The world’s leading climate scientists gathered in Japan on Tuesday to begin hashing out the final details of a “grim” climate report, which both leaked drafts and those familiar with its contents say will call on policy makers to take immediate action or face a climate future that will otherwise be marked by widespread ecological and human catastrophe.

Of those harsh challenges, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) report—according to a draft version of the leaked earlier this year— will show that the four degrees Celsius rise that we are currently careening towards will undoubtedly cause increasing natural disasters, including: more violent storms, forest fires, devastating droughts, flooding, widespread hunger, disease, and a rise in ocean levels by up to a meter.

However, as Kaisa Kosonen explains on the Greenpeace blog Tuesday, the difference in their latest report from previous work by the IPCC and other similar warnings from the global scientific community is its emphasis on the stark “choice” now before humanity.

This latest message from the IPCC, Kosonen writes, is that people—both inside and outside of government— either “reduce and manage the risks ahead and do what’s needed to keep warming as far below 2 degrees Celsius as possible, or we continue to do too little too late, drifting from crisis to crisis and on towards a disastrous 4 degrees world.”

“The IPCC will paint a picture of both these possible futures,” Kosonen notes.

Over the course of this week the scientists will be finalizing a summary of the 2,000 page report directed at policy makers. The report and summary will be released Monday, March 31, following the week-long summit.