Climate change: “It makes a considerable difference whether we end up with 2°C or 1.5°C of global warming”

Within its current assessment cycle, the Intergovernmental Panel on Climate Change (IPCC) has already published three Special Reports which evaluate key climate risks for terrestrial and marine ecosystems – and humanity as a whole. Now, a team of scientists including LMU’s Professor Matthias Garschagen (Department of Geography) has developed a new assessment method which was designed to enable an overall estimate of global climate risks. Garschagen is a lead author of the IPCC’s 6th Assessment Report and Synthesis Report, which will come out next year. In the following interview, he explains the results of the study, which has just been published in the journal Nature Climate Change.

Using an innovative approach, you have effectively synthesized the results of the three latest IPCC Special Reports regarding the worldwide risks associated with global warming. What was the major goal of the study?

Matthias Garschagen: We wanted to compare the risks assessed in the three special reports and aggregate the findings into a global view of climate change risks. Each of the three reports covers important aspects. The first compares the anticipated effects of a 1.5°C rise in global temperature with the expected impacts of a 2°C increase. The second considers the effects on land, and the third deals with the impact on the oceans and the cryosphere, i.e. the frozen parts of the Earth. In terms of the Paris Agreement and its mitigation as well as adaptation goals, it is vital to not only focus on single regions or sectors in isolation. We need to understand where the global community is headed as a whole. That’s why a global perspective is so important – and we have so far been lacking the kind of synthesis presented in our study, in which we look at more than 40 types of risk under various climate and adaptation scenarios.

What are the study’s main conclusions?

Garschagen: I would emphasize three points. First of all, from a global point of view, it makes a considerable difference whether, at the end of the century, we find ourselves in a world in which global warming has continued unabated, or in a world in which we have managed to mitigate the rise in temperature. We compared two emission trajectories, one in which the goals of the Paris Agreement are achieved and the rise in temperature is limited to less than 2°C, and another with much higher emissions and an increase in temperature by over 4°C in 2100. In the latter case, the number of risks that would have the most disastrous consequences is far higher than in a world with limited warming. Without effective climate change mitigation we might see irreversible damage by 2100 in more than half of the risk categories we have investigated, and many of the Earth’s ecosystems will have reached the limits of their adaptability.

What can we expect if we manage to keep emissions on the more ambitious trajectory?

Garschagen: Even then – and this is the second important point – we will still experience a sharp rise in climate-related risks. The international community must understand that, even if we succeed in mitigating climate change, some of its repercussions are already unavoidable. That means, even if emissions are substantially reduced, some ecosystems will reach the limits of their adaptability and will suffer massive damage, and this in turn will result in further risks for people.
In fact, the global risks may be even higher, because we do not yet have sufficient scientific tools that would allow us to precisely quantify the impacts of cascading risks and feedbacks between them. It’s quite possible that we are underestimating the strength of such feedbacks. In this respect, climate policies need to include a certain security margin.

And the third important point?

Garschagen: The third point relates to the effects of adaptation in different scenarios. We can now see that measures to adapt to climate change impacts can make a big difference in avoiding risk, and eventually harm. But – and this is a conclusion that one should ponder carefully – adaptation cannot eliminate climate risks entirely, even in low emissions trajectories. Even in a scenario that assumes wide-ranging adaptation – coordinated measures on a significantly larger scale, which entails much more effort and expenditure on infrastructure or new agricultural methods, for instance – we will still have to contend with some residual climate risk.

What kinds of risks will continue to increase despite adaptation measures?

Garschagen: The oceans and coastal areas are a case in point. We can confidently forecast that, in terms of their ability to adapt to the rise in sea level, some regions will reach the limits of their financial resources, even if the maximum rise projected for the high-emission scenario – a little less than 1 meter by the end of the century ­– is not reached. It‘s important to keep the global picture in mind. The risks posed by climate change are very unequally distributed around the world, and also within societies. It’s often the case that those who are exposed to the greatest climate hazards and are most vulnerable are those who have the least capacity to adapt, such as many small island states in the Pacific, where the average income is very low.

Which systems will be hit the hardest?

Garschagen: In the case of warm-water corals, for example, we can already observe very serious effects. We shouldn’t forget that we already have a temperature increase of over 1°C. Moreover, coastal regions and marine ecosystems appear to be highly sensitive to every bid of additional warming, as is confirmed by the comparison of the 1.5°C and 2°C scenarios. Both of these are relatively optimistic scenarios, which would be compatible with the goals of the Paris Agreement. Yet we see very clear differences between their projected effects, particularly in the system mentioned above.

Enlarge

So although a difference of 0.5°C seems relatively mild, it will trigger significantly different consequences?

Garschagen: Yes. It makes little difference for some risk types, but for many organisms and systems, the change from 1.5°C to 2°C warming makes a huge differences in terms of additional damage, for instance for bivalves, krill and the degradation of permafrost. Many risks will be pushed to a very high level, often with irreversible damage. This is an important finding in relation to the Paris Agreement. It is often said that it would be great if we reach 1.5°C, but we’re still on the safe side if we manage 2°C. But the global community should seriously ask itself whether we are really prepared to accept the risks that a 2°C increase would entail.

Do we still have a chance to reach the goals set out in the Paris Agreement?

Garschagen: We have already wasted a lot of time, but it’s not impossible if we drastically change course now. Important conferences are coming up, and the burning question at the UN Climate Conference in Glasgow in November will be whether or not the Member States are willing to set ambitious targets, and then work to implement them. The 1.5°C goal should not be abandoned. Risk analyses like ours show how important it is. Mitigation of climate change is a huge and difficult task, but in light of the dangers we will otherwise face, it is absolutely vital.

Interview: Monika Gödde

Alexandre K. Magnan, Hans-Otto Pörtner, Virginie K. E. Duvat, Matthias Garschagen, Valeria A. Guinder, Zinta Zommers, Ove Hoegh-Guldberg & Jean-Pierre Gattuso: Estimating the global risk of anthropogenic climate change. Nature Climate Change 2021