Most of the observed increase in global average temperatures
since the mid-20th century is very likely due to the
observed increase in anthropogenic GHG concentrations.8
This is an advance since the TAR’s conclusion that “most
of the observed warming over the last 50 years is likely to
have been due to the increase in GHG concentrations” (Figure
2.5). {WGI 9.4, SPM}
The observed widespread warming of the atmosphere and ocean,
together with ice mass loss, support the conclusion that it is extremely
unlikely that global climate change of the past 50 years can
be explained without external forcing and very likely that it is not
due to known natural causes alone. During this period, the sum of
solar and volcanic forcings would likely have produced cooling,
not warming. Warming of the climate system has been detected in
changes in surface and atmospheric temperatures and in temperatures
of the upper several hundred metres of the ocean. The observed
pattern of tropospheric warming and stratospheric cooling
is very likely due to the combined influences of GHG increases and
stratospheric ozone depletion. It is likely that increases in GHG
concentrations alone would have caused more warming than observed
because volcanic and anthropogenic aerosols have offset
some warming that would otherwise have taken place. {WGI 2.9, 3.2,
3.4, 4.8, 5.2, 7.5, 9.4, 9.5, 9.7, TS.4.1, SPM}
It is likely that there has been significant anthropogenic
warming over the past 50 years averaged over each continent
(except Antarctica) (Figure 2.5). {WGI 3.2, 9.4, SPM}
The observed patterns of warming, including greater warming
over land than over the ocean, and their changes over time, are
simulated only by models that include anthropogenic forcing. No
coupled global climate model that has used natural forcing only
has reproduced the continental mean warming trends in individual
continents (except Antarctica) over the second half of the 20th century.
{WGI 3.2, 9.4, TS.4.2, SPM}
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