Present climate and biological change - Climate Change: Biological and Human Aspects - page 228

7 billion humans, and rising. Nor was the planetary landscape so managed or the global
commons, both atmosphere and oceans, perturbed by human action. Therefore, the
biological and human ecological consequences of such climate change will be more
marked than the climate impact during the warmest of these earlier warm times.
If we carry on this century much as we did throughout the last, and so end up with
marked climatic change, the question arises as to what would happen if greenhouse
gas emissions were curbed. The IPCC looked at a range of such scenarios in each
of their assessments (1990, 1995, 2001 and 2007), which assumed various controls
of gas emissions, improvements in energy use and so forth. These scenarios were
realistic in that they took into account that the starting point was the present (1990
or 2000 depending on which IPCC report), so humanity was stuck with the then
actual current pattern of energy production and use, and could only modify matters
as old power-station plants came to the end of their economical (and even carbon-
economical) life. (By 'carbon economical' I mean if we scrapped a young coal-fired
power station and built a wind farm then the energy investment in building the
coal-fired station would not have been recouped.) The IPCC knew that the present
time also includes a growing global population and they used demographers' quite
reliable ranges of forecasts through to the middle of the 21st century and beyond. They
took into account that it would take time and investment to introduce alternatives.
Finally, their scenarios varied in that some assumed that there would be only modest
emission reductions compared to the B-a-U scenario, while others assumed that
more aggressive policies would mean greater savings. Among the strongest climate-
combating IPCC 1990 scenarios, carbon dioxide emissions would be reduced to 50%
of their 1985 levels by 2050 and even this unlikely scenario still resulted in a climbing
global temperature through to 2100 and beyond. So, every IPCC scenario shows that
global warming will continue through the 21st century. Obviously, if the cuts in green-
house gas emissions were sufficiently great there must be a point at which the global
climate would stabilise without further warming. The IPCC did look at such climate
stabilisation but did not include this as one of their detailed scenarios. The reason
was that the magnitude by which we would need to change the global economy
to realise these stabilising cuts is so great; to stabilise the climate we would need
to reduce greenhouse gas emissions planet-wide by 60-80% of 1990 levels! To do
this, against a backdrop of global economic growth and growing population (both
multiplicative and not additive factors; see discussion on population and Figure 5.10,
below) such cuts would have an even greater socio-economic impact. This cut to 60 -
80% of 1990 levels is therefore not even remotely likely. It would appear virtually
certain that we are committed to seeing the Earth warm up.
For its 2001 assessment, the IPCC not only reviewed the literature as to the latest
science and looked at improved computer models, they developed a range of more
complex greenhouse gas emissions scenarios arising from different policy assump-
tions. These they developed in a Special Report on Emission Scenarios (or SRESs),
of which six were highlighted but a full set of 35 were used in the IPCC's 2001
report to provide a high-low envelope inside which all the SRES best-estimate fore-
casts lay. Further, an outer envelope was ascertained, representing the scenarios used
by several different climatic models. This outer envelope provided the IPCC with
new high and low estimates for anticipated climate change in the 21st century (see
Figure 5.7a).