The country would endure weeks of “tropical nights”, droughts would be five times as likely and the risk of a tropical cyclone would rise, officials said.
Ministers are canvassing the public’s views on how pessimistic their climate planning should be.
In a bid to stem emissions, ministers also confirmed on Tuesday a ban on many short-haul flights across France.
Passengers are expected to travel by train rather than flying between Paris and the cities of Lyon, Nantes and Bordeaux.
The 2015 Paris Agreement calls for drastic emissions cuts to keep temperatures to below 2°C, or preferably 1.5°C, above pre-industrial levels.
However, the chance of temperatures rising beyond 2°C “cannot be ignored, even if achieving the Paris objectives remains our task and our priority,” says an eight-page French consultation brochure.
“In effect we need to give ourselves a trajectory of global warming … which will serve as a reference point for all adaptation measures we take in France.”
Scientists say Europe is warming faster than the global average, partly because of rapid changes in the Arctic.
It is estimated that a 3°C rise globally would be enough to make temperatures 4°C warmer in mainland France by 2100.
The French are being asked whether that 4°C rise should be adopted as the official worst-case planning benchmark.
If it comes about, there could be heatwaves lasting two months and as many as 40 to 50 “tropical nights” annually, the public was warned – compared to about 10 a year if the Paris targets are met.
Almost all of France’s glaciers would disappear in the worst case, it is feared, after a lack of Alpine snow forced ski resorts to close last winter.
The government is also asking for input on what “financial and technical support” should be offered to help people adapt to warmer times.
Christophe Bechu, Minister for Ecological Transition, has mooted new taxes on the wealthy to finance the necessary changes.
“We have to prepare concretely for the inevitable effects on our territory and our lives,” he said.
“Droughts and heatwaves are multiplying and becoming more intense, water resources are becoming strained, there is less snowfall and coastal erosion is threatening many areas.”
One opposition MP, Clemence Guette, accused the government of “giving up” by focusing on adaptation.
“The government’s rhetoric is aimed at justifying its inaction,” she said.