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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L16710, doi:10.1029/2007GL030144, 2007
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Entering into the ‘‘greenhouse century’’: Recent record temperatures
in Switzerland are comparable to the upper temperature quantiles in a
greenhouse climate
Martin Beniston1
Received 28 March 2007; revised 2 July 2007; accepted 31 July 2007; published 30 August 2007.
[1] This paper investigates the recent spate of recordbreaking warm seasons that have affected Switzerland in
less than a decade and compares the seasonal statistics to
those simulated for a ‘‘greenhouse-gas’’ climate by the end
of the 21st century. The peaks of minimum and maximum
temperatures observed during some the record seasons enter
well into the 25%– 75% temperature quantile range for the
scenario climate simulated by a set of regional climate
models. The anomalously warm seasons allow a ‘‘preview’’
of conditions that may occur with greater frequency in the
future. The use of current data as a form of proxy for
the future enables an assessment of the possible impacts
on the natural and socio-economic environments, and
can help in considering possible adaptation strategies to
reduce some of the associated risks of climatic change.
Citation: Beniston, M. (2007), Entering into the ‘‘greenhouse
century’’: Recent record temperatures in Switzerland are
comparable to the upper temperature quantiles in a greenhouse
climate, Geophys. Res. Lett., 34, L16710, doi:10.1029/
2007GL030144.
1. Introduction
[2] Over the past few years, large positive departures
of temperatures from their mean values have become
commonplace in many parts of the world. In Switzerland,
where the exceptional heat wave of the summer of 2003
still remains in public memory, new temperature records
have been broken in a recent past, in particular the warmest
July on record in 2006, the warmest fall in many parts of the
country in 2006, and one of the warmest winters on record
in 2006 – 2007. The spate of anomalously warm weather
over a relatively short time span has inevitably caught
public attention, with a legitimate questioning as to whether
this constitutes further evidence of global warming. Clearly,
mean temperatures are higher than they were several
decades ago. It can thus be expected that with increasing
average temperatures the upper extremes of temperature
probability density function (PDF) can reach levels rarely
experienced under cooler conditions. In some parts of
the country, temperatures have increased during the
20th century by 1.5°C or more, particularly at high
elevations, as reported by Beniston [2003].
[3] In the wake of the spate of recent record warm
months or seasons, many publications have attempted to
1
Institute for Environmental Science, University of Geneva, Geneva,
Switzerland.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030144$05.00
identify the causal mechanisms underlying such events [e.g.,
Beniston, 2004a, 2005; Schär et al., 2004; Luterbacher et al.,
2007]. While the enhanced greenhouse effect is certainly
partially responsible for the long-term increasing trends in
both maximum and minimum temperatures, it is more
difficult to attribute the recent large positive temperature
anomalies solely to the greenhouse effect. Enhancement of
low-level atmospheric temperatures by sensible heat fluxes
at the surface associated with dry soils has recently attracted
much interest [Seneviratne et al., 2006]. Vautard et al.
[2007] show that northward propagation of drought
conditions that begin in the Mediterranean region during
winter generate systematic preconditions contributing to the
intensity and/or persistence of heat waves in Europe. The
positive feedback effect of dry soils also helps explain the
severity of a heat wave.
[4] This paper revisits the Swiss observational record and
compares the data of a greenhouse climate simulated by
regional climate models (RCMs). The extremes associated
with some of the recent record events will be seen to enter
well into the medium to upper quantiles of future minimum
and maximum temperature PDFs. By analyzing the impacts
that these extreme conditions currently impose on environmental (e.g., the cryosphere, hydrological systems, and the
biosphere) and socio-economic systems (e.g., human health,
agriculture, hydropower), analogies with the future can be
drawn for timely decision-making in terms of risk-reduction
and adaptation strategies.
2. Data
[5] The Swiss weather service (MeteoSwiss) maintains a
dense network of observation stations and manages the
acquired daily data in the form of a digital data base.
Switzerland is an ideal locale for analyzing long time-series,
because the data is generally homogenous [Begert et al.,
2003] and available in digital form from 1901. The Swiss
data has been used in many statistical studies of climate and
climatic change in Switzerland [e.g., Beniston, 2004a], and
the climatological stations span a range of altitudes from
200– 3600 m above sea level. Jungo and Beniston [2001]
have shown through cluster analysis that, despite individual
site heterogeneity, there is a close resemblance between
climatological variables in different parts of Switzerland,
particularly on an altitudinal basis, such that even if some
sites may be biased by local characteristics, the long-term
trends are in general agreement among all stations.
[6] The observational record used in this investigation
comes from Swiss observational stations, distributed horizontally and vertically over the territory. For the sake of
conciseness, only two sites representative of low elevations
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results and observations for the control period, at least for
the region studied, as previously discussed by Beniston
[2004a, Figure 4]. Basel and even many locations within
the Alps thus constitute one of the geographical locales
where raw RCM data can be used with a reasonable degree
of confidence.
3. Results
Figure 1. Seasonal averages of minimum temperatures
(Tmin) at Basel (369 m above sea level), 1901 –2007.
Numbers in circles and squares give the year of the
10 warmest years for each season in the record.
(Basel, in NW Switzerland at 369 m above sea level) and
high elevations (Saentis at 2500 m in NE Switzerland)
will be discussed, and because they have proven their
quality in a number of previous studies [e.g., Beniston
and Stephenson, 2004]. The conclusions for this limited
sample are however consistent with the findings at the other
Swiss sites.
[7] Global and regional climate models allow an insight
into possible climate futures as a function of greenhouse
gas emissions and concentrations. The IPCC scenarios
developed by Nakićenović et al. [2000] are based on various
emission pathways as a function of economic and population growth, societal and technological choice, e.g., the
speed with which energy sectors reduce their dependency
on fossil fuels, and shifts in deforestation and land-use
trends. In this paper, results for the high emissions IPCC
SRES A2 scenario only will be discussed to emphasize that
even for one of the ‘‘worst-case’’ climatic change scenarios,
current extreme temperatures in the Alps exhibit close
statistical resemblance to those of the A2 climate. CO2
levels for this scenario reach about 800 ppmv by 2100 (i.e.,
three times their pre-industrial values), and are in the upper
range of climate futures published by the Intergovernmental
Panel on Climate Change [2007].
[8] The European Union project ‘‘PRUDENCE’’ (http://
prudence.dmi.dk) and the ongoing ‘‘ENSEMBLES’’ project
(http://www.ensembles-eu.org/) have used a suite of
regional climate models (RMCs) to simulate climatic
change over Europe for the last 30 years of the 21st century.
Shifts in a number of key climate variables can be assessed
through comparisons with the reference 1961 – 1990
climate. Déqué et al. [2005] report a generally close
agreement between many of the model results. For this
study, four different RCMs have been employed: the Swiss
CHRM; Danish HIRHAM; Swedish RCAO; and Italian
RegCM models (see Beniston et al. [2007] for further
references). Initial and boundary conditions for the RCMs
are derived from a two-step process involving the fullycoupled ocean-atmosphere general circulation model of the
UK Hadley Centre HadCM3 [Johns et al., 2003], followed
by the higher-resolution atmospheric HadAM3H model
[Pope et al., 2000] driven by HadCM3.
[9] Raw data from RCM simulations has been used in
this study because the bias is very small between RCM
[10] Figure 1 illustrates the evolution of seasonal maximum temperatures (Tmax) from 1901– 2006 at the highelevation site of Saentis; superimposed on Figure 1 are the
10 warmest seasons of the series, to highlight the clustering
of record-warm seasons that occurs in the most recent part
of the record, in particular during summer and winter.
Summer and autumn show strong increases in temperature,
and while springs have not generally been as warm as they
were in the 1940s and 1950s, 2007 has set another record.
Winter temperatures peaked in the late 1980s and show a
cooling trend that was abruptly reversed during the 2006–
2007 winter, which in many locations, including Saentis
was the warmest on record. Figure 1 also shows the large
year-to-year variability of mean seasonal temperatures,
where these can undergo changes by as much as ±5°C from
one year to the next. In the past, outbreaks of anomalously
warm seasons have occurred on numerous occasions, but
when such ‘‘flares’’ of temperature are superimposed on the
general century-scale temperature trends, it is not surprising
that in the most recent part of the record, temperatures
exceed earlier record high values. Figure 2 highlights the
seasonal distribution of minimum temperatures (Tmin) at
the low-elevation site of Basel, the clustering of the 10 most
extreme seasons is not always the same as for Saentis, but in
both cases it is seen that in less than a decade, new
temperatures records have been set for all for seasons.
The differences in the sequence of record-high years
between Tmin and Tmax, and between low and high
elevations, can be attributed to site differences, e.g., the
presence or absence of inversion layers in winter, differences in cloudiness, and snow cover and its feedbacks with
the atmosphere. In less than a decade, new temperatures
records have been set for all for seasons.
[11] It was shown by Beniston [2004a] and Schär et al.
[2004] that the extreme European summer heat wave of
2003 may be a forerunner of summers to come later this
century. In view of the spate of record warm seasons in
Figure 2. As Figure 1, except for maximum temperatures
(Tmax) at Saentis (2,500 m above sea level).
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BENISTON: RECORD TEMPERATURES IN SWITZERLAND
Figure 3. Curves of mean minimum temperatures (Tmin)
at Basel for the 1961 –1990 reference period, monthlymeans for 2003, and daily values for the summer of 2003,
superimposed upon the shaded curves depicting 10%, 25%,
50%, 75% and 90% quantiles of the A2 scenario
simulations for the 2071 –2100 period for this location.
Switzerland in recent years, it is of interest to assess how
close some of the latest events come to resembling statistically those that are expected in a future climate.
[12] In order to address this issue, RCM data from the
four models mentioned in the preceding section were used
to compute an inter-model average for the model grid-points
closest to Basel and Saentis. Using the daily data simulated
by the models, it was possible to compute the daily and
monthly means, and the lower and upper quantiles of the
Tmin and Tmax distributions.
[13] Figure 3 illustrates the particular case of the Tmin
distribution at the Basel grid-point, for the 30-year period
from 2071 to 2100. Superimposed upon the scenario climate
Tmin statistics are three sets of curves: the bold line is the
observed monthly-average Tmin at Basel for the 1961 –
1990 period, the bold dashed line illustrates the monthly
values for the year 2003, and the bold dotted curve provides
the daily Tmin values during the exceptional 2003 heat
wave from June – August. The following broad observations
are summarized below:
[14] 1. The average monthly values for the reference
climate of the 20th century are at or below the 10% quantile
limit of the A2 scenario climate.
[15] 2. Minimum temperatures between current and future
climates increase from 3.8°C to 4.9°C on average according
to the month.
[16] 3. For a particular year such as 2003, monthly mean
temperatures make broad incursions into the middle range
of the future Tmin quantiles, i.e., for example June and
August 2003 temperatures are warmer than the 50% quantile
of the A2 climate.
[17] 4. Certain days of the 2003 heat wave, particularly in
June and August, are seen to largely exceed the upper
quantiles of the scenario climate, i.e., isolated days of the
2003 summer are as warm or warmer than the 90% quantile
of the future climate. In other words, events such as the
2003 heat wave, have been as intense as what would
constitute the upper range of the Tmin PDF in the scenario
climate.
[18] In order to push this analysis further, a count of the
number of incursions of observed temperatures into the
quantile range of future climate has been undertaken to
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assess whether any significant trends are discernible. Figure 4
provides an example of such counts, where the exceedances
of summer Tmax at the Saentis site beyond the 10%, 50%
and 90% quantiles of future climate have been plotted from
1901– 2006. Values of Tmax beyond the 10% quantile level
are a fairly common occurrence, averaging about 80% of the
time and exhibiting a slightly increasing trend. The trend is
much more conspicuous for the number of counts beyond
the 50% quantile of future climate, and even ignoring the
strong bias imposed by the hot summer of 2003, the
exceedances have doubled since 1901 from less than 20%
of the summer period (15 days per summer) to more than
40% today (over 35 days). This is to be expected, since in
the intervening period mean summer temperatures have
warmed by about 1.5°C at this site; the threshold of the
50% quantile in the scenario climate is 13.1°C for the three
summer months, which corresponds roughly to the 97%
quantile of summers for 1901 – 1910 but ‘‘only’’ to the 85%
quantile for 1997 – 2006. Figure 4 shows that some summers
in the late 1940s exceeded the 90% quantile threshold of the
A2 climate for a small number of days; in 2003, however,
the exceedance was more than 10%. The number of counts
beyond the 90% quantile for a decade centered on the
warmest 10-year period prior to today, i.e., 1947 – 1956 is
13 at Saentis, while in the 10 years since 1997 this has risen
to 23. Similar conclusions can be reached for the other
seasons at all other time-series investigated in this study, but
are not shown here for reasons of conciseness.
[19] Table 1 shows for Tmin and Tmax for each season at
Basel and Saentis, how frequently such events may occur in
the future based on the comparisons with future quantiles of
temperature. It is seen that the record seasons observed in
the last decade could occur 50– 70% of the time in the A2
climate. This clearly suggests that Switzerland has already
experienced conditions that would occur at least 1 year in 2
by 2100.
[20] The statistics that underlie Figure 4 and Table 1 raise
the question as to whether climate model projections are not
underestimating observed trends. This is a question that has
recently attracted much attention [e.g., Scheffer et al., 2006;
Torn and Harte, 2006], and is perhaps related to the failure
by many models to reproduce the strength of certain
positive feedback mechanisms. It is not the purpose of
this paper to enter into the details of these processes, but
the results portrayed here, already at the high end of the
quantiles for an A2 climate, may need to be put into the
Figure 4. Exceedances of current summer Tmax values at
Saentis beyond the 10%, 50% and 90% quantiles of the
future scenario climate for this location.
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Table 1. Frequency of Occurrence of the Conditions Observed in
the Record-High Seasonal Temperatures in the Recent Past When
Projected Into a Future Scenario Climate
DJF
Basel Tmin
Basel Tmax
Saentis Tmin
Saentis Tmax
2007:
2007:
1990:
1990:
60%
60%
60%
65%
MAM
2007:
2007:
2007:
2007:
70%
65%
65%
65%
JJA
2003:
2003:
2003:
2003:
50%
50%
50%
60%
SON
2006:
2006:
2006:
2006:
60%
65%
70%
65%
perspective of an even warmer climate than that resulting
from A2 emissions.
4. Discussion
[21] The discussion in the previous section emphasizes
the point that the recent spate of anomalously warm seasons
observed in Switzerland may well be forerunners of climatic
conditions that regional climate models are projecting for
the latter part of the 21st century. While there is cause for
concern in view of the observed environmental, social and
economic impacts during the events themselves and often
long after (e.g., in the case of ground-water deficits or
the inertia of melting glaciers), the recent extremes have
provided a ‘‘preview’’ of future events and impacts.
[22] A succinct summary of the manner in which very
warm seasonal anomalies can affect the alpine region are
outlined below [e.g., Beniston, 2003, 2004b]:
[23] 1. Winters: the 2006 – 2007 season has been characterized by a sharp reduction in snow amount and duration at
elevations below 2,000 m. Many low to medium sites
suffered from significant shortfalls in revenue resulting from
a reduction in the number of days where skiing is possible
by a factor of 2– 3. Warm winters at lower elevations also
reduce the period of dormancy that many plants require, and
often fail to destroy the precursor condition for pest and
disease outbreaks later in the year.
[24] 2. Springs: anomalously warm springs leads to early
runoff in hydrological basins. Further downstream, if
sustained precipitation occurs, this can lead to flood
situations, as already experienced in 1995 along the Rhine
River. Warm springs also enable an early start to the
vegetation period of certain species, which can then be
disrupted with the return of a cold spell after the snow has
melted.
[25] 3. Summers: possibly the most adverse social and
environmental impacts occur during the summer. Summer
heat waves observed in Switzerland and many parts of the
alpine domain result in acute problems of public health,
large losses for agriculture, sharp reductions in river
discharge (compounding problems of water use in the
agricultural and energy sectors), a decline of glacier mass
and degradation of permafrost (with collateral effects on
slope instability events).
[26] 4. Autumns: warm, dry conditions can prolong
droughts that may have begun earlier in the year, with
further drops in river discharge. A persistently warm
autumn also delays the time of the first winter snowfalls.
[27] Without some form of economic or technological
adaptation, such impacts will increasingly be felt as climate
warms. The lessons learned from the current spate of
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anomalously warm seasons can thus be used as a proxy
for future seasons and the impacts they generate. Impacts
analyses can help in devising adaptation measures to
minimize the associated risks by the end of the century.
For example, strategies for avoiding conflicting uses of
water between hydropower and irrigation in summer, or
optimization of tourism across all seasons in the Alps taking
into account climatic change, can be formulated on the basis
of these recent previews of future climate.
5. Conclusions
[28] This paper has provided a brief overview of the
anomalously warm seasons that have broken records in
Switzerland in a very recent past. The spate of recordbreaking seasons is not surprising in view of the rise of
mean temperatures that have occurred since the beginning
of the selected time-series in 1901. However, when comparing the warm-season data with monthly or seasonal data
in the future, it was shown that the very warm seasons
observed in current climate already mimic what may occur
with increasing frequency in a greenhouse climate. Indeed,
the present investigation has shown that Switzerland has
experienced climatic conditions that models suggest may
occur at least 50% of the time by 2100, with a number of
impacts on environmental and socio-economic systems that
would be replicated whenever such temperature thresholds
are exceeded. By using the current-climate data as a form of
proxy for future conditions, risks associated with some of
the more adverse impacts could be reduced by putting into
effect appropriate adaptation schemes in a timely manner.
References
Begert, M., et al. (2003), Homogeneous temperature and precipitation series
of Switzerland from 1864 to 2000, Int. J. Climatol., 25, 65 – 80.
Beniston, M. (2003), Climatic change in mountain regions: A review of
possible impacts, Clim. Change, 59, 5 – 31.
Beniston, M. (2004a), The 2003 heat wave in Europe: A shape of things to
come? An analysis based on Swiss climatological data and model simulations, Geophys. Res. Lett., 31, L02202, doi:10.1029/2003GL018857.
Beniston, M. (2004b), Climatic Change and Its Impacts: An Overview
Focusing on Switzerland, 296 pp., Kluwer Acad., Dordrecht, Netherlands.
Beniston, M. (2005), Warm winter spells in the Swiss Alps: Strong heat
waves in a cold season? A study focusing on climate observations at the
Saentis high mountain site, Geophys. Res. Lett., 32, L01812, doi:10.1029/
2004GL021478.
Beniston, M., and D. B. Stephenson (2004), Extreme climatic events and
their evolution under changing climatic conditions, Global Planet.
Change, 44, 1 – 9.
Beniston, M., et al. (2007), Future extreme events in European climate; an
exploration of regional climate model projections, Clim. Change, 81,
71 – 95.
Déqué, M., et al. (2005), Global high resolution versus limited area model
climate change projections over Europe: Quantifying confidence level
from PRUDENCE results, Clim. Dyn., 25, 653 – 670.
Intergovernmental Panel on Climate Change (2007), Climate Change: The
IPCC Fourth Assessment Report, Cambridge Univ. Press, Cambridge, U. K.
Johns, T. C., et al. (2003), Anthropogenic climate change for 1860 to 2100
simulated with the HadCM3 model under updated emission scenarios,
Clim. Dyn., 20, 583 – 612.
Jungo, P., and M. Beniston (2001), Changes in the anomalies of extreme
temperature anomalies in the 20th century at Swiss climatological stations
located at different latitudes and altitudes, Theor. Appl. Climatol., 69,
1 – 12.
Luterbacher, J., et al. (2007), Exceptional European warmth of autumn
2006 and winter 2007: Historical context, the underlying dynamics,
and its phenological impacts, Geophys. Res. Lett., 34, L12704,
doi:10.1029/2007GL029951.
Nakićenović, N., et al. (2000), IPCC Special Report on Emissions Scenarios, Cambridge Univ. Press, Cambridge, U. K.
4 of 5
L16710
BENISTON: RECORD TEMPERATURES IN SWITZERLAND
Pope, D. V., et al. (2000), The impact of new physical parameterizations in
the Hadley Centre climate model HadAM3, Clim. Dyn., 16, 123 – 146.
Schär, C., et al. (2004), The role of increasing temperature variability in
European summer heat waves, Nature, 427, 332 – 336.
Scheffer, M., V. Brovkin, and P. M. Cox (2006), Positive feedback between
global warming and atmospheric CO2 concentration inferred from past
climate change, Geophys. Res. Lett., 33, L10702, doi:10.1029/2005GL025044.
Seneviratne, S. I., et al. (2006), Land-atmosphere coupling and climate
change in Europe, Nature, 443, 205 – 209.
Torn, M. S., and J. Harte (2006), Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming, Geophys. Res. Lett., 33,
L10703, doi:10.1029/2005GL025540.
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Vautard, R., P. Yiou, F. D’Andrea, N. de Noblet, N. Viovy, C. Cassou,
J. Polcher, P. Ciais, M. Kageyama, and Y. Fan (2007), Summertime
European heat and drought waves induced by wintertime Mediterranean
rainfall deficit, Geophys. Res. Lett., 34, L07711, doi:10.1029/
2006GL028001.
M. Beniston, Institute for Environmental Science, University of Geneva,
Geneva, Switzerland. ([email protected])
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