evidence: World leaders agreed in Paris to limit global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit). The planet has already warmed about 1 degree Celsius, compared to pre-industrial levels. [...] Riahi is quick to point out that what happens in the models may not be feasible in real life. They don't account for political obstruction, for instance, or human preferences. People may just want to drive an expensive car, rather than take public transit, even when the models says that choice isn't economically rational. But the models also can be far too pessimistic, in particular about technological innovation. Ten years ago, van Vuuren says, they never anticipated the rise of cheap solar power. "We have been in the extremely fortunate situation that the cost of renewables has declined rapidly in the past decade." This has made the task of reducing carbon emissions much easier.