Temperatures are continuing to rise
The rise in global surface temperature has averaged more than 0.15 °C per decade since the mid-1970s. Warming has been unprecedented in at least the last 50 years, and the 17 warmest years have all occurred in the last 20 years. This does not mean that next year will necessarily be warmer than last year, but the long-term trend is for rising temperatures.
A simple mathematical calculation of the temperature change over the latest decade (1998-2007) alone shows a continued warming of 0.1 °C per decade. The warming trend can be seen in the graph of observed global temperatures. The red bars show the global annual surface temperature, which exhibit year-to-year variability. The blue line clearly shows the upward trend, far greater than the uncertainties, which are shown as thin black bars. The recent slight slowing of the warming is due to a shift towards more-frequent La Niña conditions in the Pacific since 1998. These bring cool water up from the depths of the Pacific Ocean, cooling global temperatures.
1998 saw an exceptional El Niño event which contributed strongly to that record-breaking year. Research shows that an exceptional El Niño can warm global temperatures by about 0.2 °C in a single year, affecting both the ocean surface and air temperatures over land. Had any recent years experienced such an El Niño, it is very likely that this record would have been broken. 2005 was also an unusually warm year, the second highest in the global record, but was not associated with El Niño conditions that boosted the warmth of 1998.
Another way of looking at the warming trend is that 1999 was a similar year to 2007 as far the cooling effects of La Niña are concerned. The 1999 global temperature was 0.26 °C above the 1961-90 average, whereas 2007 was 0.37 °C above this average, 0.11 °C warmer than 1999.
