Information Sheet on Observed Climate Indicators

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 exhibitan increase during the last two decades.

►No statistically significant trendsin total annual and seasonal rainfall have been observed.

Introduction

Greater Beirutcoversa 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 As-Sahil, a coastal plain, borders the two mountains to the east and extends into the north from the mouth of the Nahr Al-Kalb, the DogRiver, and in the south from the entrance of the Nahr Ad-Damur, or the DamurRiver.

Beirut’s Mediterranean climate hashot, humid summers and mild winters. The city benefits from the moderating effects of the sea. In winter, the sea warms up the airmasses which are then blown over the coastal regions, whilein summer cool sea breezes contribute to thecooler 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 below11ºCat 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:

Maximum temperature (Tx) on an annual and seasonal (summer) basis
The number of hot summer days (Tx 31°C)
The number of hot summer nights (Tn 24°C)
Total annual rainfall
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 examinedby comparing themto 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, 1950-2001.

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 forseveral levels of confidence using the Student t-test.Likelihood ranges were used to assess the probability of occurrence, adoptingthe IPCC classification.

Summer maximum daily temperatures anomalies

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 periodduring 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 notto 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 temperaturefor 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).

Why is this important?

This indicator is of prime importancefor 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. Heat-related mortality at moderately hightemperatures 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 semi-arid 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 indicatorof 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 (closeto the 95th percentile).

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 isvirtually certain that the number of hot days has increasedduring the second half of the last century, at a rate of about two days per decade (Figure 2). The largestnumbers of hot days occurmainly from the late 1980s onwards, and an increasing trend is clear from the early1990s onwards.

Why is this important?

Variability and change in maximum temperature extremes can havea considerableimpact 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.

Number of hot nights

What is it?

A threshold of 24°C (about the 95th percentile) for summer minimum temperatures (Tn) wasused to define a hot night.

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 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 islandeffect and some sheltering from the wind (CAL, 1982). This additional urban heat is caused by high absorption of solarradiation by concrete surfaces and roads in addition to anobstruction of wind flowbybuildings. Together thesefactorscontribute to keeping hot air trapped in the city (Chaaban, 2007).

Annual Rainfall

What is it?

Total annual rainfall for the Aéroport International de Beyrouth for the years1950 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.10-year moving average of total annual rainfall (right axis, solid line) for the AIB station

What does this show?

During the second halfof 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 recordedin 1954-1962. The years 1958and 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 trendsare 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 patternscombined 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.

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 10-year moving average of the absolute annual maximum 3-day rainfall series(Figure 5).

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.

Why is this important?

Anyincrease 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 increasingproblem in Beirut (Korfali and Jurdi, 2009); an increase in intense rainfall events wouldheighten the contamination risk forurban drinking water supplies and water-borne disease.

3. Risks of current climate hazards

In general, Beirutis considered to have a hot microclimate that is most probably caused by the urban heat islandeffect combined withsome 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 Beirutis 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 totals755 mm and the mean wet day amount (average rain per rain day) is 9.8 mm.

In Table 1, lineartrends 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 (+0.03oC) / AIB station / 1950-2001 / About as likely as not
Summer maximum temperature / increase (+0.12oC) / 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 twomillion 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.

Acknowledgements

CIRCE (Climate Change and Impact Research: the Mediterranean Environment) is funded by the Commission of the European Union (Contract No 036961 GOCE) This information sheet forms part of the CIRCE deliverable D11.3.2. Metadata can be accessed from

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: 311-314.

►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, 89pp.

►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., LeSagerP., BindiM., MoriondoM., KostopoulouE., GoodessC., 2009: Climatic changes and associated impacts in the Mediterranean resulting from a 2ºC Global Warming. Global and Planetary Change, 68 (3):209-224.

►Haylock, M.R., HofstraN., Klein TankA.M.G., KlokE.J., JonesP.D., NewM. 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 management in the Greater Beirut Area. Water Resources Management, 19 (6):791-812.

Authors:M.Hatzaki1 ()1, C. Giannakopoulos1 (), P.Hadjinicolaou2 () and E. Kostopoulou1,2 ().

1 National Observatory of Athens, Greece

2 Cyprus Institute, Cyprus.

Editors: Maureen Agnew () and Clare Goodess (), Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, UK

Date: August2010