Future US megadroughts set to be the worst in 1,000 years

Southwest and Great Plains expected to see to significantly drier conditions by end of century.

By Alexandra Witze .Nature

Decades-long droughts are likely to ravage the US Southwest and Great Plains within the next century, a study suggests. This drying could be worse than any other in the past 1,000 years, including a 'megadrought' seven centuries ago that helped drive an ancient cvilization to collapse.


The work, published on 12 February in Science Advances, is among the first to rigorously compare the climate record of the deep past with long-term projections of today’s warming climate.

“These future droughts are not only going to be bad compared to what we’ve experienced over the historical period, but also really bad compared to the past millennium,” says Benjamin Cook, a drought researcher at NASA’s Goddard Institute for Space Studies in New York City, who led the work. “It’s going to be a pretty much fundamental shift.”

Much of North America has a long and detailed climate history, thanks to tree rings that preserve records of temperature and rainfall. Many scientists have used these to piece together the story of decades-long droughts, like one that gripped the US Southwest in the thirteenth century and probably contributed to the disappearance of ancient Pueblo peoples. Others have used global climate models to study the region’s future, and found that it may already be transitioning to a fundamentally drier state.

Cook’s team aimed to bridge past and present. The scientists compared 1,000 years of North American climate history with future projections from 17 different climate models — “as many as we could get our hands on that gave us the data we needed”, Cook says.

Drying out

Among other metrics, the researchers looked at a measure known as the Palmer Drought Severity Index, which is an indicator of soil moisture. Some scientists criticize the Palmer index because it can overestimate future drying if it is calculated on the basis of temperature projections alone. To get around this problem, Cook’s team used a different method of calculating the index, one that incorporates humidity and energy from sunlight.

Kevin Anchukaitis, a palaeoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts, says that the revised method gives a much more accurate projection of how dry things will really get. “This is the first convincing demonstration I’ve seen that it is both possible to seamlessly connect past, present and future, and to then be confident that they are on comparable scales,” he says.

Cook’s team modelled conditions through to the end of the twenty-first century, using both a scenario that assumes that greenhouse-gas emissions will continue on current trends — staying relatively high — and a more moderate scenario that would require major emissions cuts. Models run using both scenarios projected significant dryness between the years 2050 and 2099. These droughts would persist for many years and be even worse than the medieval megadroughts, says Cook.

California is currently in the grip of a drought that has lasted for several years, and the Southwest has been in another for more than a decade. “Imagine those droughts lasting 20, 30, maybe even 50 years — that gives you a sense for what the future droughts will look like,” says Cook. “They will be very strongly amplified by anthropogenic warming.” Tens of millions of people live in the regions at risk.

The next step is to work out exactly when this shift to a drier baseline climate might come. Cook and his team want to use models to tease out when the effects of human-driven warming might start to exceed natural climate variability.

Future US megadroughts set to be the worst in 1,000 years

12:46 PM drought, environment, featured, megadrought, US drought

Mad Cow Disease Still Menaces U.K. Blood Supply

Just as the measles outbreak in the U.S. shows what can happen when the public gets too complacent about a disease thought to be safely a thing of the past, so might the British public be letting down its guard too soon regarding an even more devastating illness: mad cow disease's human incarnation.

 donor gives blood in London in 2003. Blood banks in North America turn away potential donors who spent three months or more in the United Kingdom between 1980 and 1996 to minimize the risk of spreading the human form of mad cow disease.

The worry is that the disorder,variant Creutzfeldt-Jakob disease(vCJD), has left an unknown mark on the U.K. blood supply, and that a hidden population of carriers might lead to another wave of cases.

"For all we know, the storm may well be ongoing," noted the British Parliament's Science and Technology Committee in a report last July. They urged more precautions against vCJD in the blood supply.

vCJD, which first appeared in England in 1996, is a brain disorder linked to consuming meat from cattle infected by bovine spongiform encephalopathy (BSE), informally known as mad cow disease.

At first vCJD causes psychiatric symptoms such as depression and anxiety, and neurological symptoms such as difficulty walking. The brain then deteriorates rapidly, and death occurs a little over a year after the onset of symptoms. There is no cure.

New food supply safeguards in the U.K. and elsewhere have dramatically cut down the incidence of mad cow disease, and only a handful of new vCJD cases have emerged over the past decade. But the misshapen proteins, or prions, associated with vCJD likely can be transmitted through blood from an asymptomatic donor, which is what continues to worry public health officials.
 

Silent Carriers

At least three vCJD cases out of 229 worldwide since 1980 are believed to have been contracted via blood transfusion rather than by eating contaminated meat. To minimize risk, blood banks in North America for about 15 years have turned away potential donors who spent three months or more in the United Kingdom between 1980 and 1996.

For obvious reasons, this restriction is impossible to carry out in the United Kingdom, but the risk is still there. A study of appendix samples published last year found that 1 in 2,000 people in the U.K. might be carrying the prions linked with vCJD.

Why, then, has there been no major outbreak of mad cow disease in Great Britain? That's one of many questions that puzzle researchers including  neurologist R. G. Will, founder of theNational CJD Research & Surveillance Unit at the University of Edinburgh. At the rate of 1 in 2,000 carriers, he said, one would expect a "very large outbreak" of vCJD in the U.K., but "that simply hasn't happened." In the past 35 years, Great Britain has seen a total of 177 cases—and just one since 2012.

Nonetheless, "we should be cautious about variant CJD," said Will, "because we can't be sure that we have seen the last of either the primary epidemic or of secondary transmission through blood transfusion."

"100 Percent Fatal"

Will refers to the "primary epidemic"—the one that broke out in 1996—because some researchers believe there could still be people who were infected many years ago who simply haven't come down with symptoms yet.

With an incubation period that could stretch to decades, "it's hard to say this is the end of the outbreak and it's all over," according to Ryan Maddox, an epidemiologist with the U.S.Centers for Disease Control and Prevention(CDC).

If there are people who are carrying the agent of vCJD, "it may be years and years before we know exactly what their fate is going to be," Maddox said. "Hopefully, the fate will be that nothing ever happens. But we just don't know."

In the meantime, those same carriers can be donating blood, potentially adding danger to the supply. Officials have no good way to screen for this.

Creating a reliable way to test blood for the prions linked to vCJD would be a "major achievement," said Will. But "it's been a very, very difficult technical scientific challenge." In response to the Parliament's report from last summer, British government health officials promised in October to "explore the possibility" of carrying out further research on developing such a test.

Any resurgence of the human version of mad cow disease is likely to be relatively small, especially when compared to other blood-borne infections, such as HIV, which affects millions worldwide.

Still, the prospect of a new outbreak is scary considering that, as Maddox notes, once symptoms develop, the disease is 100 percent fatal.

"Even if there are just two cases in a given year, or four cases," he said, "you don't survive it."

Mad Cow Disease Still Menaces U.K. Blood Supply

8:47 AM blood tranfusion, diseases, health, MAD COW, Medicine