How reliable are climate scenarios?

What are the results from different climate models, and how much uncertainty is there regarding the future scenarios? A new SMHI report describes the changes in temperature and precipitation in Sweden.

There are many uncertainty factors in the climate scenarios. For one there is no way of knowing the volume of greenhouse gas emissions in the future, and moreover, the climate varies naturally. The projection models for the future climate also have inherent uncertainties, and they differ in their mathematical descriptions of land, sea and atmosphere.

Comparing models

A new SMHI report compares the results from different climate models for temperature and precipitation in Sweden. In total over a hundred scenario projections based on more than 20 global climate models have been analysed. Furthermore, around a dozen more detailed projections have been analysed using the Rossby Centre´s regional climate model and compared with the global models. Rossby Centre is the climate research unit at SMHI.

Six different emission scenarios for the period up to 2100 were included in the analysis. All the scenarios have a significantly higher concentration of greenhouse gases in the atmosphere than at present.

The analysis may, for example, provide guidance for anyone who uses scenarios, or for model developers.

"There used to be only a few simulations to compare. The new ensembles display similar results to before, but show the distribution between the models better, and thereby illustrate the uncertainty span," says climate researcher Erik Kjellström.

Distinct increase in temperature

The results clearly show that climate change will make itself felt even in the near future. Within just 40 years a distinct increase in temperature is projected for the whole of Sweden, and this will be most noticeable during the winter. Annual precipitation will also increase nationwide. There is, however, more uncertainty regarding precipitation in southern Sweden during the summer, as some models indicate a decrease and others an increase.

By the end of the century the effects of emissions will be greater, with further rises in temperature. The mean temperature for northern Sweden during the winter could, for example, increase by between just over 4°C and 8°C, depending on which emission scenario is selected.

The temperature rises in all the scenarios are greatest in the north and during the winter. This is mainly due to a decrease in snow coverage, but also to the expected change in large-scale circulation in the atmosphere, with more westerly winds resulting in a milder winter climate.

New report published

The report entitled Temperature and precipitation changes in Sweden, a wide range of model-based projections for the 21st century is by Petter Lind and Erik Kjellström. The work has been carried out within the framework of the EU project Ensembles and the Swedish Mistra-SWECIA research programme.

Temperature and precipitation changes in Sweden; a wide range of model-based projections for the 21s