Mega-droughts caused by long-term climate cycles devastated the Levant 120,000 and 10,000 years ago, research shows. Now scientists predict it’s going to happen again
By Ruth Schuster Jan 21, 2018

Around 120,000 years ago, droughts began to strike the eastern Mediterranean. They would last for thousands of years. Storms from the tropics would briefly flood the land, but they were rare and the water would not penetrate the baked, hardened earth. The same happened 10,000 years ago. Now scientists suspect severe and frequent droughts, driven chiefly by anthropogenic climate change, are going to happen again. This time, they warn, the droughts will end only when humans find a way to reverse climate change.

Humans are changing the climate in ways we’re just beginning to understand, which makes prediction somewhat challenging. We still don’t fully know what we’re causing, let alone how to cure it. Even so, scientists argue that rising greenhouse gas concentrations in the atmosphere will mean progressively less rain on the Levantine plain throughout the 21st century and beyond, explain professors Yochanan Kushnir of the Lamont-Doherty Earth Observatory at Columbia University and Mordecai Stein of the Hebrew University of Jerusalem.

“This would go on as long as the trend of ever more greenhouse gases in the atmosphere continues,” Kushnir warns. “In prehistory, the reprieve from droughts came from the changing orbital position of the Earth around the Sun. Now humanity is responsible, and humanity has yet to find a way to alleviate the situation.”

The secrets of salt

The marks left by prehistoric mega-droughts in the Levant, worse than anything in the recorded history of civilization, were found around and beneath the Dead Sea, which straddles a crack in the earth between Israel and Jordan.

Millions of years ago, the Mediterranean Sea, the Sea of Galilee in Israel’s north and the Dead Sea in the south were all connected, allowing seawater into the Dead Sea basin. One result was a thick deposition of sequences of salts, kilometers in depth. This seawater ingression is called the Sedom Lagoon.

When water levels fell and the Sedom Lagoon became disconnected from the open sea, a series of lakes filled the tectonic depressions along the Dead Sea Rift Valley. During glacial periods, the lakes expanded and rose; in the integlacial periods, they receded and shrank.

During the last glacial period, around 25,000 years ago, the Dead Sea rose as high as around 160 meters below sea level (It now measures 432 meters below sea level). It was huge, stretching from the Sea of Galilee to the Arava Valley in the south.

Dr. Yael Kiro of Columbia and her colleagues found evidence of historic Dead Sea levels as high as 160 meters and low as 500 meters below sea level. They also found the marks of the prehistoric regional mega-droughts in the sediment layers forming the cliffs surrounding the Dead Sea, and in salt deposited in cores extracted from the lake bottom.

Wet times are characterized by thin layers of silt and aragonite precipitation (more water means lower salt concentration, so it stays dissolved in the water). Drought is characterized by thick layers of salt precipitation.

Kiro found massive, sudden precipitation of salt onto the Dead Sea floor between 120,000 to 117,000 years ago and again, starting around 10,000 years ago. She interprets this as a period of droughts – bad droughts that were worse than anything we have known.

These days the level of the Dead Sea is falling by more than a meter a year, though not because of drought. It’s falling so quickly because the people living in its watershed – Israelis, Palestinians, Jordanians and Syrians – are overexploiting the freshwater sources feeding the lake, mainly the Jordan River and its tributaries.

The modern contraction of the Dead Sea, therefore, is mostly man-made. But in a warmer world, the east Mediterranean and Dead Sea watershed region are expected to get drier.

How bad could future drought in the Mediterranean be? How fast might it develop? After checking historic precedent, the next step is to input predicted data (higher temperature, less rain and so on.) into a lake model that successfully “predicts” these terrible past droughts. Stein and Kushnir say they are in the process of doing just that. But meanwhile, they explain to Haaretz why the Dead Sea is a model for the whole region, and why it is cause for acute worry.

When models fall silent

If we are to extrapolate to the future based on the past, we should start by how the conditions today are like the past conditions that led to the mega-droughts.

Now, like 120,000 years ago, when the Dead Sea dried out, the Earth’s orbit was such that the northern hemisphere was nearest to the sun in winter and farthest in the summer. Summers were relatively cool while winters were relatively warm. Also, the level of atmospheric carbon dioxide was quite high, says Kushnir: around 280 to 290 parts per million.

Under these conditions, the Mediterranean region reached an extremely dry state.

We now have similar conditions, but the level of carbon dioxide has passed 400 ppm, which is uncharted territory.

We also know that the Earth has experienced five ice age cycles, the first of which occurred 460 million years ago. The fifth and last ice ages cycle – the one on which all those cartoons featuring Manny the Mammoth are based – is termed the Quaternary Ice Age, and it began in the northern hemisphere around 2.5 million years ago.

Every 100,000 years or so, between periods of ice expansion, the planet was apparently ice-free. Today’s polar ice and Greenland ice sheet are shrunken relics from this last ice age (spanning roughly from 70,000 to 14,000 years ago) and Earth seems to be barreling like an ironic Titanic towards another ice-free era.

Seems to be? If we’re so smart, why don’t we know more? Stein and Kushnir say that one difficulty in making predictions, aside from the inestimable complexity of the systems involved, is the absence of a precedent.

Previous climate changes through Earth’s last four-plus billion years were leisurely affairs, unfolding over thousands or millions of years. This time it’s happening within decades, because of unnatural intervention in the planet’s chemistry (the sudden, vast greenhouse gas emissions).

Fluctuations by Trump

Climate models are tested for their ability to simulate the observed changes in the climate from the mid-19th century to the present, and then used to project the future based on estimated industrial activity.

Critics like U.S. President Donald Trump rebut that Earth’s climate has changed dramatically before and will again, and what we’re seeing are mere “natural fluctuations.”

The Earth indeed experiences climate fluctuations – short-term ones like the seasons, and very long-term swings like Milankovitch cycles (changes in the Earth orbit around the sun). Short-term cycles are the reason the Levant isn’t getting its usual rain this winter. Long-term cycles explain why, from 8,000 years ago to 5,000 years ago, the Sahara was lush, sporting mighty rivers and huge lakes and frolicking crocodiles, and then dried out again.

“The reason for the Saharan desertification is changes in orbital forcing,” Kushnir explains, referring to the Earth’s position in its orbit. “Earth moved from being closest to the sun (perihelion) in summer and furthest (aphelion) in winter 10,000 years ago, to being closest in winter and furthest in summer today. The northern hemisphere became gradually cooler because of changes in the perihelion and aphelion position during the year. As the northern summer cooled, so did the Sahara and its ability to draw moisture from oceans diminished and it dried out.”

The processes the Sahara underwent can also be seen in the geology of the Dead Sea: Drought can be deduced from bands of yellow sand and dust blown in by wind and storms coming from the Sahara. Wet periods feature thicker bands of dark sediment laid down by winds and storms coming from Europe.

Now adding to normal short-term and long-term cycles is anthropogenic climate change.

Because of climate change, scientists have been projecting that Mediterranean rainfall could decline by as much as 20 percent by the year 2100. Based on the discoveries of the past droughts and the conditions when they happened, Kushnir and Stein suspect that this assessment is conservative.

How bad will it get? Science has been trying to estimate this through an international collaboration called the Climate Model Intercomparison Project, which started in the mid-1990s. It is not trivial to reach concrete conclusions about future regional drying, Stein and Kushnir say.

Mega-droughts of yore were apparently chiefly a function of long-term cycles. Now the concurrence of circumstances that led to such a drought 120,000 ago is recurring because of climate change. It’s already begun, they stress: Desertification in the Middle East is advancing fast. “We are well on our way in our drying tendency,” Kushnir says. “It is a gradual drying trend that gets exacerbated by occasional natural dry spells, like the one that occurred in Syria from 2007 to 2010.”

He also notes that the Mediterranean region has been warming faster than the rest of the world since the 1990s.

“By now, the Mediterranean is half a degree Celsius warmer than the globe. As the globe continues to warm, the Mediterranean could outpace the planetary warming by increasing amounts,” Kushnir tells Haaretz. “A recent study concerned with extreme temperatures and looking separately at night and day has estimates that if the entire globe warms by 1.5 degrees, daytime maximum temperatures in the Mediterranean (that is, the temperature at around 2-3 P.M.) will increase by 2.2 degrees on average compared to pre-industrial temperatures.

The driest place on Earth

Israel sits at the northern edge of the Saharan-Arabian desert belt. While Israel’s north is Mediterranean in weather – rainy in winter and dry in summer – the south is a desert, with less than 20 centimeters of rain a year. That’s bountiful compared with the southern mountains by the Red Sea, the driest place on Earth: It has zero humidity on the surface and gets rainfall short of half an inch a year, says Stein. And that could be nothing compared with future aridity.

Israel gets rainfall chiefly from storms sweeping in from the west, when cold air from Europe slams into the Mediterranean, extracting water vapor from the sea. As global warming advances, these storms are likely to become rarer, though when they do arrive, they might be more intense.

Drought in the Middle East is already intensifying because global warming is causing cyclonic activity to move toward the North Pole and away from the Mediterranean, Stein explains. In fact, as storms move north, so are the temperate zones at the mid-latitudes, from Spain to Turkey to Morocco, and the whole Saharan desert belt is spreading north. Vast regions will get warmer and experience fewer storms in winter.

There is precedent to this scenario. Back 120,000 years ago and 10,000 years, storms from Europe all but halted, the Dead Sea cores show. Sediment and sand that reach the Dead Sea by water or wind stay forever in the lowest place on Earth. The geology of the Dead Sea is like a tape recorder for the region because its watershed is rivers originating in Israel, Syria, Lebanon and the Palestinian territories from north and floods in the Sinai from south, Stein explains. That is how “reading” the layers of sand and sediment led the teams to realize that there had been the mother of all droughts 120,000 and 10,000 years ago.

Unless action is taken to reduce greenhouse gases in the atmosphere, local summer temperatures may become unbearable. Israel can continue to desalinate seawater for its hydration needs and hope for the best. Meanwhile the Dead Sea will continue to shrink from its peak 25,000 years ago, when it was 250 meters higher than it is today. Oh well. Hardly any water reaches it any more, which brought about the idea of building a canal to reconnect the Dead Sea with the Red Sea. This article will not dwell on the merits of this lunacy.
Stein in the southern Jordanian desert, already one of the driest places on earth. Courtesy Lamont-Doherty Earth Ob

As an aside, the development of sinkholes, thousands of them, around the Dead Sea in the last 30 years isn’t mainly because of drought but because the lake’s sources are being commandeered by humans. Not all geologists were rocked to their salty cores when the ground around the lake began to collapse, forming sinkholes that have devastated Dead Sea tourism, not least by swallowing a gas station. And this is before climate change does its worst.

https://www.haaretz.com/israel-news/.premium.MAGAZINE-endless-drought-predicted-for-the-mediterranean-in-a-warmer-world-1.5747182