A landmark report released Sunday from the world’s top climate change group said “rapid, far-reaching and unprecedented changes in all aspects of society” are required to ward off the worst impacts of global warming.

The report, released in South Korea by the United Nations’ Intergovernmental Panel on Climate Change (IPCC), said that the world’s economies must quickly reduce fossil fuel use while at the same time dramatically increasing use of clean, efficient energy. These transitions must start now and be well underway in the next 20 years.

The goal is to cap global warming at 1.5 Celsius (2.7 Fahrenheit) above pre-industrial levels, which the group said may prove nearly impossible unless swift action is taken.

Limiting warming to that level would significantly reduce the risk of climate impacts such as water scarcity, flood and drought, extreme heat, tropical cyclones, biodiversity loss, and sea level rise, the report stated.

Global warming, aka climate change, is caused by the burning of fossil fuels such as oil, gas and coal that release greenhouse gases like carbon dioxide, which has warmed the planet to levels that cannot be explained by natural causes.

This report shows the longer we wait, “the more difficult, the more expensive and the more dangerous it will be,” said Bill Hare, a physicist with the nonprofit group Climate Analytics.

World governments requested the report be prepared after the Paris Agreement was adopted in December 2015 to combat climate change. Nations agreed to reduce emissions of greenhouse gases in order to hold the increase in global average temperature to well below 2 degrees Celsius above pre-industrial levels and aim to limit the increase to 1.5 Celsius.

“For some people this is a life-or-death situation, without a doubt,” said Cornell University climate scientist Natalie Mahowald, a lead author on the report. Halting global warming would mean half as many people on Earth would suffer from lack of water and that there would be fewer deaths and illnesses from heat, smog and infectious diseases, according to the report.

Panmao Zhai, co-chair of the IPCC working group, said that “one of the key messages that comes out very strongly from this report is that we are already seeing the consequences of 1 degree Celsius of global warming through more extreme weather, rising sea levels and diminishing Arctic sea ice, among other changes.

Every country in the world except the United States has agreed to be a part of the Paris Agreement.

“The IPCC report is a sobering reminder that we’re still not on track to achieve the goals outlined in the Paris Agreement,” said Will McGoldrick of the Nature Conservancy. “If we’re serious about keeping global warming well below 2 degrees Celsius and striving for no more than 1.5 degrees, we cannot afford further delay.”

More than 90 scientists wrote the 728-page report, which is based on more than 6,000 peer-reviewed references.

One expert – Appalachian State University environmental scientist Gregg Marland – said limiting global warming to either 1.5 or 2 degrees seems unlikely. He likened the report to an academic exercise wondering what would happen if a frog had wings.

Yet despite the pessimism, some report authors said they remain optimistic.

“The good news is that some of the kinds of actions that would be needed to limit global warming to 1.5 degrees Celsius are already underway around the world, but they would need to accelerate,” said Valerie Masson-Delmotte, another co-chair of the IPCC’s Working Group I.

“We have a monumental task in front of us, but it is not impossible,” Mahowald said Sunday. “This is our chance to decide what the world is going to look like.”