A casual glance at plots of global mean temperature show something of a hiatus and yet many places are still showing warming. Russia had record heat wave in 2010, the USA in July 2012 and Australia continues to beat records. So what's up with that?
The climate undergoes internal variability in the form of various oscillations like the El Nino Southern Oscillation. During an El Nino, warm waters shift to the central or eastern Pacific and pressures drop there while waters cool in the western Pacific and pressures rise (for the more technically minded, westerly winds in the western Pacific produce waters in the ocean that transport warm water to the east and change the temperature structure of the ocean at and below the surface). During a La Nina, things are the other way around. Normal or neutral years represent a weak La Nina.
El Nino years are more warm globally. The shift of warm seas and lower pressures generates rainfall. As cloud forms, the atmosphere is heated and waves form, effecting weather patterns over north America. Heat is transferred away from the tropics more strongly in the northern hemisphere winter than during its summer. Hence, if the eastern Pacific is cooler than average (La Nina), northern hemisphere winters will be cooler because of the transport of that colder air. This transport of colder air can offset the warming produced by greenhouse gases.
When Kosaka and Xie (Nature, 2013,Published online) looked at global temperatures for 2002 to 2012, they saw that there was cooling during the northern hemisphere winter but warming during the northern hemisphere summer. To finish with a quote:
'All the climate models project a tropical Pacific warming in response to increased greenhouse gas concentrations [that's an increase in El Nino, MP]. We conclude that the recent cooling of the tropical Pacific and hence
the current hiatus are probably due to natural internal variability
rather than a forced response. If so, the hiatus is temporary, and
global warming will return when the tropical Pacific swings back to a
warm state. Similar hiatus events may occur in the future and are
difficult to predict several years in advance owing to the limited
predictability of tropical Pacific SST.'
So this is not good news or an overturning of ideas on climate change, merely the observation that the climate systems is always more complicated than we imagine. The 150+ year old science that burning fossil fuels increase the temperature, stands.