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Why is CO2 responsible for global warming?
Carbon dioxide, CO2, is a so-called greenhouse gas that promotes the greenhouse effect in the atmosphere. And it works like this: The sun radiates onto the earth, most of the sunlight is converted into heat radiation on the earth's surface and radiates back into space. Now several components of the atmosphere, namely greenhouse gases, are permeable to short-wave sunlight - but not to long-wave heat radiation. This heat therefore remains in the atmosphere. This is actually a good thing, because without the greenhouse effect the average temperature on earth would be around minus 18 degrees, there would be no or almost no life on it. Only the clearly increased release of CO2 and other greenhouse gases by humans increases the effect rapidly and makes the atmosphere noticeably warmer.


What other greenhouse gases are there?
The most important are methane and nitrous oxide (N2O) - also known as nitrous oxide. Methane is mainly produced in the stomachs of ruminants such as cows - which is why our meat consumption plays a significant role in climate change - but also in rice cultivation, waste dumps and sewage treatment plants. N2O is mainly released from agriculture, where it is used in fertilizers. Compared to CO2, methane has a 25-fold higher greenhouse effect, N2O even about 300-fold. Nevertheless, CO2 is by far the most important greenhouse gas, simply because of the quantity in which we release it. In order to be able to compare the different gases, one calculates in "CO2 equivalents".


What is weather and what is climate?
Put simply: climate is the average weather over a longer period of time - usually 30 years. Weather is a snapshot, the climatic condition at a certain place at a certain time - it depends on permanently changing conditions such as air pressure, cloud fields or wind direction. A rainy summer is no proof against climate change, a warm autumn is no proof.


How much warmer has it got?
Since about 1880, temperatures have been measured in a structured way, since then they have risen by 0.8 to 1 degree Celsius on average worldwide, in Germany even by 1.4 degree Celsius. Most of this increase has occurred over the past 50 years. 17 of the 18 warmest years ever measured fall into the 21st century.


How warm is it gonna get?
There is disagreement about this - simply because it depends on too many factors and you only have model calculations on the computer as a basis. After all, there is no test ground for research purposes where you can try it all out. In the very best case, scientists believe, warming can be limited to 1.5 to 2 degrees. To do this, people would have to reduce their greenhouse gas emissions to such an extent that the CO2 content in the atmosphere would remain stable by 2070 at the latest. Negative scenarios predict an increase of 4 to 5 degrees by the year 2100 - or even higher if certain "tipping points" are reached or self-reinforcing effects occur.


What are tipping points and self-reinforcing effects?
Tipping points are sensitive points in the global ecosystem whose change cannot be reversed after a certain extent. For example, it is feared that the North Atlantic Current or the Indian monsoon will be intensified, slowed or stopped by climate change. The ice sheets at the poles and the Greenland ice are also tipping points. If it gets too warm, melting processes can be set in motion that can no longer be stopped. This also shows the self-reinforcing dynamics of climate change: While the bright ice absorbs little solar radiation, it is well absorbed by darker soils or seawater. The more ice disappears, the more the sunlight has a "surface of attack" to heat up the surroundings of the ice and accelerate the melting process. Another example: If the permafrost soils of Russia and North America gradually thaw as a result of global warming, microorganisms can decompose the fossil animal and plant remains in the soil more quickly, releasing CO2 and methane.


Is climate change causing sea levels to rise?
Yes, the onshore ice at the poles and the mountain glaciers are melting faster and faster due to the heat. Greenland alone loses between 250 and 300 billion tons of ice per year. And the sea level is already rising: in the 20th century it was a total of 20 centimeters, since the beginning of the 1990s already a good 3 millimeters on an annual average, and it is assumed that it will rise even more strongly per year. If the two-degree target is met, researchers expect an increase of about half a metre by 2100. If the earth warms up by 3 to 4 degrees, it could, depending on the scenario, be almost one meter or even more. The exact effects are difficult to determine because the increase is not uniform everywhere: In the Philippines, for example, trade winds ensure that sea levels rise at an above-average rate. In Scandinavia, on the other hand, the sea level is even falling because the mainland has been slowly rising out of the water since the last ice age, free of the weight of the ice masses.


What's the two-degree target?
It describes the limitation of the rise in temperature to a maximum of 2 degrees Celsius - calculated from the start of industrialization, so it should only add about one degree at the moment. The two-degree target was first formulated in 1975 by climate economist William D. Nordhaus. In the 1990s, a report by the German Federal Government's Scientific Advisory Council stated that 2 degrees was the limit of what our ecosystem could still cope with. Since all climate impact models only work with approximate values, the target is above all a symbol - but an effective one: in 1996 the EU set the two-degree target as a guideline for European climate policy. At the World Climate Conference in Paris in 2015 it was enshrined in an international treaty. Some states, such as the Marshall Islands, had even demanded 1.5 degrees. However, the current commitments of the signatory states on climate protection are not even sufficient to achieve the two-degree target.


Were there any climate fluctuations in the Earth's history in the past?
Yes, there were. Possible causes include volcanic activity, changes in ocean currents due to continental drifts and fluctuating solar activity. From a geological point of view, we even live in rather cold times. According to current research, the majority of the last 500 million years were spent in warm climates, i.e. periods in which the earth's poles are not iced up. The average surface temperature of the earth was sometimes 15 degrees higher than today's 15 degrees. At that time, the CO2 concentration in the atmosphere was also significantly higher. During the last ice age, however, it was around 5 to 6 degrees colder.


Is the current warming of the earth perhaps not man-made?
The climate fluctuations that have occurred so far have taken place over periods of tens to hundreds of thousands of years. An increase of about one degree within less than 150 years strongly suggests that an additional factor is at play - man. Approximately 97 percent of climate researchers assume this to be the case, and the concentration of CO2 in the atmosphere also suggests this. At present it is about 0.04 percent or 410 ppm (410 millionths) - sounds little, but about 250 years ago it was only 280 ppm and in the 650,000 years before never more than 300 ppm. This is due to the combustion of coal, crude oil and natural gas in the course of industrialization.


If the earth has already survived a much warmer climate, then what is our problem?
For the earth there is no problem. Only for the living creatures that currently live on it - so also for us. With increasing warming, the current ecological equilibrium is becoming more and more confused. If, for example, the dormice awaken earlier from their hibernation, they eat bird eggs that were once hatched at that time. According to a negative scenario, one sixth of all species could die out in this century due to climate change. Since the effects also reinforce each other here, the consequence could be a mass extinction, of which there were already some in the earth's history - last approx. 65 million years ago, when the time of the dinosaurs ended.

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