Early February 2024 California Floods likely influenced mostly by human-driven climate change

Press Summary (First published 2024/02/02)

• In the last two decades, floods affecting coastal California have been up to 15% wetter.

• The February 2024 California floods were similar to several events in the past.

• We mostly ascribe the changes in precipitation to human driven climate change and natural climate variability likely played a modest role.

Event Description

On February 01, 2024, the initial of two atmospheric rivers unleashed substantial snowfall at high altitudes, accompanied by strong winds and persistent rainfall throughout a significant portion of California. Over 20 million individuals were under flood advisories due to the looming threat of flash flooding in major cities such as Los Angeles, San Diego, Sacramento, San Francisco, San Jose, and Oakland. As rain relentlessly pounded Southern California, floodwaters began to rise, prompting the closure of sections of the Pacific Coast Highway and the 710 Freeway. Additionally, several water rescues were conducted.

Long Beach, situated south of downtown Los Angeles, experienced severe flooding, as depicted in videos circulating on social media, showing submerged roadways and vehicles navigating through high waters. A few submerged vehicles had to be rescued by fire crews in Long Beach, with no reported injuries, although one car was submerged up to its windshield. Long Beach Airport recorded 58 millimeters of rain overnight, while Los Angeles International Airport received 61 millimeters. In nearby Orange County, first responders rescued a man trapped in a storm channel turned swiftly flowing river due to heavy rain. The individual was taken to the hospital in stable condition, according to fire officials. By late morning, the storm had shifted southward into the San Diego area. However, local officials reported that it resulted in only minor disruptions, contrasting sharply with the historic rains of the previous week that triggered destructive flash floods in the downtown region. Prior to the storm's arrival, San Diego authorities had issued evacuation warnings for communities in the Chollas Creek watershed, including neighborhoods still reeling from recent flash flooding. Despite reports of several flooded or closed roads, the day in the city remained predominantly calm. 

While there may be a temporary break in the weather after this storm saturates Southern California, another, potentially more powerful atmospheric river is poised to traverse the region starting on Sunday. Rainy conditions are anticipated to persist well into February as a more typical El Niño pattern takes hold. El Niño, a natural occurrence in the tropical Pacific affecting global weather, alters the jet stream, directing storms toward California. These storms can tap into an exceptionally potent moisture source from the tropics known as an atmospheric river.

The Surface Pressure Anomalies reveal a large depression area over the analyzed domain of the order of 10 hPa or larger. This configuration led to high Temperature Anomalies over inner regions (up to 10 °C) and along the coast (up to 4 °C). Precipitation data indicate that the majority of the area experiencing heavy precipitation is along the Pacific Coast. The coastal area experienced extreme precipitation, reaching up to or exceeding 100 mm/day. Windspeed data also indicate moderate winds, sustained mainly over the coast.

Climate and Data Background for the Analysis

According to the IPCC AR6 report, there is robust evidence, based on both detailed modeling and simple physics, that precipitation associated with atmospheric rivers will increase in the future in most areas of the world (high confidence), as the content of water vapor increases over the oceans of about 7% per 1°C of warming. Looking more in detail at the case of California, Chapter 8 states that "Under continued global warming, more intense moisture transport within atmospheric river events is projected to increase the magnitude of heavy precipitation events on the west coast of the USA.". Moreover, "the frequency, magnitude and duration of atmospheric rivers making landfall along the North American west coast are projected to increase".

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 very similar to other past events in the data record.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the low pressure system leading to the February 2024 California Floods have 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 [-125°E -112°E 27°N 38°N]. The Surface Pressure Changes indicate no changes in the strength of the low-pressure system in the Pacific compared to the past, slightly increasing in distant areas from the center of the system. Temperature Changes also show no changes along the coast and in the inner areas. Precipitation Changes show that similar events now produce heavier (up to 14 mm/day) precipitation in the present than in the past in some areas of the North-East part of the Pacific coast. Windspeed Changes indicate windier conditions (up to 8 km/h) on the coastal areas analyzed. We also find that Similar Past Events occur most frequently in February in the present, while they previously mostly occurred in January or March. Considering the affected urban areas, Santa Barbara and San Francisco experienced an increase in precipitation, with up to 10 mm/day more rain in the present than in the past while in California atmospheric rivers have the same intensity than in the past. 

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation, may have only partly influenced the event. This means that the changes we see in the event compared to the past may be mostly due to human driven climate change.

Conclusion

Based on the above, we conclude that events similar to the early February California floods are up to 14 mm/day (up to 15%) wetter and up to 8 km/h windier over the Pacific coast in the present than they have been in the past. We interpret the early February 2024 California Floods as an event whose extreme characteristics can mostly be ascribed to human-driven climate change and natural climate variability likely played a modest role.

Quotes & Contact Authors

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

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