Water sector benefits from considering more extreme climate scenarios

Time and again, weather extremes, and the speed at which our climate is changing, take us by surprise. Those extremes are unexpectedly fast for both experts and the general public – and even for climate scientists who have been making tremendous effort for years and continuously developing new knowledge about climate change. To what extent can the water sector continue to base strategic decisions on IPCC and KNMI climate scenarios such as the recently published KNMI’23 scenarios? We discuss this question in the new BTO/DWSI trend alert ‘Climate scenarios, tipping points and extremes’.

Scenarios provide an accurate picture of future climate development

The trend alert describes the latest insights into the significance and limitations of climate model predictions, as well as guidance on how to respond. For the water sector, the IPCC and KNMI scenarios set out a clear direction for the climate of the future. These models generate plausible scenarios that provide additional incentives to take mitigation action, and also describe approaches for climate adaptation. They are excellent for these purposes.

Gaps and uncertainties

However, it is important to understand that the IPCC and KNMI do not provide a complete set of the relevant plausible scenarios. The climate models used for these scenarios are a tremendous achievement. Nevertheless, they include a number of uncertainties and gaps relating to a range of climate, ocean and ice processes, and tipping points.

The melting of the ice sheets and changes in large-scale ocean currents are some examples. These processes are difficult to model because we do not fully understand them and because small-scale structures play a major role.

Some of these processes and/or the activation of tipping points have been described as ‘High-Impact-Low-Likelihood’. In fact, the latest science cannot properly determine how probable such events actually are.

Exploring other plausible scenarios

For the water sector, it is also important to explore which other plausible scenarios can be used to make plans for their sources, treatment plants and mains infrastructure: water infrastructure is built to last several decades. The inclusion of new extraction locations, and the planning, design and construction of infrastructure, take a long time. That makes it difficult for the water sector to move quickly and so it is essential to have the broadest possible picture of potential futures in order to be properly prepared. Other plausible climate-change scenarios of this kind may differ significantly from the ‘mainstream’ climate scenarios. They therefore complement those scenarios.

Considering additional, more extreme scenarios

The researchers argue that the water sector should look at additional scenarios based on the extremes in the simulation results. The scientific report accompanying the KNMI’23 climate scenarios sets out suggestions for scaling the KNMI’23 scenarios towards this more extreme side.

Considering climate processes that are currently left out of the equation

In addition, the water sector should also look at additional, more storyline-like, scenarios in which the climate processes that are currently left out of the equation are activated. Examples might be the weakening or collapse of the Atlantic Meridional Overturning Circulation, which includes the Gulf Stream. This could lead to Europe becoming cooler and drier. In this way, a clearer picture is established of the margins the water sector can expect to encounter (for example, in terms of water availability and the distribution of water demand over the year), as well as a better idea of the time that may still be available to implement any adaptation measures.

By factoring in other scenarios and climate processes as well as possible, the water sector can reduce the risk of ‘maladaptation’ in the future: adapting to potential conditions that turn out to be not relevant at all because, in the meantime, we find ourselves facing more extreme, or less likely, events that we have not foreseen.