Where is the water that Europe and聽China are missing from rivers, reservoirs and underground this year? It must be somewhere.
Hillary Shaw
Newport, Shropshire, UK
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This water is elsewhere on Earth. A dried-out reservoir is striking, locally, but not in terms of the whole planet.
Just 2.5 per cent of Earth鈥檚 water is fresh. The rest is in the oceans. About 99 per cent of that fresh water is groundwater or in glaciers and ice caps, such as those over Antarctica.
Even of the remaining 1 per cent of fresh water, less than a quarter is in lakes and rivers 鈥� most is permafrost. So, for every 16 tonnes of water on Earth, just 1 kilogram is in lakes and rivers.
And the biggest reserves of river and lake fresh water are outside Europe and China, in the Great Lakes and the Mississippi, Amazon, Nile and Congo rivers, plus the large Siberian rivers (the Ob, Yenisey and Lena).
So even if half of Europe鈥檚 lake and river water were missing, the rest of the world wouldn鈥檛 notice the excess, any more than we see a deficit when one country experiences bad floods.
Guy Cox
Sydney, Australia
There does seem to be a certain irony that this question appeared in the same issue (3 September) that reported floods in Pakistan, with huge loss of life and homes. South-east Australia has also had two series of catastrophic floods this year. My home still isn鈥檛 fully habitable. Yes, the water does go elsewhere.
Stephen Johnson
Eugene, Oregon, US
The water missing from rivers, reservoirs and aquifers in Europe and China has been distributed around the world, primarily by the atmosphere.
At any time, the atmosphere contains approximately 12,900 cubic kilometres of water. With changing weather patterns, atmospheric moisture is falling as rain and snow in different places and at different rates than previously.
In 2022, this has given us droughts in China and the west of North America, and flooding in the southern US.
As well as this, many rivers are fed by glaciers and snowpacks that are diminishing or vanishing due to warmer temperatures.
Additionally, aquifers are diminishing because usage rates often exceed the recharge rate. Aquifers can recharge as slowly as 130 centimetres a century, but can be pumped out at rates of metres per year.
Water being pumped out of the ground in California today is often as much as 20,000 years old.
Graham Smith
Melbourne, Australia
The total amount of water in the world is unchanged, it is the distribution of the water that has changed. While China and Europe have been in drought, Pakistan and Australia have had record floods.
Most rain originates as evaporation from the sea, and this evaporation increases strongly with higher air or sea temperatures. The evaporated water is dropped as rain when the air cools below its saturation point (the maximum amount of water it can hold at a given temperature). For example, a cubic kilometre of saturated air 鈥� a small cloud 鈥� at 20掳C (68掳F) holds 18,000 tonnes of water. If the temperature drops to 10掳C (50掳F), then it can only hold 9000 tonnes, so the other 9000 tonnes have to fall as rain.
A 2掳C rise in air temperature increases the amount of water in saturated air by about 10 per cent, meaning that it can hold more moisture before it drops its rain. As a first approximation, this might be expected to result in a 10 per cent global increase in rain, which would be considered beneficial in many parts of the world, but climate models also predict changes to wind patterns, which will change rainfall patterns.
It is therefore not surprising that extreme 鈥渙nce in 500 years鈥� floods and heatwaves are occurring somewhere in the world more often.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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