Last week I, along with an international group of scientists, published a study in the journal Climatic Change in which we found that the hottest summer days (24 hour periods) in the Central Valley were twice as likely to occur due to climate change. Heat waves in California’s Central Valley have become progressively more severe in recent decades due to higher humidity and warmer nighttime temperatures. Observations obtained from NOAA’s National Climatic Data Center show that Central Valley nighttime temperatures were nearly 2°F (1°C) warmer in the 2000s compared to the 1901-1960 average and even higher for the whole of California (see plot below).
The departure from average for nighttime temperature extremes over California and Nevada matches the rise in atmospheric carbon dioxide (CO2). Temperature is based on NOAA observations with the base period of 1901-1960. CO2 is derived from NOAA Mauna Loa observations. Image by author
Impacts on the health sector
A pronounced increase in nighttime temperatures in recent decades has made heat waves more severe since individuals in the region are unable to recover from daytime heat at night. The worsening Central Valley heat extremes present health risks, especially for Latino farm workers and other socioeconomically disadvantaged communities.
For example, a particularly severe heat wave in 2006 was linked with increases in emergency department visits and hospitalizations and at least 146 deaths, many of which occurred in Fresno, California. Among the most affected was the Latino community in agricultural regions, who face relatively high exposure to extreme heat due to extensive outdoor work. The Hispanic community had the largest amount of hospital visits during the heat wave.
Nighttime heat
To illustrate the magnitude of the impact of increasing nighttime temperatures we can focus on two variables highlighted in our study: number of days above 104°F (40.6°C) in a month and the warmest nighttime temperatures. We found that the chance of 13 days in a month above 104°F doubles due to climate change and that the nights on those days were nearly 2°F (1°C) warmer in the 2000s in the presence of heat-trapping emissions from human activities.
I am particularly sensitive to heat, making me an individual from a vulnerable population. Back in 2006 I was in London, UK during a heat wave. Air conditioning is not prioritized there since, unlike many locations in America, it only seldom experiences very hot days. Most indoor places we visited had no air conditioner, allowing for very little relief from outdoor temperatures. After a few hours I experienced heat exhaustion and my company rushed me back to the hotel room. I sat in a cold water shower inside an air conditioned room for an hour and recovered.
In this case, we can think of my cold shower as cool nighttime temperatures during a heat wave in the Central Valley. With this in mind we could say that my chances of seeing a heat wave have doubled due to climate change and that, instead of having access to a cold shower to recover from the heat, I may only have shade. The shade option, like warm nighttime temperatures, would not be enough for me.
Attribution
Our study used extensive climate modeling to compare the climate of the 2000s with and without industrial heat-trapping emissions. Model simulations were conducted through a volunteer, distributed computing network organized through weather@home. We applied recently developed methods of probabilistic event attribution to determine changes in frequency and severity of an event.
It works like this: instead of having just one atmosphere to analyze (observational records), we have thousands of atmospheres (model simulations). We compared the number of days above 104°F for each type of simulation (with or without human emissions) to calculate the change in frequency of this variable.
Screen saver image of a climate model simulation used for our heat extremes studies. Image by author
The study’s findings open the door to larger questions of climate justice. The analysis provides a science-based framework to identify and propose actions against parties responsible for their contributions to climate damages (and costs of adaptation). Further research, for example, may be able to link excess deaths and other damages from extreme heat to the fossil fuel companies whose products are contributing to climate change.