Climate change is happening, it's almost entirely man's fault and limiting its impacts will require reducing greenhouse gas emissions to zero this century, the United Nation's Intergovernmental Panel on Climate Change (IPCC) said in a report published Sunday.

"Science has spoken. There is no ambiguity in their message. Leaders must act. Time is not on our side," U.N. Secretary-General Ban Ki-moon said at the report's launch in Copenhagen.

The report is meant to serve as a scientific roadmap for U.N. climate negotiations, which continue next month in Lima, Peru. The meeting will be the last major conference on the issue before a 2015 summit in Paris, where a global agreement on climate action is supposed to be adopted.

Governments can keep climate change in check at manageable costs, but will have to cut greenhouse gas emissions to zero by 2100 to limit the risk of irreversible damage, the U.N. report said.

The biggest hurdle is deciding who should do what, with developed countries calling on China and other major developing countries to take on ambitious targets, and developing countries saying the already developed have a historical responsibility to lead the fight against global warming and to help poorer nations cope with its impacts. The IPCC carefully avoided taking sides on the issue, saying the risks of climate change "are generally greater for disadvantaged people and communities in countries at all levels of development."

The report, which was the fourth and final installment in the IPCC’s climate assessment, summed up 5,000 pages of work by 800 scientists who concluded that global warming was now causing more heat extremes, downpours, acidifying the oceans and raising sea levels.

Failure to reduce greenhouse gas output, produced by the burning of fossil fuels, to zero this century might lock the world on a trajectory with "irreversible" impacts on people and the environment, the report said.

Amid its grim projections, the report also offered hope, saying the tools needed to set the world on a low-emissions path – such as solar and wind energy generators – already exist.

"We have the means to limit climate change," IPCC chairman Rajendra Pachauri said. "All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change."

U.S. Secretary of State John Kerry called the report "another canary in the coalmine."

"The bottom line is that our planet is warming due to human actions, the damage is already visible, and the challenge requires ambitious, decisive and immediate action," Kerry said in a statement. "Those who choose to ignore or dispute the science so clearly laid out in this report do so at great risk for all of us and for our kids and grandkids."

Pointing to solutions, the IPCC said the costs associated with mitigation action, such as shifting energy systems to solar and wind power and other renewable sources, would reduce economic growth only by 0.06 percent annually.

Pachauri of the IPCC said that cost should be measured against the implications of doing nothing, putting "all species that live on this planet" at peril.

Al Jazeera and wire services