Most scientists agree emissions from coal, oil and gas are causing climate change, while a few argue there is no link.

Asked if natural warming cycles, as the earth has seen in the past, could be behind climate change, the director general of the Spanish National Meteorological Institute was adamant they were not.

"No, because the time scale is different. This phenomenon is happening much more quickly," Francisco Cadarso told Reuters on the sidelines of an international meteorological conference in Madrid.

He also dismissed as over simple other theories some scientists say undermine the idea humans are to blame, such as changes in solar activity, or the theory that carbon dioxide increases are lagging behind temperature rises.

"This is too complex a phenomenon to use one single scientific argument to knock it down," he said.

Other experts at the meeting of the World Meteorological Organisation, a United Nations agency, also said they were sure humans were causing climate change.

"Scientifically it is clear," the head of Brazil's Meteorological Service Antonio Divino Mouro told a news conference.

"We've been measuring carbon dioxide (CO2) concentrations for many years ... and it's proven that these levels are rising and the explanation is that fossil fuels are getting into the atmosphere."

CO2 is not transparent to infra red light, but absorbs it and causes global warming, he said.

"There is no doubt man has done this," said Paulo Manso, head of Costa Rica's meteorological services.

"We are talking about 9 scientists against every 1," he said referring to last month's United Nations climate panel report, which concluded it is 90 percent certain that human activity is behind global warming.