Predicting extreme rainfall is a challenge, says Carlos Nobre – 02/22/2023 – Environment

In São Sebastião, a tourist town at the foot of Serra do Mar, on the north coast of São Paulo, the search for the missing has no deadline to end. Civil Defense and volunteers work under difficult conditions in the hardest-hit neighborhoods.

The heavy rains recorded in the early hours of last Sunday (02/19) left at least 46 dead. Hundreds are homeless.

The highest volume of rain recorded in the region in 24 hours —683 millimeters, in the municipality of Bertioga— is the new record in the Brazilian meteorological system. Before him, last year, 534.4 millimeters had been recorded in the Petrópolis tragedy, which left 241 dead a year ago. In São Sebastião, it was 627 mm in 24 hours last weekend.

Carlos Nobre, a retired climatologist from INPE (National Institute for Space Research), who was involved in the creation of Cemaden (National Center for Monitoring and Natural Disaster Alerts), in 2011, reinforces the warning that science has been giving for years: climate events extremes of this kind are becoming more frequent with climate change.

“All over the planet, all these extreme phenomena are happening much more frequently, and science leaves no doubt that they wouldn’t be happening with this frequency if the planet wasn’t suffering from global warming”, says Nobre in an interview with DW.

According to the climatologist, mathematical models that exist around the world cannot capture rainfall records like those on the coast of São Paulo, but only reproduce extreme events of recent decades.

“These rains on the north coast of São Paulo were three times greater than the forecast models indicated”, details Nobre. “It is a very big challenge for science to see how to make climate models able to predict these records that are happening every year across the planet.”

In the affected region on the coast of São Paulo, there is no siren system that communicates the risk to the population and that indicates the moment to leave the place.

“When the most serious alert was issued, around midnight on Sunday, if a siren had sounded, people could have left their homes. At that time, no landslides had yet occurred”, laments Nobre.

Why are the rains recorded on the north coast of São Paulo in the early hours of last Sunday, which left at least 46 dead, considered an extreme weather event? This is an extreme event because, in fact, it was a record of rainfall in the municipalities of Bertioga and São Sebastião. More than 600 millimeters fell in the space of nine hours. It is a record registered by the thousands of Cemaden rain gauges in ten years, this is the record of rain in less than 24 hours.

It is an extreme phenomenon that was predicted by Cemaden, which alerted all Civil Defenses in the state of São Paulo. There had never been such a high volume of rain in such a short space of time. At times, like at 2 am from Saturday to Sunday, it rained 120 mm in one hour. It is a record in the Brazilian meteorological system.

How much can you relate what happened to climate change? Today there are already several systems in the world —and it would be important to have one of these in Brazil— that try to do what is called attribution of cause. Could an extreme event like this be absolutely natural, that it has nothing to do with global warming, or can it only be explained in terms of warming?

These studies make simulations with mathematical models of the meteorological system. For example: they simulate what the climate would be like without any influence from the increase in the concentration of greenhouse gases in the atmosphere. So the model shows how extreme events like this one on the coast of São Paulo would happen all over the world, that is, natural, associated with the planet’s climate.

Scientists do this same type of simulation adding the increase in the concentration of greenhouse gases, which is what happened on the planet in the last hundred years. And then the simulation shows how much extreme events would increase in this scenario. It is a way of attributing cause.

This was done after the rain that caused more than 120 deaths in Recife in 2022. Immediately, a group from England ran this simulation and showed that the intensity and frequency of that type of extreme event is due to global warming. It wouldn’t happen if the planet wasn’t warming up.

We have to wait for those groups that have the capacity to carry out these studies. I’m sure that in a few weeks they will do that and say whether what happened can be attributed to natural causes or not.

From my experience, I think that an event that breaks the 24-hour rainfall record must have a lot to do with global warming. Much of this immense volume of rain that occurred on the coast of São Paulo has to do with the fact that the surface temperature of the ocean was higher, at around 27°C. This causes the ocean to evaporate an immense amount of water.

The meteorological system that was over the ocean, close to the coast, a pressure drop, threw this immense amount of steam that evaporated from the ocean into the coast. The steam went up the Serra do Mar, condensed and it rained a lot.

All over the planet, all these extreme phenomena are happening much more frequently, and science leaves no doubt that they wouldn’t be happening at this frequency if the planet wasn’t experiencing global warming.

Are the models used in Brazil for weather forecasting capable of predicting this volume of water that fell, more than 600 millimeters in one day in one location? Mathematical weather forecast models are normally calibrated to represent the physical phenomenon that has happened over the past 30 years. It is very difficult to predict that these models will be able to predict these records. Globally speaking, they cannot predict these records because they are calibrated to respond to what has happened over the last few decades.

These rains on the north coast of São Paulo were three times greater than the forecast models indicated. But Cemaden issued a very high risk alert: the models themselves were already indicating more than 200 mm, and that volume of rain in a few hours causes disaster anywhere in the world. But they fell 600 mm.

And this is something that happens all over the planet. When there is a record of rainfall, the mathematical models cannot capture the record, but reproduce the extreme events that have happened in the last 30 years.

So it is a very big challenge for science to see how to make climate models able to predict these records that are happening every year across the planet.

Cemaden itself was created in a similar context of tragedy, in 2011. How much has it helped to reduce the number of deaths from natural disasters? It was created in 2011, after the extreme event in the mountainous region of Rio, in Petrópolis and Teresópolis, which had the highest number of deaths recorded by a climatic event in the history of Brazil. More than 900 people died that January.

The then president, Dilma Rousseff, who had just taken office, visited the mayors and learned that there was no warning system to prevent those deaths. I had, a little earlier, presented to Minister Mercadante, who was in charge of Science and Technology, a proposal that I had defended for many years, which was to create a capacity for Brazil to be on alert for natural disasters.

Inpe already made quality forecasts of weather conditions, but that did not reach the Civil Defense. That rain that happened in the mountainous region had been predicted three days in advance by Inpe.

Cemaden was then created. He takes forecasts and transforms floods, landslides, flash floods, droughts, large fires into disaster risks. Today, he monitors 1,038 municipalities and calculated that more than 10 million Brazilians live in areas at risk of disasters. More than 2 million Brazilians live in high-risk areas.

Back in 2011, we had 900 deaths. Then, for many years, that number dropped to less than 100. The municipalities that received warnings from Cemaden were able to reduce the number of deaths. The mountainous region of Rio, since then, has sirens. When the alert arrives, the Civil Defense activates the sirens and the population knows that it has to leave the house and where it has to go.

Unfortunately, in 2022 that number exploded, there were more than 500 people killed by disasters. From the end of 2021 until now, that is, in 14 months, we had the record for natural disasters in the history of Brazil. We really need to understand why, if more events like this are happening or if risk alert communication is being effective.

None of the cities on the coast of São Paulo have sirens. More than 40,000 areas were mapped by Cemaden in the country as being at risk, so it would be very important for all these places to have a siren system.

When the most serious alert was issued, around midnight on Sunday, if a siren had sounded, people could have left their homes. At that time, no landslides had occurred yet.

Installing sirens is for yesterday, it’s a short-term measure. In the medium term, it has to remove the 2 million Brazilians who live in areas of extremely high risk. There are people who live in places with slopes above 25 degrees, this is very dangerous, no one should live on a slope like this.

This takes time. It has to create Minha Casa, Minha Vida, as President Luiz Inácio Lula da Silva announced in São Sebastião, but it has to be my sustainable home, my sustainable life. People cannot go back to these very high risk areas. It’s a huge challenge.

Most people returned to where they lived after the 2011 disaster in the mountainous region of Rio. This couldn’t have happened.

There is also the danger posed by extreme weather events not showing up in forecasts… Cemaden manages to alert days in advance for most cases associated with predictable meteorological events, such as what has occurred on the coast of São Paulo now. Last Friday, a detailed technical document was sent to all Civil Defenses.

The extreme event of February 15, 2022 in Petrópolis, which killed more than 200 people, was a rare event and was not predicted by mathematical models. It happens all over the world.

That event developed very quickly and generated a large amount of moisture that came out of the ocean, went up the Petrópolis mountain range and caused 230 mm of rain in three hours and those horrible floods. In that event, the Civil Defense did not receive the alert in advance, and that is why so many people died.

In the United States, models do not pick up tornadoes. They show that a storm could spawn a tornado, but once the tornado is spawned, the models cannot predict the trajectory and duration. But there are numerous monitoring systems that, when they detect the tornado forming, warn the entire population possibly affected. And everyone can take shelter and survive.

In 2021, there was that flood in the Ahr Valley in Germany. It was a huge analogy with what happened here on the coast of São Paulo now, because the most advanced models in the world did not predict that event. They predicted that there would be a lot of rain, but the volume was four times greater than predicted. [total de 150 mm]. Even in a highly developed country like Germany, that rain, with that intensity and that duration, caused a disaster that killed so many people.

Most extreme events in Brazil are predictable. So the population has to be trained to respond to this, especially with the increased risk that climate change brings. This needs to become a policy that the government has to develop.