High temperature in Brazil heatwave intensified by both human-driven Climate Change and Natural Variability

Press Summary (First published 2024/03/24)

• Heatwaves similar to the March 2024 Brazil heatwave are now up to 1°C warmer than the warmest heatwaves previously observed in the country, even though they occur much later than the southern hemisphere summer seasons. 

• This was a somewhat uncommon event which occurs more frequently now in February/March with respect to the past.

• Human driven climate change and natural climate variability both played a role in increasing the heat during the Brazil March 2024 heatwave.  

Event Description

From March 15th to 18th, 2024, a record-breaking heatwave engulfed southern Brazil, with temperatures soaring to unprecedented levels for the early autumn period in the southern hemisphere (reaching 42°C in Rio de Janeiro). The scorching heat index, peaking at 62.3 °C, marked the highest recorded in the city in the past decade. This measurement, crucial for gauging perceived temperature by factoring in humidity, underscores the severity of the heatwave sweeping across Brazil. It has compelled residents and visitors alike to seek refuge on the sandy shores of Ipanema and Copacabana beaches, where crowds gather despite the blistering conditions. Authorities have disseminated guidance on coping strategies to mitigate the effects of the sweltering temperatures. While Rio battled scorching temperatures, the southern regions grappled with torrential rains and ensuing chaos. An impending cold front exacerbated the situation, heralding more downpours and potential gales. Guaratiba, situated in western Rio de Janeiro, bore the brunt of high temperatures owing to its geographic disposition. The proximity to the ocean and warm northerly winds exacerbated the intensity of the heatwave in this locality.

The Surface Pressure Anomalies reveal a significant negative (cyclonic) anomaly over the Southeast Brazilian coast. This pattern transports warm and moist air toward the coast. Temperature data indicate warm anomalies reaching up to +5°C over continental areas, with anomalies around +1°C over the Atlantic Ocean. Precipitation data show moderate precipitation amounts over the continental area, indicating that heat is coupled with wet conditions, exacerbating the perceived temperature. Windspeed data depict extensive areas of the Eastern Atlantic experiencing northerly winds exceeding 50 km/h, particularly in the vicinity of Rio de Janeiro.

Climate and Data Background for the Analysis

The IPCC AR6 report provide a clear relationship between heatwaves and climate change in Brazil through projections of increased heat wave-related mortality, observed increases in extreme temperatures, and the likelihood of more frequent and intense heat waves in the future due to climate change In particular There is high confidence that extreme temperatures have increased in South-Eastern South America, including Brazil, over the last decades, with a human contribution to the observed increase in the intensity and frequency of hot extremes (IPCC AR6 WGI FR - Page 1454). Warming has led to an increased frequency, intensity, and duration of heatwaves in Brazil (IPCC SR CCL SPM - Page 9). Heatwaves are a growing health risk in Brazil due to climate change, with negative impacts on mortality, labor productivity, and mental health (IPCC AR6 WGII FR - Page 1057). Brazilian urban areas are projected to face increasing heatwave-related mortality in the future due to climate change ( IPCC AR6 WGII FR - Page 1720). By the end of the century, most regions in South America, including Brazil, will undergo extreme heat stress conditions much more often than in the recent past (IPCC AR6 WGI FR - Page 1454).

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the high temperature in Brazil March heatwave changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [50°W 40°W 20°S 27°S]. The Surface Pressure Changes show that similar events do not display significant changes in the present climate than what they would have been in the past. The Temperature Changes show that similar events produce temperatures in the present climate that are up to 1 °C warmer than what they would have been in the past, over a large area of the region analyzed. The Precipitation Changes show any significant variations. Windspeed Changes indicate up to 4 km/h windier conditions over Southern Western regions. We also note that Similar Past Events previously mainly occurred in November and December, while in the present climate they are mostly occurring in February and March. Changes in Urban Areas reveal that Rio de Janeiro, Curitiba, and Sao Paulo are up to 1 °C warmer in the present compared to the past. Sao Paulo also experiences increased precipitation (up to 1 mm/day).  This increase in precipitation, alongside higher temperatures, elevates the heat index.

Finally, we find that sources of natural climate variability, notably the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation may have influenced the event. This suggests that the changes we see in the event compared to the past may be partly due to human driven climate change, with a contribution from natural variability.

Conclusion

Based on the above, we conclude that heatwaves similar to Brazil March heatwave are 1 °C warmer than the warmest heatwaves previously observed in the country, even though they occur much later than the southern hemisphere summer seasons. We interpret Brazil March heatwave as a somewhat uncommon event which occurs more frequently now in February/March with respect to the past for which both human driven climate change and natural climate variability played a role.

Quotes & Contact Authors

-Davide Faranda, IPSL-CNRS, France 📨davide.faranda@lsce.ipsl.fr 🗣️French, Italian, English

-Tommaso Alberti, INGV, Italy 📨tommaso.alberti@ingv.it 🗣️Italian, English