Climate change scenarios are generated using climate models along with assumptions with regard to future greenhouse gas emissions. It is important to use models with a high resolution for Switzerland, given its complex topography. Even then, it is not possible to reliably and fully map processes occurring on a small scale, and this can lead to uncertainties. Uncertainties arise from the fact that natural fluctuations can be superimposed on the changes brought about by greenhouse gases.
How the climate of Switzerland will look in the future depends on global trends on the one hand, while on the other, regional and local factors play an important role. Climate model simulations are employed to enable climatologists to take into account the numerous influencing factors when generating regional climate scenarios. Pre-defined evolution paths for greenhouse gas emissions (emission scenarios) and their levels of concentration in the atmosphere constitute one of the framework conditions for projections of future climate scenarios (Source: MeteoSwiss).
How do climate models work?
A climate model is a system of physical and mathematical formulas and procedures that depict real-life conditions in simplified form. Climate models are constructed in a similar way to the models employed in weather forecasting. However, they are not restricted to the lower atmosphere where the weather plays out; they also simulate ocean currents and, to some extent, the interplay between snow, ice and vegetation.
Thanks to climate models and high-performance computers, our climate can be projected into the future. This allows us to identify the impacts of greenhouse gas emissions on the climate. Global models thus demonstrate how the climate could change on a global scale. However, their spatial resolution is too low to provide information on the regional climate of a small country such as Switzerland, and so regional climate models for various parts of the globe are also used. The results of these models are currently available at resolutions of between 12 and 50 kilometres.
With the help of statistical methods, the resolution of the results can be further enhanced. Where long-term, reliable measurements exist, it is possible to draw conclusions about certain meteorological measuring stations or to produce extensive maps with a horizontal grid box size of two kilometres, as for instance for temperature and precipitation.
What are uncertainties?
In spite of the relatively high resolution, it is not possible to compute small-scale effects, such as the impact of mountain topography on wind flow, local exchange processes between the ground and the atmosphere, or the local effects of clouds on irradiation. These influences have to be described more simply, based on empirical data. The nature of this description varies from model to model, and is one of the main reasons why different models generate different climate projections. To take into account this kind of uncertainty, several different regional and global-scale climate models are normally evaluated together (Source: MeteoSwiss). The comparative analysis of several simulations allows the uncertainties associated with the Climate Scenarios to be estimated.
Beside these model uncertainties, there are also natural variabilities. Even though climate is a representation of the “average weather” in a location, it is still subject to natural fluctuations. Climate variables such as temperature and precipitation can vary widely over decades. The Climate Scenarios provide estimations of the future climate situation, including the long-term trend, on the basis of greenhouse gas emissions as well as natural fluctuations.
Fluctuations in climate variables can be greater over decades than the changes due to a longterm trend. One such example is the temporary attenuation of the global temperature rise between 1998 and 2012, widely known from controversial debates in the media as the «hiatus». By the same token, a temporary intensification of the long-term trend is also possible.
Theoretical considerations on climate change, climate models and previous measurement series largely agree with one another. It can therefore be assumed that the simulated trends will prevail in the long term. This also holds true even if measurements over the next few years take a different course in the short term because of natural fluctuations.
Last modification 01.11.2018