World leaders reached a historic agreement in Paris on Saturday as they met to discuss the constant threat of climate change. According to a report from the Washington Post, making the agreement was the easy part – countries pledged that they would work in unison to limit the planet’s average temperature increase to 1.5 degrees Celsius, well below the previously accepted safe average increase of 2 degrees Celsius.

Living in a world that only warmed 1.5 degrees Celsius may not even be so bad for people, property, and biodiversity on Earth. The problem is, however, that the globe has already warmed an average of 1 degree Celsius since monitoring first began in the 1800s. Scientists have been saying it for quite some time now; the only way to cut emissions enough to prevent catastrophic climate change is to significantly cut down on our use of fossil fuels.

It may still be possible to keep warming below an average level of 1.5 degrees Celsius, but it won’t be easy. In addition to cutting carbon emissions from nearly all point sources across the board, nations may need to employ new technologies more rapidly than anticipated to meet the ever-growing energy demand for a global population approaching 7.5 billion.

Major carbon emitters including the U.S., the European Union, China, and India, among others, will all need to come together in order to lower global carbon emissions. The U.S. would need to pledge to slash carbon emissions 45 percent below its current pledge’s level of reducing emissions 28 percent below 2005 levels by 2025, by 2030. If that’s hard to follow, just know that such a drop would require massive lifestyle changes on the parts of many Americans, in addition to a much more rapid advancement of renewable energy technologies.

One of the world’s other biggest emitters, China, would have to begin lowering its emissions by 2025 instead of 2030. India would have to reach peak emissions by 2027. While these goals are certainly lofty and in line with the idea that developed and developing nations alike must work together to address climate change, it remains largely unclear how the countries involved in the deal would actually achieve these reductions.

While it’s possible to meet these climate goals, it is anything but certain that the world will. There are a number of analyses that tell different stories with the same data under the pledge’s goals, and the outcome is far from predictable.

A press release from Climate Interactive describing a study by researchers at MIT Sloan that addresses the uncertain effects of the climate pledges on the global climate can be found here.