-Earth
Science and Technology, Yokohama, Japan;
G. Kurata: Department of Ecological
Engineering, Toyohashi University of Technology,
Toyohashi, Japan; now at Department of Urban and
Environmental Engineering, Kyoto University,
Kyoto, Japan;
T. Shirai: Earth Observation Research and
Application Center, Japan Aerospace Exploration
Agency; also at National Institute for
Environmental Studies, Tsukuba, Japan;
D. R. Blake: Department of Chemistry,
University of California, Irvine, California,
M. Koike: Department of Earth and Planetary
Science, University of Tokyo, Tokyo, Japan.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2006GL028943, 2007
Southern Ocean simulation yields key details

In a new study, scientists have
characterized parts of the Southern Ocean with
unprecedented detail and tracked trends and
disturbances of an important surface layer of
water near Antarctica. Although general ocean
circulation models increasingly produce realistic
representations of the deep, well-mixed surface
layer known as the Subantarctic Mode Water
(SAMW), detailed distributions have been
unreliable. Moreover, verification of such
distributions is not always available for remote
regions of the ocean during winter months. Aoki
et al. investigate the layer during winter, when
the SAMW forms from convection north of the
Subantarctic Front of the Antarctic Circumpolar.
The authors use an ocean general circulation
model substantiated by floating-instrument
measurements. They find that sudden transitions
from a deep to shallow mixed layer were effected
by near-surface flows associated with
gravitational forces and the Earth’s rotation.
Underwater topography steers these flows,
implying a link between underwater features and
the formation of thick SAMW.

Title: Formation regions of Subantarctic
Mode Water detected by OFES and Argo profiling

Authors: Shigeru Aoki and Humio Mitsudera:
Institute of Low Temperature Science, Hokkaido
University, Sapporo, Japan;
Mieko Hariyama: Graduate School of
Environmental Science, Hokkaido University,
Sapporo, Japan; Now at NNT Communications, Tokyo,
Hideharu Sasaki: Earth Simulator Center,
Japan Agency for Marine-Earth Science and
Technology, Yokohama, Japan;
Yoshikazu Sasai: Frontier Research Center
for Global Change, Japan Agency for Marine-Earth
Science and Technology, Yokohama, Japan.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029828, 2007
Energy goes down the tubes in North Atlantic

Ocean eddies can behave like inverted
chimneys that drain a form of sea-borne energy,
called near-inertial energy, into the watery
depths, a new study finds. Near-inertial waves,
such as Rossby waves, travel through the bulk of
the ocean and respond to rotational forces. While
theoretical studies characterize near-inertial
waves in an idealized way, Zhai et al. sought to
model them in a more realistic setting by using a
model of the North Atlantic that is driven by
varying wind forcing and that has
1/12-degree-of-longitude resolution. The authors
find that near-inertial energy is strongly
influenced by eddies. In particular, the interior
of the subtropical gyre, an eddy system driven by
the trade winds and westerly winds, shows very
low levels of near-inertial energy, contrary to

Title: Spreading of near-inertial energy in
a 1/12° model of the North Atlantic Ocean

Authors: Xiaoming Zhai and Richard J.
Greatbatch: Department of Oceanography, Dalhousie
University, Halifax, Nova Scotia, Canada;
Carsten Eden: Leibniz Institute of Marine
Sciences at the Christian-Albrechts University of
Kiel (IFM-GEOMAR), Kiel, Germany.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029895, 2007
Predicting sea surface temperatures with artificial neural networks

Scientists have now used sophisticated
computational networks to generate sea surface
temperature maps for a time period before
widespread satellite data was available.
Artificial neural networks-which are numerically
complex modeling techniques inspired by
biological nervous systems-operate by invoking
‘training’ algorithms whereby a set of input
variables produces a specific set of output
targets within a prescribed error level. Such
networks have been previously used to identify
ocean features in satellite imagery and to
forecast sea surface temperatures. Using similar
techniques, Garcia-Gorriz and Garcia-Sanchez have
recently trained neural networks to compute sea
surface temperatures in the western Mediterranean
Sea using meteorological information as inputs
and concurrent satellite-derived, sea surface
temperatures as targets. Their networks predicted
both the seasonal and interannual variability of
sea surface temperatures, reproducing the impact
of the 2003 European summer heat wave. Turning
next to the western Mediterranean of 1960 to
1981, when satellite data were scarce, the
authors applied the network technique to predict
sea surface temperature maps for that period.
Those are providing insights into sea circulation
patterns through time and are contributing to the
validation of circulation model results.

Title: Prediction of sea surface
temperatures in the western Mediterranean Sea by
neural networks using satellite observations

Authors: Elisa Garcia-Gorriz: European
Comission-DG Joint Research Centre, Institute for
Environment and Sustainability, Global
Environment Monitoring Unit, Ispra, Italy;
Joan Garcia-Sanchez: Universitat de
Barcelona, Departament d’Astronomia i
Meteorologia, Barcelona, Spain; now at European
Commission-DG Joint Research Centre, Institute
for the Protection and Security of the Citizen,
Ispra, Italy.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029888, 2007
Extreme drought parched ancient Upper Colorado River Basin

While reconstructing the precipitation
history of the Upper Colorado River Basin back to
the 8th century A.D., scientists have uncovered
evidence of a six-decades-long drought in the
mid-1100s. To estimate yearly streamflow in a
region’s past, researchers can evaluate tree ring
widths, where narrow rings mean dryer years.
Previously, tree-ring analyses in the basin had
determined annual streamflow patterns only back
to the A.D. 1400s. Meko et al. have now used a
newly developed network of tree-ring sites
composed of living trees and dead remnant wood to
extend the basin’s precipitation history back to
A.D. 762. That expanded record includes a period
of widespread drought known as the Medieval
Climate Anomaly. The anomaly persisted sometime
between A.D. 900 and A.D. 1300. It was
accompanied by a 20 percent precipitation decline
in areas of the western United States. The timing
of the newfound extreme drought of the 12th
century is consistent with Medieval Climate
Anomaly conditions inferred from tree ring data
elsewhere in the western United States.
Understanding changes in streamflow in the Upper
Colorado River Basin on long timescales is
critical to water resources management in the
United States’ desert southwest.

Title: Medieval drought in the Upper Colorado River Basin

Authors: David M. Meko, Christopher A.
Baisan, Troy Knight, Malcolm K. Hughes, and
Matthew W. Salzer: Laboratory of Tree-Ring
Research, University of Arizona, Tucson, Arizona,
Connie A. Woodhouse: Department of
Geography and Regional Development, University of
Arizona, Tucson, Arizona, U.S.A.;
Jeffrey J. Lukas: Institute of Arctic and
Alpine Research, University of Colorado, Boulder,
Colorado, U.S.A.

Source: Geophysical Research Letters (GRL)
paper 10.1029/2007GL029988, 2007


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Vol. 316. no. 5830, pp. 1412 – 1415
June 8, 2007:

Pushing the Scary Side of Global Warming

By Richard A. Kerr

Greenhouse warming might be more disastrous than
the recent international assessment managed to
convey, scientists are realizing. But how can
they get the word out without seeming alarmist?

Climate modeler James Hansen knows all about
sounding the alarm. In the summer of 1988,
drought wracked the country, fire was consuming
Yellowstone National Park, and the nation’s
capital sweltered. Even the Senate hearing room
where Hansen was testifying was warm and
stuffy–the Democrats had opened the windows the
night before. Then Hansen, dubbed NASA’s top
climate scientist by the media, shouted “Fire!”
in the crowded theater: “With a high degree of
confidence,” he declared, greenhouse warming had
arrived. Although many of his colleagues agreed,
none chimed in with support; they could not share
his high degree of confidence. Still, Hansen’s
lone authoritative voice was enough to send the
media into a years-long brouhaha over global

That uproar quieted within a few years, but
Hansen, still the director of NASA’s Goddard
Institute for Space Studies (GISS) in New York
City, finds himself at the head of an informal
movement to again rouse the public and
policymakers. This time he worries that sea level
could rise several disastrous meters by the end
of the century, as the warming he heralded sends
the great ice sheets rumbling toward the sea. If
nothing is done to rein in greenhouse gas
emissions, he says, “I just can’t imagine that
you could keep sea-level rise under a meter.”
Then the sea would flood many kilometers inland
along the world’s low-lying coasts, from Florida
to Bangladesh.

That was Hansen’s warning to Congress in late
April, but it’s not the message that came out of
the U.N.’s Intergovernmental Panel on Climate
Change (IPCC) in early February. Many news
reports gave the impression that the prestigious
international assessment actually downgraded the
risk of imminent sea-level rise to a small
fraction of a meter.

So Hansen seems to be out on a limb, again. This
time, however, he’s got company. No longer
reticent, other scientists are going public about
how bad things might get by the end of the
century. “The IPCC has been overly cautious in
not wanting to give any large number to [future]
sea-level rise,” says climate researcher Stefan
Rahmstorf of the Potsdam Institute for Climate
Impact Research in Germany.

Scientists are still trying to strike a balance
between their habitual caution and growing
concern over uncertain but disastrous greenhouse
outcomes. “Most scientists don’t want to, but I
think we need a way to explore” the extreme end
of the range of possibilities, says glaciologist
Robert Thomas of NASA contractor EG&G at Wallops
Flight Facility in Virginia. Thomas says
scientists need “a better way” than IPCC’s
consensus approach, “so we can communicate with
the public without becoming scaremongers.”

Naturally cautious

Seldom have mainstream climate scientists spoken
out about the scary possibilities of global
warming. “Most people [in the field] realize this
really is an extremely serious problem we’re
facing” in sea-level rise, says Thomas. But no
one understands just why the great ice sheets of
Greenland and West Antarctica have accelerated
their slide to the sea in recent years (Science,
24 March 2006, p. 1698). Will the acceleration
continue? Speed up? Slow down? Stop? In the face
of such uncertainty, most climate scientists have
traditionally let IPCC speak for them. When
they’ve gone public, it was usually to counter
greenhouse contrarians arguing for an
inconsequential warming with trivial impacts.

In the latest report, its fourth since 1990, the
IPCC spoke for scientists in a calm, predictably
conservative tone (Science, 9 February, p. 754).
It is, after all, an exhaustive, many-tiered
assessment of the state of climate science based
exclusively on the published literature. In
IPCC’s Working Group I report on the physical
science of climate, 600 authors contributed to an
11-chapter report that drew 30,000 comments from
reviewers. The report was in turn boiled down to
a 21-page “Summary for Policymakers” (SPM). Its
central projection of sea-level rise by the
century’s end–0.34 meter–came within 10% of the
2001 number. And by getting a better handle on
some uncertainties, it even brought down the
upper limit of its projected range, from 0.89 to
0.59 meter.

The SPM did add that “larger values [of sea-level
rise] cannot be excluded.” Whatever has
accelerated ice-sheet flow to the sea, the report
said, might really take off with further
warming–or not. “Understanding of these effects
is too limited” to put a number on what might
happen at the high end of sea-level rise, it
concluded. Lacking such a number, the media
tended to go with the comforting 0.34-meter
projection or ignore sea level altogether.

Some scientists believe IPCC did as well as it
could in assessing the sea-level threat. “Since
2001, nature has revealed some pretty remarkable
behavior in the ice sheets,” says glaciologist
Waleed Abdalati of NASA’s Goddard Space Flight
Center (GSFC) in Greenbelt, Maryland, who manages
NASA’s ice-observation program. That behavior has
included the catastrophic collapse of the Larsen
B ice shelf–which triggered glacier
accelerations–and the galloping glaciers
draining the Greenland ice sheet, which have
doubled their pace. But “we just don’t have the
capacity to quantify” that sort of ice-sheet
behavior, he notes, so “the best you can do is
point to some red flags. The language of the SPM
does that, if you’re looking for it.”

Quantifying ice-sheet behavior does indeed have
its limitations, says climate and sea-level
modeler Jonathan Gregory of the University of
Reading, U.K., who coordinated the production of
the sea-level section in the IPCC projections
chapter. A predictive model cited by IPCC would
melt more Greenland ice as the air warms, he
says, because that is a well-understood and
quantifiable process. However, the model would
not include the effect of that glacial meltwater
lubricating the base of the Greenland ice sheet.
Although researchers have seen signs that such
lubrication speeds the ice sliding into the sea,
they aren’t yet able to model it. “If there are
no models to give us some numbers,” says Gregory,
“all you can do is make numbers up. It wouldn’t
be appropriate to make up numbers.”

Going beyond such physically based models–for
example, by extrapolating from past
trends–wouldn’t be such a good idea either,
Gregory says. Lessons taken from how sea level
rose as the 20th century warmed, he says, would
be useless in predicting sea-level rise in this
century if the underlying causes change. “If you
don’t know what is causing the relationship”
between warming and sea-level rise, he says, “is
that really a good basis for making projections?”

A bolder assessment

Scientists are well aware of the hazards of
straying far from the hard science of climate
change, but some are eager to change the IPCC
process and even move beyond it. They would begin
with wording. “IPCC gets an A+ for scientific
assessment,” says climate modeler Richard
Somerville of the Scripps Institution of
Oceanography in San Diego, California, “but a
gentleman’s C for communication.” The
communication problem is largely a matter of
structure, says geoscientist Michael Oppenheimer
of Princeton University. “All the facts are there
in the [main-report] chapter,” he says, “but the
SPM didn’t tie those facts together in a coherent
statement of risk that would allow a policymaker
to make an informed decision.”

Beyond the IPCC’s language, a number of climate
scientists think the report missed an opportunity
to broaden public appreciation of the risk of the
most dangerous climate change. “If you don’t
understand the physics, your uncertainty is
larger,” says Thomas. That greater uncertainty
extends the range of possible ice losses to
higher, more dangerous levels. But IPCC didn’t
capture that increased risk, says climate modeler
Michael MacCracken of the Climate Institute in
Washington, D.C.

A big part of IPCC’s problem, say MacCracken and
others, was its strict adherence to the use of
models. By IPCC standards, “if it’s not in a
model, it’s speculation,” says Rahmstorf. By
ignoring factors that can’t yet be modeled, he
says, IPCC came up with deceptively reassuring

More numbers

Although forewarned, some researchers are
generating numbers for public consumption by
going beyond physics-based models. In a paper
published in Science in January, too late for the
IPCC to consider it, Rahmstorf took “a
semiempirical approach to projecting future
sea-level rise.” He determined how much sea level
rose in the 20th century per year per degree and
projected that rate through the 21st century,
with its expected warming. That projection
produced a sea-level rise in 2100 of 0.5 to 1.4
meters above the 1990 level, well above the
IPCC’s projection of 0.18 to 0.59 meter.

Then, Rahmstorf and six co-authors, including
Hansen, published a paper in Science on the day
the IPCC report was released. They pointed out
that warming had been running toward the high
side of IPCC projections during the past few
decades, while sea levels rose at the upper limit
of projections. “These observational data
underscore the concerns about global climate
change,” the authors wrote. IPCC had clearly not
exaggerated sea-level rise, they said, and may
even have underestimated it. Reinforcing their
message, news stories published a few days before
the IPCC report’s release quoted Rahmstorf and
other scientists lamenting the expected
shortcomings on sea-level projections.

That was more media attention than suited some
climate researchers. “When we speak to the
public, we should not rely on the new result,”
argues Hans von Storch of the GKSS Institute for
Coastal Research in Geesthacht, Germany. “The
newest results are not necessarily the best ones.
The IPCC should represent a certain filter. That
every taxicab driver knows about [the latest
result] is a bit premature.”

Some scientists would have IPCC reach even
farther back to try to deal with “factors that
you don’t understand,” as MacCracken puts it. He
notes that paleoclimatologists and geologists
have extracted records of ancient sea level for
times when Earth was warmer or colder than today.
In the case of the penultimate warm interglacial
120,000 years ago, the globe was only about 1°C
warmer–a temperature we could reach by 2100–but
sea level was 4 to 6 meters higher. Even though
that warmth had millennia to shrink the great ice
sheets back then, MacCracken says, history still
suggests that the world’s ice is more vulnerable
than IPCC’s modeling implies.

Another way

Gregory calls projections drawing on such studies
“a scientific hunch.” Hansen prefers “insight,”
but whatever it is called, Hansen says, you won’t
find much of it in an IPCC report. IPCC “overall
does a good job,” he says, but “there are
limitations on that process. Everybody in
[sea-level] research is much more concerned than
6 or 7 years ago” when the previous report came
out, he says; yet the latest message from IPCC
was seemingly unchanged. “There is a role for
something in addition.”

As an example of an alternative to the IPCC
report, Hansen cites the U.S. National Academy of
Sciences’ climate change report of 1979. Chaired
by the late meteorologist Jule Charney, then at
the Massachusetts Institute of Technology, the
small committee delivered, among other things, a
best estimate and range for the sensitivity of
climate to greenhouse gases, a central figure in
climate science. IPCC never ranged far from those
numbers, and this year it confirmed them. “There
were huge uncertainties back then,” says Hansen,
“yet the Charney report came up with an estimate
by coming at it from different angles. That’s
what you need to do with sea level.”

Hansen practices a multipronged approach himself.
With colleagues at GISS, he draws on several
lines of evidence–climate modeling, recent
observations, paleoclimate records, and the basic
physics of the greenhouse–to “gain insight into
how the world works.” That approach helped him to
see greenhouse warming under way in 1988, he
says. Now it is revealing positive feedbacks in
the ice-climate system that can allow modest
warming to accelerate losses from the ice sheets.
The world is on a “slippery slope,” Hansen has
written, that could lead to meters of sea-level
rise in the next century or two unless people
take immediate actions to cut greenhouse gas

As in 1988, Hansen’s pursuit of such insights has
put him at odds with many in the climate
community. “Maybe he’s still within the error
bars,” says glaciologist Robert Bindschadler of
GSFC, but “I’m not prepared to put centuries on
[the timing] rather than millennia.” No matter.
Hansen has again taken to the bully pulpit as
NASA’s top climate scientist, publishing in
peer-reviewed journals, testifying to Congress,
writing op-ed articles, and appearing in
documentaries. Only last week in a GISS press
release announcing a new publication, Hansen
warned of disastrous effects–including
increasingly rapid sea-level rise–if greenhouse
gas emissions continue apace for even a couple
more decades. And a few days later, he took his
boss, NASA Administrator Michael Griffin, to task
for publicly questioning the need to tackle
global warming. Hansen’s take: “remarkably

Besides a streamlined IPCC process and individual
scientist activism, Oppenheimer sees another
approach: using expert elicitations to broaden
the assessment of uncertainty. This survey
technique grills selected experts in private on
the state of the science. Without the drive to
reach a consensus, the experts “give a different
view of the probability of various outcomes,”
says Princeton’s Oppenheimer. Ultimately, these
and other methods will be needed, he says. “You
really need a broad, thorough, and comprehensive
assessment of uncertainty and risk,” he says.
“There is no one answer to the assessing of

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