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."
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.
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"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.