KNMI Climate Scenarios
Current KNMI'06 scenarios methodology
01-09-2008
How were the KNMI'06 scenarios constructed?
In formulating the KNMI'06 scenarios, recent results from a large number of climate models have been analysed. These models are used by scientists to examine the human influence on the worldwide and regional climate. During the construction of the scenarios that were published in 2000 only the results of a limited number of climate models were available. Those only gave good information on global warming and sea level rise. The change in precipitation in the Netherlands in these scenarios was estimated using the statistic relation between precipitation and temperature in historic observational series. It was assumed that this relation would not change in the future and neither the air circulation patterns.
For the KNMI'06 scenarios, the relation between global warming, changes in air circulation above Western Europe and climate change in the Netherlands was mapped systematically. For the first time this was done by combining results from a large number of global and regional climate models and observational series. The global temperature rise, as calculated by the most important global climate models (GCMs), has been used as starting point for constructing the new climate scenarios for the Netherlands. Additionally, it was analysed how the air circulation patterns above Western Europe could change based on data from the same climate models. These global large-scale projections have been 'translated' to more detailed changes in temperature, precipitation, evaporation, wind, and sea level in the Netherlands. For this, a large number of regional climate models (RCMs) for Europe were used in addition to long-term observational series from Dutch stations. The methodology is illustrated schematically in Figure 20.
Figure 20 Schematic presentation of the methodology used for the construction of the KNMI'06 climate scenarios. The blue rectangles describe the sources of the scenario information in the green rectangles. The arrows symbolise the information flow. Information about the climate system at global, regional, and local scales was used for the climate scenarios.
The quality of the GCMs and RCMs used in this work was examined by comparing the accuracy and consistency of current climate simulations against observations (Figure 21). Knowledge of the climate system, published in scientific publications, forms the basis for the new scenarios. An elaborate description with all relevant sources and references to the scientific literature can be found in the KNMI'06 scenario report (see Further reading).
Figure 21 Comparison between simulated (blue) and measured (black) precipitation on summer days in the Rhine river basin. For the simulations the KNMI regional climate model RACMO2 was used. The figure shows the relation between the precipitation amount and a certain return period. The precipitation amount for a return period of 100 indicates that once every 100 days this amount is exceeded. Model calculations for 1961-1990 exhibit a good match with the observations. Therefore, the simulations for the future (red) are also considered reliable. The simulations for 2071-2100 show a slight decrease in low precipitation amounts, and a strong increase in larger precipitation amounts. Other climate models project the same trends. This information is the basis for the increase of the extreme precipitation in summer in all scenarios.
Dealing with uncertainty
Climate models differ considerably in their calculation of global temperaturerise. This is caused by uncertainty regarding future emissions of greenhouse gasses and aerosols, which, in turn, depend on such factors as future population growth, and economic, social and technological developments. Besides this, our understanding of the complex processes in the climate system is still limited. For example, the influence of water vapour, clouds, snow and ice on the Earth's solar and thermal radiation balance and hence the temperature still requires improvement. Some processes are not included at all. None of the climate models used in this study have an active carbon cycle. Besides, there are fundamental limits to the predictability of complex systems, such as the climate system. Uncertainty further increases when we go from a global scale to a regional scale, such as Western Europe or the Netherlands. At this scale, air circulation plays an important role. Most climate models calculate a clear change in circulation patterns above Western Europe. However, the magnitude and direction of change differ between models.
To be able to deal with these uncertainties, KNMI selected four climate scenarios from the broad range of possible futures. These scenarios are considered most relevant for Dutch policies. For these situations, as far as possible, a complete picture of our future climate is presented. Each of the specific scenarios is plausible. However, with our current knowledge it is not possible to indicate which scenario is most probable. Each of the four climate scenarios gives only one number for the change per variable, except for sea level rise where a range is presented (see Table 1). These numbers are uniform for the Netherlands: there is no differentiation by province or county. These numbers can also be used as indicative values for the river basins of the Meuse and Rhine (except for the Alps).
Choice +1°C and +2°C
The uncertainty band width in global temperature rise in 2100 (+1°C up to +6°C compared to 1990) of recent GCM simulations is slightly broader than the band width presented in the third report of the Intergovernmental Panel on Climate Change (IPCC, 2001). For the new KNMI scenarios, we used global temperature increases in 2100 of +2°C and +4°C (with associated temperature increases in 2050 of +1°C and +2°C, respectively). These temperatures lie within the GCM range.
The global temperature rise of +2°C in 2050 or in 2100 should not be confused with the +2°C target that is specified by the Dutch government and the EU as the maximum admissible global temperature rise to prevent dangerous anthropogenic interference with the climate system. This policy objective refers to a stabilisation of 2°C above the pre-industrial level, whereas the KNMI'06 climate scenarios present changes with respect to 1990. Also IPCC uses the same baseline year (1990) in its reports.
The global temperature increase of +2°C in 2100 compared to 1990 is the same as used in the old 'central' KNMI scenario, published in 2000. The increase of +4°C in 2100 is consistent with the old 'high' scenario.
Choice changed or unchanged air circulation patterns
Most of the used GCMs (selected on the basis of a good representation of the current air circulation patterns above Europe), show either hardly any change in air circulation in summer and winter, or a clear change in both seasons. On the basis of this finding, a choice was made for two scenarios with change in air circulation and two scenarios without. The last two scenarios link up well with the old scenarios, which also had no change in air circulation.
What is not included in the scenarios?
The new KNMI scenarios do not account for any possible occurrences associated with abrupt climate change, for example as a result of a complete collapse of the 'Gulf Stream' or the unexpectedly fast melting off of large ice sheets in Greenland and West Antarctica. The simulation of these types of events is relatively poor in climate models due to incomplete scientific knowledge about these phenomena. Besides, the indications for abrupt changes in the observations are also very uncertain. The scenarios neither include phenomena where it is not clear if they are physically realistic, such as 'super' storms that are much heavier than ever occurred in Europe. Whenever new insights become available on this topic, they will be included in the next update of the scenarios.
In several figures, the year-to-year variation is represented with grey bands. These bands have been made assuming that the variation in the past will remain the same in the future. The year-to-year variation is not included explicitly in the scenarios. The questions as to whether this year-to-year variation will change in the future still needs further analysis.
