Most of the world’s nations have promised to avoid dangerous interference in the Earth’s climate system. That’s often taken to mean preventing global average temperatures from rising more than 2° Celsius (3.6° Fahrenheit) above pre-industrial levels. Temperatures have already risen 0.8°C so far.
This 2°C limit has a long, tangled history. By some accounts it was pushed by a German advisory panel back in the early 1990s, who argued that letting temperatures rise more than 2°C (3.6°F) would bring us outside the temperature range that allowed human civilization to flourish in the first place. Subsequent research detailed a range of adverse impacts that would occur if temperatures rose more than 2°C, from increased risks of severe weather to adverse impacts on agriculture.
Still, by its nature, the 2°C limit is arbitrary. Any single limit would be. Some scientists have noted that we could see a range of significant impacts long before we hit 2°C — coral reefs could start dying, or tiny island nations like Tuvalu could get swallowed by the rising seas. Conversely, other impacts, such as declining crop yields in the United States, might not happen until we go above the threshold. Deciding how to weigh all that is a political judgment as much as a scientific one.
For now, international climate negotiations tend to center around 2°C. At the Copenhagen climate talks in 2009, almost every nation in the world agreed to endorse 2°C as an upper limit for allowable global warming. The main dissenters (particularly those island nations) were arguing for an even lower limit.