Crazy Hot Days, Crazy Warm Nights: A New Study on Climate Change in California’s Central Valley

, former scientist and Kendall Science Fellow | August 21, 2015, 10:36 am EDT
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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

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.


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 [email protected]. 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

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.

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  • Taiko69

    Gallilao (did you deliberately Galileo’s name to dishonor scientists?) — Why not try to learn some physics first? I have yet to encounter an AGW denier in these discussions who knows any physics, even the basics such as F=dp/dt. If you don’t have any knowledge of thermodynamics you are unqualified to comment on the science.

    Truth or Drought is spot on regarding the inefficient farming practices in the Central Valley. Alfalfa takes a lot of water — I know first-hand from growing up on a dairy farm in Wisconsin; growing alfalfa in a semi-desert is madness. Most of the alfalfa that isn’t used for feed is sold to China. We are exporting our lifeblood.

  • Mark Henry

    Roberto: I applaud your work and heartily agree that “a science-based framework to identify and propose actions against parties responsible for their contributions to climate damages (and costs of adaptation)” is absolutely necessary. But I submit that list should include UCS.

    For example, I live on the Central Coast and we have been dealing with climate change issues, not quite as severe as the Central Valley, but still a lot. We share the same drought. We also have a 2200MW nuclear facility nearby that many environmentalists want to close down. Its production will be replaced with four 550MW natural gas plants. This is what UCS has to say about it:

    “Yet, the Diablo Canyon Nuclear Power Plant continutes to operate [in light of seismic exposure]. As it has at other plants with similar issues, the NRC should enforce its seismic regulations at Diablo Canyon. The potential consequences of inaction are severe.” UCS
    The potential consequences may be severe, but UCS fails to mention that their likelihood is extremely remote. Fukushima suffered a magnitude nine earthquake and according to USGS the likelihood of a magnitude 8 in this area is less that 5%; magnitude nine is not even on the chart. All of the reactors in the Fukushima area shut down as designed. The “catastrophic nuclear disaster” was the result of a loss of coolant due to the effects of a record tsunami resulting in a loss of power, not by earthquake damage. Diablo Canyon has a reservoir of gravity flow water so loss of power is not an issue. The Fukushima reactors were to handle less than .5g of momentum; Diablo Canyon is designed to .75g.
    So UCS is implying that it is in the best interest of mankind that the plant be shut down until the “potential severe consequences” are properly addressed. Meanwhile the burning of natural gas to replace the production of Diablo will put some 9 million tons of CO2 in the atmosphere every year, along with thousands of tons of other noxious gases and particulates, and who know how much fugitive methane lost in the production of the gas. Methane has a feedback loop: the warmer it gets the more methane is released from natural resources, including the ocean, which we have plenty of nearby. Meanwhile the anti-nuclear activists can cite UCS in their campaigns, which they frequently do.
    So, for now, you need to put UCS on your list. Thanks.

  • The Central Valley contains the vast majority of California’s millions of cows, and is not so coincidentally the 2nd largest methane “hot spot” plume in the US. Methane is one of the most potent greenhouse gases, along with nitrous oxide, which is also produced by these millions of cows via their manure and the fertilizer used for their feed. 70% of CA’s thirstiest crop, alfalfa (75% of which is fed to dairy cows and nearly 100% of which is irrigated with precious groundwater) is grown in the Central Valley to feed these millions of cows, and also not so coincidentally, the ground there is sinking at historic rates. 2/3 of CA’s water losses are attributable to the Central Valley. Where, again, we are accommodating millions of water- and irrigated feed-guzzling bovines.

    When we breed, feed, water, and slaughter millions of methane-emitting, water guzzling ruminants, they bake the Earth while draining it dry. All livestock are an inefficient use of crops, water, and energy. Why is it so hard for articles like this to even acknowledge that? It’s illogical to continue to use animals as an archaic food source when so many more efficient plant-based options exist. It would be wonderful if more scientists would start addressing this. What’s stopping them? We have a weapon to fight climate change and water depletion every day 3x a day, and that’s our fork.

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    • rjmera

      Thank you for your comment. Land use accounts for a very large share of global emissions and should be dealt with appropriately. There are many options available to do so. It comes down to education, first of all, for everyone in the world, about the choices we make day in and day out.

    • CB

      “When we breed, feed, water, and slaughter millions of methane-emitting, water guzzling ruminants, they bake the Earth while draining it dry.”

      Yes and no.

      Exclusive of fossil inputs, animal agriculture is 100% carbon neutral. The carbon comes from CO₂ in the sky, goes into plants, goes into animals that eat them, comes out as methane when they fart or burp, and that methane turns right back into carbon dioxide in the sky.

      It’s a closed loop. The greenhouse forcing in it is precisely the same from year to year.

      That said, animal agriculture requires far more resources per calorie than plant agriculture, and since the energy component of those resources is currently fossil-based, going vegan, vegetarian or even consuming less meat is one of the easiest ways for an individual to reduce her carbon footprint (and saves water too, as you said).

      Animal agriculture can actually be a way to fix the mess we’ve made of the atmosphere, through carbon negative techniques like rotational grazing and pyrolysis.

      It’s incredibly frustrating that people don’t understand the short and long carbon cycles, because that’s critical to both understanding why the problem of climate change exists and how we are going to fix it.

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  • Obviously “Gallilao” doesn’t know what he is talking about, because if he knew anything about science, he would know how to spell his name. And Taz, fossil fuels are going the way of dinosaurs. Perhaps if you lived under a methane cloud the size of Delaware and had young family members breathing in volatile organic compounds from the industry, you might think differently. There is a future, and it doesn’t include fossil fuels.

    • Gallilao

      Thank you for that disinformation.

      • CB

        “Thank you for that disinformation.”

        lol! No, ranger is correct!

        That’s really not how you spell “Galileo”… and Gallilao, you’re no Galileo.

        “Born on February 15, 1564, in Pisa, Italy, Galileo Galilei was a mathematics professor who made pioneering observations of nature with long-lasting implications for the study of physics.”

  • Gallilao

    We can think of this kind of model as being like a maze. We are placed at the center with about 1000 exits. One of those exits “may” lead to the future but we have no idea of which one. A lot of good that does. Especially when their own science proves their fundamental theory wrong.

    • Taiko69

      “We can think …” Who is thinking? Obviously not you.

      • Gallilao

        It’s OK!..It’s OK!…Don’t get excited!.. You don’t have to think!

  • Taz Talks

    I believe the very fossil fuel companies you mention power the internet and the computer used to publish this electronic communication. Are you prepared to sue yourself? If not, are you prepared to be a hypocrite? Take your choice.

  • Gallilao

    Climate science is a joke! Anyone who would put any store or faith in computer models, has either a vested interest or knows nothing about computer models.

    • CB

      “Climate science is a joke!”

      …and you aren’t?

      “Our results indicate that future reductions in Arctic sea ice cover could significantly reduce available water in the American west”

      “With California facing one of the most severe droughts on record, Governor Brown declared a drought State of Emergency in January”

      • Gallilao

        What are you trying to say? Come on, spit it out!

      • CB

        “What are you trying to say?”

        I’m trying to say you’re a joke.

        Why is that the path in life you’ve chosen for yourself?

        “Arctic Sea Ice Volume Anomaly… Monthly averaged ice volume for September 2014 was 6,970 km³… 59% lower than the maximum in 1979”

      • Gallilao

        Now you’ve really perked my interest! So what is this chosen life path you dreamed up? And please, ….. spit it out! I am unable to read minds, no matter how simple.

      • CB

        “what is this chosen life path you dreamed up?”

        The path of suggesting climate science is a joke.

        Is that likely given that scientists actually predicted the precipitous decline in Arctic sea ice would give rise to drier conditions in the American west?

        If you don’t turn to scientists for your information, where are you getting your information?

        “Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere… may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.”

      • Gallilao

        What a pile of crap! Your propaganda needs an upgrade!

      • CB

        “Your propaganda needs an upgrade!”

        Are you looking in a mirror as you type that?

        Did you think you were the first Climate Denier to talk about herself very specifically in the wrong person?

        If Climate Denialism isn’t a mental disorder, why should that be?

        “Stroeve explains that highly reliable data on the extent of Arctic sea ice has been collected since 1978. From then until now she has found clear evidence of a 30-year melting trend, which, she says, “cannot be easily explained away by natural variability.” “

      • Gallilao

        I’ll bet your daddy is one of those idiot climate scientists, that can’t admit they don’t have a working theory, even when their own data proves the fact!

      • Hey folks, just dropping in here. Please be civil per our comment policy or we’ll need to close this comment thread. Thank you.

      • CB

        “Please be civil per our comment policy or we’ll need to close this comment thread.”

        Sorry mom! I’ll try to be nicer.

        “State water officials had planned to make the trek back to the Sierra Nevada to conduct their snowpack measurement Friday. But Thursday they announced they wouldn’t bother. For the second consecutive month, there won’t be any snow to measure.”