Geography Reference
In-Depth Information
a similar reduction in the GWP of another gas (see
Box 2.3). This is very similar to the 'bubble'
principle of pollution credits in the USA (Elsom
1992) and an integral part of the 1997 Kyoto
Agreement on emission regulation.
temperature data, have been basic requirements of
the scientific community in its task of
accumulating evidence for climate change. Since
few instrumental records of land surface
temperatures existed until about 1850, it is current
practice to emphasise the trends in global
temperatures from 1861 onwards, when surface
observing networks were becoming established in
many parts of the world. In the late 1980s, three
research groups produced similar analyses of
hemispheric land surface air temperature
variations—Jones
et al
. (1986a and b) at UEA, UK;
Hansen and Lebedeff (1987) at GISS, USA; and
Vinnikov et al. (1987) of SHI in the former USSR,
of which the updated and re-analysed data set of
Jones
et al
. has been used in recent studies (IPCC
WGI 1996).
Internal feedbacks in the climate system—
examples
The debate about internal changes is largely
concerned with feedback processes, which may be
initiated or enhanced through radiative forcing.
Positive feedback tends to increase the rate of a process
such as global warming. Long-term positive feedback
mechanisms could prove destructive. The IPCC
Report (1990) provides an example of a Sahelian
drought-type positive feedback, where a drier surface
resulting from rising temperatures leads to reduced
evaporation, which in turn reduces humidity and
cloud cover, promoting greater warming and yet drier
surface conditions. If there were no restraining or
counter-processes taking place, the Earth might be
faced with a 'runaway' greenhouse effect as has
occurred on Venus, where surface temperatures of
about 525°C have been recorded by the Russian space
probes (Houghton 1997). Negative feedback occurs
when there are constant checks to the rate of a process.
For example, rising sea-surface temperatures (SSTs)
would increase evaporation, which in turn would
increase the water vapour content in the lowest layers
of the atmosphere. This could lead to greater spatial
cloud coverage, decreasing the amount of incoming
radiation reaching the surface, which in turn would
lead to cooling of the Earth's surface. The climate
system is dominated by numerous positive and
negative feedbacks, some not operating until a
threshold is passed such as an SST of 27°C for tropical
storm development, others operating in a non-linear
or quasi-stochastic manner.
Global data sets: oceans
Since the oceans comprise over 60 per cent of the
Northern Hemisphere and over 80 per cent of the
Southern Hemisphere, it is important to assemble
an accurate data set. The collection and correction
of global SST data have been undertaken most
recently by Folland and Parker (1995) of the UK
Meteorological Office. They have produced
improved adjustments to temperature records
where canvas and wooden buckets were used to
sample sea water temperatures from ships from
about 1860 to 1941. It is now thought that
sampling of SSTs at night might be more reliable.
This would avoid any bias due to daytime heating
of ships' decks. There are also spatial gaps in the
data, particularly over the southern oceans, and
temporal gaps, especially in the nineteenth century
and during the world wars. Except since 1975,
SSTs appear to have lagged behind changes in land
temperatures by several years (IPCC 1990).
Global data sets: combined land and ocean
The global data record of surface land and sea
temperatures is shown in Figure 2.4. It indicates
the two main warming periods that have taken
place since 1860, the first from 1910 to 1941
and the second from 1978 to the present
EVIDENCE FOR GLOBAL WARMING
Global data sets: land
The collection and quality control of climatic
records, especially the long-term near surface
