6. Climate sensitivity. The modern temperature record
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This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
6. Climate sensitivity. The modern temperature record
To the best of present knowledge, the so-called equilibrium surface
warming, also known as the `climate sensitivity', is likely to sit
somewhere between 1.5 and 4.5 o C for a CO2 doubling, with a best
estimate of 2.5 o C [IPCC 95, p 34, 48].
Since 1890, average global surface temperature went up by about
0.5 o C with an uncertainty of roughly 0.15 o C both ways: the true
warming is likely to lie somewhere between 0.3 and 0.6 o C. This
estimate takes into account any known error sources, including urban
heat island bias, relocation of stations, changes in measuring prac-
tices and varying coverage of the globe. About 0.3 o C warming until
1940 and 0.1 o C cooling until 1975 were followed by renewed warming.
[IPCC 90, chapter 7.4] [IPCC 95, p 26-8, 141-6]
Surface and low to mid-tropospheric temperature are often confused,
but they are not interchangeable. For tropospheric temperatures, the
radiosonde and satellite record go back to 1958 and 1979, respectively.
Both records are similar since 1979. On average, both the surface and
lower-to-middle troposphere warmed by about 0.1 o C per decade since
1960. From 1979 to 1995, however, the surface warmed by 0.13 o C per
decade, while the lower-to-middle troposphere cooled by 0.05 o C per
decade. Gaps in the southern oceans surface data and errors in the
tropical satellite record may contribute to the difference, but there
are physical reasons as well. Surface and tropospheric temperatures
responded differently to El Nino-Southern Oscillation, to volcanic
eruptions, and probably also to deep Aleutian (1976-88) and Iceland
(~1980-95) winter lows. [Hurrell 96/97] [IPCC 95, p 146-8, 165-6]
Since 1960, the lower stratosphere cooled markedly by roughly -0.35 o C
per decade. Both rising CO2 levels and stratospheric ozone depletion
tend to cool the stratosphere. Initial model results suggest that,
at the moment, stratospheric ozone loss may play the lead. It may
also have a hand in the slight cooling of the upper troposphere over
the past decades. [IPCC 95, p 109-11, 148-9] [Ramaswamy] [Santer]
It is currently hopeless to draw conclusions from the observed tempe-
rature record about the present or future amount of greenhouse gas
induced warming. (Nonetheless, this is attempted time and again ;-)
Apart from the amount of the eventual warming, its speed is uncertain
as well. A given rate of warming does not by itself reveal when and
at what level the warming is eventually going to stop. Moreover, the
effects of several factors cannot yet be disentangled. Among these,
the presumably most important three are:
human-made greenhouse gases warming
human-made tropospheric aerosols cooling
natural climatic variability cooling or warming
The geographic and vertical pattern of the temperature changes suggests
an influence from human-made greenhouse gases and aerosols as well as
from stratospheric ozone depletion [IPCC 95, chapter 8] [Ramaswamy]
[Santer] [Tett]. This is a far cry from quantifying the human influen-
ce, let alone the extent of future climate change. Taking into account
numerous factors that can affect climate, climatologists can only say
that the observed changes are consistent with (though no proof for)
the estimated range of climate sensitivity to greenhouse gases.
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