The United Nation's head of climate change issues stressed Wednesday that the earth's warming trend, almost certainly caused by people, can still be eased.

Rajendra Kumar Pachauri, who chairs the Intergovernmental Panel on Climate Change (IPCC), told a small group of journalists that societies have to decide whether to take a path to deterioration or to make moves that will conserve the planet.

"Yes, of course it is difficult," he said on the sidelines of an IPCC gathering, being held this week in Japan for the first time.

"But what is going to be even more difficult, substantially more difficult, is to deal with alternatives."

"In other words, if we don't stabilise the climate of this planet, then these impacts are going to be progressively more serious," he said.

Some 550 scientists and officials from around the world are meeting behind closed doors this week to hammer out a 29-page summary of a massive report, which is expected to detail impacts and ways to reduce risks associated with climate change.

The report will serve as the second volume in the long-awaited Fifth Assessment Report, which is likely to shape policies and climate talks for years to come.

Global warming has resulted in reduced yields of wheat, rice and corn, increasing food security risks as the world's population expands, a draft of the summary says.

The draft, seen by AFP, also spells out the possibility of increased floods, drought, conflict and economic damage if carbon emissions continue unchecked.

"This report has a substantial amount of recent data, looking at the impacts on human health, the impacts on urban as well as rural systems," Pachauri said.

"The cost of inaction is going to be substantially higher and certainly make life much more difficult than the difficulty of taking actions," he said.

The final summary, to be released on Monday, should help improve understanding among decision-makers about the urgent need to take action against, he said.

Pachauri shrugged off suggestions that election cycles push politicians to focus on short-term steps linked to local economies, rather than long-term measures.

"If you inform the public and if you tell them about scientific reality of climate change, I am sure in democracy... you will see action based on knowledge based on informed scientific assessment."

He urged the public not to confuse weather patterns and climate change, as recent winter chills in North America have prompted many to joke about global warming and have fuelled scepticism.

Extreme heat waves and precipitation are on the rise as climate change progresses, bringing heavy rain and snow falls in "upper latitude" regions during winter, Pachauri added.

"The belief that there has been slowing down of warming is really misplaced," he said.

"Yes there has been a lot of extreme events. Heavy snow falls in the US and so on. But we need to distinguish changes in the weather from changes in climate," he said.

Pachauri called for robust policy action now as inertia in the global climate system will continue to heat the earth for "several decades", even if humans began to reduce emissions now.

"Clearly, it's going to be progressively difficult, and it's going to be essential that we start taking the right steps today," he said. "The sooner the better."