Although sea level has been rising since 1998, as the oceans have continued to absorb heat from the sun (see figure), the temperature of the atmosphere has risen little since the late 1990s. This reduced temperature rise is contrary to the expectations of many climate scientists whose models predict global warming as more and more carbon dioxide is emitted into the atmosphere. Climate sceptics claim the reduced rise means that global warming has ground to a halt. Now scientists have come up with explanations for the pause, or hiatus, in atmospheric warming.

Figure 1. Sea-level rise from 1870 until 2001 over a range of 270 mm (from National Oceanographic and Atmospheric Administration (http://www.realclimate.org/index.php/archives/2013/04/the-answer-is-blow...).

The simplest explanation by climate scientists is that the hiatus is down to natural variation in the Earth system. Sometimes natural variation can hide systematic trends in climate, which only become evident after two or three decades, but this argument had begun to look less convincing after 16 years. Nor were climate models able to simulate the observations.

A recent explanation is that the sites at which the surface temperature is observed are biased geographically. There is less dense coverage of ground observations from polar regions (and Africa) yet these polar regions, and particularly the Arctic, are warming faster than elsewhere. When better methods of filling in the gaps between sites and satellite data are also included it was found that the rising temperature trend is two and a half times more than previously thought. Even so, not everyone is convinced; one American scientist commented ‘Yet the effect is only small, and the analysis eliminates neither the slow-down in global warming nor the growing discrepancy between climate model simulations and surface temperature observations.’

Attempts have also been made to explain the lack of global warming by a reduced output from the sun since 1998 and by the cooling effect of atmospheric aerosols –the sort of tiny particles emitted from diesel exhausts.  However these factors are said to be able to explain only up to 20% of the hiatus.

But the research on biased observations also hinted at another influence, the strong ocean warming phenomenon in the equatorial East Pacific known as El Niño which occurred in 1997-1998 and influenced weather globally. Following the El Niño event the Pacific ‘flipped into a cool state that has continued more or less to this day.’This is a result of the variable behaviour of the equatorial Pacific known as the El Niño Southern Oscillation. Scientists believe the current cooler state of the Pacific Ocean may help to explain the hiatus in warming. Climate modelling also suggests that when the equatorial Pacific eventually reverts to its warmer state the Earth may experience decades of rapid warming.  Indeed, models thattakes account of the warmer El Niño ocean surface waters result in an excellent fit to the observed changes in global mean temperature.

Climate scientists are now beginning to better understand how heat is transferred within the oceans, particularly the Pacific Ocean, and into the atmosphere and how this affects global climate. It is becoming clear that the pause in the rise in global temperature since 1998 is a real phenomenon, although perhaps slightly less marked than at first appeared, and may be an expression of natural variation on a time-scale of decades. Eventually, perhaps after 25 years, the hiatus will be succeeded by decades of warming.

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