Thanks in large part to the record-setting El Niño of 1997–98, the year 1998 was the warmest year, globally, in the 20th century. Since 2001 the global trend has been relatively flat (see graph). However, temperatures continue to run warmer than in previous decades. The global average from 2000–09 exceeds the average for 1990–99, which in turn was warmer than 1980–89. And the average for 2010–12 topped the 2000–09 average.
Although scientists are confident that global temperatures will rise further in the coming decades, there could still be occasional "pauses" in warming that last a few years, like the one we've been seeing since the late 1990s. The Intergovernmental Panel on Climate Change (IPCC) devoted a section of its 2013 physical science assessment to analysis of the 1998–2012 warming hiatus. Among the possible factors involved:
Two of the main vehicles for ocean-atmosphere heat exchange are El Niño (which tends to warm the global atmosphere) and La Niña (which tends to cool it). La Niña events have predominated over El Niños since 2006, which helps explain the lack of a new global record in the decade thus far.
As the IPCC notes, natural variations can either amplify or mute longer-term warming trends over periods of 10 to 15 years. This is one reason why 30 years is the typical reference period in determining climate. In your local weather forecast, for example, the average daily temperatures and precipitation posted alongside today's measurements were calculated based on the period from 1981 to 2010. This was a warmer period for most U.S. cities than the previous 30-year range, from 1971 to 2000.
Meanwhile, decision makers and planners often need guidance on shorter time scales than three decades. Researchers are responding by experimenting with outlooks for global temperature in the 10- to 15-year range. The most recent decadal outlook from the UK Met Office calls for a good chance that the upcoming five years (2013–17) will average slightly warmer globally than the record year of 1998. The IPCC expects that, for periods without a major volcanic eruption, most future 15-year periods will show a stronger warming trend than 1998–2012.