Grateful to have the opportunity to speak at the 3rd Annual Heat and Health Summit at UC San Diego today. There were so many interesting issues raised, but today I am going to focus on one single issue that is very important but often overlooked: gaps in heat-related health data.
Heat-related deaths and illnesses are highly underestimated in the U.S. If a farmworker goes to the Emergency Department with a heat-related rash, it will most likely be attributed to a skin allergy. If a farmworker or a construction worker dies due to heart failure while working outdoors, it will probably be attributed to cardiovascular disease, not to heat. The same goes for many other diseases. Whether mental illnesses, kidney failure, immune diseases, or reproductive diseases, almost all would be attributed to something else even if they were triggered or caused by heat.
And then there are data gaps between federal, state, and county levels. For example, the National Weather Service database shows that there were only 207 heat-related deaths in 2023 in the entire U.S. However, Maricopa County in Arizona alone reported 645 heat-related deaths in 2023. Who is going to harmonize these databases? Nobody?
When Europe is reporting 50,000–60,000 heat-related deaths in a single summer, the U.S. database shows about 4,000 annual heat-related deaths. It is beyond comprehension.
There is also the issue of data accessibility. Healthcare data has been overly protected in the name of privacy. De-identified heat- or asthma-related ED visits and death data are publicly available from portals such as Tracking California, but if you go and ask your federal or state Department of Public Health to provide the same data at a more granular level (weekly), you will be asked to go through such a lengthy process that most people give up at one stage or another. Researchers, students, and modelers rely on these data to develop better forecasting models and tools, but in this situation that is not going to happen.
There is a lot to do! But it seems nothing significant is going to happen unless people in position or authority actually care.
California has been much cooler this summer. Most of us have felt it. Haven’t we?
There have been several headlines on this topic, but I haven’t been fully satisfied with the explanations provided so far. A general consensus is that it is mainly caused by a high pressure system that has persisted over the northeastern Pacific since May, which is pushing cool winds toward California. While this is a plausible immediate explanation, there is much more to this story because ultimately it is the sea surface temperature of the eastern Pacific that drives California’s weather across different time scales. But for now, I just wanted to look at the historical temperature for a better context.
I have plotted the average daily maximum temperature at San Diego International Airport for July since 1998. Below are my takes from the graph:
Average daily maximum temperature in July at San Diego International Airport (1998-2025).
1. Our experience is relative. Last summer was relatively hot so we are perceiving this year as a cooler year in comparison. The year 2022 was also cool, like this year, but we’ve already forgotten that experience.
2. The years 2006 and 2018 stand out as the hottest years, in which the temperatures were well above 6°F than this year. We must not forget these hottest years because they will for sure return. The heatwaves of 2006 killed more than 600 people in a single month (July) in California. These numbers are highly underestimated because deaths and illnesses due to heat are not properly accounted for, for various reasons, in the US.
3. It appears that the coolest year was 2010, in which the average temperature was about 3°F cooler than this year.
Wonder how it looks in the Northern California? I have the plot for Sacramento as well below. Northern California looks the coolest in the last 20 years this year.
Average daily maximum temperature in July at Sacramento Executive Airport.
Is there anything else worth noting in the graphs? As a Californian, which of these years do you remember and why?
Our research article, “Heat-related rest-break recommendations for farmworkers in California based on wet-bulb globe temperature” is out today, in Nature Communications Earth & Environment: https://lnkd.in/gJK5MiFi
The heat season is around the corner and we are hoping that this paper will provide a starting point for discussing heat-related policies in California and elsewhere.
What did we do?
We determined rest-break requirements for the farmworkers of the Imperial and Coachella Valleys in southern California, using high-resolution outputs from a validated Weather Research and Forecasting Model (WRF) at 1-km grid.
How? Please be more specific.
We calculated how often thresholds of heat stress indicators were exceeded in farmworkers’ working environments and translated those exceedances into rest breaks needed for their safety. We considered three existing policy guidelines that use wet-bulb globe temperature (WBGT), heat index (HI), or dry-bulb temperature (DBT) thresholds to specify rest-break requirements.
What is the finding?
WBGT was the most protective among the three heat stress indicators so we used it to derive recommended rest duration considering the acclimatization status of the workers, seasons, and work-shifts. Recommended rest breaks range from 2 to 32 minutes per work-hour between April and August. These recommendations can be directly incorporated into regulations such as the California Code of Regulations (T8 §3395 CCR) or OSHA’s proposed new heat rule.
This is the shortest research article I ever wrote but it was the most satisfying experience. We talk a lot about interdisciplinary research but this is a real example showing how a marriage between weather modeling and the public health sector can create something totally new—a policy child. Thanks to our PI Trent Biggs for persisting on this idea.
Science progresses slowly so it is important to capitalize on the existing scientific knowledge for the benefit of society. That is exactly what we have done here—utilized available Earth science modeling tools, for developing policy guidelines, to protect lives.
On a side note, air pollution and extreme heat are two real issues we should be concerned about right now. More than 7 million people around the world die every year due to air pollution, which is now the second leading risk factor for death in children (after malnutrition). Another half million die due to extreme heat, the majority of whom are our own parents (called seniors in professional language). I continue to work on these important health issues using my skills in weather prediction and air pollution modeling. I welcome any collaborations on these issues.
How does irrigation impact heat stress on farmworkers?
We answer the above question in quite detail in our freshly pressed article in Nature Communication Earth & Environment. It is an open-access article and thus freely accessible to everyone:
We used a regional climate model (WRF) set up at 1-km resolution to calculate Wet Bulb Globe Temperature (WBGT), a standard metrics of heat stress, using ECMWF’s thermofeel python library, and examined the impact of irrigation on heat stress in Southern California’s key agricultural region, the Imperial Valley. Our main conclusions are:
1. Irrigation reduces heat stress in the daytime but increases it at night.
2. Urban and fallow areas adjacent to the cropped fields also experience increased heat stress due to moisture advection from irrigated areas.
Some background about this work
According to a recent OEHHA report, heat related illnesses among farmworkers are the highest in the Imperial County in the entire California.
Imperial Valley is not a small area, the entire county is a cultivated field. That is roughtly the size of 7, 37, 000 football fields. No wonder, Imperial Valley produces about two-third of the winter vegetables and up to one-third of the fruits and nuts consumed in the US.
There are nearly a million farmworkers employed in the Central and Imperial Valley region. They are exposed to high heat stress everyday as they get involved in various agricultural activities from planting to harvesting to packaging, between late Spring to early Fall season.
One of the unique thing about the Imperial Valley is that irrigation is heavily applied in its cropfields – thanks to the All American Canal built in the 1930s. The amount of irrigation applied here (~ 5 ft) is more than 20 times greater than its annual rainfall (~3 inches).
Given such a large amount of irrigation applied in its cropfields, a natural question came in our mind. Does the irrigation affect heat stress experienced by farmworkers? If yes, when, where, and how? We try to answer these questions in the above article.
Please stay tuned for what is next. We are working on a follow-up article in which we calculate rest-break requirements for farmworkers using the same high-resolution climate model outputs. We are trying to help redefine California’s heat related policies to protect the outdoor workers.
Hats off to our food producers, the toughest and the most resilient, the farmworkers!
Thank you for reading this! Merry Christmas and happy holidays to you all!
Parajuli, S.P., Biggs, T., de Sales, F. et al. Impact of irrigation on farmworker’s heat stress in California differs by season and during the day and night. Commun Earth Environ5, 787 (2024). https://doi.org/10.1038/s43247-024-01959-7
Some 100 miles east of San Diego, lives a big agri-town called Imperial Valley (IV) that supplies 2/3 of the winter vegetables and 1/3 of the fruits/vegetables consumed in the entire US. Majority of IV residents are hispanic with 25% living below poverty line and the majority of farmworkers are migrant workers coming from Mexicali across the US-Mexico border. IV has high rates of asthma (one in five children) and the highest heat-related illness rate in the entire state. Several environmental threats contribute to these issues, including the harsh desert climate, the shrinking Salton Sea, blowing dust storms, pesticides-laden runoff from highly irrigated lands, agricultural activities that produce dust and smoke, rapid urbanization, industrial expansion, and now the lithium exploration activities.
State and Federal government representatives, local NGOs and community members, researchers, and most importantly, the farmworkers, met to discuss about how to deal with the emerging environmental challenges of the region in an event called EJSummit. The Environmental Health Leadership Summit (EJSummit) is an annual event organized by a local community organization called Comite Civico del Valle now in its 13th year. The event occured on October 22/23 this year, right next to the agricultural fields of the IV in El Centro, CA.
Following points were noted:
The hot brine below the Salton Sea (not the Salton Sea water) has enough Lithium to meet 1/3 of today’s global demand. The proposed Direct Lithium Extraction method extracts lithium from brine directly using more environmental friendly technique using a adsorption material thereby rejecting the brine which is reinjected underground, but the technology has not been implemented in industrial scale so the reality of anticipated ‘Lithium Valley’ is yet to be realized.
The Salton Sea continues to shrink due to high rate of evaporation combined with reduced inflow of water caused by the Quantification Settlement Agreement (QSA) of 2003 that diverted some 15% water to San Diego region. The local people mentioned that many people have lost their jobs due to reduced agricultural activities, caused by the reduced water inflow to the IV crop fields.
Speakers included 84-year old living legend Preston Arrow-Weed (in picture), a member of Kumeyaay tribe, who lived in this region for thousands of years. Who else can better teach how to reduce our environmental footprint and to live in harmony with nature?
I recently carried out some model experiments using WRF to evaluate how our weather forecasts perform in terms of simulating the diurnal cycle of air temperature and humidity, which are the two key parameters determining heat stress experienced by humans – thanks to our collaborators at National Weather Service (NWS), San Diego.
The NWS provides daily weather forecasts using The Global Forecast System (GFS) for upto 16 days into the future at 0.25km spatial resolution, which can be freely downloaded from the following website:
While the GFS forecast is one key forecast product among several other weather forecasts adopted by the NWS, it is also typically used to drive regional climate models such as WRF at fine resolution using them as initial and boundary conditions. The GFS provides forecast over the entire globe but at a compromised coarse spatial resolution of 25km because of computational constraints. Thankfully, these data allows us to run regional model simulations at fine grid resolution, typically at 1-3km, suitable for making weather forecast and other climate studies on a local scale, which is more relevant to the public. Such fine resolution simulations better resolve the topographic and vegetation details and thus can provide better forecast on a city scale.
So here is what I did. I basically downscaled the GFS forecast (GFS-25km) using my WRF set-up over the Imperial Valley at 1km resolution and compared the downscaled results with the observations for a recent time period.
Fig. 1. (a) WRF model domain configuration showing the region of interest (d03) over the Imperial Valley and (b) the agricultural regions of the Imperial/Coachella valleys.
I also conducted another set of simulations by using ERA5 reanalysis (ERA5-WRF) instead of using GFS forecast (GFS-WRF). ERA5 reanalysis, which is also a global product at 31km resolution, is not a forecast but a hindcast with a 5-day latency. These reanalysis data are nothing but similar model results calibrated using actual observations so they are supposed to be more accurate.
I compared the above two sets of downscaled results from WRF and the original 25km GFS results with the actual observations recorded at four of our recently installed stations in the Imperial/Coachella valleys. Basicallly, the idea was to see if there is any value of running a high-resolution model at 1-km resolution, instead of just using the original forecast at 25km spatial resolution.
Above I show two figures comparing the above four sets of results in terms of 2m air temperature and 2m relative humidity. Here are my key takes from these results:
– The diurnal cycle of air temperature is simulated quite well by the 25km GFS forecast in the Imperial Valley region and WRF downscaling doesn’t seem to add much value.
– The performance of GFS-WRF is comparable to the ERA5 although the latter assimilates observations. The European Centre for Medium-Range Weather Forecasts (ECMWF) also provides similar forecast product at 25-km resolution but they require paid subscription. So, GFS forecast is a clear win here.
– There is some added value of WRF downscaling in the Coachella date palm region, where vegetation is taller. The GFS forecast underestimated the temperature but WRF did a much better job. This makes sense because tall vegetation clearly plays a strong role in this region in modulating the local weather, whose effect becomes more apparent at higher resolution.
– In terms of humidity, it seems to be a major challenge in forecasting. WRF downscaling appears to improve it somewhat but there is a lot to work upon in terms of getting the humidity right.
– Interestingly, the GFS forecast performance does not seem to downgrade with the forecast lead time as one would generally expect. The longer the forecast time into the future, less the reliability of the forecast because the model results depends upon the accuracy of initial/boundary conditions which become more uncertain as we go into the future. As a matter of fact, forecasting diurnal cycle is easier for models because they have repetitive patterns. The main challenge for our weather and climate models is to simulate the non-repeating weather and climate events such as extreme events.
As extreme events (e.g., extreme heat, extreme precipitation, extreme air pollution) are becoming more frequent, prediction of which is much more important to the society, comparisons in terms of extreme climate would be more meaningful. However, such evaluation requires multi-year simulations, and thus requires much more resources, time and effort, which is beyond the scope of this simple investigation.
I almost dozed off on a red light yesterday. What could be the reason?
Myself during the installation of heat stress sensors (black globe thermometer, temperature/humidity sensors, and pyranometer) in Imperial Valley, CA in May, 2024.
I don’t turn on my A/C to save cost, energy, or environment whatever one might think of, unless it is unbearably hot. I only turn on the fan. Yesterday was not an extreme day anyway. Just a normal weather day in the upper 70s. And my car is always parked indoor whether at home or office. And I had enough rest for sure. So what else could be the reason?
May be CO? We know that exposure to carbon monoxide can make us sick and even unconscious depending upon the concentration. Where else can be its greatest risk, other than in a car?
May be Oxygen? We know that low oxygen level can also make us sick. Again, where else can it be lower than in an enclosed box? Oh sorry, I forgot about the high mountains.
May be heat? We all know this is also a possibility. We see more people dozing off in summer everywhere, even in meetings and conferences. :).
May be many of you have also experienced something similar. If it happens during a 15-minute drive, imagine what those long haul drivers have to go through, who have no choice but to drive for more than 12 hours everyday. I am 100% sure, coffee can’t solve this problem, however big your cup may be. This is especially critical in summer.
It could be any or all of the above factors. But summer heat is probably the most likely factor here.
With the above context, the question I would like to ask today is why the heat-related deaths are much higher in the developed countries than in developing regions such as Africa or India? As far as I know, the most intense heatwave in the recent history in India occured in 2015 and only about 2500 people died when the maximum temperature rose close to 50 degree celsius. In contrast, more than 60,000 people died during the heatwaves of 2022 in Europe when the maximum temperature rose to something similar.
Humidity has a role to play on heat stress, and the data from India may not be accurate for various reasons, but these factors can’t explain such an order of magnitude discrepancy.
What could be the main reason?
The answer has to lie in our modern way of life. We are spending most of our times indoor which is interfering with our body’s ability to acclimatize. We, the city dwellers, have become a new species that can only survive within a very narrow range of temperature T±ΔT.
The term acclimatization probably needs to be redefined. Traditionally, it referred to how people adapt to different climate regions based on geography. But now, because of our HVAC way of life, the exposure to high temperature is more based on indoor vs. outdoor temperatures than the temperature in region1 vs. region2.
The way we define heatwaves also needs to be reassessed. Normally, it is defined based on how far is the daily maximum temperature from the historical average value in a given location. But, with our indoor way of life, the average base temperature needs to represent our ‘indoor’ temperature where we spend or live most of our time. Obviously, it is going to be different for different people. It might even be associated with social structure and poverty level—just my naive guess.
It is not only about heat, it is also about oxygen level. Although oxygen is the most vital element of life, it’s study has received the least attention for some unknown reasons. One such eccentric study (I wonder how they received the funding) in a Journal of American Chemical Society, the oxygen level is declining in large cities globally, and that the increase in heatwave occurrence was correlated with reduced oxygen level.
Increasing number of studies are showing that air pollution, oxygen level, extreme weather events, occurrence of heatwaves, energy demand all of these are correlated. Our environmental problems have become much more complex nowadays. We need to rise above our traditional area of expertise and think differently if we want to solve these problems. We have to put all these pieces of the puzzle together to know whether the puzzle is a tiger or an elephant.
We have deployed two sets of instruments to our field sites at Westmorland, Imperial Valley in coordination with the University of California Cooperative Extension (UCCE) office at Holtville, CA. Each station has a black-globe sensor, relative humidity and temperature sensor, and pyranometer. Together, these measurement will be used to calculate Wet Bulb Globe Temperature (WBGT), a key indicator of heat stress on humans.
Installation of heat stress sensors at Westmorland, Imperial Valley.
We have also installed one station at an urban site in El Centro, Imperial Valley in cooperation with the Air Pollution Control District (APCD), El Centro office. The station is located on the roof of the APCD building.
Station at APCD, El Centro building.
Working for ~4 hour in the sun for the instrument installation in this early May was not easy. Imagine farmworkers working in the field everyday regardless of heat, wind, or dust.
Foods do not come to our table so easily. They do not grow in the grocery stores either. A lot of sweat, hardwork, and endurance involved. We must treat farmworkers and the soil with respect if we want quality food on our table.
PS: We are grateful to our collegues at UCCE, Hotville and APCD, El Centro for facilitating installation of our instruments in their sites.
I have lived in the southern edge of California, but the Central Valley keeps coming on my head. It has remained as a place of wonder in my heart too.
Anything that is at the center has power, it pulls you towards it. Central Valley is California’s center. It is California’s womb. Womb is where creation takes place. It is where growth takes place. No wonder, Central Valley produces more than 2/3 of the fruits and nuts consumed in the entire US.
When you truly wish for something, the entire universe conspires to make it happen.
I got invited to Sacramento for an event at the California State Capitol. I took this opportunity to steer through the fertile lands of the Central Valley. But something got my plan changed. I blamed the weather. And I decided to fly to Sacramento, instead of driving. I now planned to drive back, through the Central Valley.
The stormy weather this season lasted almost a week in the first week of February 2024 due to multiple atmospheric rivers. The flight took off on time despite the bad weather. Clouds were beautiful on their own but that is not what I had wished to see. Sierra Nevada mountains were fully covered with snow too. That was still not what I had wished to see. I had wished to see the green agricultural fields of the Central Valley from above. I couldn’t see much. Only a part of the green fields was visible near Sacramento, as the plane descended for landing. Those too, were flooded due to the heavy rain.
I atteded a CCST event at the California State Capitol in the afternoon the same day. It was a pleasure presenting my current research in the Imperial Valley to the Assemblymembers Mike Fong and Christopher Ward. There were othere legislative staff, people from California Energy Commission, California Air Resources Board, and also a astrophysics group from University of California (UC), Santa Cruz attending the event. It was specially enlightening to chat with professor Sandra Faber, an astrophysicst at UC, Santa Cruz, who had interesting views on a range of topics from energy and environment to policies. It was quite an experience presenting my research, repeatedly, in an elevator-pitch style, to multiple people from different backgrounds, packed in a small meeting room at the Capitol.
My return trip started from Oroville. Central valley can’t be understood without looking at its water infrastructures. That is why I decided to start my journey from the Oroville dam, the tallest dam in the US, on the Feather River, north of Sacramento. The dam was built between 1957-1968. To highlight the significance of this project to the entire California, a sackful of gravel and sand was used in the first concrete used in this dam.
View of Oroville lake from th Dam. View of the Oroville lake and the dam with the road on top.
My first stop was at the Antioch Bridge that lies in the south of the Sacramento County. This is the place where the northward flowing San Joaquin river meets the southward flowing Sacramento river. Such places are considered auspicious — they are, indeed. A short video clip of the area that I took is here. As I stood on the fishing pier, a useful guide to fish eating captured my attention (included below).
View of the Antioch bridge from a distance.A guide to fish eating posted in the fishing pier near the Antioch bridge.Google map view of the Antioch Bridge. The Sacramento and the San Joaquin rivers can be seen on the left and the right of the bridge.
After seeing the Antioch Bridge area, I headed south through the Byron highway. I stopped near the UC Davis Fish conservation center to see the beginning part of the California Aqueduct in the Sacramento-San Joaquin river delta, which was built to supply water from the northern California to the southern California. This was an engineering masterpiece because it involves pumping of water against gravity by huge pumps and dumping the water to the distribution aqueducts and canals. At the moment, these canals and aqueducts never run to their full capacity because the federal and state regulations now restrict pumping of water from the delta region to protect the endangered delta-smelt and other local fish species.
The green meadows on the little hills seen in the west of the Byron highway were spectacular. The view definitely reminded me of the natural beauty of Switzerland. The US as a country is huge and it indeed has so many beautiful landscapes waiting for us to explore.
I intentionally took California highway 99 avoiding the more usual I-5 route, although it would take much longer time to reach San Diego. As I drove through the Central Valley cities of Modesto, Merced, Fresno, and Tulare, I saw several railway lines that ran through the cropfields. Most of them were part of the Union Pacific railroad, which is the second largest railroad in the US. This centuries-old railroad network which was first built in 1806 made it possible to transport the excess produce of the Central Valley to the rest of the US. There are probably other smaller trolleys as well operated by the individual farm owners in the central valley.
Union Pacific trolley as seen from the Byron highway.
There were crop fields on both sides of the highway. I drove for hours through these fields while stopping occasionally to take some pictures. The almond trees were just beginning to flower. I couldn’t really tell whether they were amond trees or peach (apricot?) trees because I know that both of them look similar. We had a lot of peach trees in our village so I can recognize them. But it is hard to tell what it is in this flowering season.
A close-up view of an almond tree south of Fresno. Almond fields south of Fresno.
At the end of the crop fields near Bakersfield, the green medows looked spactacular on the west side of highway 99. I also saw a huge water pumping station (visible in the picture below). It was State Water Project’s Wind Gap pumping plant, the last major water lift from the California Aqueduct over the Tehachapi Mountains, which was renamed the Ira J. Chrisman Wind Gap Pumping Plant in his honor.
Vineyards near Bakersfield with the Green meadows in the background. Also seen on the right is the Ira J. Chrisman Wind Gap Pumping Plant. Vineyards near Bakersfield with the Green meadows in the background.
When I looked on the left, the Sierra Nevada mountain was smiling with considerable amount of snow.
The history of California is so intriguing. The Central Valley was transformed from desert to a rich land producing fruits, nuts, and vegetables some 100 years ago. Engineers built one of the most complex network of Aqueducts to irrigate the Central Valley farmlands. Dams were built, canals were laid, wells were digged. The California Aqueduct that begins at the Sacramento–San Joaquin River Delta was built in 1963. That is well before our era of computer and internet. We are living in the 21st century, but infrastructure-wise, we are only enjoying what our ancestors built. We haven’t built anything such significant in our times. Of course we have done a lot in the IT and computing sector.
The older generation got busy working. They got busy building and growing. They developed and used technologies that defied even the worst droughts. They adapted. They adapted from one crop to another. From rivers to wells. From west to east.
Central Valley farmers used technology as it became available to foster their yields. They knew all the details, when to plant, how to plant, and even how to better pollinate. To them, it didn’t matter, whether water was available or not. It didn’t matter whether soil was good or not. They grew pistachios when the water was saline. When winds didn’t blow, they knew how to pollinate. They knew how to make the fruits puffy too.
I am ever fascinated by its story of transformation. California is a miracle. Not a single person made it. But it has only known growth from the very beginning. It has never remained. And will never stand still.
Many things in California happened more than 200 years ago. California developed, flourished, and a part of it even got perished much before we were even born. California has always been in a rush, from gold rush, to water rush, to now lithium rush.
California still looks beautiful as it moves, and transforms. It is wonderful to live here and be a witness of California’s growth. California is indeed unbeatable.
California is not just about Silicon Valley; it is the Central Valley and the Imperial Valley (IV) that truly define California’s richness, as they produce more than half of the fruits, vegetables, and nuts consumed in the United States. The Imperial Valley, which lies on the south side of the Salton Sea, alone provides more than 80% of the nation’s fresh winter fruits and vegetables. But have you ever wondered how those succulent oranges, almonds, and lettuces come to your table every day?
WRF modeling domain over the Imperial and Coachella Valleys.
Unfortunately, the farmworkers in the Imperial and Coachella Valleys experience a high number of cardiovascular diseases due to heat stress. Not only this, the IV region is recently going through tremendous environmental stress due to water and air pollution; extreme droughts and flooding; and land use change.
I have built a high-resolution, high-performance weather model set up centered around the Imperial and Coachella Valleys that can be used to address such environmental concerns. As an example, the model can simulate how the microclimate will change when croplands are converted to solar fields. Similarly, the model can be used to forecast of heat stress experienced by farm workers at a specific time and location using Wet Bulb Globe Temperature (WBGT).
What question intrigues you the most about this region?
