Exactly how much will the sea level off the Dutch coast rise? Are we really getting more summer downpours for sure? And what about droughts?
The temperature on Earth will continue to rise for the time being, that’s for sure. The result is more extreme weather. But how this will play out regionally is uncertain. The KNMI report published earlier this week Climate signal ’21 is full of those uncertainties. The institute translates the latest findings of the IPCC, the climate agency of the United Nations, into the Dutch situation. To give an example: in the year 2300 the sea level off the Dutch coast may have risen by 30 centimeters. But it is also possible 17 meter turn into. The uncertainties are also great for summer downpours and drought. How is that possible? And how do you, as a policymaker, deal with these uncertainties?
Roughly speaking, the uncertainty lies in two things. It is not certain how greenhouse gas emissions will develop. In addition, it is not yet fully understood how climate change affects local weather processes. In any case, there is a large element of chance in these processes – the weather is a chaotic system. That makes it inherently difficult to predict.
There has been a lot of discussion about future emissions in recent years. Do the scenarios that the IPCC uses for this all still apply?
In its 2013 report, the IPCC introduced the representative concentration pathways (rcps). In these scenarios, each path corresponds to a different warming in 2100. In the worst scenario, greenhouse gas emissions will continue to rise this century (rcp8.5 – warming by about 5°C in 2100). In the three other paths, emissions peak in 2080 (rcp6, warming around 3°C), in 2040 (rcp4.5, around 2.5°C) and in 2020 (rcp2.6, around 1.5°C ). The IPCC replaced the RCPs three years ago with the shared socioeconomic pathways (ssp’s), whereby the emissions are translated into socio-economic narratives – these are more comprehensible to policymakers and the general public. But they represent the same range of emissions and warming.
But which range – and therefore, how much uncertainty – do you still have to take into account? Haven’t we had the Paris Agreement since 2015, which tries to severely limit emissions? So how real is rcp8.5 for example?
In the scientific literature there are still publications that take the rcp8.5 scenario into account. But last year, two researchers complained about this in a comment Nature because in their eyes it gives a false image. In that scenario, for example, the use of coal should increase fivefold this century. While coal-fired power plants are closing in many places, and coal use is expected to peak within a few decades. How realistic is that scenario then? It makes the uncertainty unnecessarily great. All the plans that countries have now drawn up in the context of the Paris Agreement lead to a world that warms 2.5-3°C.
But the question is whether all those ambitious plans will be fulfilled and on time. It is clear that phasing out the use of coal, oil and gas is not going fast enough. The extraction of fossil fuels is expected to increase in the United States, Saudi Arabia and Russia, among others.
In reply to the comment in Nature three scientists wrote a letter. Not including rcp8.5 could actually give policymakers a false sense of security. Leading to costly adjustments as the world develops in unexpected ways. “It is actually good to include that wide range of uncertainty,” says Marjolijn Haasnoot, who co-wrote the letter and is associated with Deltares and Utrecht University as a water and climate researcher.
In any case, the KNMI has simply included the rcp8.5 scenario in its climate report, as the IPCC has also done. “Because if you look at greenhouse gas emissions so far, it follows the 8.5 path very nicely,” explains KNMI climate scientist Rob van Dorland, who coordinated the report.
That is why the spread in the graphs is large. Certainly with regard to sea level rise up to the year 2300. But that is not only due to the uncertainty in greenhouse gas emissions. It is also in the uncertainty surrounding certain processes in the melting ice sheets in Greenland and Antarctica. Especially about the so-called ice cliff instability, explains Van Dorland, there is “deep uncertainty”. The ice caps consist of glaciers, part of which ends in the sea. Those hundreds of meters thick foothills, so-called glacier tongues, can thin and break up due to warming. “At the edges of the fragments you have cliffs that can collapse under their own weight,” says Van Dorland. This process has already been observed in Greenland. The crumbled ice melts faster. In addition, the broken-up glacier tongue does not slow down the glacier as it slides into the sea. This speeds up the melting process even more.
If this process is also on the horizon for Antarctica, it will have enormous consequences. The KNMI report states: “This could accelerate the erosion of Antarctica extremely, but this theory is still controversial. If the theory is correct, we could be the first to observe this phenomenon in 20 to 30 years at the currently rapidly retreating Thwaites Glacier, which flows into the Amundsen Sea.”
A robust signal
There is also process uncertainty in the prediction of drought. “Because we are at a turning point between two areas,” explains Van Dorland. Northern Europe will become wetter, even in summer. “For Southern Europe, there is a robust signal that the desiccation is continuing.” The Netherlands is somewhere in between, and it is not clear which way our country is tilting.
Central to this is the changing balance between evaporation and precipitation. When it gets warmer, evaporation increases and the soil dries out. But at the same time, the air can hold more water vapor, which basically leads to more precipitation, which keeps the soil moist.
There are analyzes that indicate that we will get wind from the east more often in the summers. “And that wind is drier than the west wind that comes from the sea.” In such a case, there will be more drought. But the analyzes are not very hard. The KNMI report states: “Climate models do not provide an unequivocal answer to the question of whether the circulation – and thus the wind directions – will change in the future.”
People tend to steer towards one outcome
Marjolijn Haasnoot water and climate researcher
Then the downpours. You would expect more, because of the simple fact that warmer air can hold more water vapor. That is also measured. Above the sea, absolute humidity increases by 7 percent with every degree of warming. But the sea warms up less quickly than the land. Because the westerly wind dominates in the Netherlands – which supplies wind from the sea – the moisture content over land rises by less than 7 percent per degree of warming. Over land, relative humidity (how humid the air actually is relative to its maximum potential) has actually decreased. A decrease in relative humidity leads to less showers, according to the KNMI report.
In addition, climate models predict that the air at a height of a few kilometers warms up faster than the air at the Earth’s surface. The mutual temperature difference decreases. That also inhibits the occurrence of showers. Because they arise when the surface is relatively warm and the air above is cold. “Both effects lead to a smaller increase in lighter showers, compared to a situation in which, among other things, the upper air warms up little,” says Van Dorland. The KNMI does expect the heaviest showers (more than 50 mm of precipitation in an hour) to become more extreme. But here too there are uncertainties. Locally, showers are subtly dependent on many factors – topography, position of high and low pressure areas, vertical temperature and moisture profiles. They can have a damping or strengthening effect. The complexity makes it difficult to predict how showers will change locally.
Very different approach
What should policymakers make of all this uncertainty? What should they send to? “People tend to want to steer towards one outcome,” says Haasnoot, who specializes in climate adaptation policy. “But due to the great uncertainty, the climate problem requires a completely different approach.” In fact, policymakers need to prepare for a wide range of possible outcomes. She calls it adaptive planning. “You explore what all these different outcomes mean. You explore how you can adapt to that, and how much time you need for those adjustments.” It requires a lot of flexibility. “You can already start with the things that you will certainly not regret.” Think of sand nourishment on the beaches, in the fight against sea level rise. “Longer term options, if the climate changes more, you have to keep it on hand.” Good monitoring is essential, she says. “So that you recognize in time that you need to adjust your plan.”
This signaling rests on three pillars, says Van Dorland. Like Haasnoot, he is part of the Delta Program Signal Group. This group reports on developments that are important for the implementation of the current Delta Programme, which aims to keep the Netherlands safe and to continue to provide sufficient freshwater. Those three pillars are: is it in the observations? Is it in the model projections? And do we understand the process? Van Dorland: „If all three are ‘yes’, then we have to do something about it. If it is ‘yes’ twice, we are extra alert.”
According to Haasnoot, the Netherlands, together with the United Kingdom and New Zealand, is an international leader in adaptive planning. But in some areas it could still be better. She is thinking in particular of the construction of homes and infrastructure. “They have been there for hundreds of years.” That is why such investments must take greater account of the long term, of rising sea levels, subsidence and downpours. Where can you be guaranteed to build safely, and where not? Should you build more houses on stilts? Or easily buildable and degradable houses? “You see plans to build in peat meadow areas and floodplains. We need to think more about that.”