From Hansen’s interview with Bloomberg:
Q: The warmest temperatures since 1998, which were in 2005 and 2010, have all been pretty similar. How can we explain that we’ve not had a significantly warmer year than 1998 since then?
A: When you look at a short period, it’s hard to have statistically significant warming. But the rate has been less in the past decade than the prior three decades.
It’s normal. There’s no reason to believe that the temperature is going to be linear. There are a couple of reasons to believe it would be less.
Since the 1970s we have been measuring the sun very precisely, and we know that this last solar cycle is the weakest of them all.
But there are other factors involved, some of which are not measured very well, including human-made aerosols [which cause cooling].
Then there’s just a natural variability. We’ve had in the last few years two strong La Ninas. That’s just a natural oscillation of tropical temperature. The 1998 El Nino was a record one, and that causes warming.
The La Ninas cause a global cooling. When you have a big warming at the beginning and two La Ninas at the end, that tends to give you a negative trend.