The Earth’s average surface temperature has already risen 0.8° Celsius over the past century (1.4° Fahrenheit) as a result of human activity.
And scientists expect average temperatures to rise an additional 1°C to 4°C in the century ahead if greenhouse gas emissions continue to rise (between 2.7°F and 7.2°F). That’s according to the IPCC’s 2013 assessment:
This chart shows temperature projections under a variety of future scenarios for greenhouse gas emissions (referred to as RCPs).
Why the huge range of projections? There are two key variables here:
Climate sensitivity: Climate scientists still aren’t sure exactly how much the world will warm if we keep pumping greenhouse gases into the atmosphere. Currently, the best estimate is that the Earth’s average temperature will increase between 1.5°C and 4.5°C every time we double the amount of carbon dioxide in the atmosphere.
That estimate, known as climate sensitivity, is derived from a variety of sources. Scientists can study how temperatures have swung in the past in response to changes in carbon dioxide levels. They can look at how the global climate responds to volcanic eruptions. And so on. That helps narrow the range, but there’s still some uncertainty.
The future growth of emissions: The other big variable here is how much carbon dioxide, methane, and other greenhouse gases humans will actually put into the atmosphere in the years ahead. That’s much, much harder to predict. It depends on how our economies grow and what sorts of energy sources we’ll use.
So scientists often lay out scenarios, as the IPCC chart above does. If we manage to cut global emissions immediately and drastically (that’s the blue line), we have a shot at keeping additional global warming to around 1ºC. But if emissions keep rising unchecked (the red line), then temperatures could rise an extra 4ºC or more.
Add up past global warming and future global warming, and the IPCC says that by 2100, the world will likely be at least 1.5°C hotter than it was in the pre-industrial era. The key question is whether we’ll exceed 2ºC of total global warming (compared with pre-industrial levels). That’s the level most world leaders have said is unacceptable.