Sizzling Hot Summers in Southern Europe: Record Highs Aren’t Due to Climate Change Alone

4 August 2022, by Dr. Leonard Borchert

Photo: pixabay, julian-hacker

Rising temperatures are an increasing burden on life in southern Europe. More accurate heat forecasts could offer people support.

Even though our weather apps often promise otherwise, reliable weather forecasts can only be made for the next few days. That’s not good enough to predict hot summers. Conversely, even though good climate models can predict several decades into the future, they’re too imprecise for such seasonal forecasts. The outcomes of these models are only really useful to identify trends, e.g. in which temperature range the changes generally take place, but they don’t show the variations from year to year. Accordingly, what I’m interested in are forecasts that lie right between these two methods and can cover timeframes of up to ten years.

Dr. Leonard Borchert ©privat

For agricultural or forestry planning, and for aid organizations, these decadal forecasts are tremendously important. That’s why I’m working together with my colleagues at Universität Hamburg’s Center for Earth System Research and Sustainability (CEN) to make such forecasts more accurate and reliable. To that end, we often have to analyze huge amounts of data. While doing so, I made a discovery, almost by chance: that before high summer temperatures developed in Southern Europe, the water in the eastern North Atlantic was often unusually warm in spring. Since the North Atlantic has a major impact on the weather in Europe, I suspected there might be a connection. In the future, should we expect record highs in the summer whenever Atlantic water temperatures are above average in the spring?

In order to test this theory for Southern Europe, I proceeded step by step. First, I analyzed data from more than 190 model-based simulations of the past – for the years 1970 to 2014. These “backward” forecasts showed me, among other things, the annual temperatures in the North Atlantic, in the Mediterranean and over Southern Europe. I then integrated the meteorological records for the regions in question. Lastly, I analyzed the data using a statistical model. And the results confirmed my theory. When the water in the North Atlantic and western Mediterranean was unusually warm in the spring, there were more often hot summers in Southern Europe; that’s what not only my calculations but also the actual meteorological records show. Generally speaking, the temperature curve for Southern Europe would seem to be characterized by variations that oscillate at a frequency of roughly 25 to 45 years. We’re currently in a climbing phase that will likely continue until mid-century.

My analysis also revealed: the frequent hot summers we’ve seen in the past several years are, to ten percent, due to the internal variability of the climate system. In other words, they’re not only the result of the rising CO₂ concentration in the atmosphere, but also part of natural variations in the climate system. In the years to come, the climate system’s internal variability will continue to affect the rising temperatures in Southern Europe. It won’t be until mid-century that these natural variations are topped by the rising CO₂ concentration.

The technique I developed can offer early warning signals for the future: if I use a climate model to predict the future springtime water temperatures in the North Atlantic – which such models can do very reliably – extremely hot summers in Southern Europe can also be accurately predicted for the next few decades.