Over the last 4,500 years, the Jerusalem area’s groundwater has depended almost exclusively on the amount of rainfall, the researchers were surprised to find. That may change
Ruth Schuster Sep. 10, 2021
For the last 4,500 years, it turns out, the renewal state of groundwater in the Jerusalem area has been a function of rainfall. It hasn’t been affected by changes in temperature and vegetation, to the surprise of science. The working assumption was that groundwater replenishment would also be correlated to fluctuations in temperature and plant mass, not just precipitation, but it wasn’t.
Yet that long-term hydrological pattern is likely to change with the rising atmospheric carbon dioxide and temperatures associated with climate change, according to a new paper published in Science Advances, “Climate change may soon disrupt groundwater recharge in Jerusalem and other arid regions.”
As climate change intensifies, groundwater replenishment in the semiarid Jerusalem region could decrease by 20 percent compared with the present and the last 4,500 years, according to an analysis of annual precipitation and other data, say Prof. Simone Fatichi of the National University of Singapore, Prof. Nadav Peleg of the University of Lausanne and colleagues.
That 20 percent assumes unchanged average rainfall. However, as rainfall is expected to decrease in Israel, groundwater recharge could fall further – to levels unequaled in the last 4,500 years, Fatichi says.
Once temperature and carbon dioxide increase beyond a certain point – 1.5-2 degrees Celsius and a bit over 450 parts per million for the CO2 (which could happen around 2060 or 2070 as estimated by the current trajectory) – a decline in the relationship between rainfall and aquifer recharge is expected, the team predicts. As Fatichi explains it, the recharge will also become a function of the heat and vegetation mass. Why? Mostly because of heightened evaporation from the soil and plants.
Israel is located in a climatic band stretching from North Africa throughout the Middle East that is expected to experience extreme, possibly irreversible desertification. While storms may become stronger, and the region may even experience increasingly violent “Medicanes,” the region is expected to receive less total precipitation. Asked about extreme storms and flooding, Fatichi and Peleg say their analysis doesn’t address extreme weather events but annual precipitation.
Their model is based on reconstructing the paleoclimate of the Jerusalem region. Jerusalem doesn’t have an aquifer: The nearest one is Yarkon Taninim to the west. Jerusalem’s water supply depends on precipitation and local springs, which come from groundwater – so why study the relation between climate, vegetation and aquifer recharge there of all places? Because paleo-precipitation research under Prof. Efrat Morin of Hebrew University provided a wealth of data on that region, and there’s none on other areas, Peleg notes.
Strange deviation
In any case, the principle of groundwater and aquifer is the same, and the Jerusalem region is an appropriate model for other arid regions – including Israel itself, Fatichi says.
So, in contrast to the scientists’ expectations, they found no change in the relation between the amount of rainfall and groundwater recharge despite changing patterns of temperature and vegetation over the last 4,500 years. Nor has this changed even though the global mean temperature has already increased by about 1.2 degrees Celsius. But we’re close to deviating from the pattern, the researchers say.
“In the paper, we did sensitivity analysis of scenarios of temperature and CO2, which show that as they rise, at some point we will deviate from this linear relationship between precipitation and groundwater replenishment,” Fatichi says.
In other words, on top of whatever hell climate change will wreak on the Middle East and Jerusalem, there may be less groundwater from the springs.
Why, for the period the researchers examined – 4,500 years – was the relationship between rainfall and groundwater recharge a function of rain but not temperature and plant life?
It’s all in the leaves’ pores
The scientists expected, CO2 being equal, that more rain means more vegetation. More vegetation would remove more rainfall, leaving less to trickle down below the soil and replenish the groundwater (again, they’re talking about long-term rain, not extreme events).
But increases in vegetation mass have turned out not to be a significant factor, so far. Plants have been reacting to the increase in atmospheric CO2 and rain by increasing their leaf area, but this has been compensated by greater “efficiency,” the scientists found. In conditions of high CO2 the pores on the plants’ leaves, their stomata, don’t need to open as wide to obtain the CO2 they need for photosynthesis, and they lose less water through these narrowed stomata, Fatichi explains.
But at some point in the future, as temperatures rise more and more, this compensation mechanism doesn’t help anymore and there will be more evaporation from the soil and from plants too, at the expense of groundwater recharging, he says. The plants can narrow their stomata to their hearts’ content but the heat will suck out their water.
So far we haven’t seen that effect because at a global level, the increase in CO2 has by and large done well by plant life, as satellite pictures show. But according to the researchers’ model, that won’t last.
“The key point,” Fatichi concludes, “is that in the past 4,500 years aquifer recharge in the Jerusalem area was controlled by precipitation, and in the future, this is not going to be the case because temperature and vegetation will change the picture, and not for the better.”
It ain’t pretty to begin with. According to the Intergovernmental Panel on Climate Change report for policymakers published in August 2021: “Climate change is already affecting every inhabited region across the globe.” The current worst-case scenario indicates an increase in global mean temperature approaching 5 degrees Celsius (9.7 Fahrenheit) by 2100.
As this panel observes, the change will not be globally uniform; according to some models, the Northern Hemisphere may even undergo significant cooling from 2081 to 2100. Not all agree about that, and in any case it doesn’t apply to the Middle East, according to projections. It’s just going to get hotter and drier.