* Roads long enough to encircle Earth 600 times to be built
* Haphazard planning needs to stop - study
* Roads can help farms and protect environment
By Alister Doyle, Environment Correspondent
OSLO, Aug 27 (Reuters) - New roads long enough to girdle the Earth 600 times are expected to be built by 2050 and better planning is needed to protect the environment while also raising food production, a study showed on Wednesday.
The study in the journal Nature showed that roads can aid farmers, especially in developing nations where food production is held back by a lack of access to markets or to fertilisers and other technologies.
But too often, new roads slice through remaining wildernesses in the Amazon, New Guinea, Siberia or the Congo Basin, which are home to valuable species of animals and plants and help to slow climate change by storing greenhouse gases.
"While new roads can promote social and economic development, they can also open a Pandora's box of environmental problems," a team of scientists from Australia, Malaysia, the United States, Britain and Costa Rica wrote in the study.
The report's maps showed that 12 percent of the world land area could benefit from roads to help raise farm output with little environmental damage, such as areas of India, central Europe and Asia, North America and the Sahel in Africa.
New roads likely to be built by 2050 would total 250 million kms (155 million miles), a 60 percent gain from 2010 and long enough to encircle the planet more than 600 times, it said.
Lead author William Laurance, of James Cook University in Australia, told Reuters the maps were "just a starting point" for a wider debate about the economic impact of roads.
Many other local factors, such as keeping costs down by taking the shortest route, usually determine routes. Shorter roads also mean less pollution by vehicles.
"What we're attempting is to put road-building into a wider context," Laurance said. "There are a lot of local factors that will come into actual road planning -- but one of the things that's been ignored so far is the 'big picture'."
Many other studies have shown that roads boost growth. A World Bank report this year about an expansion of the road network in Brazil since the 1960s found that "roads are shown to account for half of per capita GDP growth".
The maps tried to value the animals and plants in each region and gauge the amount of carbon stored in vegetation, a natural buffer against climate change. They also created an index to value agricultural production.
Stephen Perz, an expert at the University of Florida, said the quality of data available for creating such maps varied a lot from country to country but that the study could help "a broader effort to improve such maps".
"Governments routinely plan roads without adequate consultation with local people and construction often goes ahead with insufficient attention to minimizing the environmental effects," he wrote in a comment in Nature.
For study: dx.doi.org/10.1038/nature13717
For maps: www.global-roadmap.org/ (Editing by Gareth Jones)