Rains in RS: State will still experience many extreme events, say Brazilian scientists who collaborated with IPCC

The latest events “in a way confirm something we have been saying for some time: that, in addition to the natural variability that leads to extreme events, there is a human contribution or influence”, says the mathematician, who currently serves as president of the Steering Committee of the Global Climate Observation System, to BBC News Brasil.

Still according to Krug, although the so-called science of climate attribution – which studies the impact of human activity on the probability of occurrence of specific phenomena – is still very new, the relationships supported by the IPCC indicate that heavy rainfall such as those currently observed may become more recurrent. .

“Unfortunately, I believe there is a very high probability that these events will occur again in a more frequent and intense way,” he says.

The IPCC is a group of scientists defined by the United Nations that monitors and evaluates science related to climate change.

In its report, the IPCC points out the human contribution to the increase in rainfall in the region called Southeast South America (SES), which encompasses not only Rio Grande do Sul, but also other states in the southern region of Brazil and some areas from nations such as Argentina and Uruguay.

The SES is the only one that encompasses Brazil where the IPCC detected evidence of heavy rainfall related to human action.

The panel classifies its conclusion as “low confidence”, but according to Krug this is the highest level of evidence currently available for the region due to the difficulty of the calculations involved.

Mercedes Bustamante, professor at the University of Brasília (UnB) and collaborator on some of the IPCC reports, also sees strong evidence of the influence of climate change caused by human activities on the rains that caused 83 deaths and affected 345 of the 497 municipalities in Rio Grande do Sul.

According to the ecologist, member of the Brazilian Academy of Sciences (ABC), Rio Grande do Sul has always been the meeting point of tropical systems and polar systems, which creates a pattern that includes periods of intense rain and others of drought.

And the tendency is for this alternation to continue to be repeated, but with increasing intensity.

“This is a region where we are going to experience much more extremes, according to climate models,” says the expert.

The heavy rains currently hitting Rio Grande do Sul can be explained by a combination of risk factors, including a mass of hot air over the central area of ​​the country, which blocks the cold front in the South region and causes instability over the State, causing intense and continuous rains.

In addition, the period between the end of April and the beginning of May 2024 is still influenced by the El Niño phenomenon, responsible for warming the waters of the Pacific Ocean, also contributing to areas of instability over the State.

This combination of several factors at once is considered rare by experts.

However, according to Mercedes Bustamante, the greatest frequency of these “compound risks” is highlighted in the compilation of data on climate change by the IPCC.

“There is a convergence of different variables that act in synergy and increase this risk factor”, he says. “Many of the discussions about preparedness have been about risks in isolation, but we need to look at cascading effects and risks in an integrated way.”

Bustamante explains that large-scale deforestation of the Cerrado in recent decades has increased surface temperatures and reduced the amount of evapotranspiration, or the return of water to the atmosphere, in the central region of the country.

With less moisture return, the atmosphere becomes hotter and drier. In convergence with El Niño, it is this hot air mass that is blocking and maintaining the area of ​​instability over Rio Grande do Sul.

“There is a regional phenomenon, which is El Niño, but also an issue associated with the transformation of our biomes”, he says.

At the same time, this same mass of hot air blocks the so-called ‘flying rivers’ of the Amazon, a kind of invisible watercourse that circulates through the atmosphere. This is the humidity generated by the Amazon and which is dispersed throughout the South American continent.

If this watercourse found a less dry environment in the central region of Brazil, part of it would precipitate there. But under current circumstances, moisture is forced to divert around the edges of the hot, humid mass, so that it bumps into the Andes and is channeled to the south of the country.

“We had cold fronts that were unable to ‘rise’ and masses of humid air that were unable to spread to central Brazil and ‘leaked’ to the sides”, he summarizes.

According to the researcher, this context made the rains recorded in the last week more extreme and widespread than those that hit Rio Grande do Sul in September 2023.

At the same time, according to Thelma Krug, there is increasing evidence in science that links climate change to longer-lasting and more intense periods of El Niño.

“We already saw El Niño extending over a longer period of time last year,” he says.

“And now we have a composition of very hot days with implications for the surface temperature of the ocean, a scenario that has an influence on all this change in relation to rainfall.”

According to mathematics, it is quite complex to make associations between human actions and heavy rainfall – unlike heat waves, which are more easily linked to climate change caused by human activities.

“But what we know unequivocally is that human influence has warmed the entire climate system: it has warmed the ocean, it has warmed the atmosphere, the cryosphere. In other words, all the elements of the terrestrial biosphere”, he says.

“And it is impossible not to imagine that this warming that has hit the climate system in its entirety will not have consequences in several areas.”

Krug and Bustamante are categorical in stating the need for adaptation actions suited to new climate models to avoid new tragedies in cases of future extreme events.

“Brazil needs to expand its environmental data monitoring network”, highlights the professor at the University of Brasília.

According to Mercedes Bustamante, the risk mapping prepared by the National Center for Monitoring and Alerts of Natural Disasters (Cemaden), linked to the Ministry of Science, Technology and Innovation and created after the climate tragedy that left more than 900 dead in the mountainous region of Rio de Janeiro January in 2011, needs to be revisited.

For Thelma Krug, planning must be done at federal, state and municipal levels and with the support of public-private partnerships.

“The frequency of these events in Rio Grande do Sul and the intensity of what is currently happening – which could possibly even be one of the biggest in the country – is worrying and requires us to take actions not only to resume life, but to return to living from different way,” he says.