Research Spotlight: T-REX and the Quest for Sustainable Almonds

USDA photo of an almond orchard in Livingston, CA. Photo by Lance Cheung.

A meteorological field project is partnering with orchards to reduce water use in California’s dry climate

California is one of the world’s key agricultural regions. However, the highly variable “boom-and-bust” rainfall pattern of its Mediterranean climate, combined with massive overuse of groundwater regionally, threatens both the state’s farmers and the population’s drinking water. Irrigation accounts for nearly 70% of groundwater withdrawals in the state.

The Tree-Crop Remote Sensing of Evapotranspiration Experiment (T-REX) aims to help farmers monitor and sustainably reduce water use in California, with lessons for farmers in Mediterranean climate regions around the world. You can read about the project in this BAMS paper.

We spoke with Nicholas (Nico) Bambach, PhD, T-REX project co-lead, to find out more.

Bambach canoeing in the Amazon. Photo courtesy of Nico Bambach.>>

Nico Bambach canoeing

What are the aims of T-REX?

Mediterranean climate regions, such as Chile’s Central Valley and California’s San Joaquin Valley, are only about 3% of the Earth’s land surface [yet] are notable for their high biodiversity and productive agriculture. Most crops grown in this climate rely on irrigation to satisfy spring and summer crop water demands.

Alarming warming trends and unprecedented droughts have challenged farmers in Spain, Portugal, Italy, Australia, South Africa, and Chile since I started college in the mid-2000s. As an Agricultural Engineering student, I kept hearing about the need to increase irrigation efficiencies and better manage water resources. … Near two decades later, I found my place within a diverse team, tackling questions related to these themes. [We are] using holistic and interdisciplinary research approaches … to identify how we can help and work with stakeholders interested in using new technologies to become more efficient with their water use.

How did the project begin, and how does it work?

With strong support from Bill Kustas, Martha Anderson, and Andrew McElrone, Kyle Knipper and I grew the T-REX project from our postdoctoral work on a similar UC Davis project focused on vineyards—GRAPEX, the Grape Remote Sensing Atmospheric Profile and Evapotranspiration eXperiment.

GRAPEX is a hands-on research project aiming to help farmers manage vineyards using satellites, so they can reduce the amount of water needed to produce California wines. We have developed tools that use satellites and other technologies to monitor how much water vineyards are using and if the plants are under stress. To do this, we collect data from different wine-growing regions.

Up close, we measure how leaves and vines take in and release carbon and water and how much moisture is in the soil at various depths. Using special sensors on a tall tower, we can track how wind moves and how much water vapor or heat goes up or down. This helps us determine how much water plants are using, how much heat the ground is giving off, or how much carbon dioxide plants absorb. We also use planes or drones to gather information from above, covering areas as small as a few centimeters to many kilometers. Then, we work with wineries to identify the best way to provide this information for their decision-making.

The T-REX project is an expansion of GRAPEX. We use what we learned from wine grapes to apply to almonds, pistachios, and olives. … Considering the importance of the almond industry in California, we decided to focus our efforts [at the beginning] on developing accessible tools to inform irrigation decisions [for almond orchards] based on satellite observations. … We are learning more about how different environments, crop types, and agricultural management can impact the potential of these satellite-based tools.

What is it like partnering with farmers?

Working in commercial farms is challenging yet also rewarding. Every farm has its own way of operating machinery, and we must constantly adapt so that our field research team and equipment are not in their way. Summer days can be scorching, but we are doing many campaigns to ensure we understand the impact of using satellite-based irrigation recommendations at every level. We are quantifying plant stress, orchard productivity, fruit quality parameters, soil-health parameters, and many others. Our results are promising, and we are excited about that. Also, working directly with farmers gives us huge motivation; we hear the need for better tools to support sustainable farming. Growers are trying their best to run their farms not only for profit but also because they are proud of what they do and want to do it well.

<< A researcher climbs a micrometeorological flux tower at the T-REX field site in Woodland, California. Photo courtesy of Nico Bambach.

Every crop is managed differently, and plant species respond differently to such management; finding methods to model that well can be challenging. Ecological studies usually deal with ecosystem responses to certain environmental conditions. In our case, we [also need] to understand how human decisions affect the crop and agroecosystem responses observed.

… We are evolving to better integrate our understanding of the carbon and nitrogen cycles in California’s agroecosystems. Adapting theory and tools to these landscapes is more challenging than it might seem. Yet we are excited to work with farmers on how to grow food, promote soil health, and identify potential climate mitigation opportunities.

How has the project grown, and what are your hopes for the future?

We have expanded into what we envision as a long-term regional-scale observatory of California’s agroecosystems, [becoming] the Crop Sensing Group hosted by the ARS-USDA Sustainable Agriculture Water System Unit in Davis, California. We [are] a diverse, fun, and motivated group of more than thirty people working on critical fundamental and applied research questions revolving around how to do agriculture sustainably in Mediterranean climate regions. We aim to advance our capabilities to integrate across spatial and temporal scale crop-sensing datasets for climate resilience and solutions.

Providing growers with timely and readily available access to crop water use data will help them use water more efficiently. Given how scarce water has become in California, we hope that our project releases some pressure on such a critical resource as water. We believe that will be better for the environment and our communities[—although the pathways are not always clear]. For example, such “saved water” could [be used to grow] more food instead of [becoming] available for the environment or other uses.

The T-REX project intertwines with my personal and professional growth. It is like a kid I get to co-parent with many other people I enjoy having around. It’s also “cool” to think that we are following the path that people like John Norman, Bill Kustas, Martha Anderson, and many others started. … [It] is a huge responsibility and a privilege.

Photo at top: USDA photo of an almond orchard in Livingston, CA. Photo by Lance Cheung.

Southeasterners Perceive Tornado Risk Dangerously Different Than They Should, Especially at Night

While a major winter storm last month was plastering the United States from Texas and New Mexico to New England with heavy snow and ice, volatile conditions in the Southeast (SE) spawned damaging and deadly tornadoes. One of these overnight Monday, February 16, tragically took the lives of 3 people and injured 10 in coastal North Carolina. Such nocturnal tornadoes are common in the Southeastern U.S.—a unique trait—and represent an extreme danger to sleeping residents.


Compounding this problem, new research in the AMS journal Weather, Climate, and Society suggests there may be a deadly disconnect between tornado perception and reality in the region right when residents instead need an acute assessment of their tornado potential.
The article “Do We Know Our Own Tornado Season? A Psychological Investigation of Perceived Tornado Likelihood in the Southeast United States,” by Stephen Broomell of Carnegie Mellon University, with  colleagues from Stanford and NCAR, notes the tragic results of the regional misperception:

The recurring risks posed by tornadoes in the SE United States are exemplified by the significant loss of life associated with recent tornado outbreaks in the SE, including the 2008 Super Tuesday outbreak that killed over 50 people and the devastating 27 April 2011 outbreak that killed over 300 people in a single day.

Their survey of residents in seven states, from Louisiana and Arkansas to Georgia and Kentucky, representing the Southeastern region, finds that the residents perceive their tornado likelihood differently than meteorologists and experts familiar with Southeastern tornado risk. This puts them at great risk because residents’ experiences don’t match what actually happens where they live.

Broomell and his fellow researchers contend that Southeast residents may be misusing knowledge of Great Plains tornado events, ubiquitous in tornado chasing reality shows and social media videos, when determining their own risk. A fatal flaw since tornado behavior is different between the two regions.
WCAS SE tornado season survey2For starters, unlike in infamous “Tornado Alley” states of Texas and Oklahoma north through Nebraska and Iowa into South Dakota, the Southeast lacks a single, “traditional” tornado season, with tornadoes “spread out across different seasons,” Broomell along with his coauthors report, including wintertime. The Southeast also endures more tornadoes overnight, as happened last week in North Carolina. And they spawn from multiple types of storm systems in the Southeast, more so than in the Great Plains. This makes knowledge about residents’ regional tornado likelihood especially critical in Southeastern states.

Another recent study published in the Bulletin of the American Meteorological Society, “In the Dark: Public Perceptions of and National Weather Service Forecaster Considerations for Nocturnal Tornadoes in Tennessee,” by Kelsey Ellis (University of Tennessee, Knoxville), et al., surveyed residents of Tennessee and came away with similar findings about tornado timing: about half of Tennessee’s tornadoes occur at night, and yet less than half of those surveyed thought they would be able to receive nighttime tornado warnings.

Local forecasters and broadcast meteorologists as well as emergency managers are tuned into the mismatch. In the BAMS study, NWS forecasters said they fear for the public’s safety, particularly with nighttime tornadoes, because they “know how dangerous nocturnal events are”—fatalities “are a given,” some said.

Ellis and her colleagues recommended developing a single, consistent communication they term “One Message” to focus on getting out word about the most deadly aspect of the tornado threat. Forecasters, broadcasters, and emergency managers through regular and social media would then be consistent in their messaging to residents, the researchers state, decreasing confusion. For example:

Nighttime tornadoes expected. Sleep with your phone ON tonight!

With severe weather season ready to pop as spring-like warmth quickly overwhelms winter’s icy grip in the next couple of weeks, the nation’s tornado risk will blossom across the South and Southeast. And nocturnal tornado threats will only increase, particularly in the Southeast, as February turns into March, and then April—a historically deadly month.
For residents in places more prone to nighttime tornadoes, Ellis et al. say the ways to stay safe are clear:

Have multiple ways to get tornado warnings, do not rely on outdoor sirens, sleep with your phone on and charged during severe weather, and do not stay in particularly vulnerable locations such as mobile homes or vehicles.

Not Just in Their Heads: New Research Confirms Connection Between Weather and Chronic Pain

Stormy weather increases pain. That fact is actually good news for people suffering from chronic pain whose complaints are often dismissed by family, their friends, and even their doctors.

Speaking, and likely complaining, about the effect of weather on their pain for millennia, chronic pain sufferers finally have a large-scale scientific study to back up their claims. Its findings reveal that, at least in the United Kingdom where the research was conducted, days with higher pain levels correspond with lower pressure and its attendant adverse weather conditions, including higher humidity, precipitation and increased wind. Conversely, low pain days where fewer experience a pain event are dominated by higher pressure with weaker winds and drier air.

In “Weather Patterns Associated with Pain in Chronic-Pain Sufferers,” recently published in the Bulletin of the American Meteorological Society, author David Schultz of the University of Manchester’s Centre for Atmospheric Sciences and Centre for Crisis Studies and Mitigation, along with colleagues, detail their 15-month smartphone study, “Cloudy with a Chance of Pain.” The UK-wide initiative involved more than 10,000 participants suffering from chronic pain who used a special phone app daily to answer 10 questions about their level of pain, related symptoms, mood and physical activity, and other questions about their pain. GPS sensors in their phones enabled concurrent tracking of their location’s average weather conditions, pairing observations with their responses for analysis.

Using an epidemiological method to confine comparisons to individual participants, their research found on any given day that 16 percent of chronic-pain sufferers experience a pain event at least one level above their typical pain level, using a 5-point scale: “no pain,” “mild pain,” “moderate pain,” “severe pain,” and “very severe pain.” This increased to 23 percent on high pain days, and dropped to 10 percent on low pain days. Weather patterns, they found, explain part of the changes, modulating pain in people. It remains to be determined who suffers most, but Schultz et al. have confidence in the findings of the largest and longest-term study of its kind on the weather–chronic pain relationship.

“This result confirms the anecdotal evidence from the three-quarters of those who suffer from chronic pain who say that their pain levels are related to poor weather,” Schultz says in a video he produced to discuss the topic and the study’s findings. [See below.]

He notes that because prior research had inconsistent results and disagreement over findings, reactions to their research have been mixed, with both high praise and disdain.

“It’s perhaps not surprising that the public reaction to our study is one of two things: depending on who you listen to, people either thank us for showing what they already knew and giving support to their strongly held beliefs, or they say it was a waste of time and money because they already knew that poor weather was associated with their pain.”

By the very nature of the smartphone survey, Schultz notes that participants had the opportunity to really look at their pain and the conditions accompanying it. And with evidence in hand, the opportunity now exists for scientists to move toward forecasting such conditions in tandem with people’s pain levels to provide understanding in addition to potential welcome relief.

For people who suffer from chronic pain and believe weather influences their daily pain levels, scientists including them in the study and then conclusively demonstrating this relationship on a large scale gives their suffering meaning and validates their beliefs.

“We’ve received so many emails and tweets of support from those who participated telling us how much they appreciate being able to participate in a study like this where they actually felt their contributions were valued and they were contributing to answering a scientific question that was important to them personally.  And, we do value their contributions.”

An Anticipated Increase in Earth's Strongest Storms

GoniA little more than two weeks ago, Supertyphoon Goni blasted ashore in the Philippines with top sustained winds of 195 mph, becoming the strongest landfalling tropical cyclone on record. It topped STY Haiyan’s 190 mph land strike just seven years ago. With Hurricane Iota in the Caribbean explosively intensifying 100 mph in under 24 hours to reach Category 5 intensity Monday, it set a new record of five consecutive years of Cat 5 hurricanes in the North Atlantic tropical cyclone basin. Among the seven catastrophic hurricanes, starting with Matthew in 2016, were Dorian and Irma, packing 185 mph and 180 mph steady winds, respectively, with peak gusts well over 200 mph.
Goni is the latest formidable example of an increasing trend in tropical cyclone intensity. While Goni established a new landfall wind intensity record, Iota and other recent major hurricanes Eta, Zeta, and Delta set or challenged records for most intense hurricanes so late in the season.
PercentilesJames Elsner of Florida State University says this is to be expected. His research stated in 2008 that there was an upward trend in the intensity of the most intense tropical cyclones. Rising ocean temperatures, as theory predicted, were driving the trend. And with oceans continuing to warm along with Earth’s climate since then, Elsner anticipated the continuing upward trend. New research published in the Bulletin of the American Meteorological Society confirms his prediction, finding that another 3.5 to 4.5 percent increase in intensity has occurred with the strongest tropical cyclones during the period 2007-19.
Globally, all basins show upward trends, Elsner says, with the North Atlantic and Western North Pacific revealing the steepest and most consistent upticks.

An Anticipated Increase in Earth’s Strongest Storms

GoniA little more than two weeks ago, Supertyphoon Goni blasted ashore in the Philippines with top sustained winds of 195 mph, becoming the strongest landfalling tropical cyclone on record. It topped STY Haiyan’s 190 mph land strike just seven years ago. With Hurricane Iota in the Caribbean explosively intensifying 100 mph in under 24 hours to reach Category 5 intensity Monday, it set a new record of five consecutive years of Cat 5 hurricanes in the North Atlantic tropical cyclone basin. Among the seven catastrophic hurricanes, starting with Matthew in 2016, were Dorian and Irma, packing 185 mph and 180 mph steady winds, respectively, with peak gusts well over 200 mph.

Goni is the latest formidable example of an increasing trend in tropical cyclone intensity. While Goni established a new landfall wind intensity record, Iota and other recent major hurricanes Eta, Zeta, and Delta set or challenged records for most intense hurricanes so late in the season.

PercentilesJames Elsner of Florida State University says this is to be expected. His research stated in 2008 that there was an upward trend in the intensity of the most intense tropical cyclones. Rising ocean temperatures, as theory predicted, were driving the trend. And with oceans continuing to warm along with Earth’s climate since then, Elsner anticipated the continuing upward trend. New research published in the Bulletin of the American Meteorological Society confirms his prediction, finding that another 3.5 to 4.5 percent increase in intensity has occurred with the strongest tropical cyclones during the period 2007-19.

Globally, all basins show upward trends, Elsner says, with the North Atlantic and Western North Pacific revealing the steepest and most consistent upticks.

We May Be Able to Further Improve Hurricane Track Forecasts After All

ZetaZeta blossomed into the 11th hurricane of this hyperactive season Monday and its forecast track takes it ashore on the Gulf Coast by midweek. The National Hurricane Center (NHC) on Monday said computer models had become more tightly clustered with the forecast in the Gulf, “resulting in an increase in track-forecast confidence,” and Hurricane Zeta is expected to become the United States’ record 11th storm landfall in a single season.

Such confidence is a reflection that NHC’s tropical storm and hurricane track forecasts, which go out 5 days, have substantially improved in the last 25 years. But, a 2018 essay in the Bulletin of the American Meteorological Society (BAMS) found that such improvements in track forecasts have slowed, raising concern about making them any better and extending them out beyond 5 days with any skill. While that study suggested improvements may have reached a limit, new research also published in BAMS finds a way to further refine them in the coming decades. Using a different interpretation of track position errors than the earlier article, the research moves the predictability limit for tropical cyclones out a day per decade to 6-8 days in the next 10-30 years.

Online Climate Science Course Keys Success to Frequent Forum Discussions

While colleges pre-COVID-19 were already designing and implementing courses for online instruction, the pandemic has pushed entire academic course offerings into this rapidly evolving virtual environment. A new article in the Bulletin of the AMS about an online climate science course for undergraduates, which was developed, offered, and honed to near-perfection based on postcourse surveys before coronavirus, provides this tip for virtual success: Have students engage each other often, one-on-one, in a discussion forum. The result, the instructors are finding, is improved comprehension, with a high percentage of students successfully absorbing and accurately communicating course material.

The online course is titled, “Climate and Climate Change,” and has been offered through the Department of Atmospheric and Oceanic Sciences (AOS) at the University of Wisconsin—Madison since 2013. “Students enrolled in this course learn the physical principles governing Earth’s climate and climate change within the broader context of societal impacts and global political considerations,” writes lead author Andrew M. Dzambo and colleagues in the article.

The goal, they write, is to improve student science literacy and address misconceptions by implementing a key learning tool: “the weekly discussion forum where students engage with each other while testing their own knowledge.” The result is an increased knowledge of climate science and the Earth-climate system, as the surveys showed.

The course—AOS 102—has been improved since its inception, and it grew in popularity when it was moved to summer semester in 2016. It expanded its capacity in 2018 to accommodate a growing waitlist of interested students. The course is delivered through weekly worksheets, quizzes, and a final project, but it’s the weekly forum discussions that instructors credit with students’ retaining and being able to discuss climate science.

In their article, the authors present a template of the course for implementation with other atmospheric or Earth-related science coursework:

Although the discussion forums were monitored by course instructors, every student engaged other students at least once a week and freely expressed their own fact-based feedback to one another. By having the majority of the weekly course grade centered around discussion forums and worksheet assignments, complemented with weekly quizzes and an independent final project, the majority of students leave the class with a fundamental understanding of climate science (as evidenced by the course surveys) and with the confidence that they feel well informed about climate change.

Syllabus

"Sleep with your phone on!": Messaging for Nighttime Tornadoes

With Hurricane Delta poised to strike Louisiana today, the risk of embedded tornadoes will increase as rainbands spiral ashore, along with the primary threats of storm surge and damaging winds. Delta is forecast to plow well inland Friday night into the weekend, continuing a low risk for tornadoes, some of which could occur at night in Alabama, Mississippi and into southern Tennessee. The threat is more than a bit worrisome as new research in BAMS finds through phone surveys and followup interviews in Tennessee that people are woefully unprepared for nocturnal tornadoes.
In their article,” Kelsey Ellis and colleagues found a host of poor practices by residents when it comes to tornadoes at night. The authors recommend forecasters narrow their messaging about nocturnal tornadoes in the Southeast to a single important message to limit confusion.
Almost half of Tennessee’s tornadoes occur at night, as in other Southeast states with large numbers of nocturnal tornadoes, and are two-and-a-half times as deadly as daytime tornadoes. This creates detection, warning, and public response challenges. Yet, respondents in the western part of the state overestimated tornado occurrence at night while those in the east substantially underestimated the number.
Additionally, nearly half of participants in the survey say they rely on sirens to receive tornado warnings. This is despite the fact that sirens are not designed to warn people inside nor be loud enough to wake anyone up. Instead, Wireless Emergency Alerts (WEAs) “should be a constant,” the authors say. Also, people mentioned they rely on TV and social media for receiving warnings even though generally neither will wake you up.
The authors felt it was “dangerous” that even the more tech savvy and tornado aware respondents answered they were compelled to look outside for evidence of a tornado—even in the dark. Interviewees explained they were “checking for sounds instead of visual cues.”
NWS forecasters were also surveyed about nocturnal tornadoes. The forecasters mentioned the lack of ground truth and fear for public safety among challenges to the nighttime warning service. They noted few spotter or social media reports inform them if “the storm is actually showing the signs on the grounds that radar is indicating aloft.” Forecasters said they felt “fearful, worried, or nervous for the public during nocturnal tornadoes because fatalities ‘are a given.’”
The survey responses moved Ellis et al. to recommend a single-emphasis message be presented to residents to combat the nighttime tornado problem:

One strategy that may improve public safety during a nocturnal tornado event, and which addresses the forecaster challenge of communication prior to and during an event, is to develop “One Message”—a consistent message that EMs and the media use throughout broadcasts, briefings, and social media. Examples of messages could be: “Nighttime tornadoes expected. Sleep with your phone ON tonight!” or “Tornadoes will form quickly! Make plans now where you will take shelter!” or “If you live in a manufactured home, you may not have much time to seek shelter tonight!” One Message may decrease confusion for receivers, making them more likely to make safe decisions. Messages could similarly be used to dispel misconceptions about local geography in ways relevant to the specific listening area, for example: “You are not protected by nearby hills. Seek shelter immediately!”

With Hurricane Delta’s nighttime tornado threat ramping up, the authors suggest people use multiple ways to receive warnings, keeps phones on and charged, don’t rely on tornado sirens, and if possible relocate ahead of the weather from “particularly vulnerable” situations, such as mobile homes and vehicles.

“Sleep with your phone on!”: Messaging for Nighttime Tornadoes

With Hurricane Delta poised to strike Louisiana today, the risk of embedded tornadoes will increase as rainbands spiral ashore, along with the primary threats of storm surge and damaging winds. Delta is forecast to plow well inland Friday night into the weekend, continuing a low risk for tornadoes, some of which could occur at night in Alabama, Mississippi and into southern Tennessee. The threat is more than a bit worrisome as new research in BAMS finds through phone surveys and followup interviews in Tennessee that people are woefully unprepared for nocturnal tornadoes.

In their article,” Kelsey Ellis and colleagues found a host of poor practices by residents when it comes to tornadoes at night. The authors recommend forecasters narrow their messaging about nocturnal tornadoes in the Southeast to a single important message to limit confusion.

Almost half of Tennessee’s tornadoes occur at night, as in other Southeast states with large numbers of nocturnal tornadoes, and are two-and-a-half times as deadly as daytime tornadoes. This creates detection, warning, and public response challenges. Yet, respondents in the western part of the state overestimated tornado occurrence at night while those in the east substantially underestimated the number.

Additionally, nearly half of participants in the survey say they rely on sirens to receive tornado warnings. This is despite the fact that sirens are not designed to warn people inside nor be loud enough to wake anyone up. Instead, Wireless Emergency Alerts (WEAs) “should be a constant,” the authors say. Also, people mentioned they rely on TV and social media for receiving warnings even though generally neither will wake you up.

The authors felt it was “dangerous” that even the more tech savvy and tornado aware respondents answered they were compelled to look outside for evidence of a tornado—even in the dark. Interviewees explained they were “checking for sounds instead of visual cues.”

NWS forecasters were also surveyed about nocturnal tornadoes. The forecasters mentioned the lack of ground truth and fear for public safety among challenges to the nighttime warning service. They noted few spotter or social media reports inform them if “the storm is actually showing the signs on the grounds that radar is indicating aloft.” Forecasters said they felt “fearful, worried, or nervous for the public during nocturnal tornadoes because fatalities ‘are a given.’”

The survey responses moved Ellis et al. to recommend a single-emphasis message be presented to residents to combat the nighttime tornado problem:

One strategy that may improve public safety during a nocturnal tornado event, and which addresses the forecaster challenge of communication prior to and during an event, is to develop “One Message”—a consistent message that EMs and the media use throughout broadcasts, briefings, and social media. Examples of messages could be: “Nighttime tornadoes expected. Sleep with your phone ON tonight!” or “Tornadoes will form quickly! Make plans now where you will take shelter!” or “If you live in a manufactured home, you may not have much time to seek shelter tonight!” One Message may decrease confusion for receivers, making them more likely to make safe decisions. Messages could similarly be used to dispel misconceptions about local geography in ways relevant to the specific listening area, for example: “You are not protected by nearby hills. Seek shelter immediately!”

With Hurricane Delta’s nighttime tornado threat ramping up, the authors suggest people use multiple ways to receive warnings, keeps phones on and charged, don’t rely on tornado sirens, and if possible relocate ahead of the weather from “particularly vulnerable” situations, such as mobile homes and vehicles.

Welcome to the Sticky Skies: Costlier Air Travel in a Warmer, Wetter World

COVID-19 has upended air travel for now, but if the growth in global aviation resumes, one real drag on flying is going to be increasing energy needs due to global warming. New research published in the Bulletin of the American Meteorological Society by Diandong Ren (Curtin University, Perth) et al. shows that Earth’s warming climate is going to have an often overlooked—but costly—impact on the fuel consumption of airplanes. The reason: increased viscosity, or “stickiness,” of the air.

For starters, a warming atmosphere will expand and become less dense, reducing the lift produced by aircraft wings, which means planes must increase speed and burn more fuel to maintain carrying capacity. This disadvantage counteracts any fuel advantages of flying in thinner (less resistant) air. There are other small effects on engine efficiency. In all, there is some ambiguity about the direct effect of the warming atmosphere on fuel needs at cruising altitudes. But Ren et al. point to a much larger fuel impact due to increasing atmospheric viscosity.

In a warmer world, more water can evaporate into the air. The extra molecules of water increase the drag on aircraft and that in turn will cause planes to fight harder to cruise through the air, requiring additional fuel. The increasing drag turns out to be the dominant issue—and could become very expensive.

Fuel increaseRen et al. use an ensemble of 34 climate models to project that aviation fuel requirements by the end of the century could be an extra 160 million gallons per year due to viscosity, approaching an extra $1 billion per year in costs more than today in a scenario in which fossil fuel use is basically unabated.

The findings take account of regional and altitudinal variations in warming for different projections based on different amounts of emissions predicted. They also take into account the most trafficked flight paths, based on recent airline data. For example, some high altitude cooling at high latitudes would mitigate the effects of drag in near-polar routes, but few jets fly these paths. Flights at mid- to low-latitudes experience the biggest increases in drag—less than 2% per century, but enough to have  consequences in fuel usage. Overall, an air viscosity increase leads to about a 0.22% increase in fuel consumption by the year 2100 over 2010.

While these costs are still a small fraction of the total aviation fuel usage, they are yet another incentive for the industry to mitigate global warming through emissions reductions, as well as to pursue adaptations and efficiencies in aviation technology.