Climate Science

2020 has been an unusual and challenging year in many ways. One was the record-breaking number of named tropical cyclones in the North Atlantic (and the Carribean Sea). There has been 30 named North Atlantic tropical cyclones in 2020, beating the previous record of 28 from 2005 by two.

A natural question then is whether we can expect this high number in the future or if the number of tropical storms will continue to increase. A high number of such events is equivalent to a high frequency of tropical cyclones.

But we should expect fewer tropical cyclones generally in a warmer world according to the IPCC “SREX” report from 2012, and those that form may become even more powerful than the ones that we have observed to date:

There is generally low confidence in projections of changes in extreme winds because of the relatively few studies of projected extreme winds, and shortcomings in the simulation of these events. An exception is mean tropical cyclone maximum wind speed, which is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged…

So how does this conclusion relate to the number of tropical cyclones in the North Atlantic with a new record this season? One reason to look in more detail at the North Atlantic is because its observational record is believed to be more complete and more reliable than for other regions around the world.

The observational record may also suggest that the number of tropical cyclones in the North Atlantic has increased slowly over the 50 years in addition to year-to-year fluctuations around this trend (black symbols in Fig 1).

We know that the number of cyclones is sensitive to the time of the year (hence, hurricane seasons), phenomena such as El Niño Southern Oscillation (ENSO) and the Madden Julian Oscillation (MJO), and geography (the ocean basin shape and the latitude). We also know that the sea surface needs to be warmer than 26.5°C for them to form.

The role of sea surface temperature is indeed an important factor, and from physical reasoning, one would think that the number of tropical cyclones depends on the area of warm sea surface (sea surface temperature exceeding 26.5°C).

One explanation for why the area is a key factor may be that the probability of finding favourable conditions with right ‘seed’ for organised convection (e.g. easterly waves) and no wind shear increases when there is a greater region with sufficient sea surface temperatures.

The area of warm sea surface is mentioned in the IPCC SREX that dismisses the expectation that an increase in the area extent of the region of 26°C sea surface temperature should lead to increases in tropical cyclone frequency. Specifically it says that there is

a growing body of evidence that the minimum SST [sea surface temperature] threshold for tropical cyclogenesis increases at about the same rate as the SST increase due solely to greenhouse gas forcing.

On the other hand, there has also been some indication that the number of tropical cyclones does seem to be proportional to the area to the power of 5: (Benestad, 2008). When this relationship is extended to recent years, as shown with the green and blue curves in Fig 1, we see an increase that this crude estimate more or less follows the observed number of evens.

Global warming implies a greater area with sea surface exceeding the threshold of 26.5°C for tropical cyclone genesis. Also, the nonlinear dependency to implies few events and little trend as long as is below a critical size. The combination of a nonlinear relationship and a critical threshold area could explain why it is difficult to detect a trend in the historical data.

There is some good news in that is limited by the geometry of the ocean basin. Nevertheless, a potential nonlinear connection between the number of tropical cyclones and is a concern. If this cannot be falsified, then the tropical cyclones represent a more potent danger than anticipated by the IPCC SREX conclusions. So let’s hope that somebody is able to show that the analysis presented in (Benestad, 2008) is wrong.

Fig 1. Observed (black symbols) and estimated (green and blue curves) number of named tropical cyclones in the North Atlantic and the Caribbean Sea after (Benestad, 2008). Source: “demo(tropicalcyclones)”.

References

1. R.E. Benestad, "On tropical cyclone frequency and the warm pool area", Natural Hazards and Earth System Sciences, vol. 9, pp. 635-645, 2009. http://dx.doi.org/10.5194/nhess-9-635-2009

The point that climate downscaling must pay attention to the law of small numbers is no joke.

The World Climate Research Programme (WCRP) will become a ‘new’ WCRP with a “soft launch” in 2021. This is quite a big story since it coordinates much of the research and the substance on which the Intergovernmental Panel on Climate Change (IPCC) builds.  

Until now, the COordinated Regional Downscaling EXperiment (CORDEX) has been a major project sponsored by the WRCP. CORDEX has involved regional modelling and downscaling with a focus on the models and methods rather than providing climate services. In its new form, the activities that used to be carried out within CORDEX will belong to the WCRP community called ‘Regional information for society’ (RifS). This implies a slight shift in emphasis.

With this change, the WCRP signals a desire for the regional modelling results to become more useful and relevant for decision-makers. The change will also introduce a set of new requirements, and hence the law of small numbers.

The law of small numbers is described in Daniel Kahneman’s book ‘Thinking, fast and slow‘ and is a condition that can be explained by statistical theory. It says that you are likely to draw a misleading conclusion if your sample is small.  

I’m no statistician, but a physicist who experienced a “statistical revelation” about a decade ago. Physics-based disciplines, such as meteorology, often approach a problem from a different angle to the statisticians, and there are often some gaps in the understanding and appreciation between the two communities.

A physicist would say that if we know one side of an equation, then we also know the other side. The statistician, on the other hand, would use data to prove there is an equation in the first place.

One of the key pillars of statistics is that we have a random sample that represents what we want to study. We have no such statistical samples for future climate outlooks, but we do have ensembles of simulations representing future projections.

We also have to keep in mind that regional climate behaves differently to global climate. There are pronounced stochastic variations on regional and decadal scales that may swamp the long-term trends due to greenhouse gases (Deser et al., 2012). These variations are subdued on a global scale since opposite variations over different regions tend to cancel each other.

CORDEX has in the past produced ensembles that can be considered as small, and Mezghani et al., (2019) demonstrated that the Euro-CORDEX ensemble is affected by the law of small numbers.

Even if you have a perfect global climate model and perfect downscaling, you risk getting misleading results with a small ensemble, thanks to the law of small numbers. The regional variations are non-deterministic due to the chaotic nature of the atmospheric circulation.

My take-home-message is that there is a need for sufficiently large ensembles of downscaled results. Furthermore, it is the number of different simulations with global climate models that is key since they provide boundary conditions for the downscaling.

Hence, there is a need for a strong and continued coordination between the downscaling groups so that more scientists contribute to building such ensembles.

Also, while CORDEX has been strong on regional climate modelling, the new RifS community needs additional new expertise. Perhaps a stronger presence of statisticians is a good thing. And while the downscaled results from large ensembles can provide a basis for a risk analysis, there is also another way to provide regional information for society: stress-testing.

References

1. C. Deser, R. Knutti, S. Solomon, and A.S. Phillips, "Communication of the role of natural variability in future North American climate", Nature Climate Change, vol. 2, pp. 775-779, 2012. http://dx.doi.org/10.1038/nclimate1562
2. A. Mezghani, A. Dobler, R. Benestad, J.E. Haugen, K.M. Parding, M. Piniewski, and Z.W. Kundzewicz, "Subsampling Impact on the Climate Change Signal over Poland Based on Simulations from Statistical and Dynamical Downscaling", Journal of Applied Meteorology and Climatology, vol. 58, pp. 1061-1078, 2019. http://dx.doi.org/10.1175/JAMC-D-18-0179.1

Many of the earlier predictions of climate research have now become reality. The world is getting warmer, sea levels are rising faster and faster, and more frequent heat waves, extreme rainfall, devastating wildfires and more severe tropical storms are affecting many millions of people. Now there is growing evidence that another climate forecast is already coming true: the Gulf Stream system in the Atlantic is apparently weakening, with consequences for Europe too.

Recently, a so-called “white coat summit” gave me a sense of dejavu. It was held by a group that calls itself ‘America’s Frontline Doctors’ (AFD) that consisted of about a dozen people wearing white coats to the effect of achieving an appearance of being experts on medical matters.

The AFD apparently wanted to address a “massive disinformation campaign” (what irony) and counter the medical advice from real health experts. This move has a similar counterpart in climate science, where some individuals also have claimed to be experts and dismissed well-established scientific facts, eg. that emissions of CO2 from the use of fossil fuels results in global warming.

Climate science is not the only discipline where we see confusion sown by a small number of “renegades”. A few white-coated scholars have disputed the well-established danger of tobacco. We see similar attitudes among the “Intelligent Design” community and the so-called “anti-vaxxers”.

Statistically speaking, we should not be surprised by a few contrarians who have an exceptional opinion within a large scientific community. It is to be expected from a statistical point of view where there is a range of opinions, so there should be little reason to make a big deal out it.

On the other hand, there are some fascinating stories to be told. Sometimes there are individuals who can be described as “crackpots” and “quakesalvers” (e.g. a scholar believing in dowsing rods among the climate renegades and some within the AFD who talk about demons). Hollywood has even realized that some scientists may be mad, which has given us the familiar term “mad scientist”. But all “renegades” may of course not necessarily be mad.

Nevertheless, according to Snopes, the background of the individuals of the AFD is rather colourful. And there is nothing in the background provided about them that gave me any confidence in their judgement. On the contrary.

A sign that should trigger a big warning is that Snopes found it difficult to see who the AFD really are or where their conclusions really come from. The transparency is lacking and their story is murky. Especially so if the results have not been published through renowned peer-reviewed scientific journals. This is something we have seen time and again with climate change contrarians.

Any claim would be more convincing if colleagues independently are able to replicate the work and get the same results (without finding anything wrong with the process). This would require transparency and openness.

Another sign that should make you skeptical is if the claims have a dogmatic character. The AFD address is all dogma. This is also typical among the science deniers.

It’s also typical that the extreme fringes cannot falsify the established science and therefore move on to conspiracy theories. In the case of AFD, it is the alleged “massive disinformation campaign”.

Should we take such fringe views seriously? This type of “infodemics” seems to become a growing problem as described in a feature article in Physics World July 2020: ‘Fighting flat-Earth Theory’. The term “infodemic” reflects the fact that false information is just as contagious as an epidemic. Imposters dressed in white coats peddling false information can cause harm if people take them seriously.

The damage caused by erroneous information and conspiracy theories is discussed in the HBO documentary ‘After truth’, and the wildest claims can spread like a rampant disease as shown in that film.

We have witnessed how misinformation and lack of trust of true medical sciences have caused bad situations in some countries, while in others (eg. New Zealand, Canada, and some Nordic countries) the pandemic has been kept under control because the general public in general has followed the scientific health advice.

There is a common denominator when it comes to the AFD, anti-vaxxers, flat-earthers, “intelligent design”, chem-trail evangelists and those dismissing climate science. I think it may be useful to join forces within the broader scientific community to help the general public understand the real issues. This effort should also be on more general terms. People have a right to reliable and truthful information. Everybody should understand that anyone who spreads bullshit or lies also shows you a great deal of disrespect. The same goes for platforms spreading disinformation.

So what can we do to make people understand how science works and enhance the general science literacy? Is it better to teach people how to spot these “alternative scientists” (the term is inspired by “alternative facts”), conspiracy theories, and falsehoods, if we show a range of examples from different disciplines? We can probably learn from each others. There seems to be a lesson to be learned from the pandemic.