INSIGHTS | PERSPECTIVES
476 30 JANUARY 2015 • VOL 347 ISSUE 6221 sciencemag.org SCIENCE
tion computed by Laliberte et al. can be used
to infer all thermodynamic transformations
in the atmosphere, based solely on standard
The authors apply their methodology
to the global thermodynamic cycle using
data from meteorological reanalysis and
from a global climate model. The results
confirm that the hydrological cycle reduces
the amount of kinetic energy generated by
the large-scale atmospheric flow by about
one-third. As much as we may associate the
hydrological cycle with severe weather and
heavy precipitation, it is also responsible for
nice, sunny weather, which occurs when air
masses gradually regain some of the water
vapor lost during an earlier rainfall (see
the figure). The hydrological cycle reduces
the average intensity of the winds around
Earth mostly by generating pleasant weather
around large portions of the globe.
The study by Laliberté et al. offers a blueprint for the systematic analysis of the thermodynamic transformations in the climate
system. The method can be applied based
solely on the standard output of numerical
models and can easily be used to compare
the thermodynamic cycles simulated by different global climate models. It also opens
up two important questions.
First, what are the contributions of convection and other atmospheric motions at
small scales in the atmospheric heat engine?
The approach presented by Laliberté et al. is
limited to motions resolved by their data set,
which excludes anything with a horizontal
scale of less than 100 km. Yet, many thermodynamic transformations occur within
clouds, at scales that are not typically resolved in an atmospheric or climate model.
Second, how are the thermodynamic processes affected by climate change? As Earth’s
climate warms, the air will be able to contain
more water vapor on average. Laliberté et al.
indicate that this would lead to a slight reduction in the work done by the atmospheric
circulation. This result, if confirmed, could
offer important insights into what Earth’s
global engine could do in the future. ■
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By Achim Kramer
inhibition is relieved and a new cycle can
start. The key finding of Larrondo et al. is
that the circadian cycle is not one of synthe-
sis and destruction as the prevailing model
reflects, but instead, a cycle of synthesis and
progressive posttranslational modifications.
In other words, degradation of the negative
element does not seem to be required for
generating circadian rhythmicity. Rather,
progressive and sequential phosphorylation
of negative elements occurs in a characteris-
tic temporal pattern, thereby controlling the
negative element’s activity and clock speed.
According to their model, the circadian loop
is closed by phosphorylation-induced inac-
tivation of the negative element rather than
by its degradation (see the figure).
Larrondo et al. base their model primar-
ily on observations made in the filamentous
fungus Neurospora crassa, a well-estab-
lished genetic model of eukaryotic clocks.
A protein called Frequency (FRQ) is the key
circadian negative element of Neurospora.
FRQ expression is rhythmic and controlled
by the transcriptional activator White Col-
lar Complex (WCC). FRQ represses WCC
by bringing along casein kinase 1a (CK1a)
that—with other protein-phosphorylating
enzymes—phosphorylates and inactivates
WCC. FRQ itself is progressively phosphory-
lated at over 100 sites, and hyperphosphory-
When the circadian clock
The conceptual model of circadian oscillator
function may need revision
determines circadian clock speed
Closing the loop. Clock speed is determined by progressive phosphorylation (green) of the negative element (FRQ
for N. crassa) rather than by its degradation rate. A second class of phosphorylation (red) inactivates FRQ, to which the
circadian clock then becomes blind.
Your alarm clock rings in the morning —it’s time to get up, but you feel like it’s the middle of the night. If this is happening regularly, your circadian clock may not be adjusted to your so- cial life. Circadian clocks are endogenous oscillators that coordinate not only our
sleep-wake behavior with the environmental
24-hour light-dark cycle but also a myriad of
rhythmic physiological and metabolic processes. A set of so-called clock genes comprises a regulatory network that generates
self-sustained molecular ~24-hour rhythms
of gene expression. For eukaryotic clocks,
the prevailing conceptual model proposes a
negative transcriptional-translational feedback loop (1). On page 518 of this issue, Larrondo et al. (2) challenge the molecular basis
of this view.
In the course of 1 day, a negative element
(a clock protein) is synthesized, progressively phosphorylated and, after a delay of
several hours, shuts down its synthesis by
acting negatively on its own transcription.
Once the negative element is degraded and
cleared from the cell, the transcriptional
Laboratory of Chronobiology, Charité Universitätsmedizin
Berlin, Germany E-mail: firstname.lastname@example.org