48 / Basics of Environmental Science
of 2.5°C, and at the current rate of increase in greenhouse-gas concentrations these temperatures
would be reached by around 2100. During the same period, warming of the oceans is calculated to
cause them to expand, producing a rise in sea level of 2–4 cm per decade (IPCC, 1992).
It is not certain that sea levels have risen, although predictions of global warming include an assertion
that they have risen world-wide by about 25 cm over the past century. In 1841, the explorer Sir James
Clark Ross visited Tasmania, where he met Thomas Lempriere, an amateur meteorologist. The two
men installed an Ordnance Survey Bench Mark, chiselling it into a rock face at a place called the Isle
of the Dead, near Port Arthur. It was positioned with great care and precision and was meant to act
as a sea-level gauge. The gauge has been rediscovered and the Tasmanian climatologist John L.Daly
visited it in August 1999. He found that it remains visible above the water line, despite the supposed
rise in sea level. It is uncertain whether the gauge was set at a level close to the high tide mark or at
the mean tide level. If it was close to the high tide level it shows that sea level has not changed since
- If it was at the mean tide level it shows the sea level has fallen. (www.vision.net.au/~daly/)
The mean global temperature increased by 0.37°C between 1881 and 1940. The temperature fell
from 1940 until the 1970s, since when it has risen again, but there is no clear evidence of any
warming between 1980 and 1998. The total warming from 1881 to 1993 amounts to 0.54°C. Two-
thirds of the warming occurred before 1940—and before the main rise in the atmospheric carbon
dioxide concentration—and 1881 was an unusually cold year (BALLING, 1995).
Mean temperatures are calculated from three sets of data. Weather stations and ships record surface
temperatures, balloon sondes record temperatures above the surface, and TIROS-N satellites operated
by NASA on behalf of the National Oceanic and Atmospheric Administration (NOAA) measure
temperatures from space (www.atmos.uah.edu./essl/msu/background.html). Surface measurements
are difficult to interpret over long periods. This is because weather stations that are established in
open country may gradually be affected by nearby urban development and road building, which will
raise the temperature by an ‘urban heat island’ effect, producing an illusory warming trend. There is
also the possibility that, over the years, changes in staff may lead to unrecorded changes in the time
of day when measurements are made. Ships measure the temperature of sea water below the surface,
but different ships do so at different levels. Their measurements of air temperatures are unreliable
for similar reasons. Over the course of this century, ships have become larger, so their decks from
Figure 2.14 IPCC estimates of climate change if atmospheric CO 2 doubles