INFORMATION SHEET ON OBSERVED CLIMATE INDICATORS
FOR THE CIRCE URBAN CASE STUDIES: BEIRUT, LEBANON
Summary
► The 50+ year-long surface air temperature and rainfall record for Aéroport International de Beyrouth has
been analysed to determine any significant deviation from the long-term average conditions in Beirut.
► The analysis of the complete series reveals a tendency towards warmer summers.
► The number of hot nights and days indicates a ‘virtually certain’ increasing trend, while hot nights exhibit
an increase during the last two decades.
► No statistically significant trends in total annual and seasonal rainfall have been observed.
1. Introduction
Greater Beirut covers a wide range of
geographical zones. The city is situated atop two
hills, the Al-Ashrafiyah and the Al-Musaytibah,
which meet to create a triangular peninsula
protruding into the Mediterranean Sea. The AsSahil, a coastal plain, borders the two mountains
to the east and extends into the north from the
mouth of the Nahr Al-Kalb, the Dog River, and
in the south from the entrance of the Nahr AdDamur, or the Damur River.
Beirut’s Mediterranean climate has hot,
humid summers and mild winters. The city
benefits from the moderating effects of the sea.
In winter, the sea warms up the air masses which
are then blown over the coastal regions, while in
summer cool sea breezes contribute to the cooler
coastal temperatures. Most of Beirut’s annual
rain (about 800 mm on average) falls in the rainy
season, which occurs from mid-autumn to early
spring. In mid-winter, the average midday
temperature rises to 18ºC and falls below 11ºC at
night. Beirut’s average summer temperature is
approximately
25.5ºC,
while
maximum
temperature rises to 29ºC.
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 summer days (Tx >31°C)
3. The number of hot summer nights (Tn >24°C)
4. Total annual rainfall
5. Greatest rainfall total falling on three
consecutive days during the year.
Daily Tx and Tn and annual rainfall were used to
calculate the indicators. Data are from the
Aéroport International de Beyrouth (AIB)
meteorological station and cover the period
February 1931 to February 2002 for temperature
and May 1931 to September 2002 for rainfall.
Due to concerns about the reliability of the early
data only the period 1950-2001 is retained for
analysis. Splines interpolation was used to
estimate missing monthly values for the years
1976 and 1982. Due to the Lebanese Civil War
(1975-1990), there are concerns about data
quality and homogeneity for all the Lebanese
stations. From the metadata available, AIB
seems to be less affected, though we still need to
be cautious. The ability of AIB data to describe
the Beirut climate was examined by comparing
them to the gridded observational datasets
(Haylock et al., 2008) of daily rainfall, Tx and
Tn (E-OBS; 1950-2007). The two datasets
compare well for the common period, 19502001.
For each indicator, anomalies from the
1971-2000 (common period) were calculated,
and a 10-year moving average of the absolute
values is shown. Linear trends for the indicators
have been calculated and their statistical
significance examined for several levels of
confidence using the Student t-test. Likelihood
ranges were used to assess the probability of
occurrence, adopting the IPCC classification.
1
Summer maximum daily temperatures anomalies
1.00
0.80
0.60
0.40
29.4
anomalies
29.3
10-year Mov.Avg.
29.2
29.1
0.20
0.00
-0.20
29
28.9
-0.40
-0.60
28.8
10-year Mov. Avg.
28.7
-0.80
-1.00
28.6
1950
1952
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
Summer Tx anomalies (oC)
What is it?
Observations of the Aéroport International de Beyrouth (AIB) meteorological station are the only
series available for Beirut. Annual and summer (June-August) maximum temperature anomalies (from
the 1971-2000 average) are calculated for the years 1950 to 2001.
Figure 1: Summer maximum temperature (Tx) anomalies from the 1971-2000 average (left axis) for the AIB
station (bars). 10-year moving average of the absolute summer Tx (right axis) for the AIB station (line)
What does this show?
The main features of the annual maximum
temperature (not shown) include a cooler period
during the first 5-6 years of the examined period,
followed by a phase of warming until the late
1960s, and an absence of trend in subsequent
years. Annual maximum temperature is about as
likely as not to have increased during the 50+
years of the record, at a rate of 0.03°C per
decade. Summer maximum temperature (Figure
1) exhibits similar behaviour to annual
maximum temperature for the first half of the
observation period, but shows an intense
warming during the last decade. An increase of
0.12°C per decade in the AIB record is virtually
certain to have occurred (>99% likelihood of
occurrence).
2
Why is this important?
This indicator is of prime importance for a
Mediterranean city such as Beirut, since it
determines thermal comfort and cooling
demands. Urban temperature changes may have
critical implications for surface water resources,
peri-urban forestry, infrastructure, industry, and
most notably human heat stress and health
(Giannakopoulos et al., 2009). There is a strong
relationship between the stress experienced by
organisms and daily temperature extremes. Heatrelated
mortality
at
moderately
high
temperatures can be a significant public health
issue in countries with warm climates, as in
Lebanon (El-Zein et al., 2004). Beirut, a highly
congested and overpopulated city with a semiarid climate may be exposed to high temperature
stress and heat island effects. The rate of energy
demand depends on the ambient temperature and
hence, summer maximum temperature is a
valuable indicator of energy consumption for air
conditioning of buildings where it is installed.
Number of hot days
What is it?
Hot days are defined as the number of days each summer that daily maximum temperature (Tx)
exceeds 31°C (close to the 95th percentile).
26
20
anomalies
24
15
10-year Mov. Avg.
22
1998
2000
1994
1996
1990
1992
10
1986
1988
-15
1982
1984
12
1980
-10
1976
1978
14
1972
1974
-5
1968
1970
16
1964
1966
0
1960
1962
18
1956
1958
5
1954
20
1950
1952
10
10-year Mov. Avg.
Hot Days anomalies
25
Figure 2: Number of ‘hot days’ (Tx > 31°C) anomalies from the 1971-2000 average (left axis, bars) for the AIB
station. 10-year moving average of ‘hot days’ (right axis, solid line) for the AIB station
What does this show?
It is virtually certain that the number of hot days
has increased during the second half of the last
century, at a rate of about two days per decade
(Figure 2). The largest numbers of hot days
occur mainly from the late 1980s onwards, and
an increasing trend is clear from the early 1990s
onwards.
Why is this important?
Variability and change in maximum temperature
extremes can have a considerable impact on
many sectors relevant to urban centres such as
human health, energy demand for space cooling
(where air conditioning equipment is installed)
and tourism. Heat-wave days in particular, have
negative effects on human comfort and
contribute significantly to heat stress especially
if associated with high levels of humidity.
3
Number of hot nights
What is it?
A threshold of 24°C (about the 95th percentile) for summer minimum temperatures (Tn) was used to
define a hot night.
70
anomalies
40
30
60
10-year Mov. Avg.
50
20
10
0
40
-10
-20
20
30
10-year Mov. Avg.
Hot Nights anomalies
60
50
10
-30
-40
1998
2000
1994
1996
1992
1988
1990
1984
1986
1980
1982
1978
1974
1976
1970
1972
1966
1968
1964
1960
1962
1956
1958
1954
1950
1952
0
Figure 3: Annual number of ‘hot nights’ (Tn > 24 °C) anomalies from the 1971-2000 average (left axis, bars)
for the AIB station. 10-year moving average of ‘hot nights’ (right axis, solid line)
What does this show?
It is ‘virtually certain’ that the number of hot
nights (Tn > 24°C) has increased during the last
century at a rate of 10 nights per decade, while a
particularly strong rate of increase is noted for
the last decade of the record, as derived from the
AIB series (Figure 3).
Why is this important?
Hot nights act synergistically with hot days to
contribute to human discomfort during a heat
wave. Warm nights following a hot day and
accompanied by high humidity can be
4
particularly uncomfortable to urban residents.
This can have important implications for human
health but also for energy demand levels during
a heat wave.
In Beirut, there is a hot microclimate that is
most probably caused by the urban heat island
effect and some sheltering from the wind (CAL,
1982). This additional urban heat is caused by
high absorption of solar radiation by concrete
surfaces and roads in addition to an obstruction
of wind flow by buildings. Together these
factors contribute to keeping hot air trapped in
the city (Chaaban, 2007).
Annual Rainfall
500
400
300
200
1000
950
anomalies
10-year Mov.Avg.
900
850
800
750
700
-200
-300
650
600
-400
-500
550
500
1950
1952
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
100
0
-100
10-year Mov. Avg.
Total annual rainfall anomalies (mm)
What is it?
Total annual rainfall for the Aéroport International de Beyrouth for the years 1950 to 2001 is
presented. The deviation from the common baseline (1971-2000) is shown overlain with the 10-year
moving average of the absolute total annual rainfall series (Figure 4).
Figure 4: Total annual rainfall anomalies from the 1971-2000 average (left axis, bars) for the AIB station. 10year moving average of total annual rainfall (right axis, solid line) for the AIB station
What does this show?
During the second half of the 20th century,
several periods of positive rainfall anomalies are
succeeded by negative ones. Wet periods are
identified during 1962-1971, and 1974-1981. In
contrast, an earlier dry period is recorded in
1954-1962. The years 1958 and 1990 are the
driest in the record. It is evident from the 50-year
annual rainfall AIB record that there is no
statistically significant trend.
Similarly, no significant trends are found for
neighbouring stations in Israel and Cyprus as
documented in the study of Alpert et al. (2002).
Why is this important?
Although Lebanon has a favourable share of
water resources relative to other countries in the
region (Bou-Zeid and El-Fadel, 2002),
temporally
inconsistent
rainfall
patterns
combined with seasonal water shortages and
saline intrusion, increase the danger of water
resource scarcity in the case-study area. Water
resources in Lebanon are particularly vulnerable
to higher temperatures and changes in patterns of
precipitation. Water shortages can constrain
economic growth and present a complex
challenge for future development.
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 overlain with the 10year moving average of the absolute annual maximum 3-day rainfall series (Figure 5).
150.00
130
anomalies
100.00
120
10-year Mov.Avg.
110
50.00
100
0.00
90
80
-50.00
70
60
1950
1952
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
-100.00
Figure 5: Greatest 3-day rainfall anomalies (mm) from the 1971-2000 average (left axis, bars) for the AIB
station. 10-year moving average of total annual rainfall (right axis, solid line) for the AIB station
What does this show?
There is no statistically significant trend for the
greatest 3-day rainfall over the examined period
(Figure 5). This implies that rainfall intensity has
not changed systematically during the record.
The time series is, however, dominated by the
extremely large 1969 value. Since 1977 there
have been no large (> +40mm) positive
anomalies and in the last two decades negative
anomalies are more frequent than positive ones.
6
Why is this important?
Any increase in the incidence and intensity of
extreme rainfall events may provoke natural
disasters such as localised flash flooding in
urban areas. This is particularly true for urban
areas with inadequate and antiquated drainage
systems. 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. Water-related morbidity is an
increasing problem in Beirut (Korfali and Jurdi,
2009); an increase in intense rainfall events
would heighten the contamination risk for urban
drinking water supplies and water-borne disease.
3. Risks of current climate hazards
In general, Beirut is considered to have a hot
microclimate that is most probably caused by the
urban heat island effect combined with some
sheltering from the wind (CAL, 1982; Chabaan,
2007). The average summer (June-August)
maximum temperature at Beirut (AIB) is 29°C
while the 90th/95th percentiles correspond to
30.8°C/31.4°C respectively (for the 1971-2000
period). The all-time maximum temperature
recorded for the metropolitan area of Beirut is
40°C, while the all-time record minimum
temperature is −0.2°C. Summers in Beirut are
rain-free (the mean summer rainfall for the years
1971-2000 is 2.3 mm (± 0.6 mm), while the
mean annual rainfall totals 755 mm and the
mean wet day amount (average rain per rain day)
is 9.8 mm.
In Table 1, linear trends in the examined
climate indicators are summarised for all data
records used in this study. Results from this table
should be used with caution, bearing in mind
that there is concern about the data quality /
homogeneity of all Lebanese stations as
discussed in the Introduction. In the last column
the confidence level is given following the
approach developed for the IPCC 2007 report.
Table 1: Change in the climate indicators (hazards) for Beirut and associated likelihood of occurrence
Climate Indicator (hazard)
Change (per decade)
Region
(or stations)
Time period
Likelihood§
Annual maximum
temperature
increase (+0.03 oC)
AIB station
1950-2001
About as likely
as not
Summer maximum
temperature
increase (+0.12 oC)
AIB station
1950-2001
Virtually certain
Annual total rainfall
No statistically significant
trend
AIB station
1950-2001
About as likely
as not
Greatest 3-day rainfall
No statistically significant
trend
AIB station
1950-2001
About as likely
as not
Hot days (Tx > 31°C)
increase (+2 days)
AIB station
1950-2001
Virtually certain
Hot nights (Tn > 24°C)
increase (+10 nights)
AIB station
1950-2001
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
Greater Beirut is the largest metropolitan area in
Lebanon with around two million inhabitants;
nearly half of the country’s population (Yamout
and El-Fadel, 2005). It is a densely populated
urban area renowned for problems of water
shortage and is particularly vulnerable to climate
change due to an anticipated increase in water
demand
for
domestic
and
industrial
consumption. Sensitivity to climate change is
heightened by poverty, poor access to clean
water and sanitation, overexploitation of
groundwater, saline intrusion and a lack of
environmental monitoring and regulation.
7
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.
The authors would also like to thank the Lebanese Meteorological Service, the Ministry of Environment and
UNDP Lebanon for the meteorological data for Beirut. Special thanks also go to Dr. Elena Xoplaki for fruitful
comments and suggestions when preparing this information sheet.
References
►
Alpert. P., Ben-Gai T., Baharad A., Benjamini Y., Yekutieli D., Colacino M., Diodato L., Ramis C.,
Homar V., Romero R., Michaelides S., Manes A., 2002: The paradoxical increase of Mediterranean
extreme daily rainfall in spite of decrease in total values. Geophysical Research Letters. 29: 311314.
► Bou-Zeid E. and El-Fadel M., 2002: Climate change and water resources in Lebanon and the
Middle East. Journal of Water Resources Planning and Management, 128, 5: 343-355.
► CAL 1982: Atlas climatique du Liban, Tome 1. Service Météorologique, Ministère des Affaires
Publics, Beirut.
Chaaban F.B., 2007: National Capacity Self-Assessment for Global Environment Management,
NCSA Project, Thematic Assessment Climate Change, Beirut, Lebanon, 89 pp.
► El-Zein A., Tewtel-Salem M., Nehme G., 2004: A time-series analysis of mortality and air
temperature in Greater Beirut. Science of the Total Environment, 330: 71-80.
► Giannakopoulos C., LeSager P., Bindi M., Moriondo M., Kostopoulou E., Goodess C., 2009:
Climatic changes and associated impacts in the Mediterranean resulting from a 2 ºC Global
Warming. Global and Planetary Change, 68 (3): 209-224.
► Haylock, M.R., Hofstra N., Klein Tank A.M.G., Klok E.J., Jones P.D., New M. 2008: A European
daily high-resolution gridded dataset of surface temperature and precipitation. Journal of
Geophysical Research (Atmospheres), 113, D20119, doi:10.1029/2008JD10201
► Korfali S.I. and Jurdi M., 2009: Provision of safe domestic water for the promotion and protection
of public health: a case study of the city of Beirut, Lebanon. Environ Geochem Health, 31: 283-295.
► Yamout G. and El-Fadel M., 2005: An optimization approach for multi-sectoral water supply
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►
Authors: M.Hatzaki1 (marhat@phys.uoa.gr)1, C. Giannakopoulos1 (cgiannak@meteo.noa.gr), P.Hadjinicolaou2
(hadjinicolaou@cyi.ac.cy) and E. Kostopoulou1,2 (ekosto@meteo.noa.gr).
1 National Observatory of Athens, Greece
2 Cyprus Institute, Cyprus.
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: August 2010
8