I want to be honest, it's way too warm and I actually don't want to write at all. The heat makes me slow in the head. Actually I only want one thing right now: Go to the lake, get out of these clothes and into the water to cool down. Instead, I'm writing about a question I as a meteorologist hear a lot recently: Is this climate change? Is it so hot at the moment because humanity has emitted so much CO2 ?

Is it so hot today, because we humans emit so much CO2 ?

As a scientist, I have to answer this question with a clear "it's not that simple". Because unfortunately, in science, things are usually more complicated than you would imagine at first. We know this problematic as attribution. It involves two steps:

1. Discover climate change, i. e. find that climate has warmed.

2. Determine the reason for the observed climate change.

For both of these two steps, a statistical is analysis is needed so one can clearly state that climate has warmed, and that it is due to human activities.

But let's be honest

Let's be honest. We all know that climate has been warming and that we humans are to blame. If anyone wants to state otherwise, just turn a blind eye on the truth. As scientists we just have a hard time stating this exactly in this way. It is virtually impossible for us to say things such as "we are 100% sure."

The reason for that is simply the nature of science itself: The scientific method which has been practiced since the 17th century is based upon a large amount of skepticism. A hypothesis is made, and to test it, experiments are conducted or other data are gathered, the results are evaluated statistically and interpreted critically. We are practically educated to doubt everything. This is why one can often read about a high "confidence" when it comes to climate change. What we actually mean is that we're pretty damn sure that climate change exists and that it's mankind's fault.

Sometimes I could rant about the cautious use of language in science. I think that we could and should be a little more bold at times.

But, back to the subject: Is the current heat wave part of climate change?

To answer this question, we have to go back to the two steps of "attribution":

1. Did climate change?

A single hot day or one heat wave is not yet climate change. This means that you cannot say after a hot day that "this is climate change". Because climate implies that you consider a long time period, typically 30 years. For example we could look at the climate statistics for the month of June in Leipzig.

climate data from Leipzig

Here we can see that the mean value of temperature for June 2019 is 22.2 °C. The mean of the years 1970 to 1999, i.e. the climatological reference period of 30 years, is only 17.1 °C.  This means that the month of June 2019 is - until now - 5.1 degrees warmer than the average.

Apart from that we can see that the highest temperature measured in June ever is only a few days old: On 26th of June we had incredible 37.3 degrees in Leipzig! This is the highest temperature, which was measured in June since the beginning of our records.

Our institute has even made a tweet on this occasion.

But not only Leipzig cracked all the temperature records this week. Stations all over Germany announced that the past Wednesday was the warmest June day since the beginning of measurements.

The second step is determining the reason for the increased temperature. For that, scientists use climate models. For example, it is possible to determine the temperature increase due to CO2 emitted by humans by modeling the climate system one time with this additional CO2 and another time “without humans”. By these means, one can estimate how strong the human effect is on climate and better understand the physical processes which play a role in this.

This means: Single hot days cannot be attributed to climate change. However, heat waves happen more often due to the warming climate. When looking at the trend of temperature, it is clear that climate has been warming noticably. I now quite popular example are the warming stripes of the Scientists for Future (siehe unten).

The pictures in this post were taken from Julia D'Alkmin and Atte Grönlund on unsplash. Thanks a lot!

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