Chartres said that 70% to 80% of fresh water the world extracts every year is used for growing food.
"We expect to see 2 billion more people on the planet by 2050. Water requirements for agriculture will also increase dramatically," he said, not least because higher consumption of meat and milk requires more water than cereals.
"We are going to have to come up with ways of making water go much further if we are going to grow 70% more food by 2050 -- on about 10% less water than we use today."
A world away from Sindh, in Marin County, north of San Francisco, David Little is reviving the old technique of "dry farming." In an area where farms must often rely on their own wells, he grows potatoes and squash without irrigation by repeated tilling in the spring and then rolling to lock moisture into the soil rather than allow it to evaporate.
The practice encourages crops to grow deeper roots, and less water means a fuller flavor. His potatoes are favored by upscale restaurants in the area.
Little says that he learns something every year and that his sandy loam is better suited than most soils to the practice.
"Dry farming" is labor intensive, but in the American West, plentiful, cheap water for irrigation is a distant memory.
The open refrigerator
Such initiatives may be all the more important because the underlying causes of more volatile weather cannot be addressed in decades -- maybe even centuries. Climate scientists attribute that to a complex combination of long-term trends that include warming oceans and shrinking Arctic ice cover. This summer, the ice cover is at its lowest ever, declining by about 35,000 square miles a day (an area bigger than South Carolina) in August.
Walter Meier of the National Snow and Ice Data Center at the University of Colorado says that Arctic sea-ice has been shrinking since satellite observations began in 1979.
"Then it dropped off the cliff in 2007, and this year we have surpassed that level. And in addition to the extent of sea-ice, what remains is thinner than it used to be."
Measurements from submarines, drilling and satellites suggest that the ice is about one-third of its thickness 30 years ago, a change Meier describes as "remarkable." Almost all the ice that melted used to be first-year ice," he says; now, multi-year ice 10 to 12 feet thick is breaking up and melting.
The consensus among scientists is that the declining Arctic ice cover is now irreversible and accelerating. Some models predict that the North Pole will be ice-free in the summer within a decade; others project sometime between 2030 and 2050.
"It used to be -- 10 years ago -- that we spoke about 'if' rather than 'when' the Arctic would be ice-free in summer. Now, it's the other way round," Meier said.
It may seem a long way from the fields of Iowa or Ukraine, but what happens in the Arctic doesn't stay there. The region is often called the Northern Hemisphere's refrigerator.
"Sea-ice reflects much of the sun's energy back into space," Meier said. "When it's no longer there, dark ocean water absorbs the energy, which then warms the water and further melts the ice."
It's like leaving the fridge door open. The only way to restrain the process would be to moderate temperature increases, which in turn would depend on lowering carbon dioxide emissions.
Just how that affects weather patterns is only now being explored, but in 2009, the National Snow and Ice Data Center noted that "some numerical simulations indicate that the loss of the sea-ice cover may lead to changes in storm tracks and rainfall patterns over Europe or the American West."
Research this year by Jennifer Francis at the Institute of Marine and Coastal Sciences at Rutgers University shows that enhanced Arctic warming slows atmospheric jet streams, which tends to prolong weather patterns.
In other words, it entrenches drought in some areas.