Bridging the gap between user needs and science capability: dealing with uncertainty in climate scenarios for adaptation
Synthesis and Integrative Research Program
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Traditionally, in carrying out research on human-induced climate change (global warming) and its impacts, scientists have followed a linear pathway of activity, starting from the specification of greenhouse-gas emissions, and finishing up with impacts and possible response strategies. From the perspective of those working at the right-hand end of this chain, i.e., policy- and decision-makers, the problem is that each step in the chain has an associated uncertainty.
Thus, for example, it isn’t possible to be certain about the amount of warming that will take place in response to an increase in atmospheric concentrations of greenhouse gases (i.e., moving from Step 2 to Step 3 in Figure 1), for reasons which include:
- Uncertainties in future demographic, social, economic and technological trends.
- Uncertainties around the carbon cycle response. What proportion of those greenhouse gases will remain in the atmosphere, and what proportion will be taken up by the land and ocean?
- Uncertainties connected to the climate sensitivity. What is the response of the earth system to these increased concentrations? Different climate models have different sensitivities.
Different uncertainties influence each step of the knowledge cascade in Figure 1. More importantly, these uncertainties compound at each step, so that by the time we have reached the step of projecting climate change impacts at spatial scales relevant for decision making (Step 4 in Figure 1) they have become large. This scoping document relates to the total sum of uncertainties from all sources, whether human or biophysical. The desire of the scientific community to be honest about these uncertainties means that statements in scientific papers and reports about future climate change and its impacts are expressed in terms of a range of confidence, and not as a single number.
Thus, a footnote in the IPCC Working Group I Summary for Policymakers states that:
“In general, uncertainty ranges for results given in this Summary for Policymakers are 90% uncertainty intervals unless stated otherwise, that is, there is an estimated 5% likelihood that the value could be above the range given in square brackets and 5% likelihood that the value could be below that range. Best estimates are given where available. Assessed uncertainty intervals are not always symmetric about the corresponding best estimate.”
It then goes on to say:
“...the best estimate for the low scenario (B1) [of warming] is 1.8°C (likely range is 1.1°C to 2.9°C), and the best estimate for the high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C).”
End Users, Decisions Makers and Practitioners
End users find this mode of expression around uncertainty hard to deal with in their thinking around actions on climate change. They seek firm numbers, at clear time points and fine spatial scales about the amount of warming and the associated effects on other climate variables such as rainfall, wind and windstorm. This is not something that science can deliver. Moreover, the absence of this precision can provide a justification for inaction. This desire for certainty as a prerequisite for action flies in the face of human experience. As a species, we have always been able to make decisions and plan for the future in the context of uncertainty in fields as diverse as defence, finance and insurance.
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