|Climate Protection Policies: Can We Afford to Delay? (WRI, 1997, 44 pages)|
Although economic and political issues are currently generating the most heated debate, certain scientific issues also bear on the timing of policy response. One concerns the rate at which greenhouse gas concentrations increase. Final concentrations are not the only relevant measure of environmental impacts. Figure 1B shows that the concentration levels over the next century vary between the two paths. In practice, this variance implies differences in the rates of change of radiative forcing and global average temperature en route to stabilization.
RISKS OF SURPRISE
Radiative forcing is the process by which gases in the atmosphere trap heat and raise global temperature. Even if increases in radiative forcing and concentration levels were gradual and continuous, the non-linear nature of the climate system means that future changes may involve surprises (IPCC, 1996a, p.7).10 By definition, the unexpected nature of these effects makes them impossible to guard against and so potentially more costly to respond or adapt to. Though we do not know what rate of concentration build-up would generate surprises, clearly higher concentrations in the near term, and hence higher radiative forcing, raise the risks of surprise (Schneider, 1997).
10 The unanticipated appearance of a hole
in the ozone layer is an example of such a surprise, though caused by a
RATES OF TEMPERATURE CHANGE
Even without surprises, different rates of build-up will generate different rates of temperature change. Although ecosystems possess a natural resilience to change, too rapid a rate of change may overwhelm certain species' ability to adapt. Climatic zones may shift faster than natural forest migration processes (Davis, 1989). Coral reefs may die out as sea level rise outpaces the upward growth of reefs that keeps them near the surface (Harvey, 1996; Gleick and Sassin, 1990). Human responses may also be slow. The spread of malaria into developing countries may outpace the development of health care systems that might otherwise contain the disease (Harvey 1996; Martens et al., 1994). For such reasons, some argue that it is more appropriate to focus on the rates of change of key variables, or to impose a "speed limit" on climate change (Alcamo and Kreileman, 1996; Amano, 1996).
One variable of interest is the rate of change of global average temperature. Unfortunately, little is known about what constitutes a safe rate of change. Moreover, it is the rate of change sustained over several decades that is important. Nonetheless, delayed reduction paths with higher near-term emissions may sustain higher absolute rates of temperature change over the first few decades of the next century than is ever achieved under a path of immediate action. (See Figure 4.)
Source: Alcamo and Kreileman
Predictions of rate of temperature change are sensitive to assumptions about aerosol emissions, which have a cooling effect that offsets greenhouse gas emissions. In a scenario where aerosol emissions are coupled to greenhouse gas emissions, the pattern may be somewhat reversed. Figures from another model show that both paths may have low rates of temperature change early on, before the delay path rises above a path of immediate reductions. For example, under these assumptions, between 2040 and 2100, the sustained rate of change for the WRE-550 path is 0.17C° per decade as opposed to 0.14C° per decade under the WGI-550 path (Wigley et al., 1996).
The scope for surprise impacts and the uncertainty regarding
the effects of higher sustained rates of temperature change may mean that
ecosystems and human health face higher threats under a path of delayed
Though there is uncertainty regarding the impacts of different rates of change, such figures represent potential risks imposed by delay and add another dimension to near-term policy decisions. The scope for surprise impacts and the uncertainty regarding the effects of higher sustained rates of temperature change may mean that ecosystems and human health face higher threats under a path of delayed reductions. Taking account of these effects also undermines the notion that some emissions are "free" in environmental terms, if used early on. Even though early emissions may not bear on final concentrations, they do influence near-term rates of radiative forcing and temperature change.