Climate trends in Israel over a 30 year period
Authors: Hendrik J. Bruins and Hemu Kafle
This summary is based on the following article: Kafle, H. and Bruins, H.J. (2009) Climatic trends in Israel 1970-2002: Warmer and increasing aridity inland. Climatic Change 96:63-77.
Israel is situated in the subtropical drylands of south-western Asia at a very sensitive climatic position. All four dryland zones are present in the country, ranging from the hyper-arid and arid deserts in the south (Negev) and east (Judean desert) to the semi-arid and dry sub-humid zones in the west and north of the country. Israel’s climate shows remarkable variability and change over small distances and the country is very suitable for the study of climate change. Therefore, a systematic study was conducted to investigate climatic trends in Israel for the period 1970–2002, based on a detailed analysis of monthly temperature and annual precipitation data from 39 meteorological stations.
The climatic trend analysis for the above 30 year period includes three different parameters: annual temperature, annual precipitation and the calculated annual aridity index P/PET (P is precipitation; PET is potential evapotranspiration). A drier or wetter climate in bio-climatic or agricultural terms is not just a matter of changes in rainfall (input), but also of possible changes in potential evapotranspiration (output), in which temperature is an important factor. The Thornthwaite method was used to calculate PET, as recommended by the United Nations Environment Program (UNEP) and the UN Convention to Combat Desertification (UNCCD).
The climate in Israel is Mediterranean in character with a dry summer season from May to October. The rainy season and agricultural year begins in September–October and the rainfall ends in April–May. Since the common calendar year, beginning in January, cuts the rainy season in the middle, the climatic analysis used an annual division from September 1st to August 31st.
Temperature trends during the period 1970–2002
All meteorological stations in Israel showed a distinct warming trend for the above period. Eilat along the Red Sea, Beer Sheva in the northern Negev and the Sedom Pans at the southern part of the Dead Sea displayed the most significant trends in temperature increase at the 99.95% confidence level.
Concerning annual variations in the yearly average temperatures, the coolest years at all meteorological stations were 1982–1983 and 1991–1992, whereas 1998–1999 was the warmest year during the period 1970–2002.
Precipitation trends during the period 1970–2002
The study noted an interesting difference between meteorological stations in the coastal plain along the Mediterranean coast, which showed a slight increase in precipitation trends, and stations more inland (both eastward and southward) that showed an overall decreasing rainfall trend.
For example, the average annual precipitation at Kefar Blum (eastern Galilee, upper Jordan valley) declined from ca 530 to 480 mm, according to the linear regression trend line. Likewise at Har Kena’an in the upper Galilee precipitation decreased from ca 695 to 665 mm. The average rainfall at Beer Sheva during the period 1970–2002 dropped from ca 230 to 180 mm, at Arad from ca 140 to 115 mm and at the Sedom Pans south of the Dead Sea from ca 58 to 28 mm, and at Eilat in southernmost hyper-arid part of the Negev desert from ca 35 to 20 mm, according to linear regression. The decline is statistically most significant for the driest parts of the country, for example at the 97.5% confidence level at the Sedom Pans.
Our results suggest that precipitation along the Mediterranean coast tends to show a slight increase, while all studied stations inland (eastward and southward) show a decrease in rainfall.
P/PET aridity index trends during the period 1970–2002
The annual P/PET aridity index analyses showed mixed trends for the meteorological stations in the coastal plain along the Mediterranean coast, as some stations became slightly more humid and others slightly more arid. Concerning the average annual P/PET values, Ein HaHoresh has an index of 0.50, placing it on the boundary between the semi-arid and dry sub-humid zone. Tel Aviv-Sede Dov, located more to the south, is situated in the semi-arid zone, having an average annual P/PET value of 0.40.
However, the meteorological stations in the inland parts of Israel, showed an increase in aridity, i.e. the climate became drier during the period 1970-2002. For example, the P/PET value for Kefar Blum in the upper Jordan valley (eastern Galilee) declined, according to the linear regression trend line, from ca 0.43 to 0.34, both values being in the semi-arid zone. The P/PET value for Har Kena’an (upper Galilee) declined from ca 0.72 to 0.63, indicating that the climate in this area changed from wet sub-humid to dry sub-humid. In Beer Sheva (northern Negev), the P/PET index declined significantly from ca 0.16 to 0.11; both values are within the arid zone. Sede Boqer in the central Negev also became more arid, as the P/PET index declined from ca 0.08 to 0.06, approaching the boundary with the hyper-arid zone (0.05). Eilat in the southern Negev became extremely hyper-arid, as the P/PET index decreased from ca 0.015 to 0.007.
Three meteorological stations in the Negev desert, Eilat, Beer Sheva and Sedom Pans showed statistically the most significant increase in aridity in Israel during the period 1970–2002.
A significant warming trend is evident in all meteorological stations in Israel. Precipitation was found to increase in most stations in the coastal plain along the Mediterranean coast. The more inland stations in Israel showed a decline in precipitation. Though the individual significance values for each station vary, the collective downward trend of the inland meteorological stations gives an undeniable collective weight of significance that should not be underestimated.
The trend analysis of the P/PET index in the Mediterranean coastal plain of Israel showed mixed results. Increasing PET, due to the warming trend, was countered by increasing P (precipitation) in several stations. However, away from the Mediterranean coast, all inland stations in Israel showed both a decrease in precipitation (P) and an increase in PET, resulting in increased aridity.
Except for the Mediterranean coastal plain, the climate has become more arid in Israel during the period 1970–2002.
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