Geoscience Reference
In-Depth Information
25.4 Current global and regional trends in temperature and precipitation
Global mean land temperatures have increased by roughly 0.7
°
C during the past 100
years, and sea surface temperatures have increased by 0.9
C since 1900 (Raupach and
Fraser,
2011
). Between 1961 and 2008, the upper few hundred metres of the oceans
absorbed about 1
°
10
22
Joules of energy, which has also helped slow the rate of
warming in the lower atmosphere (Braganza and Church,
2011
). The rise in temper-
ature on land has fluctuated, but the overall trend is upwards. The oceans have also
become progressively warmer during that time, resulting in thermal expansion and a
rise in global sea level at a rate that in 2010 amounted to 3.2 mm/year (Braganza
and Church,
2011
). Between 1880 and 2010, mean global sea level rose about
20 cm, about half of which is attributable to thermal expansion of the warmer ocean
surface and about half of which is the result of the melting of ice stored in mountain
glaciers and ice caps, including Greenland and Antarctica. We saw in
Chapter 13
that
the mountain glaciers in Africa have been retreating rapidly over the past fifty years,
and this is true of the majority of mountain glaciers in both hemispheres. Global sea
level rise is likely to continue as the surface of the oceans continues to warm and ice
caps and glaciers continue to melt. Given that more than 150 million people today live
within one metre of mean sea level, this is a sobering trend. Should the Greenland ice
cap melt entirely, as it has in the geologically recent past, it is estimated that global
sea level would rise about 7 m, and if the entire West Antarctic ice cap melted, there
would be an additional 5 m rise (Braganza and Church,
2011
). During the peak of the
last interglacial 125,000 years ago, mean sea level is estimated to have been 6-9 m
higher than it is today, reflecting meltwater contributions from both Greenland and
West Antarctica (Dutton and Lambeck,
2012
).
The synchronous increase in both land and sea surface temperatures during the last
hundred years points to a global cause, with the warming of the lower atmosphere
as a result of the enhanced greenhouse effect being the most probable cause. Global
climate models that only take into account natural forcing factors, such as volcanic
eruptions, solar variability and ENSO events, fail to reproduce the monitored changes
in global temperature. When the models include both natural and anthropogenic
forcing factors, the match between model outputs and observations is much improved
(IPCC,
2007a
), suggesting that anthropogenic emissions of the greenhouse gases such
as CO
2
,CH
4
and N
2
O are indeed a significant factor in the global warming trend.
Warming of the land due to an increase in the carbon dioxide content of the
lower atmosphere is entirely consistent with basic atmospheric physics, as Svante
Arrhenius demonstrated more than a hundred years ago (Arrhenius,
1896
). Building
on the solid experimental work of John Tyndall and on Joseph Fourier's identification
of '
l'effet de serre
', or the greenhouse effect, Arrhenius calculated that a reduction
in the atmospheric CO
2
concentration to 0.67 of the late nineteenth-century value of
about 300 ppmv would lower the mean surface temperature by 3
×
°
C, while increases
