INFORMATION SHEET ON OBSERVED CLIMATE INDICATORS
FOR THE CIRCE URBAN CASE STUDIES: ATHENS
Summary
►
The 100+ year-long surface air temperature and rainfall record of the National Observatory of
Athens (NOA) has been analysed to determine indications of significant deviation from long-term
average conditions in the city of Athens.
►
The analysis of the complete record reveals a tendency towards warmer years, with significantly
warmer summer maximum temperatures of about 1.8 °C from the beginning of the record.
►
The number of hot days and nights shows a ‘virtually certain’ increasing trend in the order of 10
more days per year since the late 19th century, especially in the last decade.
►
A slightly increasing trend for total rainfall has been observed.
1. Introduction
Greece has a temperate Mediterranean climate
with a remarkable range of micro-climates due
to its complex geography. Ancient Greeks
established their residence in dry locations
securing in this way healthier living conditions
and so most major Greek cities were founded in
dry sites. Athens, lies at the southeasternmost
part of the mainland of Greece and enjoys a
prolonged warm and dry period during the year
with July and August being the hottest and driest
months. The topography of the city, surrounded
by mountains, favours the formation of air
pollution episodes during periods of anticyclonic
circulation. The city is prone to summer heat
waves and flash floods. In the present study, we
have analysed the climatic characteristics of the
urban area of Athens by analysing the
observational record of the National Observatory
of Athens station in the centre of the city.
2. Indicators of observed climate
Five indicators are presented:
1. Maximum temperature (Tx) on an
annual and seasonal (summer) basis
2. The number of hot days and very hot
days (Tx >35 °C and >37 °C) during
summer
3. The number of hot and very hot nights
(Tn >20 °C and >26 °C) during summer
4. Total annual rainfall
5. Greatest rainfall total falling on three
consecutive days during the year.
Daily records of maximum and minimum
temperature and rainfall were used to calculate
the indicators. Data are obtained from the
National Observatory of Athens (NOA)
meteorological station and cover the period
1897-2007 for temperature and 1899-2007 for
rainfall. Anomalies of the indicators from the
1971-2000 (common period) were calculated,
and a 10-year moving average of the absolute
values is shown on each plot. Statistical
significance has been examined at several levels
of confidence using the Student t-test, and
likelihood ranges were used to assess the
probability of occurrence, following the IPCC
classification.
1
Maximum temperatures anomalies
What is it?
The meteorological station of NOA is the oldest in Greece and one of the oldest in the Balkans. The
first meteorological observations date from 1839, but were performed in a non systematic way and
cannot be considered homogeneous before 1890 because of successive site relocations. Therefore we
use the uninterrupted maximum and minimum temperature series from 1897 to 2007.
4.0
34
3.0
33.5
2.0
33
1.0
32.5
3
0.0
32
2005
2001
1997
1993
1989
1985
1981
1977
1973
1969
1965
1961
1957
1953
1949
1945
1941
1937
1933
1929
1925
30
1921
-4.0
1917
30.5
1913
-3.0
1909
31
1905
-2.0
1901
31.5
1897
-1.0
10-year Mov. Avg.
Summer Maximum Temperature Anomalies (oC)
anomalies
10-year Mov Avg.
Figure 1: Summer maximum temperature (Tx) anomalies from the 1971-2000 average (left axis) for the NOA
station (bars). 10-year moving average of the absolute summer Tx (right axis) for the NOA station (line)
What does this show?
The main features of the annual maximum
temperature (not shown) include the cold period
in the beginning of the 20th century, the warm
period 1940–1960, cooling until around 1975,
followed by another phase of warming until
2007. It is ‘virtually certain’ (>99% likelihood
of occurrence) that the annual maximum
temperature has increased during the 100+ years
of the record, with a rate of 0.12 °C per decade.
The summer maximum temperature exhibits
similar behaviour to the annual Tx (Figure 1).
There is a constant increase in Tx of up to 0.2 °C
per decade and this increase is virtually certain
(>99% likelihood of occurrence). The warmest
summers on record occurred in 1998, 1999,
2000 and 2007.
2
Why is this important?
This indicator is of prime importance, especially
for a Mediterranean city such as Athens, since it
determines the thermal comfort and cooling
demands. There is a strong relationship between
the stress experienced by organisms exposed to
high temperatures and daily temperature
extremes. Moreover, the rate of energy demand
depends on the ambient temperature and hence,
summer maximum temperature is a valuable
estimator of changes in energy consumption for
air conditioning of buildings. Changes in
temperature may have critical implications in an
urban area for surface water resources, periurban forestry, infrastructure, industry, and most
notably population heat stress and health
(Giannakopoulos et al., 2009).
Number of hot and very hot days
What is it?
Hot days (and very hot days) is defined as the number of days each year that maximum temperature
(Tx) exceeds 35°C (equivalent to the 84th percentile of Tx) or 37°C (equivalent to the 95th percentile).
14
anomalies
10-year Mov. Avg.
2005
2001
1997
1993
1989
1985
1981
1977
1973
1969
1965
1961
1957
1953
1949
0
1945
-10.0
1941
2
1937
-5.0
1933
4
1929
0.0
1925
6
1921
5.0
1917
8
1913
10.0
1909
10
1905
15.0
1901
12
1897
20.0
10-year Mov. Avg.
Hot Days anomalies
25.0
Figure 2: Number of ‘very hot days’ (Tx > 37°C) anomalies from the 1971-2000 average (left axis, bars) for
the NOA station. 10-year moving average (right axis, solid line) for the NOA station
What does this show?
It is ‘virtually certain’ that hot days have
increased by 1.5 days per decade over the data
record (not shown). It is ‘about as likely as not’
that the number of very hot days has increased
during the last century, at a rate of about 0.12
days per decade (Figure 2). The two highest
number of very hot days occurred in the years
2007 and 2001. An increasing trend is evident
from the mid-1970s onwards.
Why is this important?
An increase in the frequency of heat waves has
been observed, particularly in southern Europe
(IPCC, 2007). These events intensify in urban
areas such as Athens, and are a familiar feature
of Greek summers. During anticyclonic
conditions with large-scale subsidence there is
advection of warm air masses from northern
Africa and heating due to changes in pressure.
Heat-wave days have negative effects on human
comfort and contribute significantly to heat
stress especially if associated with high levels of
humidity. In addition, stagnant air masses
encourage the buildup of air pollutants, which
act synergistically with higher air temperature to
increase mortality.
3
Number of hot and very hot nights
What is it?
Hot nights act synergistically with hot days to contribute to human discomfort during a heat wave. For
example, the persistently high nocturnal temperature of the 2003 heat wave in France prevented
respite from the heat of the day, and was a crucial factor in the sudden increase in mortality. Here, a
threshold of 26°C for minimum temperature (Tn) was used to define a very hot night (>95 th percentile
of summer Tn for the complete NOA record), and a Tn threshold of 20°C was used to define a warm
night (22nd percentile of summer Tn for the complete NOA record).
25
16
anomalies
10-year Mov. Avg.
14
20
12
10
10
8
10-year Mov. Avg.
Very Hot Nights Anomalies
15
6
5
4
0
2
2005
2001
1997
1993
1989
1985
1981
1977
1973
1969
1965
1961
1957
1953
1949
1945
1941
1937
1933
1929
1925
1921
1917
1913
1909
1905
1901
0
1897
-5
Figure 3: Annual number of very hot nights, anomalies from the 1971-2000 average (left axis, bars) for the
NOA station. 10-year moving average of very hot nights (right axis, solid line)
What does this show?
It is ‘virtually certain’ that the number of very
hot nights has increased during the last century
at a rate of 0.5 nights per decade, as derived
from the NOA series (Figure 3). Moreover, it is
also ‘virtually certain’ that the number of hot
nights has increased at a rate of 1.4 nights per
decade over the last century (not shown).
4
Why is this important?
Warm nights following a hot day and
accompanied by high humidity can be
particularly uncomfortable to urban residents.
This can have important implications for human
health but also for energy demand levels during
a heat wave.
Annual Rainfall Anomalies
What is it?
Total annual rainfall amount for the National Observatory of Athens for the period 1899 to 2007. The
deviation from the common baseline (1971-2000) is shown (Figure 4) together with the 10-year
moving average of the absolute annual rainfall series.
650
anomalies
10-year Mov. Avg.
600
470
550
500
450
400
350
430
300
250
410
200
150
100
390
10-year Mov. Avg.
Total annual precipitation anomalies (mm)
450
50
0
370
-50
-100
-150
350
-200
2007
2003
1999
1995
1991
1987
1983
1979
1975
1971
1967
1963
1959
1955
1951
1947
1943
1939
1935
1931
1927
1923
1919
1915
1911
1907
1903
-300
1899
-250
330
Figure 4: Total annual rainfall anomalies from the 1971-2000 average (left axis, bars) for the NOA station.
10-year moving average of total annual rainfall (right axis, solid line) for the NOA station
What does this show?
From the late 19th century to the 1940s there are
successive positive and negative anomalies, with
higher positive anomalies from the middle of the
1900s to the late 1910s and during the 1930s.
During the second half of the 20th century
several positive rainfall anomalies are succeeded
by many negative ones. Wet periods are
identified during 1962-1969, 1975-1980, 19871988 and 2002-2004. In contrast, dry periods are
detected in 1970-1971, 1989-1990 and 19921993. The years 1989 and 1990 are the driest in
the record. From the examination of the 100+
year NOA annual rainfall, it is evident that the
rainfall amounts do not exhibit a statistically
significant change. The year 2002 recorded the
highest total rainfall (987 mm) and twice the
average annual total. Although it rained almost
every day from 20th August to 15th September,
the impacts were minimal since there were no
flash flood events.
Why is this important?
Rainfall is a limiting factor in biological
systems, so the examination of the total rainfall
on an annual and seasonal basis is important,
particularly for Mediterranean regions. Changes
in annual total rainfall may have important
implications for the availability of water
resources, which in turn can affect agriculture,
forestry and water supply.
5
Greatest 3-day rainfall
What is it?
The greatest rainfall total (mm) falling on three consecutive days during the year is indicative of
rainfall intensity. The deviation from the common baseline (1971-2000) is shown (Figure 5) together
with the 10-year moving average of the absolute annual maximum 3-day rainfall series.
100.0
90.00
anomalies
10-year Mov. Av.
85.00
80.0
75.00
40.0
70.00
20.0
65.00
60.00
10-year Mov. Avg.
Annual max total rainfall over 3d anomalies
80.00
60.0
0.0
55.00
-20.0
50.00
-40.0
45.00
2007
2003
1999
1995
1991
1987
1983
1979
1975
1971
1967
1963
1959
1955
1951
1947
1943
1939
1935
1931
1927
1923
1919
1915
1911
1907
1903
40.00
1899
-60.0
Figure 5: Greatest 3-day rainfall anomalies from the 1971-2000 average (left axis, bars) for the NOA station.
10-year moving average of total annual rainfall (right axis, solid line) for the NOA station
What does this show?
It is ‘likely’ that the greatest 3-day rainfall
amount has increased during the last century
(Figure 5), even though the total annual rainfall
remains unchanged (Figure 4). This implies that
rainfall has become more intense since total
rainfall remains the same, and the number of wet
days (daily rainfall total >1mm) is virtually
certain to have decreased (not shown).
6
Why is this important?
The increase in the incidence and intensity of
rainfall events may provoke natural disasters
such as localised flash flooding in urban areas.
Intense rainfall events lead to greater erosion
rates and a higher risk of flash flooding in urban
areas with sometimes serious losses of lives and
property.
3. Risks of current climate hazards
The average summer (JJA) maximum
temperature at Athens (NOA) is 32.1°C while
the 90th /95th percentiles correspond to 35.8°C /
36.9°C respectively (for 1971-2000 period). The
all-time maximum temperature (Tx) recorded for
the metropolitan area of Athens is 44.8°C, while
the all-time minimum temperature is –6.5°C.
Rainfall is sparse during summer (mean rainfall
for the years 1971-2000 is 20.4 mm (± 2.6 mm)
and falls in the form of heavy showers.
The summer of 2007 in Athens provides an
example of the risk of current climate hazards.
Hot days and warm episodes are not unusual in
Athens during summer. The statistics for the
summer of 2007, however, illustrate the rareness
of the temperatures recorded with respect to the
observed series. The summer of 2007 was the
hottest summer of the instrumental record with
respect to summer mean, maximum and
minimum temperatures (Table 1). The all-time
record of 44.8°C was measured at the NOA on
26 June 2007 (Founda and Giannakopoulos,
2009).
In Table 2, the trends of the examined
indicators are summarised for all data records
used in this study. In the last column the
confidence level is given following the approach
developed for the IPCC 2007 report. In addition,
it was estimated that the decade 1998-2007
(+2.1oC relative to the 1961-90 period) was
warmer (probability >99.99) than any other
decade of the record. Another important feature
of Athens’ recent climate is the increase in the
frequency of hot days and heat wave episodes as
well as the longer duration of these episodes
(Founda et al., 2004).
Table 1: Temperature statistics for the summer of 2007 compared to temperature statistics for the
periods 1897-2006 and 1971-2000, National Observatory of Athens.
(1897-2006)
(1971-2000)
2007
Previous record
Average daily mean summer
temperature (oC)
26.3
26.2
29.1
28.6 (2003)
Average daily maximum
summer temperature (oC)
31.8
32.1
34.9
34.4 (2003)
Average daily minimum
summer temperature (oC)
21.8
21.9
24.4
24.0 (2003)
Maximum daily temperature
(oC)
43.0
42.8
44.8
43 (June 1916)
Maximum nocturnal
temperature (oC)
31.2
31.2
30.8
31.2 (July 2000)
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Table 2: Change in the climate indicators (hazards) for Athens and associated likelihood of
occurrence
Climate Indicator (hazard)
Change (per decade)
Region
(or stations)
Time period
Likelihood§
Annual maximum
temperature
increase (+0.12 oC)
NOA station
1897-2006
virtually certain
Summer maximum
temperature
increase (+0.2 oC)
NOA station
1897-2007
virtually certain
Annual total rainfall
increase (+1 mm)
NOA station
1899-2007
unlikely
Greatest 3-day rainfall
increase (+1.25 mm)
NOA station
1899-2007
likely
Very heavy rainfall days
(>95th percentile)
decrease (-0.05 days)
NOA station
1899-2007
unlikely
Hot days (Tx >35°C)
increase (+1.5 days)
NOA station
1897-2007
virtually certain
Very hot days (Tx >37°C)
increase (+0.12 days)
NOA station
1897-2007
about as likely
as not
Hot nights (Tn >20°C)
increase (+1.4 nights)
NOA station
1897-2007
virtually certain
Very hot nights (Tn >26°C)
increase (+0.5 nights)
NOA station
1897-2007
virtually certain
The terminology for likelihood of occurrence is based on the standard terms used in the IPCC 2007 report: Virtually
certain > 99% probability; Extremely likely > 95% probability; Very likely > 90% probability; Likely > 66% probability;
More likely than not > 50% probability; About as likely as not 33 to 66% probability; Unlikely < 33% probability; Very
unlikely < 10% probability; Extremely unlikely < 5% probability; Exceptionally unlikely < 1% probability
§
4. Integrating case study themes
The Greater Athens area, i.e., the city with its
suburbs and the Attica peninsula in general,
comprises almost half of the Greek population
(circa 5 million inhabitants) and consequently
represents a major part of the economic, social,
cultural and administrative activities of the
country. The occurrence of a severe heat wave in
Athens can therefore paralyse all these sectors
and culminate in a substantial increase in energy
demand (for air conditioning needs) and a
8
deleterious effect on human health (due to the
high population density). In Athens, urbanization
constitutes an additional aggravating factor since
the urban fabric retains night temperatures at
high levels and hinders biological cooling.
Additionally, the poor air quality in Athens
contributes to the increased health risks.
Persistent high summer temperatures also
increase significantly peri-urban forest fire risk
(Giannakopoulos et al., 2009).
Acknowledgements
CIRCE (Climate Change and Impact Research: the Mediterranean Environment) is funded by the Commission of
the European Union (Contract No 036961 GOCE) http://www.circeproject.eu/. This information sheet forms part
of the CIRCE deliverable D11.3.2. Metadata can be accessed from http://www.cru.uea.ac.uk/projects/circe.
References
►
Founda, D., K.H. Papadopoulos, M. Petrakis, C. Giannakopoulos and P. Good, Analysis of mean,
maximum and minimum temperatures in Athens from 1897 to 2001 with emphasis on the last
decade: trends, warm and cold events, Global and Planetary Change, vol. 44, 27-38, 2004
► Founda D. and C. Giannakopoulos, The exceptionally hot summer of 2007 in Athens, Greece: A
typical summer in the future climate? Global and Planetary Change, accepted, vol 67, Issues 3-4,
227-236, 2009.
► Giannakopoulos C., P. LeSager, M. Bindi, M. Moriondo, E. Kostopoulou, C. Goodess, Climatic
changes and associated impacts in the Mediterranean resulting from a 2º C Global Warming, Global
and Planetary Change, vol 68, issue 3, 209-224, 2009.
Authors: C. Giannakopoulos (cgiannak@meteo.noa.gr), M.Hatzaki (marhat@phys.uoa.gr) and E. Kostopoulou
(ekosto@meteo.noa.gr) , National Observatory of Athens, Greece.
Editors: Maureen Agnew (m.agnew@uea.ac.uk) and Clare Goodess (c.goodess@uea.ac.uk), Climatic Research
Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK
Date: November 2009
9