by Dr Johannes Bühl, Professor, Harz University, Wernigerode
Firefighting plane in the area of Ancient Olympia, Peloponnese, Greece, 2021
Recently, notable and impactful weather events have brought the topic of weather and climate extremes back to our attention. But what’s behind them? Why are we apparently seeing more extreme weather? Is it only an observational bias because social media connects us to even the most remote parts of Earth or are we really observing a new powerful natural phenomenon? Unfortunately, the latter is the case.
Extreme weather increase – not a fiction
The Intergovernmental Panel on Climate Change (IPCC) states in its latest assessment report quite unambiguously that “the frequency and intensity of hot extremes will continue to increase and those of cold extremes will continue to decrease, at global and continental scales and in nearly all inhabited regions with increasing global warming levels”.
While to date the global temperature on Earth has increased by about 1.1°C compared to the pre-industrial age, the global temperature on the European continent has already risen by more than 1.5°C, with a particularly rapid increase observed since about the year 1990. We may remember the winters of our childhood being colder and longer with more intense snowfall and extremely cold temperatures. On the other hand, heatwaves or forest fires were only a rare nuisance. That observation is correct and confirmed by recent scientific observations.
Our human senses alone are not capable of detecting a change in average annual temperature of only 1.5°C. However, we do quite easily observe changes in extreme events because we can perceive their frequency, intensity and – especially – their devastations.
A simple statistical effect
This shift in the character of extreme events has its root cause in the annual distribution of temperatures in our part of the world. The mid latitudes, in which the European continent is located, is privileged to experience quite diverse weather during the annual cycle. You can have it all: skiing in winter and relaxing at the lake shore in summer. In between, flowering fields and a beautiful Indian summer may be enjoyed. Figure 1
shows the corresponding annual temperature distributions, one from the pre-industrial age and one assuming a climate warming of 3°C. These distributions are bell-shaped, because weather is formed by an interaction of different processes – some strengthening and some weakening each other. For extreme weather to occur, a lot of effects must add up, so the more you deviate from the centre of the distribution, the rarer an event becomes.
As it turns out, climate change is shifting the distribution, but keeps its bell-shaped form untouched (red curve in Fig. 1). You can see what was described before in a subjective way: on average, we do not feel a big difference in temperature, but at the extremes edges of the temperature distribution large relative changes happen. In effect, extremely low temperatures are suppressed while extremely high temperatures are greatly increased. In the end, the tiny shift in mean temperature is amplified for extreme events.
Impacts – only the beginning
As a result of shifting weather patterns, an increasing amount of local ecosystems are pushed beyond their sustainable limits. Some notable recent examples include:
- In the Alps, glacial buildup in winter is reduced due to warmer temperatures and less snowfall, leading to less buffering of water and to low levels of the river Rhine in summer.
- A heavily increased occurrence of extreme heat and drought in summer puts stress on agricultural production and forests leading to increased failure of crops and wildfires.
- A warmer atmosphere can store more water vapour and extreme precipitation events occur more frequently and more violently.
This principle also applies to other problematic areas, like sea level rise, global precipitation and tropical storms. In all of these fields, a new class of uncommonly violent events are observed, and even more unusual events can be expected in future. The takeaway message is that there is no buffer. Climate change will push global temperatures higher and in its wake, new and so far unknown extreme weather events will happen.
No excuse for inaction
The small and gradual change in global temperature that has happened so far has already measurably affected weather extremes. In all its simple mathematical beauty, the statistical effects illustrated above show how merciless climate change is manifesting itself. By now, we should really hear the atmosphere’s warning bells ringing faster and louder. Extreme weather events which until now have occurred once in 100 years may now occur once in 10 years. This acceleration will put ever increasing strain on our infrastructure, with both an increasing magnitude of extreme weather and shortening recovery times. But we still have an ace or two up our sleeve, because these changes do not come unexpected and have been projected by the IPCC long ago. The recent reports of the IPCC give detailed recommendations for each region of Earth and allow us to get ahead of this seemingly dire situation. The contents of the IPCC reports certainly count into the greatest achievements of humankind so far and their multiplicative value is immeasurably high. The reports and their suggestions are meanwhile so elaborate, that taking them literally provides more chances of success than using its remaining small mathematical uncertainties as an excuse for inaction.
