Heavy precipitation causing Texas floods likely influenced by both human-driven climate change and natural variability

Press Summary:

• Weather systems similar to that producing the Texas floods are warmer (+2/3°C), and locally wetter (3-11 mm/day, up to 15% wetter) than they were in the past.  

• The depression causing Texas floods was similar to others observed in the past.

• We mostly ascribe the  heavy precipitation that caused Texas floods to human driven climate change and natural climate variability likely played a modest role.

Event Description

In line with the peak of severe weather season for the Midwest and the Southern Plains of the US, the last decade of April has seen intense outbreaks of thunderstorms accompanied by tornadoes, hail and flooding especially in Nebraska and Oklahoma. The severe weather risk has continued through the beginning of May. Starting from May 3rd, several systems have brought abundant precipitation over several areas of Texas. On May 5, a lower than normal pressure area was located over Mexico extending through Texas, although lacking organization. Areas from around Houston to the West were located at the convergence between the outflow from a storm system moving into Western Louisiana and a warm, moist flow from the Gulf of Mexico. This air mass was unstable and rich in water content, due to the temperatures over the Gulf being 2°C above average. As a result, regenerating thunderstorms fired along the outflow boundary, and remained stationary for several hours over hydrologically sensitive areas, prompting flood warnings. The areas of concern were indeed affected by intense precipitation, bringing rivers to hurricane Harvey levels, causing catastrophic damage and threatening the life of thousands of people, hundreds of which had to be rescued. 

The Surface Pressure Anomalies show a very small negative (cyclonic) anomaly over Central-Eastern Texas, in correspondence of the affected area. Temperature anomalies also show positive values over most of Texas and in the North-Western Gulf of Mexico. Precipitation data show patches of both positive and negative trends across the affective region, with variations up to 11 mm/day, that is the 15% of the precipitation amounts recorded during the event.

Climate and Data Background for the Analysis

In the IPCC AR6 report, Chapter 12, assigns high confidence to positive observed trends in extreme precipitation over Central and Eastern North America. Moreover, there is high confidence that high precipitation will increase over the same area with ongoing global warming.

Intense convective episodes over this region are not uncommon in the spring, when convective activity is also heavily influenced by natural sources variability, with the greatest contribution coming from ENSO. In particular, a situation of decaying El Niño as observed during spring of 2024, is linked to an enhanced occurrence of severe weather outbreaks in this region. It is worth noting that the ENSO index we use does not distinguish a decaying El Niño from other neutral phases. 

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have high confidence in the robustness of our approach given the available climate data, as the event is similar to other in the database

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the pressure system leading to Texas Floods 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 [102°W, 90°W; 27°N, 38°N]. The Surface Pressure Changes show that similar events are very slightly deeper in the present climate than what they would have been in the past over Central-Eastern Texas. The Temperature Changes show that similar events produce temperatures in the present climate that are up to 3ºC warmer than what they would have been in the past, over a large area of Texas. The Precipitation Changes areas of significant but irregular trends of different signs, with a positive change up to 11 mm/day in the affected areas. We also note that Similar Past Events previously occurred in April, May or June with exactly the same frequency observed in recent decades. Changes in Urban Areas reveal that Houston, Dallas and Shreveport (Louisiana) are 5-7 mm/day wetter than in the past during similar events, with also modestly higher temperatures.

Finally, we find that sources of natural climate variability, notably the Pacific Decadal 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 convective systems similar to those producing Texas Floods are up to 11 mm/day (up to 15%) wetter over some areas, and up to 2-3 °C warmer in the present than they would have been in the past. We interpret Texas Floods as a common event for which both human driven climate change and natural climate variability likely played a role. We remark that the influence of ENSO might be present but not detected, due to the particular phase involved.

Quotes & Contact Authors

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

-Flavio Pons, IPSL, France 📨flavio.pons@lsce.ipsl.fr 🗣️Italian, English, French