Download impact of climate change on the management of district heating

IMPACT OF CLIMATE
CHANGE ON THE
MANAGEMENT OF
DISTRICT HEATING
SYSTEMS
Authors : C. Déandreis (IPSL), P. Braconnot (IPSL), S. Planton (CNRMGAME).Study performed for the DALKIA company
State of the knowledge on Diurnal
temperature range
Summary
Definition
This fact sheet proposes a literature review as of
May 2009 on diurnal temperature range. Studies to
date describe the past evolution of diurnal temperature range and give projections for the next century.
The global annual mean (oceans and continents) of
this temperature range has fallen by about 0.07°C per
decade since 1950. This fall is the result of a more rapid rise in daily minimum temperatures (Tmin) than in
daily maximum temperatures. At the global scale, the
diurnal temperature range should continue to shrink
over the next century due to the intensification of
global warming (Meehl et al., 2007).
At the regional level the trend is more variable. In
particular, southern Europe, eastern China and the
south-eastern United States are showing an increase
in their diurnal temperature range. These contrasts
are expected to become more intense over the 21st
century.
To date, no study has been conducted on changes in
extreme values and diurnal temperature range variability.
The daily temperature range or diurnal temperature range (often written DTR) corresponds to the
maximum variation in temperature over a period of
24 hours in a given place. It is expressed in degrees
Celsius (°C) or kelvin (K)1.
It is defined as the difference between the daily
maximum temperature (Tmax) and the daily minimum
temperature (Tmin):
DTR = Tmax - Tmin
Factors influencing diurnal
temperature range
The analysis of diurnal temperature range first requires an examination of its two terms: Tmax and Tmin.
Their respective variations are not controlled by the
same physical mechanisms. Tmax is reached during daylight hours. It depends on the amount of solar radiation absorbed by the Earth’s surface. The influence of
infrared radiation on Tmax is thus negligible. The level of
daytime heating depends on the following factors:
❚❚ the level of insolation: this is higher in the middle
of the day and in summer;
❚❚ cloud cover: clouds reflect varying amounts of solar
radiation back into space depending on their thickness (very thick cloud cover will prevent any daytime heating);
❚❚ wind: it dilutes the layer of warm air near the
ground into unheated air and thereby prevents its
warming. Strong winds can prevent any daytime
heating. Wind can also rapidly replace a cold air
mass with a warm air mass, or vice versa;
1. Conversion formula °C to K: K = °C + 273.15.
IMPACT OF CLIMATE CHANGE
ON THE MANAGEMENT OF
DISTRICT HEATING SYSTEMS
❚❚ surface albedo2: a surface such as snow has a high
albedo, as it reflects almost all the solar energy hitting the surface and thereby prevents daytime heating of the air near the ground;
❚❚ atmospheric particles (or aerosols): these absorb
and reflect solar radiation. Their mean effect tends
to reduce maximum temperatures;
❚❚ air humidity: this can also play an important role in
absorbing some near-infrared radiation;
❚❚ thermal mass: this depends on the soil type and
moisture. The higher it is (very wet soil, for example), the lower the maximum temperature.
The Tmin, on the other hand, is reached during the
night. The surface cools through the emission of infrared radiation and lowers the temperature of the layer of
air in contact with it. The level of cooling depends on:
❚❚ cloud cover: clouds reflect infrared radiation back
towards the ground. Thick cloud cover prevents
nocturnal cooling;
❚❚ air humidity: it absorbs some near-infrared radiation and prevents nocturnal cooling;
❚❚ wind: it dilutes the thin layer of cold air into uncooled air. The stronger the wind, the less the temperature will fall. Strong wind prevents nocturnal
cooling;
❚❚ surface emissivity: the more the surface radiates
(infrared), the more it cools. This is the case of snow;
❚❚ thermal mass: this depends on the soil type and
moisture. The higher it is (very wet soil, for example), the lower the minimum temperature.
From these observations regarding Tmax and Tmin,
we see that cloud cover tends to reduce the diurnal
temperature range (lower levels of nocturnal cooling
and daytime heating). But this range will be wide on
sunny, windless days.
Other factors may influence the diurnal temperature range (DTR):
❚❚ Liquid water vs. continental surface: the more
continental the region, the wider the DTR will be.
On the contrary, the presence of oceanic masses reduces this range, since water acts as a temperature
regulator. The presence of lakes or groundwater
(see thermal mass above) is enough to reduce this
temperature range;
❚❚ latitude: the closer a place is to the equator, the
more the solar energy provided varies between day
and night;
2. The surface albedo is the ratio of incident radiation to reflected radiation.
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❚❚ relief: the DTR increases at higher altitudes. Atmospheric density decreases as the altitude increases.
Thus, in the daytime, the atmospheric layer reflects
less incident solar radiation back into space (the
Tmax increases more easily). On the other hand, at
night, infrared radiation is trapped less by greenhouse gases (very cold nights).
Table 1 summarises the impact of these different
factors on the Tmax, Tmin, and DTR.
Factors
Tmax
Tmin
DTR
Clouds
Wind
Albedo
Emissivity
Latitude
Liquid water*
Relief
∕
+
∕
+
+
∕
+
∕
+
-
+
+
TABLE 1 Impact of different factors on maximum, minimum and
diurnal temperature range variations. *Oceans, lakes, groundwater, etc.
Recent changes in diurnal temperature
range
Uncertainties associated with studies of diurnal
temperature range are generally high, as they include
both uncertainties about the estimation of maximum
temperatures and those about minimum temperatures. These uncertainties are all the greater when the
zone studied is small (regional versus global). However, several studies have revealed significant trends
from a statistical point of view, at least at the global
level.
Annual trends:
Figure 1 presents the evolution of annual anomalies
for the maximum temperature, minimum temperature and DTR, relative to the period 1961-19903.
Analysis of maximum and minimum land surface
temperatures logged daily over the 1950-2004 period shows that the maximum temperature range
observed over the course of a day is noticeably
decreasing (-0.0666°C per decade). Globally, we are
seeing a warming of both minimum and maximum
3. The annual mean maximum temperature anomaly corresponds to the difference between the annual mean maximum temperature and its mean value
calculated for the reference period 1961-1990. This definition also applies to the
minimum temperature anomaly and the diurnal temperature range anomaly.
The analysis of anomalies (rather than gross values) is preferred in climatology
as it makes it possible to study the trend of a variable without the mean bias
of the models.
a) 1950-2004
a) 1950
- 2004
a) 1950 - 2004
b) 1979
b) 1979-2004
- 2004
b) 1979 - 2004
FIGURE 1 Evolution of global annual mean anomaly (oceans and
continents) of the maximum temperature (top), minimum temperature (middle) and diurnal temperature range (bottom) for the
period 1950-2004 relative to the reference 1961-1990 (from Vose
et al., 2005).
temperatures. However, minimum temperatures are
rising around 1.5 times faster than maximum temperatures (0.204°C compared to 0.141°C per decade). Over
this period, the decrease in diurnal temperature range
is comparable to mean warming.
For the period 1979-2004, the temperature range
measured over the course of a day stabilises. For
this period, the Tmax and Tmin trends are very similar
(0.287°C compared to 0.295°C per decade). The trend
(-0.001°C per decade) is not statistically significant at
the 5% level over this period (Tmax and Tmin trends too
similar).
No suitable data sets exist to extend this study to
the beginning of the 20th century.
Geographical distribution:
The planispheres in Figure 2 represent the regional
diurnal temperature range anomalies for the period
1950-2004 and for the period 1979-2004. The anomalies were calculated relative to the period 1961-1990.
For the period 1950-2004 (Figure 2.a), we see that the
decrease in the temperature range over the course of a
FIGURE 2 Trend of DTR anomalies for the periods 1950-2004 (a) and
1979-2004 (b). Grid resolution 5x5 (from Vose et al., 2005).
day concerns all parts of the world, with the exception
of a few regions (north-eastern Canada, southern Argentina, south-eastern Australia). These findings from
the study by Vose et al. (2005) have been confirmed
by several regional studies (Bonsal et al. 2001; Wibig
and Glowicki, 2002; Kruger and Shongwe, 2004, for
Canada, Poland and South Africa respectively).
However, for the period 1979-2004 (Figure 2.b), the
trend of the diurnal temperature range is far more
heterogeneous. We even see an increase in this range
in several regions, including the south-eastern United
States, many parts of Europe and south-eastern China.
Physical causes of the decrease in DTR
The physical causes of this decrease in diurnal
temperature range are still uncertain. In particular,
few studies have been carried out on this subject. In
addition, they have often been conducted using limited sets of data. The results presented below are therefore to be approached with caution; they are likely to
evolve rapidly in response to forthcoming studies on
this subject.
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IMPACT OF CLIMATE CHANGE
ON THE MANAGEMENT OF
DISTRICT HEATING SYSTEMS
The decrease in diurnal temperature range has been
linked to the increase in anthropogenic pollutant
emissions (greenhouse gases and aerosols).
An aerosol is a suspension of particles in the atmosphere, which reflect the incident solar radiation.
They thereby reduce the diurnal surface temperature
(Tmax). The increase in the number of these aerosols
due to human activity has long been an argument for
the decrease in diurnal temperature range. However,
one study (Stone and Weaver, 2002) shows that the effect of aerosols is negligible (their direct effect being
offset by their water vapour feedback). The decrease
is this temperature range is therefore only the result
of the increase in atmospheric greenhouse gas concentrations.
Stone and Weaver (2003) also showed that the diurnal temperature range is more sensitive to climate
change feedback than to the direct effect of increasing greenhouse gas concentrations (a rise in Tmin
caused by the increasing greenhouse effect). In a statistical study, they show that increasing cloud cover
and soil moisture are the main causes of the decrease
in the diurnal temperature range for most of the cases
studied.
The increase in cloud cover (higher impact of low
rain clouds) causes a reduction in solar energy reaching the surface (reduction in Tmax) and captures the
infrared energy emitted from the surface overnight
(increase in Tmin).
The increase in soil moisture permits faster cooling of the surface during the daytime through evaporation and also moderates temperatures during the
daytime by increasing the heat capacity of the ground.
There is one exception to this rule: winter at midlatitudes when snow covers the ground. It thus insulates the surface (preventing the effect of soil moisture) and increases its reflective properties (which
reduces the impact of the reflection of solar radiation
by clouds).
The question of the effect of land use change has
also been raised (Stone and Weaver, 2003). It seems
that this factor may have a considerable impact on
diurnal temperature range at the local and regional
scales.
To date, there are no explanatory elements in the literature about the stabilisation of diurnal temperature
range from 1979.
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Model performance – validation over the
historical period
The observed change in diurnal temperature
range is greater than the change simulated by most
models (-0.08°C compared to -0.02°C per decade for
the period 1950-1993).
The study by Stone and Weaver (2002) shows that
in the northern hemisphere, their model underestimates the trend of the diurnal temperature range
mainly due to an overestimation of the Tmax in summer and autumn and an underestimation of the Tmin
in spring and winter. In the southern hemisphere, the
conclusions are more uncertain due to a lack of data
(few monitoring stations).
In this study, the model/observation variations concerning the trend of the diurnal temperature range
were mainly attributed to the misrepresentation of
the increase in cloud cover by models.
Diurnal temperature range and future
climate change
The maximum temperature range measured in
the course of a day should continue to decrease in
the future due to the intensification of global warming (Figure 3.a).
However, this decrease does not concern all parts of
the world. The southern United States, Latin America,
South Africa, southern Europe, eastern China and
southern Australia will all see a considerable increase
in their diurnal temperature range. Changes in this
temperature range (positive or negative) could be as
much as 0.5°C by the end of the century in certain
regions (results consistent between the models used
in this study).
As with historical trends, future changes in the
diurnal temperature range are closely linked to the
evolution of total cloud cover (Figure 3.b). This range
decreases over regions where cloud cover becomes
denser, and increases in regions where cloud cover
diminishes.
Where Europe is concerned, there is a strong contrast
between southern Europe (considerable growth in diurnal temperature range) and northern Europe (decrease
in this range). The results are consistent between models
for the extreme south and north of the region.
Over China, we also see significant regional disparities between the Tibetan Plateau (decrease in diurnal
temperature range) and eastern China (growth in this
range). However, the model response is less reliable in
this part of the world.
a) Diurnal temperature range (°C)
b) Total cloud fraction (%)*)
Figure 10.10.
Change of diurnal temperature range (a) and of total cloud fraction (b) between the periods 2080-2099 and 1980-1999. Results
obtained from all models of the fourth IPCC report for the A1B scenario. Above the stippled areas, the result is consistent between the
models (model mean exceeds standard deviation). Figure from IPCC, Third Assessment Report, chapter 10. * Cloud fraction is the fraction of
a model grid that is covered with clouds, as opposed to the clear sky fraction (cloud free). It is expressed as a percentage.
FIGURE 3
To date, no detailed study has been conducted to explain these contrasts, but in Europe, changes in cloud
cover should play an important role, as suggested in
Figure 3.
Further comments (in relation to the
project)
This literature review raises three points of interest
in the study of the vulnerability indicator defined in
the fact sheet related to this case study and entitled
“Vulnerability indicator: development and analysis”:
1) Europe and China can be divided into two zones
in which the diurnal temperature range shows trends
with opposite signs (north versus south for Europe,
and east versus west for China). It will be important
to take this contrast into account when analysing the
vulnerability indicator concerning the diurnal temperature range. An analysis of this contrast will undoubtedly be necessary to understand the regional
evolution of our climate indicator in the future.
2) All of the studies that were used to produce this
fact sheet deal with changes in the mean value of the
diurnal temperature range. However, in the context
of the INVULNERABLe project, studying the mean
767
alone is not enough. It must be supplemented by the
analysis of extremes (95th quantile, for example), the
study of the signal variability, and of its annual cycle,
etc. Such studies do not yet exist in the literature.
3) Within the framework of the INVULNERABLe
project research, we mainly focus on wide diurnal
temperature ranges, and more specifically on the probability of seeing abnormally high diurnal temperature
ranges in relation to those currently recorded. The decrease in the mean global diurnal temperature range
(DTR) does not systematically result in a reduction in
the frequency of abnormally high values. Indeed:
❚❚ the trend of the mean is not proportional to that
of extremes. If the mean diurnal temperature range
decreases, the probability of observing high DTR
values does not necessarily diminish. It is therefore
important to study more specifically the form of
the DTR distribution and its future evolution (see
point 3);
❚❚ The global mean is not representative of regional
disparities. Indeed, we see an increase in the diurnal temperature range in some of our target regions
(eastern China, southern Europe).
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IMPACT OF CLIMATE CHANGE
ON THE MANAGEMENT OF
DISTRICT HEATING SYSTEMS
Bibliography:
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2001b : Characteristics of daily and extreme temperatures over Canada. J. Climate 14: 1959-1976.
Dai, A., K. E. Trenberth, and T. R. Karl, 1999: Effects of
clouds, soil moisture, precipitation, and water vapour
on diurnal temperature range, J. Clim., 12, 2451–2473.
Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M.
Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J.
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Kruger, A.C. and S. Shongwe, 2004: Temperature
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1929-1945.
Stone D., A. J. Weaver, 2002: Daily Maximum
and Minimum Temperature Trends in a Climate Model, Geophys. Res. Lett., 29 (9), 1356,
doi:10.1029/2001GL014556.
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Stone, D. A., and A. J. Weaver, 2003: Factors contributing to diurnal temperature range trends in 20th and
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Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D.
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133
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