Global warming to increase intensity of drought in the 21st century.

The consequences of global warming that will do the most harm to humans in the coming decades? Sea level rise of 3 feet or so? It will force millions of people to higher ground. Hundreds of Trillions of dollars in real estate swallowed by the sea? No, not even close. I believe that the most horrific consequence is extended drought over large sections of the Southwestern US, Eastern Europe, Middle East, China, and Australia. Drought threatening the lives of millions will spread across half the land surface of the Earth in the coming century because of global warming. Extreme drought, in which agriculture is impossible, will affect about a third of the planet. It is one of the most dire predictions so far of the potential effects of rising temperatures around the world, and yet it may be under-estimated. 
The PDSI [Palmer Drought Severity Index] 
 James Hansen’s New York Times Op-Ed piece states that “Over the next several decades, the Western United States and the semi-arid region from North Dakota to Texas will develop semi-permanent drought, with rain, when it does come, occurring in extreme events with heavy flooding. Economic losses would be incalculable. More and more of the Midwest would be a dust bowl. California’s Central Valley could no longer be irrigated. Food prices would rise to unprecedented levels.” 

A basic prediction is that many parts of the world will experience longer and deeper droughts, due to the effect of global warming. Precipitation patterns are expected to shift. Warming causes greater evaporation and once the ground is dry, the Sun’s energy goes into baking the soil, leading to a further increase in air temperature. That is why, for instance, so many temperature records were set for the United States in the 1930s Dust Bowl; and why, in 2011, drought-stricken Texas saw the hottest summer ever recorded in the state. Many regions will see earlier snow melt, and less water will be stored on mountain tops for the summer dry season. Added to natural climatic variation, these factors will intensify droughts. Overall trends are clear. This processes of ‘desertification’  or prolonged drought will strike around the globe. This would be a drastic change in climate that will threaten food security and may be irreversible over centuries. "This is genuinely terrifying," said Andrew Pendleton of Christian Aid. "It is a death sentence for many millions of people. It will mean migration off the land at levels we have not seen before, and at levels poor countries cannot cope with." 
"Climate change largely irreversible for 1000 years, with a permanent dust bowl in America's Southwest" is the conclusion reached from research led by NOAA scientists finds that “Irreversible climate change because of carbon dioxide emissions”  will be irreversible for 1,000 years after emissions stop…. Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 400 parts per million (ppm) to a peak of 450-600 ppm over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the ”dust bowl” era.
One of Britain's leading experts on the effects of climate change on the developing countries, Andrew Simms from the New Economics Foundation, said: "There's almost no aspect of life in the developing countries that these predictions don't undermine - the ability to grow food, the ability to have a safe sanitation system, the availability of water. For hundreds of millions of people for whom getting through the day is already a struggle, this is going to push them over the precipice."  A measure of drought known as the Palmer Drought Severity Index (PDSI) is likely to increase globally during the coming century with predicted changes in rainfall and heat around the world because of climate change. It shows the PDSI figure for moderate drought, currently at 25 per cent of the Earth's surface, rising to 50 per cent by 2100, the figure for severe drought, currently at about 8 per cent, rising to 40 cent, and the figure for extreme drought, currently 3 per cent, rising to 30 per cent.  it is the figure for the increase in extreme drought that some observers find most frightening. "We're talking about 30 per cent of the world's land surface becoming essentially uninhabitable in terms of agricultural production in the space of a few decades," Mark Lynas, the author of High Tide, the first major account of the visible effects of global warming around the world, said. "These are parts of the world where hundreds of millions of people will no longer be able to feed themselves." A key drought conclusion is "serious hydrological changes and impacts known to have occurred in both historic and prehistoric times over North America reflect large-scale changes in the climate system that can develop in a matter of years and, in the case of the more severe past megadroughts, persist for decades. Such hydrological changes fit the definition of abrupt change because they occur faster than the time scales needed for human and natural systems to adapt, leading to substantial disruptions in those systems. In the Southwest, for example, the models project a permanent drying by the mid-21st century that reaches the level of aridity seen in historical droughts, and a quarter of the projections may reach this level of aridity much earlier."

This is in addition to the threat of global sea level rise. To quote the USGS’s Climate Change Science Program's sea level rise conclusion, “based on an assessment of the published scientific literature, recent rapid changes at the edges of the Greenland and West Antarctic ice sheets show acceleration of flow and thinning, with the velocity of some glaciers increasing more than twofold. Glacier accelerations causing this imbalance have been related to enhanced surface meltwater production penetrating to the bed to lubricate glacier motion, and to ice-shelf removal, ice-front retreat, and glacier ungrounding that reduce resistance to flow. The present generation of models does not capture these processes. It is unclear whether this imbalance is a short-term natural adjustment or a response to recent climate change, but processes causing accelerations are enabled by warming, so these adjustments will very likely become more frequent in a warmer climate. The regions likely to experience future rapid changes in ice volume are those where ice is grounded well below sea level such as the West Antarctic Ice Sheet or large glaciers in Greenland like the Jakobshavn Isbrae that flow into the sea through a deep channel reaching far inland. Inclusion of these processes in models will likely lead to sea-level projections for the end of the 21st century that substantially exceed the projections presented in the IPCC AR4 report".