Sciency Thoughts: Butte County

Saturday, 10 November 2018

Wildfires kill at least nine in California.

Nine people have been confirmed dead in California this week and thousands more have been made homeless, as two major wildfires sweep across parts of the state. All of the confirmed deaths occurred in the town of Paradise in Butte County, which was largely destroyed by the Camp Fire, which started in the Plumas National Forest on Thursday and rapidly spread to the town, where it is reported to have destroyed about 90% of the buildings. So far five people have been found dead in vehicles in the town, three in buildings and one outside, though another 35 people are currently missing, so it is highly likely that the number of dead will climb.

Burned out vehicles near Paradise, California, after the town was destroyed by a wildfire on Friday 9 November 2018. Peter Desilva/EPA.

Elsewhere in the state the Woolsey Fire and Hill Fire have caused extensive damage to the communities of Thousand Oaks, Casabias and Malibu, though there are no reports of any casualties due to prompt evacuations of the areas affected. Around a quarter of a million people have been evacuated from their homes across the state.

A wildfire burning in Malibu, California, this week. Reuters.

The fires have a variety of causes and have been fuelled by dry winds blowing from the mountains in the northeast of the state, but are essentially due to a prolonged drought in the state, which has been experiencing dry conditions since 2011, the longest such drought in the state's recorded history. This drought has killed vegetation, including thousands of trees, across much of central California, providing dry tinder to fuel the fires, despite high rainfall that caused flooding in parts of the state in the winter of 2016-7. The draught has been made worse by the diversion of water to suit Human purposes, such as agriculture, industry and leisure, which has taken water away from other ecosystems, resulting in the build-up of dead, dry vegetation that has produced fuel for the fires.

The extent of fires burning in California on Saturday 10 November 2018. Google/Esri/NASA/BBC.

Damage to Lake Oroville Dam leads to over 180 000 evacuations in Butte County, California.

Over 180 000 people have been evacuated from their homes after damage to the Lake Oroville Dam (America's highest dam) after damage was discovered to the main spillway of the dam on Tuesday 7 February 2017 (a spillway is an overflow channel that releases water from a dam to stop it overfilling). The flow of water was initially switched to a second, emergency spillway, but damage to this channel was discovered on Sunday 12 February, prompting the evacuations.

Damage to the auxiliary spillway at Lake Oroville Dam. Kelly Grow/California Department of Water Resources.

The damage is particularly worrying as it comes after a winter of exceptional rains as California has been battered by a succession of Pacific storms, bringing widespread flooding and triggering a number of landslip and subsidence events. This is due to exceptionally high temperatures over the Pacific Ocean off the coast of the state this year due to a La Niña weather system over the southern Pacific. This meant that the dam was already filled to capacity and regularly using the spillways to remove water when the damage was discovered, with another major series of storms due to start ariving later this week.

The La Niña weather system is the opposite of the El Niño weather system, in which unusually cold surface temperatures spread across the equatorial Pacific from the upwelling zone on the South American coast. This traps warm water from the western Pacific, preventing it from spreading east and warming the central Pacific. This leads to lower evaporation over the (cooler) east Pacific, leading to low rainfall on the west coast of South America, and higher evaporation over the (warmer) west Pacific, leading to higher rainfall over East and Southeast Asia and northern Australia.

The effects of a La Niña weather system in December-February. NOAA.

This also leads to a breakdown in surface circulation in the North Pacific, which generally rotates clockwise, so that the same body of water stays off the coast of California, where it is constantly warmed by the Sun, leading to high levels of evaporation and onshore winds that bring high rainfall and flood events to the state.