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Published online by Cambridge University Press: 28 January 2021
Section 2.1 describes the present state of knowledge about the climate. Section 2.2 discusses the difficult judgments that have to be made in formulating climate policy. Section 2.3 shows that the climate policy pursued to date can at best be seen as a first step.
KNOWLEDGE ABOUT THE CLIMATE
Our knowledge about the climate and the influence of human activity on it is still incomplete, but is increasing. This report takes as its starting point the scientific insights as set out in the Third Assessment Report (TAR) by the Intergovernmental Panel on Climate Change (IPCC 2001), supplemented by more recent insights both at global level (Levin and Persching 2006) and specifically for the Netherlands (MNP 2005; Rooijers etal. 2004). The current level of knowledge suggests that, without a climate policy, the global temperature will rise in the period to 2100 by an average of between 1.4 °C and 5.8 °C compared with 1990. The average world temperature in 1990 was already 0.6 °C higher than in 1850.
The global climate system reacts only very slowly to changes in emission patterns. There is a time lag in the reaction of CO2 concentrations to changes in emission patterns because of the long residence times of greenhouse gases (GHGS) in the atmosphere and because emissions resulting from human activity are small in relation to the total amount of CO2 in the atmosphere. Moreover, the global temperature responds slowly to changes in atmospheric concentrations of greenhouse gases because the change in the energy balance is small in relation to the accumulated warmth. There is thus a double delay in the transition from a ‘flow variable’ to a ‘stock variable’, in which the second flow is determined by the initial stock of greenhouse gases. In such a slow system, disturbances have a lasting effect and the end result of changes in emission patterns are seen only after many centuries. Even in the event of a reduction in net emissions, the temperature rise that has begun will continue for a long time. Although the year 2100 is often used as an endpoint for projections, a stable final situation will not have been reached by that year, especially as regards sea levels.
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