Temperature increase last century was 0.6°C. Most climatologists would like to see temperature increase this century stay below 1.4°C, and many would like to see a cap of 1.2°C or less.
A recent MIT study looks at the odds of this. FIrst, the no-policy scenario:
Without a policy, it is highly probable that we will see a temperature increase larger than that needed to throw us into a glacial period—about 4 – 6°C separates glacial (ice age) and interglacial period. A temperature increase of about this much is expected to change the climate dramatically.
With a policy, chances are better.
While the warming is still more than climatologists would like to see, there is a 90% chance of staying below 3°C this century, compared to the no-policy case, where the chances are only 1%.
The predicted increases are greater than predictions from 2002:
The differences are greatest for the reference or “no policy” wheels. In the previous wheel the likelihood of exceeding 5°C was about 4%, but in the new wheels that likelihood is 57%. There is no single revision that is responsible for this change. In our more recent global model simulations, the ocean heat-uptake is slower than previously estimated, the ocean uptake of carbon is weaker, feedbacks from the land system as temperature rises are stronger, cumulative emissions of greenhouse gases over the century are higher, and offsetting cooling from aerosol emissions is lower. No one of these effects is very strong on its own, and even adding each separately together would not fully explain the higher temperatures. Rather than interacting additively, these different affects appear to interact multiplicatively, with feedbacks among the contributing factors, leading to the surprisingly large increase in the chance of much higher temperatures.