*CEFE–CNRS, 1919 route de Mende, 34293
†LSCE–CEA–CNRS CE Saclay l’Orme des
Merisiers, 91191 Gif-sur-Yvette, France
‡INRA Site Agroparc, domaine Saint-Paul, 84914
Avignon Cedex 9, France
§Collège de France, 75231 Paris Cedex 05, France
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Supplementary information accompanies this communication on
Competing financial interests: declared none.
more than 27% of global land area) in
1958–99.For 1950–2000,the trends ofglobal
annual average Tminfor windy, calm and all
conditions were identical (0.19?0.06 °C per
decade; Fig. 1a). So, urbanization has not
systematically exaggerated the observed
global warming trends in Tmin.The same can
be said for poor instrumental exposure and
microclimatic effects,which are also reduced
when instruments are well ventilated5.
When the criterion for ‘calm’ was
changed to the lightest decile of wind
strength, the global trend in Tmin was
unchanged.The analysis is therefore robust
to the criterion for ‘calm’.To assess the effect
of time differences between the reanalysis4
daily-average winds and Tmin, I repeated
the analysis using 26 stations in North
America and Siberia that have hourly or
six-hourly reports ofsimultaneous temper-
ature and wind. Again, windy and calm
nights warmed at the same rate,in this case
by 0.20°C per decade.
Because a small sample was used,I com-
pared global trends for the reduced period
1950–93 with published all-conditions
trends for that period based on a sample of
over 5,000 stations6. All differences were
within ?0.02°C per decade.This robustness
arises because ofthe spatial coherence ofsur-
face temperature variations and trends7.
The global annual result conceals a
relative warming of windy nights in winter
in the extratropical Northern Hemisphere
(Fig. 1b), mainly in western Eurasia. The
observed tendency to an increased positive
phase of the North Atlantic Oscillation8
implies that the windier days in western
Eurasia had increased warm advection from
the ocean9, yielding greater warming. In
summer in the extratropical Northern
Hemisphere (Fig. 1c), there was no relative
change in Tminon windy nights.At that time
of year,atmospheric circulation changes are
less influential,but an urban warming signal
is still absent. In the tropics, calm nights
warmed relative to windy nights on an annu-
al average, but only by 0.02?0.01 °C per
decade, which is much less than the overall
tropical warming in Tmin(0.16?0.03°C).
This analysis demonstrates that urban
warming has not introduced significant biases
into estimates of recent global warming.
The reality and magnitude of global-scale
warming is supported by the near-equality of
temperature trends on windy nights with
trends based on all data.
Hadley Centre, Meteorological Office,
Exeter EX1 3PB, UK
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Competing financial interests: declared none.
NATURE|VOL 432|18 NOVEMBER 2004|www.nature.com/nature
is not urban
perature trends. Urban heat islands occur
mainly at night and are reduced in windy
conditions3. Here we show that, globally,
temperatures over land have risen as much
on windy nights as on calm nights, indicat-
ing that the observed overall warming is not
a consequence of urban development.
Observations of the minimum tempera-
ture (Tmin) over 24 hours at 264 stations
worldwide since 1950 were expressed as
anomalies, relative to the period 1961–90
where possible. Coverage of Tmindata was
good north of 20° N, in Australasia and in
the western tropical Pacific, but poor in
Africa,South America,Antarctica and parts
of southern Asia.Reanalysed4daily-average
near-surface wind components were used
to classify the Tminanomalies into ‘windy’
(upper tercile) and ‘calm’ (lower tercile)
conditions.Daily average wind speeds were
used because the timings of temperature
extremes are not known. For stations
between 140° E and the dateline, Tmin—
which occurs most frequently in the early
morning — was matched with the previous
day’s speed.This is because the early morn-
ing in terms of universal time (equivalent
to Greenwich Mean Time) is still in the
previous day in the Far East.
Annual and seasonal anomalies of Tmin
were gridded on a 5°?5° resolution for
windy, calm and ‘all’ conditions. Coverage
was at least 200 grid boxes (equivalent to
ontroversy has persisted1,2over the
influence ofurban warming on
reported large-scale surface-air tem-
Early peak in Antarctic
(or Southern Hemisphere annular mode),
represents fluctuations in the strength of
the circumpolar vortex and has shown a
trend towards a positive index in austral
summer in recent decades, which has been
linked to stratospheric ozone depletion1,2
and to increased atmospheric greenhouse-
gas concentrations3,4. Here we reconstruct
the austral summer (December–January)
Antarctic oscillation index from sea-level
pressure measurements over the twentieth
century5and find that large positive values,
and positive trends of a similar magnitude
he principal extratropical atmospheric
circulation mode in the Southern
Hemisphere, the Antarctic oscillation
Figure 1 Anomalies in Tminfor windy (red) and calm (blue)
conditions. a, Annual global data; b, winter data (December to
February) for Northern Hemisphere land north of 20° N; c, sum-
mer data (June to August) for Northern Hemisphere land north of
20° N. The linear trend fits, and the ?2? error ranges given in
the text, were estimated by restricted maximum likelihood10,
taking into account autocorrelation in the residuals. As expected
from the reduced stratification of the boundary layer, Tminis, on
average,warmer on windy nights than on calm nights.
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