Climate change scenarios in different countries

Designing future-proof cities and stormwater systems requires some forecast of what rainfalls can be expected. In this step we will go over a few examples of where this data can be found and what it may look like.

Sweden

We will show two different approaches used in Sweden. Based on work by the Swedish Hydrological and Meteorological Institute, the Swedish Water and Waste Water Association has developed guidelines that rainfall intensities (calculated from IDF curves) should be multiplied by a factor of at least 1.25 for rains shorter than an hour and at least 1.2 for rains longer than an hour. While this is a very simplified method, it does make it very easy to use for practitioners.

More detailed data is available from the Swedish Hydrological and Meteorological Institute (SMHI). Using climate models, they provide estimates of future climate conditions accounting for:

• Different scenarios for the future mission of greenhouse gasses.
• Different time horizons: until 2040, until 2070, and until 2100.
• Seasonal variability or monthly variability (for some variables).

The available projections cover many different types of data, including:

• Temperature
• Precipitation
• Soil moisture

As we saw last week, these factors can all influence the generation of runoff in urban catchments. The climate change scenario service also provides estimates of changes in runoff from natural catchments. Here the variation throughout the year is particularly interesting: runoff is expected to increase during the winter months but decrease during the summer. There is also considerable variation across the country, as shown in the example below.

The portal is available in two versions:

• Basic Climate Change Scenario Service (link)
• Advanced Climate Change Scenario Service (link)

At the bottom of the page for the Advanced service, there is also information about the uncertainty of the forecasts. For each scenario, many different runs of the climate model are made with variations in the input data based on how certain (or uncertain) each input is considered to be. The graphs about the uncertainty of the forecast show the variation between these different model runs. If there is a lot of variation it means this particular forecast is more uncertain.

The percentage increases from the service can be applied to design rainfalls calculated from IDF curves.

United Kingdom

For the UK, similar data is available from the UK Climate Projections User Interface. Like the Swedish service, forecasts consider:

• Different scenario for future greenhouse gas emissions
• Different time horizons until 2100
• Seasonal or monthly variability

Projections are also available with different spatial resolutions. For urban drainage applications the smallest resolution (2.2 km) is most relevant, but forecasts are also available for coarser resolutions up to 60 km. Forecasts with a resolution of 25 km are more extensive, e.g. they have more different climate change scenarios available.

This data is available under Open Government License, read more about the license here.

It is important to realize that all forecasts of climate change are uncertain. This is both because we do not know everything that will happen in the future (e.g. greenhouse gas emissions), but also because we cannot model the earth’s atmosphere, oceans and land with perfect accuracy.

Important to note about climate change projections

All forecasts of climate change are uncertain. This is both because we do not know everything that will happen in the future (e.g. greenhouse gas emissions), but also because we cannot model the earth’s atmosphere, oceans and land with perfect accuracy.

A lot of work is being done on gathering more and better data to support climate change models. It is therefore likely that services, such as the ones described in this article, will continue to be updated with new projections. It is therefore important to re-visit such guidelines regularly to make sure you are using the most up-to-date information.