"We can now look at climate change regionally"

Ms. Schwikowski, the partial report of a working group of the Intergovernmental Panel on Climate Change (IPCC) published in August includes the results of your research at PSI. How did that come about?

My research group and I investigate, among other things, various aerosols and how their concentration in the earth's atmosphere has increased or decreased over the past centuries. This is relevant for the IPCC because aerosols, too, have an impact on the climate. Aerosols are small particles suspended in the air. Soot is a classic example of this. You know it, that black smudge you see when a candle doesn't burn well. Soot is mainly produced by the incomplete combustion of fossil fuels.

The tiny floating droplets through which the coronavirus can spread are aerosols, right?

Yes, those are aerosols too, but they are liquids. My research group and I, on the other hand, investigate solid suspended particles, from soot or from other chemical compounds, for example sulphate aerosols, which are primarily formed from sulphur dioxide.

And these solid aerosols contribute to global warming much like the climate-active gases carbon dioxide (CO₂), methane, and so on?

With aerosols, you have to take a closer look. They clearly have an impact on the climate, but this is more complicated than the effect of climate-active gases for two reasons. First, not all aerosols have the same effect. Soot aerosols are dark and heat up in sunlight, so that the air is also warmed up. Sulphate aerosols, on the other hand, tend to reflect sunlight back into space and thus even cool the atmosphere. So we have to calculate how the effects of the various aerosols offset each other.

And the second reason?

CO₂ and the other greenhouse gases are very long-lived, so they reach every corner of our globe, and it doesn't matter whether you determine their concentration in Hawaii or in Switzerland – we have seen the same sharp increase in these gases all over the world since the beginning of industrialisation. Aerosols, on the other hand, have a much shorter life span: After several days or at the latest a week, they are removed from the air by precipitation, which is the most important cleaning process in the atmosphere. So they don't get evenly distributed in the atmosphere, and accordingly we actually have to measure them in the regions where they are released.

That sounds advantageous for the climate models. Is it?

Yes, the climate models have been improved significantly by taking aerosols into account.

When it comes to climate change, the general public mainly thinks about CO₂. What do you think non-experts should understand better?

For the time being, the fact remains that the climate-active gases are clearly the biggest factor in climate change. As you are probably aware, CO₂ is in first place. Methane then already comes in second place, but all these other gases are nowadays often converted into CO₂ equivalents and find their way into the debates in this way.

What rarely occurs to the public is the perception that the climate effect of all aerosols taken together has so far had a cooling effect on the climate. The warming aerosols are currently being outdone by the reflective, cooling aerosols.

Can that be quantified?

Yes. Currently, the global temperature has already risen by an average of 1.1 degrees Celsius compared to pre-industrial times. You will probably remember: The Paris Agreement, which was signed in 2015, stipulates that we should allow a maximum increase of 1.5 degrees in total. What we now know from research: We have already released so much of the climate-active gases into the atmosphere that we would almost have reached 1.5 degrees already. We only measure the current level of global warming as 1.1 degrees because the overall cooling effect of the aerosols comes into play.

Is there anything else to be considered besides the greenhouse gases and aerosols?

The third factor that has a strong influence on the climate and is therefore included in the climate models is the earth's reflectivity, the so-called albedo. Put simply, this has to do with the bright surfaces such as ice and clouds, which reflect sunlight during the day and thus cool the earth, compared to the dark surfaces such as oceans and forests.

What is your conclusion from the current IPCC report, and what does it mean for Switzerland?

Climate change will affect us enormously. The 1.1 degrees Celsius increase that we currently have globally represents the average value across all continents and oceans. The oceans react more slowly, whereas the land areas in contrast have already undergone correspondingly greater warming. This is precisely the strength of the current IPCC report, that it breaks down such developments on a regional basis. A global mean is too hard to grasp. In Switzerland we are already measuring a temperature increase of 1.8 degrees Celsius on average, almost twice as much as the global value.

And how could it proceed from here?

The IPCC scenario that extrapolates the current development path leads to an increase in the global average temperature of up to 6 degrees Celsius by the year 2100. Applied in turn to Switzerland, this means that without strong interventions we can expect a temperature increase of more than 10 degrees, on average. Nobody wants to imagine that. Last summer – with the forest fires in the Mediterranean area, the droughts, and the floods – reflected exactly what many experts predict, and those are just the weak beginnings. We have to avert this climate crisis.

Interview: Paul Scherrer Institute/Laura Hennemann