Scientists explain pause in global warming – temperature rise set to continue
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-isblowing-in-the-wind-the-warming-went-into-the-deep-end/).
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 that takes 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.