E. J. Steig et al
Synchronous Climate Changes in Antarctica and
the North Atlantic
Science 1998 Oct 2; 282: 92 - 95
Central Greenland ice cores provide evidence of abrupt changes in climate over the
past 100,000 years. Many of these changes have also been identified in sedimentary
and geochemical signatures in deep-sea sediment cores from the North Atlantic,
confirming the link between millennial-scale climate variability and ocean
thermohaline circulation. It is shown here that two of the most prominent North
Atlantic events--the rapid warming that marks the end of the last glacial period and the Bølling/Allerød-Younger Dryas
oscillation--are also recorded in an ice core from Taylor Dome, in the western Ross Sea sector of Antarctica. This result contrasts
with evidence from ice cores in other regions of Antarctica, which show an asynchronous response between the Northern and
Southern Hemispheres.
Thomas F. Stocker
The Seesaw Effect
Science 1998 Oct 2; 282: 61 - 62
What are the origins of past, present, and future climate change? In his Perspective,
Stocker discusses ways in which the climate system can act like a seesaw, as the
high northern latitudes push and pull against the Southern Hemisphere to create
abrupt shifts. Stocker briefly discusses a report in the same issue by Steig et al.,
showing that Northern and Southern Hemisphere climate shifts act in synchrony. A related Perspective by Cane looks at the role
of the tropics in paleoclimate. Recent observations and model calculations indicate that effects of climate change in the tropical
latitudes merit a closer look.
Mark A. Cane
A Role for the Tropical Pacific
Science 1998 Oct 2; 282: 59 - 61
What are the origins of past, present, and future climate change? In his Perspective,
Stocker discusses ways in which the climate system can act like a seesaw, as the
high northern latitudes push and pull against the Southern Hemisphere to create
abrupt shifts. Stocker briefly discusses a report in the same issue by Steig et al.,
showing that Northern and Southern Hemisphere climate shifts act in synchrony. The Perspective by Cane looks at the role of the
tropics in paleoclimate. Recent observations and model calculations indicate that effects of climate change in the tropical latitudes
merit a closer look.
H. N. Pollack and S. Huang
Climate Change Record in Subsurface Temperatures: A Global Perspective
Science 1998 Oct 9; 282: 279-281
Analyses of underground temperature measurements from 358 boreholes in
eastern
North America, central Europe, southern Africa, and Australia indicate that, in the
20th century, the average surface temperature of Earth has increased by about
0.5°C and that the 20th century has been the warmest of the past five centuries.
The subsurface temperatures also indicate that Earth's mean surface temperature
has increased by about 1.0°C over the past five centuries. The geothermal data offer an independent confirmation of the unusual
character of 20th-century climate that has emerged from recent multiproxy studies.
Richard A. Kerr
Greenhouse Forecasting Still Cloudy
Science 1997 May 16; 276: 1040 - 1042
A year and a half ago, an international panel suggested that global
warming had arrived. But as negotiators prepare to gather in Bonn in
July to discuss a climate treaty that could require nations to adopt
expensive policies for limiting emissions of greenhouse gases, many
climate experts caution that it is not at all clear yet that human
activities have begun to warm the planet--or how bad greenhouse warming
will be when it arrives.
Karen Schmidt
Coming to Grips With the World's Greenhouse Gases
Science 1998 July 24; 281: 504-506
Negotiators hammering out the details for implementing the Kyoto climate
change treaty have agreed that changes in forest cover since 1990 can be
counted for--and against--a nation trying to meet its carbon dioxide
emissions obligations. But the treaty is hazy about how to calculate
forest carbon stocks and whether nations can use forestry projects in
developing countries to claim carbon credits. Efforts to clarify the
CO2-forest connection have just begun in earnest, with a U.N. workshop
scheduled for September, and the results will be critical to the treaty's
success. Scientists are therefore mounting an ambitious effort to trace
the flux of CO2 between land and air.
Richard A. Muller, Gordon J. MacDonald
Glacial Cycles and Astronomical Forcing
Science 1997 July 11; 277: 215 - 218.
Narrow spectral features in ocean sediment records offer strong evidence
that the cycles of glaciation were driven by astronomical forces. Two
million years ago, the cycles match the 41,000-year period of Earth's
obliquity. This supports the Croll/Milankovitch theory, which attributes
the cycles to variations in insolation. But for the past million years,
the spectrum is dominated by a single 100,000-year feature and is a poor
match to the predictions of insolation models. The spectrum can be
accounted for by a theory that derives the cycles of glaciation from
variations in the inclination of Earth's orbital plane.
David Rind
Just Add Water Vapor
Science 1998 August 21; 281: 1152 - 1153.
Increased global warming can lead to greater evaporation of water into the atmosphere, and water vapor
is itself an excellent greenhouse gas. For this reason, a proper understanding of the behavior of water
vapor in the atmosphere is crucial to formulating accurate global climate models. In his Perspective,
Rind discusses recent efforts to unravel this very important and
complex feedback mechanism.
James E. Hansen, Makiko Sato, Reto Ruedy, Andrew Lacis, Jay Glasco
Global Climate Data and Models: A Reconciliation
Science 1998 August 14; 281: 930-932.
The debate over the existence of global warming and climate change has been muddled because of satellite data showing a
cooling trend in Earth's troposphere. This apparent cooling is in disagreement with measurements at surface stations and with
climate models. In their Perspective, Hansen et al. discuss a correction to the satellite data published by Wentz and Schabel in
Nature that may have profound implications for discussions of climate change. Wentz and Schabel discovered that the original
satellite data, published in 1995, was not adjusted for the natural decay of spacecraft altitude caused by atmospheric drag.
When this adjustment is made, the satellite data agree with both surface data and model calculations. The authors of the
Perspective conclude that the question now is not whether global warming exists--it clearly does--but what should be done
about it.
Richard A. Kerr
Climate Change: Model Gets It Right--Without Fudge Factors
Science 276: 1041.
To keep their computer models from drifting off into climates quite unlike today's, climate modelers have gotten in the habit of
fiddling with fudge factors, arbitrary "flux adjustments." But now researchers at the National Center for Atmospheric Research
in Boulder, Colorado, have developed a model that gets it right without flux adjustments. The first results from this model imply
that future greenhouse warming may be milder than some other models have suggested--and may take decades to reveal itself.
Thomas R. Knutson, Robert E. Tuleya, and Yoshio Kurihara
Simulated Increase of Hurricane Intensities in a CO2-Warmed Climate
Science 279: 1018-1021.
Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character
of these powerful storms could change in response to greenhouse gas-induced global warming. The impact of climate warming
on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western
Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO2 conditions.
More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model.
For a sea surface temperature warming of about 2.2oC, the simulations yielded hurricanes that were more intense by 3 to
7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.
J. D. Mahlman
Uncertainties in Projections of Human-Caused Climate Warming
Science 278: 1416-1417.
Increased atmospheric greenhouse gases resulting from human activities have produced a sharp increase in the potential to
warm the climate to levels that may have serious implications. Predictions of future climate changes in response to these
greenhouse gas increases depend almost completely upon physically based mathematical models of the climate system. The
strengths and weaknesses of these models to project various aspects of climate change are evaluated in terms of a "betting
odds" approach to provide guidance to the policy deliberation process.
Yoram J. Kaufman, and Robert S. Fraser
The Effect of Smoke Particles on Clouds and Climate Forcing
Science 277: 1636-1639.
Smoke particles from biomass burning can generate forcing of climate by modifying cloud microphysics and reflectance of
sunlight. Cloud modification, critical to an understanding of climate change, is uncertain and variable. Satellite data over the
Amazon Basin and Cerrado were analyzed for cloud reflectance and droplet size and for smoke concentration. Smoke
increased cloud reflectance from 0.35 to 0.45, while reducing droplet size from 14 to 9 micrometers. The regional variability of
the smoke effect was correlated to the availability of water vapor. During the 3 months of biomass burning in the dry season, the
smoke-cloud forcing of climate was only 2 watts per square meter in this region, much smaller than what can be inferred from
model predictions.
K. Hasselmann
Climate Change: Are We Seeing Global Warming?
Science 276: 914-915.
Measurements over the last century show that the global mean temperature has increased by about 0.5 degrees C. But how
much, if any, of that increase can be attributed to human activity is uncertain and highly controversial. In his Perspective,
Hasselmann discusses recent efforts to untangle anthropogenic climate change from natural climate variability. Improvements in
computer modeling have reduced the scatter in climate change simulations from 50 to 20%, but significant differences remain.
To resolve the contentious issues of anthropogenic warming, more work will be needed on the role of aerosols, clouds, and
ocean-atmosphere coupling in climate change.
P. J. Sellers, R. E. Dickinson, D. A. Randall, A. K. Betts, F. G. Hall, J. A. Berry, G. J. Collatz, A. S. Denning, H. A.
Mooney, C. A. Nobre, N. Sato, C. B. Field, and A. Henderson-Sellers
Modeling the Exchanges of Energy, Water, and Carbon Between Continents
and the Atmosphere
Science 275: 502-509.
Atmospheric general circulation models used for climate simulation and weather forecasting require the fluxes of radiation, heat,
water vapor, and momentum across the land-atmosphere interface to be specified. These fluxes are calculated by sub models
called land surface parameterizations. Over the last 20 years, these parameterizations have evolved from simple, unrealistic
schemes into credible representations of the global soil-vegetation-atmosphere transfer system as advances in plant
physiological and hydrological research, advances in satellite data interpretation, and the results of large-scale field experiments
have been exploited. Some modern schemes incorporate bio geochemical and ecological knowledge and, when coupled with
advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to
global change, for example, increasing atmospheric carbon dioxide.
Jocelyn Kaiser
New Network Aims to Take the World's CO2 Pulse
Science 281: 506-507.
A key question in climate change models is how much carbon is sequestered by ecosystems; researchers have attempted to
answer this question by modeling fluxes globally or looking at tree growth and other clues to carbon storage. Now a worldwide
network of 70 or so towers already running or about to come online will soon churn out a stream of data on how much CO2 is
socked away in various soil and plant types. That information should, over the long haul, help refine models of global warming
as greenhouse gases continue to build up in the atmosphere and could potentially be used to help enforce the Kyoto treaty.
Richard A. Kerr
Warming's Unpleasant Surprise: Shivering in the Greenhouse?
Science 281: 156-158.
At a conference last month in Snowbird, Utah, researchers heard overwhelming evidence that the so-called "conveyor belt"
current that warms northern Europe and adjacent Asia has repeatedly slackened and at times even shut off during the past
100,000 years, in concert with dramatic climate shifts around the hemisphere. And computer models suggest that, ironically, the
greenhouse's moister air could also squelch the conveyor belt--with potentially alarming repercussions. The prospect
underscores the oceans power over climate, also featured in the Special Section beginning on page 189.
Andrey Ganopolski, Claudia Kubatzki, Martin Claussen,
Victor Brovkin, and Vladimir Petoukhov
The Influence of Vegetation-Atmosphere-Ocean Interaction
on Climate During the Mid-Holocene
Science 280: 1916-1919.
Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during
the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of
solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly
(through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic
effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the
atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean
leads to a warming of the Southern Hemisphere.
Seong-Joong Kim, Thomas J. Crowley, and Achim Stössel
Local Orbital Forcing of Antarctic Climate Change During the
Last Interglacial
Science 280: 728-730.
During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. Large local changes in
precession forcing could have produced this pattern if there were a rectified response in sea ice cover. Results from a coupled
sea ice-ocean general circulation model supported this hypothesis when it was tested for three intervals around the last
interglacial. Such a mechanism may play an important role in contributing to phase offsets between Northern and Southern
Hemisphere climate change for other time intervals.
Richard A. Kerr
Models Win Big in Forecasting El Niño
Science 280: 522-523.
Predictions of the most recent El Niño were widely regarded as a stunning success: Forecasters warned of torrential rain in
California this winter and drought in Indonesia, and they were right. But according to a recent ranking of which computer
models do the best job of predicting this warming of the tropical Pacific and its effects on global weather patterns, if
meteorologists had relied more heavily on their models, those predictions could have been even better.
Stephen M. Griffies, and Kirk Bryan
Predictability of North Atlantic Multidecadal Climate Variability
Science 275: 181-184.
Atmospheric weather systems become unpredictable beyond a few weeks, but climate variations can be predictable over much
longer periods because of the coupling of the ocean and atmosphere. With the use of a global coupled ocean-atmosphere
model, it is shown that the North Atlantic may have climatic predictability on the order of a decade or longer. These results
suggest that variations of the dominant multidecadal sea surface temperature patterns in the North Atlantic, which have been
associated with changes in climate over Eurasia, can be predicted if an adequate and sustainable system for monitoring the
Atlantic Ocean exists.
Ganapati Mudur
Climate Change: Monsoon Shrinks With Aerosol Models
Science 270: 1922.
New Delhi, India--Global climate models based on rising carbon dioxide levels predicted wetter monsoons in south Asia. But
new models that incorporate an increase in the aerosol haze from pollution in south Asia reverse that prediction for parts of
India and China, predicting a decrease in rainfall that could hurt agriculture.
Richard A. Kerr
The Hottest Year, By a Hair
Science 279: 315b-316b.
Last week, government climatologists hailed 1997 as the warmest year of the century, citing it as another sign that greenhouse
warming is real. But although the 1990s could end up as the warmest decade on record, computer climate models have called
for the rising levels of greenhouse gases to be having a bigger effect on global climate. Reality may catch up with the models,
however. By the turn of the century, researchers say, some factors that have slowed global warming so far will dissipate, and it
will arrive in earnest.
Daifang Gu, and S. G. H. Philander
Interdecadal Climate Fluctuations That Depend on Exchanges
Between the Tropics and Extratropics
Science 275: 805-807.
The unexpected and prolonged persistence of warm conditions over the tropical Pacific during the early 1990s can be
attributed to an interdecadal climate fluctuation that involves changes in the properties of the equatorial thermocline arising as a
result of an influx of water with anomalous temperatures from higher latitudes. The influx affects equatorial sea-surface
temperatures and hence the tropical and extra tropical winds that in turn affect the influx. A simple model demonstrates that these
processes can give rise to continual interdecadal oscillations.
Simon F. B. Tett, John F. B. Mitchell, David E. Parker,
and Myles R. Allen
Human Influence on the Atmospheric Vertical Temperature Structure:
Detection and Observations
Science 274: 1170-1173.
Recent work suggests a discernible human influence on climate. This finding is supported, with less restrictive assumptions than
those used in earlier studies, by a 1961 through 1995 data set of radiosonde observations and by ensembles of coupled
atmosphere-ocean simulations forced with changes in greenhouse gases, tropospheric sulfate aerosols, and stratospheric ozone.
On balance, agreement between the simulations and observations is best for a combination of greenhouse gas, aerosol, and
ozone forcing. The uncertainties remaining are due to imperfect knowledge of radiative forcing, natural climate variability, and
errors in observations and model response.
Alan Robock
Stratospheric Control of Climate
Science 272: 972-0.
Firm evidence for a connection between variations in solar radiation and climate has been elusive. In his Perspective, Robock
discusses the results of Haigh, who reports in this same issue ( p. 981) new computer simulations of solar-climate interaction.
The computer model produces a realistic tropospheric response to solar variation, in part because the simulation takes into
account the stratospheric changes in ozone caused by the sun, which in turn influence the troposphere.
