Historians for Future on the IPCC Report: Part 3, What Are The Risks?

Lightning striking a hill in a dark landscape. The lights of a settlement can be seen in the right of the image, below the hill where the lightning strikes
Source: https://pxhere.com/en/photo/590124

This is our third post on what the Intergovernmental Panel on Climate Change (IPCC) Working Group I Sixth Assessment Report means for the global climate, past and future.

So far, we have learnt about the current problematic state of the climate (Section A) and possible climate futures (Section B). This time, we have a look at Section C of the Summary for Policymakers: Climate Information for Risk Assessment and Regional Adaptation. 

Previous Working Group 1 reports (e.g. report 5 in 2013) did not deal with “risk assessment”. Future risks were outlined, but they were not put into a separate section. This is one of the big changes in the way the IPCC report treats climate change in the 2021 report: “Risk” is now a key concept. Rather than simply naming the potential risks in the future, it gives us a more concrete outline of dangers that informed policymakers must now respond to with adaptation planning. 

In this report, risks include not just potential negative consequences of climate change such as floods, droughts or heat waves, but also the dangers that come from how humans respond to climate changes: where and how we build safe buildings, how we secure water supply, or how we try to manipulate local weather by intervening with technological inventions (e.g. through geoengineering). This new perspective on risks also includes more focus on regional and even local dimensions. The report is quite clear that climate change affects every country, town, village, coast, forest, valley, or river in some way: knowledge is key.

The IPCC report has to navigate very difficult waters: on the one hand, it has to correspond with different scientific findings and present the objectively established facts that climate change is man-made and accelerating. On the other hand, it must avoid allegations that the reporting is too alarmistic. Section C can therefore be read as the part of the report that gives policymakers a solid basis for decision-making and a “nudge” to prepare proper risk assessments on local, regional, national and global levels.

Section C is divided into three parts. Each part is dedicated to showing how climate change not only affects global warming, but also has and will have profound effects on regional and local climates. Regional changes can and will differ from general global warming, and they will be noticeably different at 2°C compared to 1.5°C global warming. And finally, regional low-likelihood events become more likely and have to be part of decision-makers’ risk assessments.

The three parts are centred around three key aspects of climate information: 

1. Variability versus global climate change;

2. Climatic impact drivers;

3. Low-likelihood and high impact outcomes. 

What do these terms mean?

C.1 Variability versus global climate change.

There have always been “natural” events that have caused global warming, such as volcanic eruptions or solar storms. These so-called “natural drivers” of climate change have long masked the long-term changes caused by humans. That means that it is difficult to assess what change in global temperature is caused by “natural” variability and what by human-made interventions. In addition, there are the effects of the two “forces” interacting with each other: They can amplify climate change, but they can also mitigate it – especially in the short term and at the regional level. 

A good example is El Niño – that is the name of the warm phase of the irregular periodic variation in winds and sea surface temperature over the tropical eastern Pacific Ocean, which affects the climate of much of the tropics and subtropics. El Niño is a so-called “internal variability,” and the report suggests that it might have amplifying but also mitigating effects on the human caused changes. However, it is unclear what impact these mutual effect amplifications and mitigations will have – especially on a regional level. Yet, the report shows that the near-term changes, for example in monsoons and their heavy rains, will be dominated by the effects of internal variability. The important thing to keep in mind is this: It is likely that at least one large volcanic eruption will occur during the 21st century. The massive explosion would reduce temperatures, alter monsoon circulation and change many climate impact drivers for a few years. But also: it would temporarily and partially mask human-caused climate change.

C.2 Climatic impact-drivers.

Before we start: what exactly are climatic impact-drivers (CIDs)? The report defines CIDs as “physical climate system conditions (e.g., means, events, extremes) that can be directly connected with having impacts on human or ecological systems.” The term covers all kinds of impact, whether detrimental or positive. For example, a flood may damage a human city while having a beneficial impact on an ecosystem; in either case it is certainly a CID. You can find the most relevant CIDs in the figure below.

If we head for a 2°C increase in global warming instead of a 1.5°C, these CIDs will change more dramatically – everywhere. All regions will experience further increases in hot CIDs and decreases in cold CIDs. But CIDs will not be single events, they tend to affect each other. 

There is only one illustration in Section C. It is an overview over the multiple climatic impact drivers projected to change everywhere:

A chart showing various climate impact-drivers (CIDs) and whether they are projected to increase or decrease. These are grouped under the headings: heat and cold; wet and dry; wind; snow and ice; other; coastal; and open ocean. The last is its own table, the others apply in land and coastal regions. The CIDs and their projected changes are as follows:
Heat and Cold
- Mean surface temperature, increase with high confidence in all regions.
- Extreme heat, increase with medium to high confidence in all regions.
- Cold spells, decrease with medium to high confidence in all regions.
- Frost, decrease with medium to high confidence in all regions.
Wet and Dry
- Mean precipitation, increase with medium to high confidence in most regions, decrease in some.
- River flood, increase with medium (some high) confidence in most regions, possible decrease medium confidence in some.
- Heavy precipitation and pluvial flood, increase with medium to high confidence in most regions, no decrease.
- Landslide, increase with medium confidence in some regions, no decrease.
- Aridity, both increases and decreases: high confidence changes show slightly more decreases than increases, medium confidence in more increases. Most regions neither increase not decrease.
- Hydrological drought, a few regions show increase, with mostly medium confidence (some high). No decreases projected.
- Agricultural and ecological drought, some regions show increases, mostly with medium confidence (some high). No decreases projected.
- Fire weather, most regions show increases, medium confidence (some high). No decreases.
- Mean wind speed, most show no change. Some decrease medium confidence (some high), fewer increase with medium confidence.
- Severe wind storm, few areas projected to change, a few show increase with medium confidence.
- Tropical cyclone, most regions show increase, medium confidence. No decreases.
Sand and dust storm, most regions show no change, a few show increases, medium confidence.
Snow and Ice
- Snow, glacier and ice sheet, almost all regions show decreases, medium to high confidence;
- Permafrost, almost all areas show increase with high confidence.
- Lake, river and sea ice, almost all areas show increase with high confidence.
- Heavy snowfall and ice storm, vast majority of areas show no change. A few show increase with medium confidence.
- Hail. No regions project change.
- Snow avalanche, vast majority show no change, a few decrease with medium confidence.
- Air pollution weather, no regions project change.
- Atmospheric CO2 at surface, all regions projected to increase with high confidence.
- Radiation at surface, most regions project no change. Some increase and some decrease, medium confidence.
- Relative sea level, almost all areas projected to increase with high confidence, a few with medium confidence and very few with no change.
- Coastal flood, almost all projected to increase with high confidence, a few with medium confidence and fewer with no change.
- Coastal erosion, almost all projected to increase with high confidence, a few with medium confidence or with no change.
- Maritime heatwave, all projected to increase, vast majority with high confidence a few with medium.
- Ocean acidity, all regions predicted to increase with high confidence.
Open Ocean
- Mean ocean temperature, all regions projected to increase with high confidence.
- Maritime heatwave, all areas increase with high confidence.
- Ocean acidity, all areas increase with high confidence.
- Ocean salinity, some regions decrease with medium confidence, somewhat fewer increase with medium confidence, some project no change.
- Dissolved oxygen, all regions project decrease with medium confidence.
This figure shows how the most important 35 CIDs affect our climate if we are heading towards a 2°C global warming increase over the next three decades.

Let’s have a look at two striking projections:

– First, heat and cold: The first four bars summarise the effects of global warming on temperature scenarios. An increase in mean surface temperature and extreme heat waves is very likely in many land and coastal regions. At the same time, cold spells and frost will decrease with almost the same likelihood. 

– The second group of CIDs that immediately catches our eye is the dramatic increase of CIDs in coastal regions. Coastal regions will not just be affected by the CIDs just mentioned, but also by sea level changes, coastal flooding and erosion, and ocean acidification. The report concludes that in any case, regional mean sea level rise will continue throughout the 21st century almost everywhere, a fact that is not just very likely, but “virtually certain”. The consequences for coastal cities will be very hard: they will have to face a combination of sea level rise, storm surges, extreme rainfall and/or riverflow events on the one hand, and more frequent hot extremes on the other.

All changes in CIDs will be more dramatic with 2°C of global warming than with 1.5°C.

C.3 Low-likelihood and high impact outcomes.

The last section deals with what are called “low-likelihood, high impact outcomes”. These are events which are unlikely or hard to predict but that could dramatically impact societies and ecosystems. Unlike CIDs, which can be predicted fairly accurately, these extreme events and combined effects that cannot be predicted but are becoming more likely. Here the report is clear that even if low greenhouse gas emissions scenarios are only slightly exceeded, the global and regional changes would also exceed their very likely ranges. 

In other words, low-likelihood, high impact outcomes will become more and more likely, more frequent, more intense and more permanent. This means that so-called tipping points such as the Gulf Stream, which is an important factor for the mild Western European climate, will most likely be affected. The Gulf Stream is itself part of a larger system called the Atlantic Meridional Overturning Circulation, or AMOC.  There’s a good chance AMOC will weaken over the 21st century – in all emission scenarios! [for more on the scenarios, see our previous post]

It is still unclear to what extent such a continuous collapse will affect other CIDs, but it is again very likely that it would cause abrupt shifts in regional weather patterns and water cycles. Finally, there are of course unpredictable and rare natural events, such as massive explosive volcanic eruptions. These low-likelihood, high impact events can always occur, but they could intensify the effects of CIDs even more.

Yes, these findings may sound familiar, but it is the structured synopsis of section C that does the trick: the IPCC report helps us understand the compound quality, the interactions, and the complexity of the various CIDs, which could and should create a solid basis for an appropriate risk assessment.


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