The choice ofstabilisation level 259of the damage that could not be quantified in money terms. But even if
only the costs that can be estimated in terms of money are considered,
in Chapter 9 it was pointed out that estimates of the cost of the likely
damage of the impacts at that level of climate change were larger than the
costs of stabilising carbon dioxide concentration at levels above about
500 ppm (Figure 9.4). We also noted that, beyond the doubled carbon
dioxide situation, the damage due to greenhouse gas climate change is
likely to rise substantially more rapidly as the amount of carbon dioxide
in the atmosphere increases. A further factor is the rate of climate change
(see Figure 10.1(c)) which, with all the profiles except possibly the two
lowest, is likely to be such that some important ecosystems may not be
able to adapt to it (see Chapter 7). Studies^18 show that stabilisation below
550 ppm should avoid some of the worst impacts; for instance, some of
the large-scale die-backof forests and the transition of the biosphere
from a source to a sink for carbon dioxide (see box in Chapter 3 on page
40) which would otherwise occur around the middle of the twenty-first
century. Considering carbon dioxide alone, these considerations suggest
that the range between about 400 ppm and 550 ppm is where further
careful consideration of the choice of the target stabilisation level should
be made.
Although carbon dioxide is the most important greenhouse gas, other
gases also make a contribution to climate change. The combined effect
of the increases to 1990 of the gases methane, nitrous oxide and the
CFCs^19 is to add a forcing equivalent to that from an additional 60 ppm
or so of carbon dioxide (see Chapter 6, page 124). The effect of these
other gases also needs to be taken into account in our overall discussion
of the Climate Convention Objective of stabilisation. Even if there were
no further increase in these minor gases, the 1990 forcing would still
require to be added to future projections of change. The effect of this,
if turned into equivalent amounts of carbon dioxide, would be that the
450 ppm carbon-dioxide-only level would become about 520 ppm and
the 550 ppm level would become about 640 ppm of equivalent carbon
dioxide.^20 This means that, if it is considered that the climate effects of
doubled pre-industrial carbon dioxide concentration should be an upper
limit, when the increases in other gases are allowed for, the stabilisation
limit for carbon dioxide only is about 490 ppm.
How realistic is it to assume that the concentration of the other gases
will not change? We saw earlier that the Montreal Protocol should ensure
that the CFCs are stabilised in concentration over the next decade or two.
We also saw, for methane, that means are available that are not costly
and that, if taken, could stabilise methane concentrations at about today’s
levels. There is more uncertainty about nitrous oxide as its sources and
sinks are not well known. However, it is only a small contributor to