How warm will it get? This question is probably on the mind of every person dealing with the issue of climate change. It is so important because all other consequences of climate change depend on it. The less the earth warms, the better mankind will cope with it. In the following, I will deal with this question and its implications. In doing so, I will focus on how we gain our knowledge of climate change in the first place, how various temperature increases affect our planet, and what options we have as individuals and as a society to protect the climate.

In very basic terms, you might think that such a simple-sounding question as “How warm is it getting?” should actually be relatively easy to answer. But in my experience, the more you think about it, the more complicated it becomes. The biggest difficulty here is that we need to understand not only how the climate system works today, but also how it will evolve over the next decades or even centuries. The various climate mechanisms on our planet naturally behave differently depending on the intensity of global warming, which in turn has implications for further warming. For example, it is relatively easy to determine how high sea level currently is. But if we want to know how high it will be in 10 years, it becomes much more difficult. Since we can’t simply see into the future, we have to make assumptions. For example, about how much carbon dioxide we will still release, how much our population will grow, or how well different countries will support each other in their climate protection efforts. Because many of these assumptions affect each other - for example, a larger population leads to more carbon dioxide emissions - the scientific community has agreed on five scenarios that comprehensively map the possible event space of future development. These range from a world in which we all work together to become more sustainable to a world entirely reliant on fossil fuels, with each country fighting for itself.

However, when we look at these scenarios today, it is clear that some of them have become less likely today than they were when they were first developed, due to the developments that have taken place in the meantime. For example, the more extreme scenarios assume that humanity will continue to intensify the use of coal through the entire 21st century. In reality, however, it appears that we are likely to have passed the peak of global coal use sometime between 2010 and 2020. Therefore, we can assume that we are at least not in the worst case scenario and that we are moving in the right direction overall.

In addition to these scenarios, however, there is also the political goal of limiting warming to a certain temperature. The target range is between 1.5 and 2°C, since most of the effects of climate change can still be well limited there. However, global warming has already progressed so far that we have to assume that the global temperature will already exceed the 1.5°C mark around the year 2040. In order to reach the 1.5°C target, we would have to be in a world where all political efforts are directed towards limiting emissions and thus warming accordingly.

Unfortunately, it doesn’t look like we are in that best case scenario either. For the best case scenario, net global carbon dioxide emissions would need to be at 0 in 2050. However, current emissions are at a plateau and have been for several years. That means we would have to reduce our emissions by the same amount in 28 years as we have increased them over the last 100+ years. While this is theoretically possible, it would require a very extreme change in the global economy. For example, the elimination of coal-fired power plants worldwide in the near future. There is little evidence that this is happening at present.

So we can see that we are on the middle ground. We will probably not be able to prevent the climate on our planet from changing decisively, but we have also already taken important steps to avert the greatest possible damage.

It is difficult to estimate how much warming we can expect along this middle path, as our understanding of how sensitive the climate system is to changes in carbon dioxide levels is improving and we need to adjust our calculations. Also, what emission control measures have already been adopted changes regularly. Despite this complex situation, there are organizations such as the International Energy Agency that regularly provides projections of future warming based on this information. In its latest report from 2021, it shows that currently adopted measures are likely to lead to a warming of 2.4 - 2.8°C. So we are still a long way from the 2°C and especially the 1.5°C target.

What do we even know?

We can gain the knowledge we need to assess what scenarios we are in and how they will affect temperatures by looking at both the future and the past. In the past, we can explore how the Earth behaved when it was warmer before. A particularly important event there is the Paleocene/Eocene temperature maximum. During this, the Earth warmed by 5-8°C within a few thousand years. This warming occurred more slowly than we expect from human-induced climate change, but it still provides good comparisons. So we can see from it that the biosphere can survive relatively short term and intense temperature shocks, but man-made climate change will be much faster again. But it is reassuring to know that similar events in the past did not cause life on Earth to cease to exist. Also, our knowledge of the past allows us to check our climate models. If the models we use to look at the future can correctly simulate the past state of the Earth, we can also be more confident that our projections for the future are correct. These are essential for us because we can only adjust to future developments if they are quantifiable for us.

What is the IPCC and why is its focus so important?

All this knowledge about future and past climate development now comprises such a large quantity of publications that no single person can have an overview there. To enable a comprehensive assessment of the current state of climate science, the Intergovernmental Panel on Climate Change was founded in 1988. This body does not conduct its own research, but coordinates thousands of scientists worldwide who jointly produce so-called state of the art reports. These reports reflect the current state of knowledge. The first of these reports was published in 1990. Since then, a new report has been published every five to seven years. In between, special reports are also published regularly, focusing research on currently important topics, such as the ozone layer or renewable energies. Because the IPCC reports have been published for so long and are so thoroughly compiled, they can serve as a reflection of how knowledge about climate change has evolved and where research priorities have been set.

This distribution of knowledge in the Intergovernmental Panel on Climate Change reports has been one of my research priorities in recent years. Science about science, so to speak. To do this, my co-authors and I took advantage of the fact that the text from the reports can be easily analyzed because they can be made machine-readable. In our first study on the topic, we focused on the then-current Fifth Assessment Report. We wanted to find out how often the report referred to specific temperatures and how likely they were to occur. The procedure here was relatively straightforward, since all we had to do was count how often the various temperatures were mentioned in the text. We could then compare these percentages, i.e. how often a certain temperature was mentioned in relation to all other temperatures, with the probability that we will reach this temperature. The hope was that both values would be about the same. Because that would mean that research would look at warming scenarios to the extent that they were likely to occur. Or in other words, the more likely something is, the more of my research time I should invest in understanding it. Unfortunately, that was not the outcome of the study. In fact, the state of the world report focused mainly on the 1.5-2°C temperature range, while higher temperatures were considerably less represented. However, as I explained a few minutes ago, it does not currently look like 1.5-2°C is the temperature range we are moving toward.

To better understand this focus in the state of the art reports, we did a follow-up study that looked at how this focus on different temperature ranges has evolved over time. As with the first study, the approach was that we evaluated how often the different temperatures were mentioned, but with the difference that all reports ever published by the Intergovernmental Panel on Climate Change were used. In total, several tens of thousands of pages of text. There it shows that the focus of the Intergovernmental Panel on Climate Change was relatively balanced until the fourth Assessment Report. In the fifth and especially the sixth Assessment Report, however, the 1.5 and 2°C temperatures are mentioned much more frequently than any others. This means that we are increasingly focused on this range in current research. This is a problem because, after all, the current expected warming is much higher than two degrees. So we have most of the research for the temperature range where we are unlikely to end up at all. But this does not mean that the Intergovernmental Panel on Climate Change or the scientific community have done a bad job. It just reflects what the scientific community as a whole is focusing on, and their focus in turn is influenced by what policy makers are allocating funding to and what is at the forefront of societal discourse. That is, if different research is to emerge, it must also be supported and funded by society.

This change in research took place after the Paris Agreement in 2015. In the Paris Agreement, 1.5°C was intensively discussed as a global target. Therefore, it can be assumed that the change in focus in the state of the art reports is also due to this. 1.5°C and 2°C are simply the values we would like to move towards and therefore most research is taking place for this.

Why are two degrees warming so bad?

So we know that we have to expect warming above 2°C, but we don’t have as much research available on this as we would like. But why is the 2°C target so important anyway? Of course, there is no magic limit in our climate system below which all is well and above which all is lost. Every little bit of extra warming that we can prevent reduces the damage that climate change will do to the Earth and to human society.

In particular, the damage will not increase linearly, but exponentially. We have all learned what this means in the past few years of the pandemic. It means that the damages that await us will increase much more with a warming from 2 to 3 degrees than with a warming from 1 to 2 degrees. So if we look at climate change from a risk management perspective alone, our current focus in research seems even more problematic. We need for the higher temperatures an amount of research that is commensurate with the magnitude of the risks. That the effects of climate change can become so extreme is also due in large part to the fact that temperatures such as 2°C, which are usually spoken of, are only global averages. Locally, the effects of climate change can be much more extreme. Antarctica, for example, will probably warm twice as much as the respective average global warming. Accordingly, in such places, the effects will also be much more drastic. In the case of Antarctica, this means that gigantic glaciers will melt there.

In the following I will point out some effects of a climate change above 2°C to make clear again how important it is that we prevent this.

• The first point, which has also been increasingly discussed in the media lately, is heat stress. People cool themselves by sweating, but this only works as long as it is not too hot. Above 35°C, it becomes increasingly difficult to cool down sufficiently via sweating, especially when humidity is high. As a result, people can die of heat stroke within a few hours. Nowadays, this is a condition that occurs relatively rarely, but has claimed victims, such as in India this year. However, at current expected emissions, this could be the normal case in many areas of Africa, South America, India and Australia by the end of the century. This would potentially affect billions of people.
• Another much discussed issue is sea level rise, probably because it is the easiest to imagine. Depending on the scenario, the prediction here varies widely. According to the most extreme scenarios, the rise could be as much as 15 meters. But it is not likely to get that far. A more realistic value is 0.5 to 1 m by the end of the century. But even that will have a big impact. Many big cities like Tokyo, New York or Dhaka already have flood problems. These will only get worse and cause many people to leave their current homes.
• The impact of climate change on our biosphere is much less clear. It is difficult to estimate how well nature will be able to adapt to the expected changes. There could be a collapse of large parts of the biosphere, or there could be a relatively good adaptation, as we observed Paleocene/Eocene temperature maximum. The main problem here is more that we do not yet understand the systems well enough to make a final assessment. What is clear, however, is that many species are already threatened by climate change. This will increasingly become more so.
• One area where we have better insight into the effects of warming above two degrees is agriculture. There is even a special report by the Intergovernmental Panel on Climate Change on this topic. It shows that we will have to reckon with considerable difficulties in the future if the earth warms by more than 2°C. In particular, securing food supplies will become more difficult. In particular, securing the food supply will be permanently disrupted. We can already see what this might look like. Crop failures in China and India combined with the war in Ukraine have caused global food prices to rise enormously. Such problems will become more frequent with each further increase in temperature. Another important point here that is often overlooked is that the higher carbon dioxide concentration in the air causes the ratio of carbohydrates to other nutrients in many crops such as wheat to deteriorate. Therefore, even in regions where there is enough food in the future, there may be problems with malnutrition.
• The last point I want to mention here is global instability. All the things I have mentioned so far have the consequence that people will have to leave their homes because they become too hot or too dry or sink in the sea. Just a half-meter rise in sea level will result in 72 million people worldwide needing new homes. Even the people who now live in areas where heat stroke will be more frequent in the future or which are increasingly drying out will probably want to move away from there. These huge migratory movements will be exacerbated by the fact that people are more aggressive under heat. In the past, it has often been shown that these are conditions that can quickly lead to armed conflict, such as civil wars. A concrete example of this is Ethiopia’s conflict with Egypt over who gets to use how much of the Nile’s water. This instability, in turn, lowers the likelihood that climate-impacting measures will be adopted and implemented at the global level.

So we see that climate change above two degrees will lead to significant problems that will get exponentially worse the warmer it gets.

What is unlikely to happen?

Fortunately, however, it’s not all bad news. While it doesn’t currently look like we will reach the 1.5°C target, recent research has also shown that it is much less likely that we will see truly extreme climate change of 4°C or more. The amount of carbon dioxide emitted and the Earth system’s sensitivity to it is simply not sufficient for that to happen.

Moreover, we have a relatively long time to adapt to the effects of climate change. While there is much to do, climate change is a slow disaster. The effects are already being felt now, and they will only get stronger in the future. However, humanity has already begun to adapt, for example with new approaches to agriculture or cities working to enable fresh air corridors. So I think there is a good chance that we could also address warming above 2°C if we fail to prevent it. But there are also so-called tipping points in our climate system. These are processes that are self-reinforcing once they get going. An example would be the drying up of the rainforest. In a warmer world, droughts will occur more frequently in the Amazon. These cause trees to die, resulting in less water evaporating, which in turn makes future droughts more likely. So there is still great pressure to act, even if we can adapt to higher temperatures, we are making extreme climate events more likely.

What can we do?

This naturally raises the question of what can and must be done at all to combat climate change. Well, a surprising amount, and it will have to take place at all levels of society, from the individual to the nation state. In my opinion, however, the focus should be on what we can do as a society, because the leverage is simply much greater there. While I’m personally happy about every person who stops eating meat or is willing to give up a car, that won’t be enough. Global players from politics, business and the military have much larger footprints here and must accordingly set a good example and refrain from shifting responsibility onto individuals. However, if you still want to do something as an individual, I believe that the best ways are personal involvement and donations. In terms of donations, you can probably achieve the most by approaching organizations that want to directly influence political decisions about energy supply and the like. This is where the potential for decisive developments is greatest.

Good examples of effective organizations here are Future Cleantech Architects and Carbon180. Such projects are particularly important because in the future we will need an energy system that is no longer dependent on fossil fuels. Wind and solar are, of course, very important in this regard, but it is difficult to use them to ensure baseload supply for our power grids. For this, we need new technologies such as deep geothermal energy or fusion power plants that can ensure a constant supply of electricity and heat.

Another important point, which will have to be developed much more in the future, are possibilities to extract carbon dioxide directly back from the air. This is already possible today, but it is still very expensive. But it is assumed in many climate scenarios that we can use this technology on a large scale. While there is also the possibility of using trees to remove carbon dioxide from the air, this cannot be scaled up to an area that would be needed, since we also need the area for arable farming. There is still a lot to be done here.

In terms of society as a whole, Germany has already made progress by setting up a national certificate trading system for carbon dioxide. However, past governments have acted too late and too timidly. For example, the prices in national certificate trading are still far too low to have any real effect. Currently, a certificate for one ton of carbon dioxide is issued for €30. This is still extremely far from the social cost. For example, the Federal Environment Agency estimates that one ton of carbon dioxide causes damage worth around €200. My personal estimate would be much higher here, as the Federal Environment Agency makes relatively conservative assumptions.

In addition, Germany can do much more to combat climate change than simply reduce its share of 2% of global emissions to 0. As a strong research location, it also has the opportunity to contribute new ideas and technologies to ensure that the consequences of climate change threaten as few people’s livelihoods as possible.

Summary and Conclusion

In conclusion, we have already done a lot to mitigate climate change, but not nearly enough. The effects of climate change will be increasingly felt in the coming decades and will affect our planet for thousands of years. Extreme events - heat waves, heavy rains, and more - that we rarely experience today will become the new normal.

I am also concerned about the research gap that I have been able to identify with my co-authors. This can only be closed by political incentives. For example, why is there a special report from the Intergovernmental Panel on Climate Change on 1.5°C warming, but not 2.5°C? 2.5°C is more likely and will cause much more damage.

Nevertheless, our situation is not as bad as it is sometimes made out to be. By narrowing down the possible values of climate sensitivity, we now know that the chance that we will experience climate change above 4°C is small. But even a climate change of 2-3°C will have very negative impacts for an extremely large number of people and cost many their lives. Nevertheless, I think that we as humanity have a good chance to survive climate change. We just have to finally bring ourselves to act decisively!

Thanks to Peter Ruschhaupt, Ekaterina Ilin, Paula Eckelmann, and Konrad Bestian for helpful comments on this presentation.