* New buildings, cars keep emissions up in a downturn
* May need economic rethink of GDP, CO2 growth-study
By Alister Doyle
OSLO, Oct 7 (Reuters) - Greenhouse gas emissions rise when economies expand but don't fall as quickly when recession strikes, perhaps because people stick with a higher-emitting lifestyle from the boom times, a study showed.
The report in Monday's edition of the journal Nature Climate Change dents many governments' hopes that recession can at least bring the consolation of a sharp contraction in greenhouse gas emissions.
Emissions of carbon dioxide, the main greenhouse gas, rose by an average of 0.73 percent for every 1 percent growth in gross domestic product (GDP) per capita, Richard York of the University of Oregon wrote in his report.
But emissions fell just 0.43 percent for every percent decline in GDP per capita, he added, based on a review of World Bank statistics of more than 150 nations from 1960 to 2008.
"Economic decline ... doesn't lead to as big a decline in emissions as a comparable amount of economic growth leads to growth in emissions," York told Reuters.
He said the difference might be because new infrastructure added during times of economic growth - new homes, roads or factories - is still used during recession. Carbon dioxide is mainly emitted by burning fossil fuels.
"When economies decline, factories don't shut down immediately, people don't stop driving (although they may defer buying a new car)," he wrote in a email. And many new buildings still needed heating or air-conditioning.
Even since 1990, when many developed nations started trying to curb their greenhouse gases under a U.N. treaty, emissions had also fallen less in recession than they rose when the economy grew, he said.
York said economists may have to re-think how they project future growth of carbon dioxide. Most studies assume that GDP and emissions move in lockstep, both up and down.
The findings "don't necessarily suggest future emissions will generally be higher or lower than current projections, but they suggest that this will depend more sensitively on how exactly economies grow (or shrink)," he said.
"It doesn't only matter how big GDP is in the future, but also how it gets there, such as by slow steady growth, or by periods of rapid growth mixed with recession," he said.
Almost 200 nations are aiming to agree a new global pact to combat climate change by 2015 that would enter into force in 2020. World leaders failed to agree a new deal at the last attempt, at the Copenhagen summit in 2009.
The U.N. panel of climate scientists says the build-up of greenhouse gases is raising temperatures and will bring more floods, droughts, heatwaves, dust storms and rising sea levels.
Its scenarios foresee the world economy expanding steadily to anywhere from $235 trillion to $550 trillion by 2100, from $21 trillion in 1990. That would raise world temperatures by between 1.1 and 6.4 degrees Celsius (2.0-11.5F), it says.
The world economy is expected to grow by 3.5 percent in 2012, including a 0.3 percent contraction in the euro zone, according to the International Monetary Fund. (Reporting by Alister Doyle; Editing by Mark Potter)