lar resolution necessary to make an image of
the jet, which would settle the issue.
Laurent et al. have not attempted to
answer all of the questions raised by these
observations. They have not carried out any
modeling to investigate whether the gamma-
ray emission is consistent with the properties
of the radio jet seen by (9) or whether—pos-
sibly more likely because the high-energy
excess is only seen at certain states of the
accretion disk (12)—the detectable gamma-
ray jet is a transient phenomenon perhaps
related to the transient emission seen at
radio wavelengths (14) or in TeV gamma
rays (15). The presence of a transient, pow-
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ancy between the jet power inferred by (4)
and the appearance of the standard steady jet
seen in radio emission, but this question will
have to be addressed by future studies. The
work of Laurent et al. represents one of the
first uses of high-energy polarimetry, and
the first to give us an appreciable improve-
ment in our understanding of the physics of
an x-ray binary. More work of this nature is
expected from existing and planned gamma-
ray and x-ray polarimeters in the future.
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A Faster Water Cycle
Gabriel J. Bowen
Fossil teeth from marine mammals suggest
that the tropical water cycle sped up during
Most of us treat water as a commod- ity, a consumable if vital substance that we access with a turn of a faucet. But water is also an integral part of the
climate system, contributing to the delicate
balance of energy and mass that regulates the
temperature of our planet. This relationship is
at the heart of the scientific community’s concern that dramatic changes may be in store
for the water cycle in the future (1). Moreover, given water vapor’s role as the most
powerful greenhouse gas in Earth’s climate
system, uncertainty about future water cycle
change underlies much of the uncertainty in
future climate projections (2). Fortunately,
for geochemists, water is not an undifferentiated commodity, but rather exhibits subtle
but measurable variations in its stable oxygen isotopic compositions. These differences
allow researchers to trace past and present
fluxes within the water cycle. On page 4 55 of
this issue, Clementz and Sewall (3) use isotopic data from fossil teeth to reconstruct past
global patterns of variation in water isotopic
composition. They fill a major data gap in the
geological record of the water cycle and provide evidence that fluxes of water through the
tropical atmosphere were more vigorous during past periods of extreme global warmth.
The tropics are the engine of the global
water cycle. Water vapor concentrations in
the tropical troposphere are substantially
higher than those in the extratropics (between
Water world. Data from the
Eocene, when the climate warmed
due to elevated CO2 levels, offer
insight into how the global hydro-logical cycle could shift in a
future greenhouse world (right).
Hypothesized changes from a
cooler “icehouse” world (left)
include enhanced vertical and
poleward vapor transport in the
Hadley cells, poleward expansion
of the subtropical arid zones associated with the subsiding branch
of the Hadley cells, and associated
increases in tropical precipitation
and subtropical evaporation rates.
Earth and Atmospheric Sciences, Purdue University, West
Lafayette, IN 47907, USA. E-mail: email@example.com
the tropical and polar regions), as a result of
the nonlinear relationship between tempera-
ture and saturation vapor pressure. Even so,
water cycles through the tropical atmosphere
about twice as fast as it does through the
mid and high latitudes. This is largely due
to high precipitation and evaporation rates
within the ascending and descending “arms”
of a circulation pattern known as the Had-
ley cell, which dominates the tropical atmo-
sphere (2). Deep convection within the trop-
ical rising branch of the Hadley cells plays
an important role in controlling the atmo-
spheric energy balance by delivering water
vapor to the upper troposphere, where it
contributes disproportionately to the green-
house effect (2). The tropics
are also a dominant source
of moisture to the higher
latitudes, and although the
strength of this tropical moisture pump is
sensitive to a number of competing influ-
ences, most models suggest that more water
vapor will be exported from the tropics in a
warmer world (1, 4) (see the figure).