A strange thing happened on the way to dealing with climate change: Advances in hydraulic fracturing put trillions of dollars' worth of previously unreachable oil and natural gas within humanity's grasp. The environmental costs -- and benefits -- from "fracking," which requires blasting huge amounts of water, sand and chemicals deep into underground rock formations, are the subject of new research that synthesizes 165 academic studies and government databases. The survey covers not only greenhouse gas impacts but also fracking's influence on local air pollution, earthquakes and, especially, supplies of clean water.

The authors are seven environmental scientists who underscore the real consequences of policy decisions on people who live near the wells, as well as some important remaining questions.

"Society is certain to extract more gas and oil due to fracking," said Stanford environmental scientist Robert Jackson, who led the new study. "The key is to reduce the environmental costs as much as possible, while making the most of the environmental benefits."

Fracking's consumption of water is rising quickly at a time when much of the United States is suffering from drought, but extracting natural gas with hydraulic fracturing and horizontal drilling compares well with conventional energy sources, the study finds. Fracking requires more water than conventional gas drilling; but when natural gas is used in place of coal or nuclear fuel to generate electricity, it saves water. From mining to generation, coal power consumes more than twice the water per megawatt-hour generated than unconventional gas does.

Unconventional drilling's water demand can be better or worse than alternative energy sources, the study finds. Photovoltaic solar and wind power use almost no water and emit no greenhouse gas, but cheap, abundant natural gas may limit their deployment as new sources of electricity. On the other hand, fracked gas requires less than a hundredth the water of corn ethanol per unit of energy.

Fracking's impact on both climate change and local air pollution is similar to its impact on water, finds the study "The Environmental Costs and Benefits of Fracking," published in the Annual Review of Environment and Resources.