WASHINGTON - Divorce can be bad for the environment. In countries around the world divorce rates have been rising, and each time a family dissolves the result is two new households.

"A married household actually uses resources more efficiently than a divorced household," 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.

The United States, for example, had 16.5 million households headed by a divorced person in 2005 and just over 60 million households headed by a married person.

Per person, divorced households spent more per person per month for electricity compared with a married household, as multiple people can be watching the same television, listening to the same radio, cooking on the same stove and or eating under the same lights.

That means some $6.9 billion in extra utility costs per year, Liu calculated, plus an added $3.6 billion for water, in addition to other costs such as land use.

And it isn't just the United States.

Liu looked at 11 other countries such as Brazil, Costa Rica, Ecuador, Greece, Mexico and South Africa between 1998 and 2002.

In the 11, if divorced households had combined to have the same average household size as married households, there could have been a million fewer households using energy and water in these countries.

"People have been talking about how to protect the environment and combat climate change, but divorce is an overlooked factor that needs to be considered," Liu said.

Liu stressed that he isn't condemning divorce: "Some people really need to get divorces." But, he added, "one way to be more environmentally friendly is to live with other people and that will reduce the impact."

Don't get smug, though, married folks — savings also apply to people living together and Shaker communities or even hippie communes would have been even more efficient.

So, what prompts someone to figure out the environmental impact of divorce?

Liu was studying the ecology of areas with declining population and noticed that even where the total number of people was less, the number of households was increasing. He wondered why.

There turned out to be several reasons: divorce, demographic shifts such as people remaining single longer and the demise of multigenerational households.

"I was surprised because the divorce rate actually has been up and down for many years in some of the countries ... but we found the proportion of divorced households has increased rapidly across the globe," he said.

So he set out to measure the difference, such as in terms of energy and water, land use and construction materials and is now reporting the results for divorce.

The research was funded by the National Science Foundation, the National Institutes of Health and the Michigan Agricultural Experiment Station.

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