Present climate and biological change - Climate Change: Biological and Human Aspects - page 227

rough-and-ready, loose approximation of the long-term climatic trend for the next
few decades at least as far as policy-makers are concerned.) From this it can be seen
that should we continue to increasingly add greenhouse gases to the atmosphere as
we have been, then the planet will end up warmer than it has been since before the
beginning of the Industrial Revolution. Even if we included the lowest of the best
estimates in the various IPCC 2001 scenarios, the forecast still would have been far
warmer at the end of the 21st century than at any other industrial time. (This can be
seen in Figure 5.5a.)
It is important to note that the temperature scale varies between IPCC assess-
ments: this is a feature that most people overlook. The 1990 assessment refers to
temperatures above pre-industrial times (Figure 5.6), taken as the temperature in
1765. Conversely, the 1995 and 2001 IPCC assessment forecasts for the 21st century
refer to temperatures above the 1990 global temperature. Different again, the 2007
assessment's forecasts were based on temperatures relative to the 1980-99 average.
Meanwhile the Climate Research Unit at East Anglia University uses the 1961-90
average as its zero temperature against which anomalies are presented. We need to
note that the warming base line is not the same for all the IPCC assessment. So what
I have done with the Figure 5.8b is to adjust all the IPCC assessments estimates
under B-a-U (or the analogous scenario depending on the respective IPCC report)
relative to the Climate Research Unit 1961-90 baseline standard. The IPCC scenario
forecasts presented in Figure 5.8b are: the IPCC 1990 Business-as-Usual forecast;
the IPCC IS92a scenario from the 1995 assessment; the IS92 equivalent to 1990 in
the 2001 assessment 3 ; and the A2 scenario from the IPCC's 2007 assessment. From
Figure 5.8b it can be seen that all the IPCC's various B-a-U (or to be strict the B-a-U
analogous) scenarios forecast a temperature rise of between 2 and 4 ◦ C above the
1961-90 mean global temperature for the year 2100. Indeed, remove the IPCC's 1995
B-a-U estimate and all remaining estimates are between 2.85 and 3.96 ◦ C above the
1961-90 mean for the year 2100. The reason why the 1995 IPCC estimate is lower is
largely because they assumed a greater cooling forcing from anthropogenic aerosols.
The take-home message from this confusing variation in the different IPCC assess-
ments is that there is actually reasonable agreement between the future greenhouse
gas scenarios. That they differ slightly is due to (1) continually improving scientific
understanding of the various climate forcing factors and (2) the (hopefully) better
appreciation of the likely additions of greenhouse gases to the atmosphere as we
progress into the 21st century, especially as, if nothing else, the future is closer.
Taking all the IPCC assessment reports together (irrespective of whether or not we
place greater emphasis on the latest), the greatest likelihood is that in the 21st century
the Earth will become warmer than at any other time in the Holocene interglacial. In
fact, using IPCC 2007 warming high estimates (as opposed to best estimates) the end
of the 21st century could easily see temperatures higher than Earth has had since the
last interglacial some 140 000 years ago, or indeed the previous interglacials during
which humans evolved. However, the Earth is a very different place today compared
to 140 000 years ago (or earlier). To begin with, back then there were not more than

3

Note that that some of 2001 assessment's other B-a-U-type scenarios were slightly higher than the one
meant to be the most analogous to the 1990 assessment and so are even more in line with the 1990
assessment.