Renate Wilcke

Climate scenarios describe how the future climate may develop and form the basis for climate adaptation in Sweden. They are produced through a chain of scientific steps: societal and emission scenarios are fed into climate models, which calculate temperature, precipitation and other climate variables at global and regional level. Scenarios are not forecasts but reference points that show possible futures under different conditions. Climate scenarios can be presented as time series or as global warming levels, and the two approaches complement each other.

On behalf of the government, SMHI has surveyed and analyzed which climate scenarios can currently be used as a basis for developing national guidelines. The analysis constitutes an interim report—for which SMHI is responsible—as part of a commission assigned to SMHI, the National Board of Housing, Building and Planning (Boverket), and the Swedish Transport Administration (Trafikverket) to develop proposals for national guidelines on the selection of climate scenarios and guidance on how climate-related risks can be assessed by government agencies, municipalities, and other stakeholders, and provides a basis for further analysis.

The climate scenarios that are available and used in climate adaptation work in Sweden include, among others, radiative forcing scenarios (RCPs), published in 2010, and newer socioeconomic scenarios (SSPs), which together comprise six future scenarios. The scenarios describe pathways with a strong emphasis on carbon dioxide removal as well as pathways with continued heavy reliance on fossil fuels. The underlying societal development differs between the scenarios and leads to different degrees of temperature increase.

Several European countries use so-called global warming levels, which are based on pooled climate data from different future scenarios. There are therefore two possible starting points for national guidelines: climate scenarios as time series, and global warming levels at different points in time. They have the following advantages and disadvantages.

Time series make it possible to follow developments over time and show how the pace can vary between periods, for example emissions that first decline slowly and then rapidly once new technology is introduced. However, the spread between models is large, both for when a given temperature is reached and for how much the climate changes at a given point in time. Global warming levels reduce the spread in the results, because simulations are compared at the same climate state, and give a clear message about which climate state, that is, which temperature increase, climate adaptation should be based on. They can also be linked directly to the goals of the Paris Agreement. The drawback is that they say nothing about developments over time, and that slow processes such as sea level rise do not follow temperature but respond with a long delay. The two approaches therefore complement each other.

Because the chain is serial and largely funded through time-limited research grants, it takes several years before new knowledge is reflected in finished climate scenarios. The climate scenarios that SMHI provides for Sweden currently consist partly of older RCP-scenarios based on regional climate models and partly of newer SSP-scenarios based on global climate models. The next generation of scenarios, produced ahead of the IPCC’s seventh assessment report, is expected to give a narrower range of warming by 2100. The first global climate data for these scenarios are expected during 2027.

The report contains no recommendations but serves as a basis for the continued work on the government assignment.