If we keep sucking down Colorado River water the way we have been (likely), and if climate change reduces the amount of water in the system (also likely) there's a fifty-fifty chance that the system's reservoirs will hit bottom by the middle of this century. That's the stark conclusion of a new study released in July by the University of Colorado at Boulder.

Other river researchers say that a 1 degree Celsius increase in average temperature will translate to a 5 percent drop in annual water discharge from the Upper Basin, which produces the vast majority of the system's water. But even small attempts at conservation could have big impacts.  A 6 percent reduction in current demand results in a 37 percent reduction in the risk that the reservoirs will dry up. (For more background on the Colorado River, see our stories "A tug of war on a tightrope", "Arizona returns to the desert", "What's worse than the worst-case scenario? Real life" and "How low will it go?".)

"Water managers are used to engineering solutions," says CU-Boulder scientist Doug Kenney, "but we've hit the limits there. Now we need political solutions and reallocation of water rights." But even that may not be enough, as witnessed by what's happening in Australia's Murray-Darling Basin.