LOS ANGELES—Desalination is among the most energy-intensive processes for treating water and could cause increases in greenhouse gas emissions, a report from the Oakland, Calif.-based Pacific Institute said.

Desalination consumes more energy per gallon than most other water supply and treatment options and exposes plant operators to volatility in energy prices, according to the report, Key Issues for Seawater Desalination in California: Energy and Greenhouse Gas Emissions.

Were all the desalination projects proposed in California to be built, the resulting increase in greenhouse gas emissions would be 0.2 percent or about one-fifth the planned reductions for the water sector...