Climate scientists are saying goodbye to the most extreme climate scenarios, we read in the media last week. The most optimal scenario, in which global warming remains below one and a half degrees for the entire century, will be in a new publication no longer considered realistic. A huge blow to the optimism with which the world seemed to be really tackling the climate crisis in 2015.
But the most pessimistic scenario is also said goodbye to: that too is no longer considered plausible. Climate skeptics raised their flags. Climate scientists would have been far too alarmist. That accusation is unfounded. Emphasizing pessimistic scenarios is not alarmist but very sensible. This exposes the risks for a future that is not very likely but does have major consequences.
An important shortcoming of the climate scenarios in most climate reports is that their authors do not consider how likely each scenario is. On the one hand, this is understandable: the purpose of the scenarios is to give policymakers and leaders a sample of possible future consequences of climate change, and for them to choose from that menu of futures what they consider acceptable and feasible. On the other hand, it hinders a proper and objective assessment of the risks of climate change. This makes it like comparing apples and oranges. Is it a celebration because we managed to prevent the worst? Or have we closed the door to a future with limited risks for our society?
A common way to calculate risk is to multiply the probability of an event occurring by the impact it has. This is also how it works with insurance. An insurance policy that covers a very small chance of a major event (building insurance against fire) is about as expensive as an insurance policy that covers a much greater chance of a not so major event (theft insurance for your bicycle). The financial risk for the insurer is approximately the same. It is therefore important for policymakers and politicians not only to know which climate scenarios are most likely, but also what the risk is of a situation that is not very likely, but still has far-reaching consequences.
In the Netherlands, people have often worked in terms of risks. For example, the KNMI has developed scenarios for: extreme sea level risebecause the small chance that sea levels will rise by much more than one meter this century is an existential risk for our society. But much more often, policymakers and media limit themselves to mentioning the most likely scenarios. If a climate report states that sea levels will probably rise by 30 to 90 centimeters this century, then the chance of both values being the same, but their consequences certainly not.
For the past ten years or so, the call for a risk approach in climate science has become increasingly louder. I illustrate how this could work with a graph of climate sensitivity. That is the expected temperature increase if you increase the amount of CO2 in the atmosphere doubles. A probability distribution is known: it is probably between 1.5 and 4.5 degrees. It can also be more than 6 degrees, but that chance is not that great (left graph). But the impact of high climate sensitivity is increasing disproportionately fast (middle graph). The risks of an impossibly high climate sensitivity are therefore the greatest (graph on the right), even greater than the risks for the more likely lower values for climate sensitivity.
It is quite a task to assign a probability to climate scenarios. The guideline for the climate scenarios is that they plausible must be, so with a non-negligible probability. Plausibility is ultimately a subjective concept, but scenario makers do take into account our current scientific understanding of the speed of natural processes and social changes. The previous extreme scenarios date from around 2010, a year before the first Tesla rolled off the production line. Now we are 16 years later and what is plausible can be refined.
The new scenarios include three semi-optimistic overshoot scenarios: in them the temperature will rise to above 1.5 degrees this century, but will later drop due to techniques that reduce CO2 take it out of the air. And it contains a new most pessimistic scenario. That is less extreme than the old scenario, but it does not make much difference. The 4.5 degree warming that was achieved in 2090 in the old scenario will occur around 2140 in the new scenario.
Climate scientists will continue to rely heavily on that pessimistic scenario in the future, because the risks are so great. That makes sense: an insurer that only looked at opportunities and not at risks would be bankrupt within a year. That is why the emphasis on extreme warming is not alarmist, but rather the most rational.
