Why is Australia exposed to extreme weather conditions

Extreme weather due to climate change More heat waves, droughts, heavy rain and hurricanes


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Is there a direct connection between extreme weather phenomena and climate change? Scientists can find out with refined climate models and faster computers.

Status: 04.03.2021

The average temperature is one thing. Of course, in our latitudes it is on average warmer in summer than in winter. How the weather is or will be determined by various properties, temperature is just one of them. The weather can also be described by the pressure and density of the atmosphere. Their interaction determines whether the sun shines here on earth, whether it is hidden by a blanket of clouds, whether we should pack an umbrella, sunglasses or a windproof jacket.

But then there are also extreme weather phenomena. These are extraordinary weather phenomena such as heavy rain, heavy thunderstorms, gale force winds or a persistent drought. Whether such extreme weather is really extreme always depends on the area observed. While snow-capped pyramids would be a sensation in Egypt, one would rather worry in this country if no snow would fall at all. On the other hand, there is only barely two dozen millimeters of precipitation per year in the Egyptian capital, Cairo, which is considered completely normal. In this country, on the other hand, not only nature in parts of northern Germany and Bavaria suffered from a drought in 2018 and 2019 with much larger amounts of precipitation in the year. That is also extreme weather.

What is extreme weather?

Extreme weather is not a meteorological term. Rather, the phenomenon describes an extraordinary weather event - torrential rain, a heavy thunderstorm, drought, storm, a tornado, extreme snowfall, slipperiness, a hurricane-like gust of wind - which is statistically rare in terms of its return, size and / or duration. In order to recognize such an extreme event, one needs valid comparison and observation data and must specify a comparison period. This normal climatological period must always be related to a clearly defined geographical area.

The German Weather Service (DWD) expects more storms, extreme rainfall and heat waves due to global warming - so "more extreme weather". It is difficult to establish a connection between a single, by definition extreme, event and the climate. But the observed accumulation of such events can now be taken as an indication that the assumption of many climate researchers is correct. This assumption is that extreme weather increases with increasing temperatures. Statistical evidence is not that easy, because short observation periods, the natural climate variability and the comprehensive recording of small-scale events - for example heavy rain or thunderstorms - are difficult. But for some years now, scientists have been investigating the connection between extreme weather and climate change with the help of refined climate models and faster computers: this science is called assignment research.

The weather machine: sun, clouds, highs, lows, jet streams ...

Everything starts with the sun ... The sun's rays hit the earth from one direction. Since the earth is a sphere, it hits with different intensities: At the equator, the angle of incidence is steeper and the radiation is more intense. At the poles, the angle of incidence becomes flatter and the radiation is distributed over a larger area. In addition, the path through the atmosphere is longer and more energy is absorbed.

Weather is a chaotic and very complex system. If a parameter changes, such as the average temperature, this has serious consequences.

Weather vs. climate: climate change and extreme weather

Heat in Hamburg: extreme weather related to climate change - or just a nice summer day?

Weather is a short-term phenomenon. It describes the short-term state of the atmosphere in a certain place at a certain time. The climate, on the other hand, is more of a long-term issue. Usually over a period of several dozen years, the climate describes the typical conditions on the basis of a large number of individual observations. Therefore, one cannot draw any conclusions about the climate from a single observation, for example a rainy day in July. Only after decades of observations every day in July and during the year can scientists say whether this one rainy day fits the typical climatic conditions of the observed region - or not.

Extreme weather is by definition an extraordinary event that rarely occurs. With a climate model, however, scientists cannot even predict whether the sun will shine the next day or not. Just like the climate itself, climate models describe rather long-term trends. It is not possible to find out whether a single year of drought or a single heavy rain event caused climate change alone and would not have happened at all without it. But to find out to what extent extreme weather events are related to global warming in the course of climate change, scientists can come up with something. This field of research is called assignment research.

Extreme weather in Germany and Bavaria

A farmer harvests potatoes in extreme drought - a dusty affair given the drought.

What extreme weather can do is shown again and again in parts of Germany: State of emergency due to torrential rain. Due to the constant rain and the resulting flooding, entire residential areas have to be evacuated. Germany has been suffering from a drought since 2018. These dry years must first be compensated for, but that is not currently the case. The soils are only superficially moist. The deeper soil layers, on the other hand, are still far too dry at the moment.

The impact of climate change on the weather

Bavaria's climate strip: The visualization shows how much Bavaria warmed up on average from 1881 to 2019.

The average temperature in Germany has already risen since the beginning of systematic weather records in 1881: From 1881 to 2019, the German weather service DWD measured a temperature increase of 1.6 degrees Celsius. An increased temperature of the air has an influence on the weather: if the temperature rises by one degree, the air can absorb seven percent more water. Especially Eastern Europe, Eastern Germany and parts of Bavaria are also increasingly coming under the influence of the Mediterranean. A low over the Mediterranean Sea can soak up water like a sponge, pass the Alps to the east and rain down in Central and Eastern Europe. According to the meteorologists, May in particular offers a high potential for severe weather. Because of the strong sunlight, the continent has already warmed up, but the moist air over the sea is still quite cold. The temperature contrasts and the humidity create low pressure areas over Central Europe.

Extreme weather and temperature records in Bavaria

2020: third warmest year With an average temperature of 9.5 degrees averaged over the year in Bavaria, as in the previous year, the year 2020 was the third warmest year since the weather records began. More than two degrees, regionally even three degrees in some places, the temperatures exceeded the long-term mean from 1961-1990, in which the temperature averaged 7.5 degrees over the year. Only 2018 were warmer than 2020 in Bavaria - almost half a degree warmer - and 2014 with 0.1 degrees more. 2020 was the second warmest year in Germany.

In Germany too, climate change does not only mean a change in the mean conditions, i.e. an increase in mean temperature. It also means a change in the extreme values: there are more hot days and fewer frosty days. The number of heat waves is increasing - and what used to be extreme is almost normal today. That is why climate change also means more extreme weather. An increased mean temperature increases the risk of droughts, forest fires and heat waves.

"The heat extremes are not increasing simply because we are warming the planet, but because climate change is also disrupting air flows that are important for the formation of our weather. The reduced daily fluctuations that we observe lead to longer-lasting weather conditions. And these create extremes that stretch over weeks. "

Dim Coumou, Potsdam Institute for Climate Impact Research

Extreme weather: heavy rain, mudslides, floods, floods

Especially with regard to heavy precipitation events - i.e. torrential rains - scientists assume that the amount of rain will increase with global warming. The DWD notes that from 2001 to 2019 almost every place in Germany was affected by a heavy rain event, and continuous rain particularly hit the mountain regions. Even in hot and dry years, there were lots of heavy rain events. In principle, extreme storms, torrential rain and floods can occur anywhere in Germany. However, the heavy rain can be particularly heavy in mountainous areas. That is why the Ore Mountains, the edge of the Alps and the Black Forest are particularly affected. This is due to the fact that rain and thunderclouds accumulate there and the precipitation falls more intensely there than elsewhere.

"If you have more energy in the atmosphere close to the ground because the greenhouse gas concentrations have risen and cause a higher heat content in the lower layers of the atmosphere, then you should assume from the point of view of physical plausibility that there will be more extreme events in the future."

Prof. Heiko Paeth, Institute for Geography and Geology, Chair for Physical Geography, Julius Maximilian University of W├╝rzburg

The frequency of extreme weather events in Germany is increasing

In August 2005, Weltenburg Abbey near Kehlheim was surrounded by floods from the Danube. Not a rare danger in recent years.

Although there are regional differences and also uncertainties as to the precise effects of individual changed parameters, it is basically clear that climate change should lead to more extreme weather events in Germany. Indeed, an increase in extreme weather has been observed in Germany. In individual cases, however, this observation is of little help at first. It is true that a storm potential can be recognized in the weather forecast days in advance. But where exactly more than a hundred liters of rain per square meter will fall can often only be said minutes in advance. The fire brigade and disaster control are networked directly with the DWD and receive constant information via their own fail-safe systems. You can reach warning meteorologists on hotlines. If extreme weather conditions can be identified, the DWD can set up extra shifts.

Extreme weather - natural or man-made?

Was this heat wave a coincidence or is it due to climate change? And what about the heavy rain, or the hurricane, the drought or the cold spell? For a single event, no matter how extreme, it is impossible to say: climate change is the cause. Any weather event could occur purely by chance or be part of natural variations in the weather. Extreme weather also existed long before climate change.

However, it is clear that climate change has an impact on weather and extreme weather. It is evident from observations and climate models that climate change makes certain extreme weather events either more probable or stronger. This mainly affects heat waves and heavy rain, but also floods. But until recently, researchers could not really say whether a single event was actually due to climate change. The relatively new research field of assignment research has been changing this for a few years.

Attribution research: assigning extreme weather to climate change

What is mapping research?

Assignment research is also called attribution research. It is still a very young field of research. Researchers working on attribution research want to assign extreme weather events by finding out to what extent man-made climate change could be responsible for them. There are different methods and approaches. One method is to create parallel climatic worlds on the computer. Virtual worlds without climate change and virtual, "real" worlds with climate change are then compared with one another in order to find out how often and how strongly certain extreme weather events occur.

With the help of complex climate models and powerful computers, it is now quite possible to make statements about certain extreme weather events as to whether these are "man-made" or "natural in origin". An example: climate researchers were able to show that a heat wave that suffered the Russian region of Siberia in 2020 - 38 degrees Celsius was measured in the city of Verkhoyansk on June 20, 2020 - would have been almost impossible without climate change. Without climate change, such an extreme weather situation would only occur approximately every 80,000 years. But humans increase the probability of such heat waves through the greenhouse gas emissions by a factor of 600.

A map of man-made extreme weather?

The climate website Carbon Brief has put together an interactive map on which it has so far collected more than 350 scientific studies that deal with the allocation of extreme weather events. Their analysis shows that 70 percent of the 405 extreme weather events examined were made either more likely or stronger by man-made climate change, while 9 percent were less likely or weaker. That means: 79 percent of all events examined were influenced to a certain extent by climate change and thereby by humans.

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