“Predicting extremes like SP precipitation is a challenge for science” – DW – 02/22/2023

In São Sebastião, a tourist town at the foot of the Serra do Mar, on the northern coast of São Paulo, there is no deadline for tracing. Civil defense and volunteers work under difficult conditions in the most affected neighborhoods. to high rain In the early hours of last Sunday morning (02/19), at least 46 people were killed. Hundreds are homeless.

The greatest amount of rain recorded in the region in 24 hours – 683 mm, in the municipality of Bertioga – is the new record in the Brazilian meteorological system. Before him, last year, 534.4 mm was recorded in Tragedy of PetropolisWhich left 241 dead a year ago. In Sao Sebastião, it was 627 mm in 24 hours last weekend.

Carlos Nobre, a retired climatologist from the National Institute of Space Research (Inpe), who co-created the National Center for Natural Disaster Monitoring and Alerts (Cemaden), in 2011 reinforces the warning that science has given for years: Weather extremes of this kind are becoming more frequent with climate change.

Nobre says in an interview with DW.

According to the climatologist, mathematical models found around the world cannot capture precipitation records like those on the Sao Paulo coast, but only reproduce extreme events that have occurred in recent decades.

“This rain on the northern coast of São Paulo was three times greater than the forecast models indicated,” Nobre details. “It’s a very big challenge for science to figure out how to make climate models able to predict these records that happen every year across the planet.”

In the affected area on the coast of São Paulo, there is no siren system that conveys danger to residents and indicates the moment to leave the place. “When the most serious alert was issued, around midnight on Sunday, if the siren had sounded, people could have left their homes. At that time, there were no landslides yet,” Nobre laments.

DW: Why was the rainfall recorded on the northern coast of Sao Paulo in the early hours of last Sunday, which left at least 46 dead, considered a severe weather event?

Carlos Nobre: This is an extreme event because, in fact, it was a record rainfall in the municipalities of Bertioga and São Sebastião. More than 600 mm fell within nine hours. It is a record recorded by thousands of Cemaden rain gauges in ten years, and this is the record number of rain in less than 24 hours.

It is an extreme phenomenon predicted by Cimaden, which alerted all civil defenses in the state of São Paulo. There has never been such a large amount of rain in such a short period of time. Sometimes, like at 2 am from Saturday to Sunday, it rained 120 mm in one hour. It is a record in the Brazilian meteorological system.

To what extent can you relate what happened to climate change?

Today there are already many systems in the world – and it would be important to have one in Brazil – trying to do what is called cause-attribution. An extreme event like this may be completely normal, and has nothing to do with Global Warmingor can it be explained only in terms of warming?

These studies simulate mathematical models of the meteorological system. For example: they simulate what the climate would be like without any effect from an increase in the concentration of greenhouse gases in the atmosphere. So the model shows how extreme events like this one on the Sao Paulo coast would occur all over the world, i.e. natural, linked to the planet’s climate.

Scientists do this kind of simulation adding in the increase in the concentration of greenhouse gases, which is what has happened on the planet in the past 100 years. The simulation then shows how much more extreme events are in this scenario. It is a way of attributing the cause.

Done yet Rains causing more than 120 deaths in Recife in 2022. Immediately, a group from England ran this simulation and showed that the intensity and frequency of this type of extreme event is due to global warming. It wouldn’t happen if the planet wasn’t warming.

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

In my experience, I think a record-breaking 24-hour rainfall event must have a lot to do with global warming. Much of the massive amount of rain that occurred on the Sao Paulo coast had to do with the fact that the ocean surface temperature was higher, at around 27°C. This causes the ocean to evaporate a huge amount of water. The meteorological system that was over the ocean, near the coast, dropping pressure, threw such a huge amount of steam that it evaporated from the ocean to the coast. The Serra do Mar steamed up, condensed and it rained heavily.

Across the planet, all of these extremes occur frequently, and science leaves no doubt that they would not occur at this frequency if the planet were not experiencing global warming.

Are the models used in Brazil to forecast the weather able to predict this volume of water that has fallen, more than 600 mm in one day in one place?

Mathematical weather forecast models are usually calibrated to represent a physical phenomenon that has occurred within the past 30 years. It is very difficult to predict that these models will be able to predict these records. Globally, they cannot predict these records because they have been calibrated to respond to what has happened over the past few decades.

This rainfall on the northern coast of São Paulo was three times greater than the indicated forecast models. But Cemaden issued a very serious warning: the same models were already indicating more than 200mm, and that amount of rain in a few hours spells 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 precipitation, mathematical models cannot capture the record, but rather reproduce the extreme events that occurred in the past 30 years.

So it’s a big challenge for science to see how climate models can predict these records that happen every year across the planet.

Cemaden itself was created in a similar context of tragedy, in 2011. How much does it help reduce the number of deaths from natural disasters?

It was created in 2011, after the extreme event in the mountainous region of Rio, in Petropolis and Tresopolis, which recorded the highest number of deaths recorded by a climatic event in the history of Brazil. More than 900 people were killed in January.

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

Inpe did indeed make good weather forecasts, but that didn’t make it to civil defence. Enppi had predicted this rain, which occurred in the mountainous region three days ago.

Then Cemaden was created. It takes forecasts and converts floods, landslides, flash floods, droughts and major fires into disaster risk. Today, it monitors 1,038 municipalities and it is estimated that more than 10 million Brazilians live in areas at risk of disasters. More than two million Brazilians live in high risk areas.

In 2011, we had 900 deaths. Then, for many years, that number dropped below 100. Municipalities that received warnings from Cimaden were able to reduce the number of deaths. The mountainous region of Rio, since then, has sirens. When the alarm arrives, the civil defense activates the siren and the residents know that they must leave the house and where to go.

Unfortunately, this number exploded and in 2022, more than 500 people lost their lives due to disasters. From the end of 2021 until now, that is, in 14 months, we have had a record of natural disasters in the history of Brazil. We really need to understand why, if more events like this happen or if the risk alert communication is effective.

There are no sirens in any of the cities on the coast of São Paulo. More than 40,000 areas in the country are designated as vulnerable, so it will be very important for all of these places to have a siren system.

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

Yesterday’s siren installation, it’s a short-term measure. In the medium term, it has to get rid of the two million Brazilians who live in high-risk areas. There are people who live in places with slopes above 25 degrees, which is very dangerous, and no one should live on a slope like this.

This takes time. She must create a Minha Casa, Minha Vida, as announced by President Luiz Inácio Lula da Silva in São Sebastião, but it must be my sustainable home, my sustainable life. People cannot return to these high-risk areas. It’s quite a challenge.

Most people have returned to where they lived after the 2011 disaster in the mountainous region of Rio. This cannot happen.

There is also the danger posed by extreme weather events that don’t show up in the forecast…

Cemaden was able to warn days in advance of most cases associated with predictable meteorological events, such as what happened on the coast of São Paulo now. Last Friday, a detailed technical document was sent to all civil defence.

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

This event developed very quickly and generated a large amount of moisture that came out of the ocean, went up the Petropolis mountain range and caused 230 mm of rain in three hours and those terrible floods. In that case, the civil defense did not receive the warning in advance, which is why many people died.

In the United States, models do not capture tornadoes. They showed that a storm can spawn a tornado, but once a tornado appears, the models cannot predict the path and duration. But there are many monitoring systems that, when they detect tornado formation, warn all residents that they may be affected. And everyone can take cover and survive.

In 2021, there was that Flooding in the Ahr Valley, Germany. It was a great analogy to what is happening here on the coast of São Paulo now, because the most advanced models in the world did not predict this event. They expected heavy rain, but the volume was four times greater than expected. [total de 150 mm]. Even in a highly developed country like Germany, this rain of such intensity and duration caused a catastrophe that took many lives.

Most extreme events can be expected in Brazil. So the population must be trained to respond to this, especially with the increased risks that climate change brings. This should become a policy that the government should develop.