Las lluvias torrenciales caídas en Libia hace dos semanas fueron 50 veces más probables por el calentamiento global provocado por la acción humana. Algo similar ocurría con las lluvias caídas en Grecia, Bulgaria y Turquía, que fueron 10 veces más probables por el cambio climático, según concluye un estudio de atribución rápido de la Word Weather Attribution (WWA). La investigación apunta a que la tragedia en Libia se agravó por otros factores humanos, como construcciones en llanuras inundables y presas en mal estado. Los autores también analizaron las inundaciones registradas en España a principios de septiembre y concluyeron que precipitaciones tan intensas se esperan una vez cada 40 años.
A damaged neighbourhood, days after storm Daniel swept through eastern Libya, in the port city of Derna. The death toll has exceeded 11,300 and more than 34,000 people are displaced across the country. EFE/EPA/STR.
In my view, the Daniel episode that affected Greece and then Libya may have been of greater intensity than would be expected with the historical climate because of the large volume of precipitable water now in the atmosphere, largely related to the huge thermal anomaly in the oceans. And with climate change, the oceans will undoubtedly continue to warm much more. Therefore, such intensities are more likely to recur in the very near future. The formation of Mediterranean cyclones is not a new phenomenon, and even assuming the possible doubts about whether they will form more frequently, what we can clearly state is that, once they do form, the intensity is directly related to the amount of precipitable water, which is increasing.
As far as the research is concerned, the first thing I must emphasise is the speed with which this study, which is so complicated from a climatological point of view, was carried out. The study is of more than sufficient quality to be taken into consideration. However, some minor comments are:
- The limitations of ERA5 for such extreme cases need to be analysed.
- Figures 7 and 8 do not look good, but it seems that in the first case there is some statistical relationship between temperature and maximum rainfall in 4 days for each year; not so in Figure 8, at least not so clearly.
The study period probably needs to be further extended to at least 100 years of historical data (observed or simulated) for the case of Libya. Mediterranean climate has so much variability that it is very difficult to find statistically significant trends in a few decades.
It is very difficult to measure the probability of such extreme events that have a frequency of less than "once in 100 years", as there are hardly any observed data to test and validate the models. Any record of previously unobserved hourly intensities is by definition an increase of "1 now versus 0 before". Therefore, it is necessary to use theoretical functions to extrapolate a probability of phenomena that have never occurred. The technique of using theoretical distributions is very difficult to verify for variables such as precipitation as they are far away from typical Gaussian behaviour, as is the case for temperature.
I think the authors should also use observed data from long weather stations (at least 50 or 100 years) to better analyse the return periods. Models and reanalyses are insufficient for these cases of extreme rainfall events [as in the case of those occurring in Spain]. In fact, the parameterisation of the models is, in general, more suited to typical rather than extreme rainfall situations, and therefore tends to underestimate sub-daily intensities.
It is indeed encouraging to see that this analysis, like our previous report, the First Mediterranean Assessment Report (MAR1) prepared by the independent network of Mediterranean Experts on Climate Change and Environment (MedECC), takes into account the factor of land use change. This reinforces our understanding that, while climate change can be seen as the underlying cause of recent catastrophes, its impacts are exacerbated by landscape management that lacks adequate preventive measures.
Preventive measures in flood management play a key role in reducing risks and protecting communities. These measures include the development of early warning systems, the construction of check dams, the improvement of drainage systems and the implementation of emergency management plans.
In addition, preventive measures based on urban planning for resilience and nature-based solutions offer a promising and economically viable approach. This includes actions such as reforestation in upstream areas, floodplain restoration and bank erosion protection, as well as appropriate agricultural practices to retain water.
In addition, proactive adaptation to sea level rise is essential for coastal areas. We are talking about basic coastal adaptation practices, such as protection, strategies like beach and coastal accretion, as well as dune and wetland restoration, which are gaining ground as more sustainable alternatives to hard structures. These measures not only protect against flooding, but also contribute to preserving biodiversity and promoting long-term sustainability.
The WWA lists some of the causes of the disasters that have occurred in Greece, Bulgaria, Turkey, Libya and Spain in recent weeks from a rigorous point of view supported by observations and predictions of scientifically validated models, although not all of them could be considered as a consequence of climate change, as they point out in their report.
The magnitude of the catastrophe in the case of Libya is not only due to rainfall, but also to the poor state of the dams, which has produced a chain disaster that could have taken place at any time and not because of the increase in global temperature or the greater accumulation of rainfall due to climate change.
The same can be said in the case of Spain, where the damage can be caused by the well-known cold drop phenomenon, independently of global warming, although it is true that global warming can cause this type of event to intensify and even occur more frequently, as the scientists who carried out the analysis point out.
The Mediterranean is a hotspot of climate change because it is warming faster than the global average but, above all, because the region is highly exposed and vulnerable to the effects of climate change.
We are not prepared or adapted for what is happening in terms of extreme events, let alone what is coming, and this study describes it in numbers. For example, the extreme rainfall event in Libya would occur once every 300-600 years in today's climate. In other words, it is very unlikely. However, this event is now 50 times more likely than without climate change. That is, unlikely but not as unlikely as before.
We need to adapt for unlikely but high-risk events, and for that we need more studies like this, so that we can adapt by knowing what level of risk we are exposed to. This allows us to design infrastructure (in this case dams) that can withstand more extreme events than before, and to have planned escape routes in the event that the event exceeds the capacity of the infrastructure.
- Research article
- Observational study