Courtesy of Slate and New Scientist, a short, well-written article explaining the primary causes of short-term variation in global temperature, and how they occasionally seem to mask the long-term warming trend:
For the past 30 years, global temperature has shown a linear warming trend of 0.16 C per decade (Environmental Research Letters, vol 6, p 044022). When looking for the cause of this warming, a physicist will look for the heat source. One possibility is that the oceans are releasing heat. But measurements show the opposite: The oceans are soaking up heat. The other possibility is that the heat is coming from above, and indeed it is: More radiation is entering the top of the atmosphere than leaving it. This is because increasing amounts of greenhouse gases in the atmosphere hamper the loss of heat into space.
Superimposed on this global-warming signal is short-term natural variability, which makes some years hotter and some colder. Some, notably 2005 and 2010, stick out above the trend line, whereas others, like 2008 and 2011, stay below it. But overall, temperatures are creeping upward within a corridor of plus or minus 0.2 C around the trend line. Climate deniers use this variability to claim there is a slowdown in global warming, by cherry-picking time intervals that happen to start in the upper part of the corridor and end in the lower. They mix up signal and noise.