How do you predict the climate?

Climate change: "We have to adapt to increasingly extreme weather"

Heat waves, heavy rain, floods: Extraordinary weather events kill thousands of people every year and cause billions in damages. The climate researcher Thomas Stocker explains why we have to reckon more and more with such catastrophes, how Germany is changing due to global warming - and what everyone can do themselves to slow down climate change

GEOkompakt: Professor Stocker, anyone who watches the news sometimes has the feeling that extreme weather events such as heat waves, floods and storms are on the increase. Is that true?

Prof. Thomas Stocker: In fact, some weather extremes such as heat waves and heavy rain have become measurably more frequent due to climate change. Think of the summer of the century in 2003, the warmest European summer in 500 years - and yet it was just the beginning. In recent years, the summers in parts of Europe have always been unusually hot. But we can clearly see that not only the frequency but also the intensity of such exceptional phenomena has increased.

What did you find in terms of rainfall?

Especially in the transition months in spring and autumn, heavy precipitation is measured, as we know it from the tropics in this intensity and frequency. This is due to the fact that the westerly wind increasingly transports moist, warm air masses from the sea to the mainland. Then there are always sudden rains which, in the worst case, overload sewage systems, flood cellars and cause rivers to overflow. In the windless summer, on the other hand, it gets hotter and drier inland - the said heat waves are the result.

Is climate change leading to an increase in storms, as is often claimed?

So far we cannot say with certainty whether violent hurricanes like “Xavier”, which swept across northern Germany and Poland at the end of 2017 and killed nine people, are already a consequence of climate change. However, model calculations indicate that as global warming increases, we can definitely expect more severe storms. This is not least due to the fact that storms form over the open sea and are fed by the warm surface water. With climate change, the water is also warming, thus supplying the storms with more energy. The hurricane season is likely to worsen as a result, especially in the USA.

Where is the boundary between normal, seasonal fluctuations and extreme weather events?

The weather is a statistical phenomenon. For each location, we can determine the range of fluctuations in temperature, precipitation and wind speeds over the years. But there are always outliers, such as exceptionally cold, warm or windy days that do not fit into the statistics for the respective season. By definition, these are extreme weather events.

If the earth gets warmer, then the fluctuation range of the measured values ​​for Germany would have to shift in the direction of warmer temperatures.

That is exactly what we are observing. We are already measuring increased mean temperatures for Central Europe. At the same time, there are significantly fewer cold waves, but more and more intense heat waves. And if the curve shifts further in the direction of warmer temperatures, some heat waves in the future will move out of the extreme range and suddenly become the new normal. The outliers will then be even hotter and drier than we are used to today.

Do phenomena like the water warming by El Niño in the Pacific or the monsoon in India count as extreme weather?

No, such phenomena, which occur with a certain periodicity every few years, are counted among the “climatic modes”. These are regional manifestations of weather events that can last for months or even years and are considered normal climatic variations. For the European area, the North Atlantic Oscillation is particularly relevant, primarily a winter phenomenon that has no fixed periodicity. There is a fluctuation in the air pressure difference between the Iceland low in the north and the Azores high in the south, which increases or decreases the westerly wind. Depending on whether the fluctuation is positive or negative, we have more or less precipitation, milder or lower temperatures in winter.

How well are we able to predict specific extreme events today?

They can be forecast for five to ten days within the framework of the normal weather forecast. For longer-term estimates, only statistical statements are possible.

Why can researchers determine how the earth's climate will develop in the next 100 years, but find it so difficult to forecast the weather for the next week?

The problem is the level of detail: general trends and developments are much easier to calculate than the precise weather conditions at an exact time in a specific location. I compare it to a saucepan: if you keep adding the same amount of water to your home, you can predict quite reliably how hot the water will be after ten minutes of heating at level three. But if you want to predict exactly where a steam bubble will next form at the bottom of the pot, you have to realize that this is impossible. What is happening in the boiling water is too complex. And so it is with the climate and the weather: We can calculate global warming very long in advance if we set the scenario for CO2 emissions. We can also estimate the future frequency of heat waves, for example, but not the weather or an individual event.

How do you know your climate projections are correct?

We work with models that are verified using metrics recorded in the past. If we succeed in simulating the climatic developments of the past 100 years, we assume that we can also apply this model to the future. However, there are limits to predictability in complex systems such as weather.

What does the warming mean for Germany? Will we have a Mediterranean climate in 2100?

No, definitely not. Because in addition to rising temperatures in summer, we can also expect increased amounts of precipitation and wind speeds in autumn and spring. In concrete terms, this means: more storms, floods, storm surges and heat waves. We assume that certain weather extremes will occur five to ten times more frequently in the next 40 years if it is not possible to keep the warming below two degrees Celsius. Then there would be a heat wave every two to three years instead of every 20 years. For Central Europe this means a completely different, even more changeable climate.

How does this affect the environment?

It would have dramatic consequences for ecosystems. Certain plants such as the spruce do not cope well with summer drought. They will withdraw and instead give way to more oaks. The landscape will then change over the decades.

How can Germany prepare for more frequent extreme weather events?

Better air circulation in cities by cleverly arranging streets and buildings helps against heat waves. Green areas also mitigate the warming, especially on and around buildings. In addition, the sewage systems would have to be expanded in order to be able to contain increasing amounts of precipitation.

If you look over a larger area, you need more floodplain landscapes and floodplain areas as a buffer zone to prevent rivers in populated regions from overflowing during persistent rainfall. Improved coastal protection can help against storm surges, and in order to reduce the effects of storms, it makes sense to remove diseased trees in the city, along roads and rails. Conversely, extensive forest areas help storms to take their power when they move from the sea over the land.

Could some regions of the world become uninhabitable due to climate change?

The island states in the Pacific are already threatened by rising sea levels because seawater is entering the freshwater reservoirs. If the warming continues, entire islands and large coastal areas will be flooded, for example in Bangladesh. Unlike Germany, the country does not have the means for protective measures. Millions of people would be forced to flee. The poorest would be hit hardest.

To prevent this, scientists are calling for global warming to be limited to two degrees Celsius. Is this realistic?

This is not what the scientists are asking for, it is what all the countries that have agreed to the Paris Climate Agreement are calling for. I have great hope that we can achieve this goal. However, this becomes more difficult with each passing year. Because in order to comply with it, we must not emit more than 770 billion tons of CO2. Annual emissions worldwide are currently around 40 billion tons. This means that if emissions remain the same, our budget will be used up by 2040. Then we would suddenly have to go down to zero. So the more we reduce today, the more time we have before we finally break the two-degree mark.

Some even want to limit it to 1.5 degrees. Why does half a degree make such a difference?

Think about your own body. Half a degree more and you will feel limp and feverish. It is no different with planet earth with its ecosystems. In addition, the 1.5 degrees Celsius is an average value that is not evenly distributed across the globe. The warming in the tropics is low, while the temperature in Central Europe and at the poles rises by several degrees Celsius. For example, whether the ice cover in the Arctic and Antarctic will survive the summer or not is decided between 1.5 and two degrees Celsius. The Intergovernmental Panel on Climate Change came to this conclusion in its last report.

What is the function of this council?

This is a UN body, composed of delegates from all over the world, which was founded in 1988. Its task is to collect scientific information on man-made climate change and to evaluate it critically. Thousands of scientists around the world are doing this work. The first report was published in 1990. On this basis, the Framework Convention on Climate Change was formulated in 1992 at the Rio conference. In it, the states committed themselves to the need to prevent dangerous effects from human-made climate change. However, it took more than 20 years until the 2015 Paris Agreement agreed to limit global warming to two or even 1.5 degrees Celsius. On the basis of the fifth climate report, the basic scientific part of which I was allowed to lead together with a Chinese colleague.

And how is that supposed to happen in practice?

The countries have made commitments on how to reduce their emissions. After five years, each country takes stock of what it has achieved and sets itself the next, even more ambitious goal. In this way we hope to collectively reach the two-degree goal step by step - or maybe even exceed it.

What happens when a country does not keep its promises?

There are no sanctions in today's regulations, because then many countries would not have accepted the agreement. So the whole thing is voluntary and is based on the willingness of the countries to participate. A single country can also withdraw from the agreement, as the USA is currently planning. But that was also the case in 1997 with the Kyoto Protocol. At that time, the industrialized countries agreed to reduce their emissions by at least five percent compared to 1990. The emerging and developing countries were excluded from the Kyoto Protocol. This time everyone is encouraged to pursue the common goal. Much has already happened, but it is not enough.

Assuming all countries kept their promises, could they achieve the two-degree target?

No, the projects are not yet ambitious enough for that. If every country implements its current goals, we will still end up with a temperature increase of around 2.7 to three degrees Celsius. So more has to be done in the future. With each round it becomes more difficult, of course, because the remaining greenhouse gas budget is getting smaller every year. Right now we're losing half a degree with every decade. Ten years from now, the two-degree target will be as ambitious and difficult to achieve as the 1.5-degree target is today.

What can each individual do to protect the climate?

First of all, every gram of carbon dioxide saved counts. For example, I myself have greatly reduced my meat consumption, as the production of animal foods is responsible for a large part of global greenhouse gas emissions. I also try to do without the car as much as possible. Unfortunately, as a scientist, I cannot avoid long-distance travel. But that's only one aspect. It is much more important that politics finally set the right course.

Which would that be?

It's time for a fourth industrial revolution. After mechanization, electrification and digitization, decarbonization must come now. If you can get away from coal, oil and natural gas, it will not only help the climate. It also creates jobs, technological development and growth because we have to build a completely new renewable energy infrastructure. This is a huge opportunity and everyone can have a say in whether it happens or not.

Do we citizens really have that power?

You have more influence than you think. Take part in political life. Put pressure on, vote, and show politicians that you care about the climate. It is no longer about positions from the left or the right, about this or that party. It's about our common future.