By Alister Doyle
OSLO (Reuters) - Six candidates are vying to become head of the U.N.'s top authority on climate change science this week, seeking to narrow down uncertainties about future warming to guide a trillion-dollar shift to greener energies.
Top scientists - all men - from Austria, Belgium, Sierra Leone, South Korea, Switzerland and the United States will seek to become chair of the Intergovernmental Panel on Climate Change (IPCC) in a vote due on Tuesday at an IPCC meeting in Croatia.
Governments have to pick a successor to Rajendra Pachauri of India, who quit the Nobel Peace Prize winning panel in February, after 13 years, when a female researcher in India accused him of sexual harassment, an allegation he denies.
The outcome of the vote is hard to predict, especially after Sierra Leone's Ogunlade Davidson, a former IPCC vice-chair, joined the race in recent weeks, scientists and officials said.
He could be well placed, if developing countries back him in the one-nation, one-vote election. "He's thrown a wild card into the race," one scientist said, asking not to be named because of the sensitivity of the vote.
Rivals include Chris Field of Stanford University and expert in the impacts of warming, who would be an eloquent advocate of climate science for Americans if a Republican who doubts that warming is man-made succeeds President Barack Obama.
Jean-Pascal van Ypersele, an IPCC vice-chair from Belgium known for an ability to build consensus, was the first candidate to enter the ring in early 2014.
And there are still many uncertainties about warming. Governments will meet in Paris from Nov. 30-Dec. 11 to agree ways to curb greenhouse gas emissions.
The IPCC reckons that a doubling of the concentration of carbon dioxide in the atmosphere would drive temperatures up by between 1.5 and 4.5 degrees Celsius (2.7-8.1 Fahrenheit) - the lower end is manageable, the upper could be disastrous.
"It would help everyone if we knew better where the real number is," van Ypersele told Reuters of the IPCC's future. The IPCC says warming is causing more downpours, heatwaves, floods, mudslides, desertification and rising sea levels.
Others candidates are Thomas Stocker of Switzerland, who helped lead the last IPCC report, Nebojsa Nakicenovic, an IPCC veteran nominated by Austria and Montenegro, and Hoesung Lee of South Korea, another IPCC vice-chair.
The post lasts 6-8 years to oversee a new report on climate change after a series in 2013-14 concluded it was at least 95 percent probable that warming has a mainly human cause.
(Reporting By Alister Doyle; Editing by Andrew Heavens)