For nine days, from May 13 to May 21, Jerusalem saw an unprecedented string of high daily temperatures above 33º C (91.6º F). The record stood all of two and a half months, until August 29.
That day, the high temperature in the city soared to a scorching 38.2º C. It would be 14 days until the high in Jerusalem would cool to a breezy 32.1º C on September 12, and it would take another 10 days until the high finally dipped below 30º C — barely — topping out at 29.4º C on September 22.
The fact that September 2020 was the hottest month in recorded history in Jerusalem likely comes as no surprise to anyone who sweated through the heatwave that blanketed the city. The unprecedented run included two days, September 3 and 4, during which the temperature hit 42.3º C and 42.7º C respectively, the highest temperatures Jerusalem had seen in over 100 years, bested only by the 44.4º C, reached on two days in August 1881, according to the Israel Meteorological Service.
While the extremeness of the temperatures was terrifying, scientists say that what is more ominous are the larger trends and how long the heat waves are lasting. A look at the daily temperature data compiled by the Israel Meteorological Service over the last 70 years provides a sweat-inducing peek at just how hot Jerusalem is becoming, and how temperatures and heatwaves once regarded as rare have become commonplace.
According to Prof. Yoav Yair, Dean of the School of Sustainability at the Interdisciplinary Center, a Herzliya-based university better known as IDC, a series of days with temperatures of 33º C (91.4º F) or higher is the threshold for a heatwave in Jerusalem.
Days of 33º C or higher are not overly common in the capital, which usually enjoys cooler temperatures than coastal Israel thanks to its elevation some 750 meters (2,460 feet) above sea level. The lack of humidity also helps make the city a refuge from the muggy climate that envelopes Tel Aviv and most of central Israel.
Most years see a few handfuls of days that reach above 33º C, and every few years the city may even see a string of six or seven days, according to an analysis of IMS data. At the same time, before 1990, it was not uncommon for whole years to pass in which the mercury only hit the milestone once or twice, if at all.
Now that has all changed.
According to data from the IMS, from 1950 to 1959 there were a total of 99 days in which the high temperature in Jerusalem hit 33º C or higher.
From 1960 to 1969 the number climbed to 119 days. The next two decades actually saw a cooling trend: From 1970 to 1979 there were only 64 heat wave days, and from 1980 to 1989 there were 98 such days.
And then the heat began to build again. From 1990 to 1999 there were 127 heatwave days. The next decade, Jerusalem experienced temperatures of 33º C or higher 137 times. and from 2010 to 2019 there were a whopping 230 days in which the temperature reached or surpassed the toasty threshold.
As of October 8, this year has seen 40 heatwave days. Assuming that the year is not a major outlier, this upcoming decade would see some 400 heatwave days in Jerusalem.
“None of this is surprising,” said Yair. “Temperatures in Israel have gone up by around 1.4 degrees between 1950 and 2017, with most of the increases happening over the past 30 years.”
And it is only expected to get worse.
The month of September, which supplied 18 of the year’s heatwave days, broke records across Israel, becoming the hottest September on record in Jerusalem and most of the rest of the country, according to a report from the IMS.
There are signs, though, that the heat seen in September was not an every year event. The extreme heat, which was felt throughout the region and in Europe as well, was registered as an anomaly, and in Israel was attributed to hot easterly winds blowing in from the Arabian peninsula, circling around a low-pressure system.
In Jerusalem, highs spiked to above 40º C (104º F) for three days in a row. The event appeared extraordinary, especially given that according to the IMS, temperatures in Jerusalem have only crossed the 40º C threshold seven times since 1950, including those three days.
It’s not unprecedented, however. A report from the service released in mid-September noted that it had happened three times before in Jerusalem’s history since 1860: June 1942, July 1888 and August 1881, when that record 44.4º C temperature was recorded. Those extremes could not be attributed to the global trends of climate change being seen now.
Part of a worldwide phenomenon
But there are many more signs that even if extreme temperatures are the result of local factors, the persistence and frequency of the hot days are part of a larger global trend.
“In recent years we’re seeing temperatures pick up and continuing to increase,” said Prof. Ori Adam, a Hebrew University climate expert. “It’s always hard to differentiate that from local effects. But it’s consistent [with] the global trends.”
A 2017 UN report on climate change challenges in the Arab world predicted there would be 40 to 80 days per year with temperatures between 35º C and 40º C by the end of the century across the region, depending on carbon emissions models.
In Jerusalem, the average high temperature in September was 33.4º C, five degrees above the average between 1995 and 2009. Nationwide, according to the IMS, only three months have ever had higher average temperatures: August 2010, July 2015 and August 2017.
And those numbers might not even tell the whole story. The IMS was forced to base much of its data for September on a secondary weather station that recorded slightly lower temperatures. That’s because on September 4, researchers were unable to reach the main monitor atop a central Jerusalem building to confirm its reading of 42.7º C, due to a work accident, leading the IMS to rely on the secondary monitor on Hebrew University’s Givat Ram campus for that month only. (Most of the data in this report is based on the main monitor’s findings, which the IMS has continued to publish.)
Of the 10 highest temperatures recorded in Jerusalem’s main weather station over the past 70 years, only two predated 2010: 40.6º C on July 30, 2000, and 39.6º C on August 22, 1954.
It’s not only the days. Temperatures in early September were so consistently high in Jerusalem, according to the IMS, that the daily low actually surpassed the average high at one point.
In May, IMS data shows that nights stayed so hot that low temperature remained above the average high for that period for a week straight.
Of the 22 times since 1950 that the low temperature in Jerusalem has remained over 28º C (82º F), only nine of them have come before 2000.
Jerusalem is far from alone in feeling the heat. A report from the European Union’s Copernicus Climate Change Service Wednesday found that the month was the hottest September on record worldwide, helped along by an extreme heatwave that blanketed Siberia for much of the summer.
According to the climate monitor, May and January 2020 were also the hottest on record globally, and every month of the year so far has been in the top four heat-wise.
“The year-to-date global temperature anomaly shows that 2020 is on a par with 2016, the current warmest year on record,” a statement from Copernicus said. “In addition, for the same period, 2020 is warmer than 2019 — the second warmest year currently on record.”
Across Europe and the Middle East, cities have sweltered for days or weeks under record temperatures.
‘Humanity-driven climate disasters’
According to Yair, the IDC expert, Israel — along with the rest of the Middle East and North Africa — is seeing temperatures rise faster than the rest of the world because the band of tropical climate to the north and south of the Equator is expanding.
Research published two years ago shows that it has been doing so by 0.5 degrees of latitude — equal to 55.5 kilometers (34.5 miles) — per decade since 1979.
According to an article published by the Yale School of Environment in October 2018, the tropics include both wet and dry regions. The wet regions are contracting and the drier ones are expanding, bringing ever-drier weather to places such as the Mediterranean.
These shifts, according to the article, are caused by changes such as the opening of the southern hemisphere ozone hole, warming black soot in polluted air from Asia, and rising air — as well as sea surface — temperatures caused by greenhouse gases [from burning fossil fuels].
As a result, between 1930 and 2013, the Sahara Desert grew in size by 10 percent, pushing north and south.
Yair, who researches atmosphere and extreme weather events, said the extreme heating in the region could not be untangled from climate disasters happening around the world, including devastating wildfires in California and Australia this year.
“What we are seeing are humanity-driven climate disasters, not natural disasters, “ he said, blaming politicians for refusing to deploy policies that would help the country catch up to the rest of the world on fighting global warming.
“The human-driven contribution is clear, strong and undeniable,” he said. “And this is only the beginning.”