California, the most populous state in the US, faces a future marked by “precipitation whiplash events” – extreme swings between drought and floods – caused by human-induced climate change, according to a team of scientists from the University of California at Los Angeles.
They warn of flooding that “would probably lead to considerable loss of life and economic damages approaching a trillion dollars”.
Their report, published in the journal Nature Climate Change, says California’s rapid shift from severe drought between 2012 and 2016 to heavy rains and widespread flooding during the 2016-17 winter offers a compelling example of one such transition in a highly populated, economically critical and biodiverse region.
It takes on global significance when considering that California, with a population of 39.5 million people in 2017, has the largest economy in the US – the sixth largest in the world – and has accounted for about 20% of the nation’s economic growth since 2010. {%recommended 4888%}
The state, covering more than 1000 kilometres north to south, has abundant water resources in the north and a complex network of infrastructure to bring it to the massive population centres in the south. The civil engineering and risk management practices that are in place have been predicated on a largely stationary climate, and most existing water storage and conveyance structures have been built under such assumptions.
The report notes that during the 2016-17 winter, hundreds of roads throughout California were damaged by floodwaters and mudslides, including an important bridge collapse, and that In February 2017 heavy runoff in the Feather River watershed contributed to the failure of the Oroville Dam’s primary spillway, culminating in a crisis that forced the emergency evacuation of nearly a quarter of a million people.
The report’s lead author, Daniel Swain, says the team used specific flood and drought events from California’s history as baselines for exploring the changing character of precipitation extremes. Taking data from the Community Earth System Model Large Ensemble (CESM-LENS) allowed them to directly quantify changes in large-magnitude extremes.
Swain says this approach gave his team a much larger sample size of data from which to draw inferences, “without making assumptions regarding the underlying precipitation distribution”.
“By selecting a wide range of wet, dry and dry-to-wet transition (that is, ‘whiplash’) events informed by historical analogues,” he explains, “we aim to provide a comprehensive perspective on the changing risks of regional hydroclimate extremes in a manner directly relevant to climate adaptation and infrastructure planning efforts.”
The researchers assessed simulated changes in the frequency of California’s precipitation extremes caused by increasing atmospheric greenhouse gas concentrations. Swain says they estimated how often these events occurred, based on direct observations or historical accounts.
They then deployed a simulated control model of pre-industrial events that occurred with comparable frequency and severity to those observed to get an idea of how often such events could occur, given the continued growth of greenhouse gas concentrations.
Their findings indicate both extreme dry seasons and whiplash events increased by more than 50% over much of the state, and extreme wet events increased by more than 100% over nearly the entire state.
Moreover, they report a substantial increase in the projected risk of extreme precipitation events exceeding any that have occurred over the past century, meaning that such events would be unprecedented in California’s modern era of extensive water infrastructure.
Taking as its benchmark California’s “Great Flood of 1862”, epic storms over just six days that caused widespread destruction across Southern California, the report says, “Few of the dams, levees and canals that currently protect millions living in California’s flood plains and facilitate the movement of water from Sierra Nevada watersheds to coastal cities have been tested by a deluge as severe as the extraordinary 1861-1862 storm sequence, a repeat of which would probably lead to considerable loss of life and economic damages approaching a trillion dollars.
“Our results suggest that such an event is more likely than not to occur at least once between 2018 and 2060, and that multiple occurrences are plausible by 2100 on a business-as-usual emissions trajectory.”