Here are some questions and answers about the new study predicting the worst drought in 1,000 years in the Southwest by the end of the 21st century.

The questions were put to scientists working on the study from reporters attending the American Association for the Advancement of Science convention in San Jose on Thursday. The study was written by researchers from Columbia and Cornell universities.

Q: At what point does something go from being a megdrought to just a new climactic environment? How can you tell the difference?

Toby Ault, assistant professor, Cornell University: I don’t know that we have a good answer, that we will be able to tell, at least not with the tools we have at our disposal.

"I’m not sure that distinction is extremely critical  . . . If we're talking about being able to cope with water shortages . . . and making the Southwest more resilient region in how it uses water from one generation to the next.

Q. Is this end of the century drought going to be continuous and never-ending, or will there be dotted points of normal or even abundant rainfall? After 35 years, will there be a 5 to 10 year period where the water will be what we think of as normal?

Ault: One thing that does come out of the projections is more extreme rainfall. One to two well placed El Nino years can make the difference  between a decade being really severely dry versus getting some relief. I don’t think I can give a really great answer on how we will know the difference.

Q. What is a megadrought?

Benjamin Cook, of NASA and Columbia University's Lamont Doherty Earth Observatory: "We're really talking about mostly the duration. Imagine a drought such as the current drought in California, which is due largely to natural variability in the past two to three years, Imagine that going on for decades. That’s a megadrought.

Ault: An example is Tucson. I went to grad school there. They've been at 80 percent of their expected rainfall since the late 1990s. At that level or lower for another two decades, it would qualify as a megadrought. What we use for our threshold is anything seen during the 20th century such as the Dust Bowl or the 1950s drought lasting 35 years or longer.

A. Marcia McNutt, editor in chief, Science Magazine, whose online journal Science Advances published the new drought paper: If you imagine that water in the Southwest is overallocated, and that if you're talking about then having rainfall decreased by 20, 25 or 30 percent of normal in a situation where existing water supplies are already overallocated -- now you can see the seriousness of the situation over the long term.

Q. Is what California is experiencing potentially a preview of what may be to come?

Cook: All evidence is that the California drought is primarily caused by natural variability. There's some discussion about whether climate change has started to affect that. That's outside the scope of our particular study.

In our study, (the area) where we saw the most severe drying does include California, that drying over California is predominantly driven by an increase in temp and an increase in evapotranspiration. Expect longer and more severe droughts on level of what we're seeing now.

Q. Is there currently a place in world in the grips of a megadrought?

Ault: I'd say the benmark for the megadrought . . .  is the Sahel drought in Africa. It's very castrophic and continues to be felt today.

Q. Is there anything about this to be optimistic about?

Ault: I think one tiny piece of optimism you can find, at least in nearer term . . . is that the records we have on past megadroughts are based mainly on tree ring estimates. That means it wasn't so bad as to kill off all the trees.

I’m optimistic that we can cope with the threat of megadrought. It doesn’t mean no water. It means significantly less water than in the 20th century.

Q. Your paper talks bout the Great Plains, the world's breadbasket. Did you consider the impacts on the world food supply?

Ault: Water security is food security. If we can manage water security, we can manage future food security in many parts of the world.