In countries around the world divorce rates have been rising, and each time a family dissolves the result is two new households.

"That really has a big impact in terms of the environment," said Jianguo Liu, an ecologist at Michigan State University whose analysis of the environmental impact of divorce appears in this week's online edition of Proceedings of the National Academy of Sciences.

More households means more use of land, water and energy, three critical resources, Liu explained in a telephone interview.

Households with fewer people are simply not as efficient as those with more people sharing, he explained. A household uses the same amount of heat or air conditioning whether there are two or four people living there. A refrigerator used the same power whether there is one person home or several. Two people living apart run two dishwashers, instead of just one.

Liu, who researches the relationship of ecology with social sciences, said people seem surprised by his findings at first, and then consider it simple. "A lot of things become simple after the research is done," he said.

Some extra energy or water use may not sound like a big deal, but it adds up.