The recent WMO press release on the climate of the 2001-2010 period highlighted that global temperature change was accelerating. Although this could be a misleading statement, should we even be expecting global temperature changes to be accelerating at present?
Although the rates of change of global temperatures are generally positive since the 1970s*, one key issue with talking about acceleration is that it is non-trivial to define and observe. Also, we do not expect an acceleration to continue indefinitely. These factors ensure that communication of this issue is especially difficult.
The rates of change of global temperatures in observations (black) and the CMIP5 simulations (colours) are shown below, and indicate a warming since around 1970, which generally accelerates until around 2000, with some variability and also dips around large volcanic eruptions. What is most interesting is the general deceleration from 2000 to 2010 in the simulations, and from around 1995 in the observations.
As for the future, RCP2.6 (low emissions) shows almost continuous deceleration and RCP8.5 (high emissions) resumes an acceleration around 2012 until 2050. The mid-range emissions scenario (RCP4.5) shows a flat rate of change until mid-century, followed by a deceleration.
So, the CMIP5 models simulate a deceleration at present which may continue, depending on our future emissions. This suggests that talking about rates of change is useful, but that discussion of acceleration or deceleration is potentially very confusing in communication efforts.
* Shorter periods for the linear trends show a similar picture which much higher levels of variability, and using the most recent 10-year period actually results in a negative rate of change.