The cost of fighting climate change will only climb if industrialized nations don't take steps to reduce greenhouse gas emissions, the United Nations' panel on the matter warned Sunday in its wrap-up report.

In its "synthesis report," the Intergovernmental Panel on Climate Change said that the hundreds of authors involved in the study were even more certain than before that the planet is warming and humans are the cause.

"If left unchecked, climate change will increase the likelihood of severe, pervasive and irreversible impacts for people and ecosystems," the report said.

U.N. Secretary-General Ban Ki-moon told reporters that action must come soon.

"Leaders must act. Time is not on our side," he said.

The report said there are solutions to keeping the rise in temperatures from crossing a 2-degree Celsius increase, the goal of many governments.

"It is technically feasible to transition to a low-carbon economy," said Youba Sokona, the co-chairman of IPCC Working Group III. "But what is lacking are appropriate policies and institutions. The longer we wait to take action, the more it will cost to adapt and mitigate climate change."

Previously the group has said that about half of the carbon dioxide released into the atmosphere since the dawn of the industrial age has been produced since 1990. On the current path, global average temperatures could go up anywhere from 3.7 to 4.8 degrees C (6.7 to 8.6 F) over pre-industrial levels by 2100.

According to the IPCC, to stay below a 2-degree C increase, greenhouse gas emissions need to fall as much as 70% around the world by 2050 and to zero by 2100.

U.S. Secretary of State John Kerry said the report is "another canary in the coal mine" and added that "ambitious, decisive and immediate action" is needed.

"We have that opportunity, and the choice is in our hands," R. K. Pachauri, chairman of the group, said in the report.

(CNN)