40°C (5, 6). These differences were driven at
least in part by atmospheric CO2 concentrations about five times the preindustrial value
(7). Researchers have long documented (8)
that the water cycle associated with Eocene
greenhouse climate must have been substantially different from that of today. However,
although studies have documented shifting
patterns and overall intensification of water
cycling, particularly at high latitudes and during short-lived, extremely hot hyperthermal
events (9– 12), strong evidence for changes in
the tropics has been lacking.
Clementz and Sewall provide an innovative approach to fill this gap by looking for
the imprint of atmospheric water cycling on
the oceans. Imbalances in evaporation and
precipitation rates over the tropical and subtropical oceans affect the chemistry of marine
surface waters, most notably producing salinity contrasts between relatively fresh tropical
waters, and salty subtropical surface waters.
The same process also leads to differences in
the stable isotopic composition of these water
masses, with the heavy isotope 18O more concentrated in subtropical regions of net evaporation, and less concentrated in the tropical
surface ocean. The strength of this isotopic
variation is directly related to the local imbalance in precipitation and evaporation fluxes,
and hence represents a proxy for the intensity
of water cycling within Hadley cells.
The primary challenge to applying the
water isotope proxy to greenhouse climate conditions millions of years ago is
that the isotopic composition of paleo–
surface waters can be measured only indirectly. The most common way to do this is
to measure the oxygen isotope composition
of fossil shells produced by marine microorganisms, which are widely available and
reflect the isotopic composition of the water
in which they formed. One problem, however, is that their isotopic composition is
also strongly influenced by water temperature. Clementz and Sewall circumvent this
challenge by taking advantage of biology:
They measure the O isotopic composition
of carbonate in the tooth enamel of fossil
sirenians (sea cows, dugongs, and manatees), which like all mammals presumably
maintained their body temperatures at constant values. Together with certain favorable behavioral characteristics, inferred
by analogy with modern sirenians—such
as large home ranges and a preference for
shallow-water habitats—the homeothermic
conditions of sirenian tooth enamel formation allow the authors to reconstruct paleo-seawater O isotope values without many of
the uncertainties inherent to other methods.
The results demonstrate enhancement of the
isotopic contrast between the tropical and
subtropical surface oceans, supporting the
hypothesis that contrasts between low-latitude regions of net evaporation and precipitation were enhanced during the Eocene CO2
greenhouse. The isotopic results are backed
up by climate model simulations designed to
replicate the climate of the Eocene, which
suggest a pattern of intensified water-cycle
change at the fossil collection sites that is
largely consistent with the isotopic data.
The new results offer compelling evi-
dence that the tropical engine of the water
cycle revved faster during past greenhouses,
but the implications for understanding past
and future climate states still hinge on a
number of unknowns. Both the natural range
of sirenians and the availability of fossil col-
lections limit the distribution of samples in
this study to coastal regions. As a result, the
isotopic data do not directly represent the
massive gyres and regions of the intertropi-
cal convergence zone where the most intense
precipitation-evaporation imbalances occur.
Records directly representing these regions
would strengthen the case for globally signif-
icant changes in tropical water cycling in the
Eocene. In addition, the surface-water proxy
approach applied here does not clarify the
dynamic role of enhanced water cycling in
the Eocene climate, and it does not account
for important factors such as the relation-
ship between faster cycling and cloudiness,
water transport to the extratropics, and pre-
cipitation intensity. These are challenging
problems that both the modern and paleocli-
mate communities will continue to struggle
with, but the work of Clementz and Sewall
should help assure us that when the wheels
of the water cycle spin faster, they are driven
by the tropics.
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2. I. M. Held, B. J. Soden, Annu. Rev. Energy Environ. 25,
3. M. T. Clementz, J. O. Sewall, Science 332, 455 (2011).
4. I. M. Held, B. J. Soden, J. Clim. 19, 5686 (2006).
5. M. Huber, Science 321, 353 (2008).
6. P. K. Bijl et al., Nature 461, 776 (2009).
7. M. Pagani, J. C. Zachos, K. H. Freeman, B. Tipple,
S. Bohaty, Science 309, 600 (2005).
8. E. J. Barron, W. W. Hay, S. Thompson, Palaeogeogr.
Palaeoclimatol. Palaeoecol. 75, 157 (1989).
9. L. Handley, P. N. Pearson, I. K. McMillan, R. D. Pancost,
Earth Planet. Sci. Lett. 275, 17 (2008).
10. C. Robert, J. P. Kennett, Geology 22, 211 (1994).
11. M. Pagani et al., Nature 442, 671 (2006).
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Gautham Nair and Arjun Raj
RNA transcription rates can randomly vary in a single cell over time.
Measurements that involve averaging values from large numbers of cells can obscure the fact that processes
such as gene expression are fundamentally
dynamic and can vary greatly from cell to
cell (1). Similarly, static snapshots of individual cells can reveal instantaneous differences between cells but cannot reveal often
important changes over time; for example,
it is impossible to tell whether a yellow light
follows a green light using only static pictures of traffic lights. On pages 472 and 475
of this issue, Suter et al. (2) and Larson et al.
(3) report on using a time-lapse approach to
document the dynamics of a key cellular pro-
cess, RNA transcription. Using carefully con-
structed experimental systems and sophisti-
Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, USA. E-mail: rajlaboratory@
cated analyses, they tracked changes in RNA
transcription in single live cells. Their results
provide new details about the dynamics of
key steps in transcription.
The transcription of a gene into mRNA
is fundamental to the parsing of the genetic
code and is highly regulated by the cell. In
eukaryotes, transcription requires a large
number of biochemical steps, including the
initiation of the process and the subsequent
elongation and maturation of the RNA molecule. The transcriptional activity of individual cells may deviate from the averages
measured in most experiments. In part,
that is because a typical cell contains only
a few copies of most genes—or even just
one copy—and these few genes determine
the production of the RNA transcript for the
entire cell. In some cells, small fluctuations
in the biochemical reactions that determine