This is the crux of the United Nations climate change science panel report that all the countries accepted on Saturday after a contentious and strenuous meeting between scientists and diplomats in Korea.
The world has seen 1C of warming so far, with consequences such as more extreme weather already being felt, and there is more to come as temperatures continue to rise, the report said.
Another recent report from the consulting firm PwC makes it clear that even limiting warming to 2 degrees C will be a stretch: "There seems to be nearly zero chance of limiting warming to well below two degrees (the main goal of the Paris Agreement), though widespread use of carbon capture and storage technologies, including Natural Climate Solutions, may make this possible", it says. For example, by 2100, global sea level rise would be 10 centimeters lower with global warming of 1.5 degrees Celsius compared with 2 degrees Celsius.
The report was requested when the worldwide community came together in December of 2015 for the Paris agreement, which aims to keep global warming within this century "well below" 2°C, with an ultimate target of 1.5°C. Holding warming to 1.5 °C would reduce that risk by half.
It will also require action to take excess carbon emissions out of the atmosphere, the Intergovernmental Panel on Climate Change report said. We need to invest in new technology and new training, and we need to ensure access to clean, reliable energy for all to lift even more people out of poverty.
What is of particular concern is that the report lowers the level after which disastrous climate change is anticipated: Previous research had set the critical threshold at 2ºC of warming. Coral reefs would decline by 70-90 percent with global warming of 1.5 °C, whereas virtually all ( 99 percent) would be lost with 2 °C.
She warned the predicted loss of all coral reefs if governments could not contain warming would mean a massive loss of fish that people rely on for food and costing lives and livelihoods.
The report stated that allowing the global temperature to "overshoot" 1.5 degrees would mean a greater reliance on removing carbon dioxide from the air.
Earth's surface has warmed one degree Celsius (1.8 degrees Fahrenheit) - enough to lift oceans and unleash a crescendo of deadly storms, floods and droughts - and is on track toward an unliveable 3C or 4C rise.
The review of thousands of scientific papers also said the spread of disease and economic damage and harm to yields of crops will be less severe at 1.5C than 2C, as will the extinction of species.
As opposed to one sea ice-free Arctic summer per decade with 2°C global warming, one sea ice-free Arctic summer is projected per century with 1.5°C of global warming.
As more greenhouse gases lead to more warming, stabilising the planet's temperature at any level will require emissions to fall to zero overall To keep temperatures from rising to more than 1.5C in the long term, countries need to cut carbon emissions by 45% by 2030 and to net zero by 2050, with steep cuts in other greenhouse gases such as methane. "Our countries are not near where they need to be on achieving the kinds of emission reduction that we need if we want to actually successfully fix the planet and reverse the changes that we've already made".
The Paris Agreement on Climate Change was expected to produce a new urgency but the U.S. pullout from the accord dented those hopes.
Fourth, the number of people affected by multiple climate change risks could double if global temperature rises by 2°C, compared to a rise of 1.5°C.
But that is a big ask, considering we released more carbon dioxide into the atmosphere a year ago than ever before. By 2050, they will need to reach "net zero" - any further CO₂ emissions due to human activity would then have to be matched by deliberate removal of CO₂ already in the atmosphere, including by planting trees.
In an IPCC press conference Monday morning in Korea, Imperial College London's Jim Skea took a question about the importance of reforestation vs. fossil fuel cuts as an opportunity to summarize the report's basic message: "S$3 aying "option x or option y" is not the way that this report is framed".
Switching away from coal, oil and gas would also be expensive but it could avoid more than 100 million premature deaths this century.