By Brien Seeley M.D., President, Sustainable Aviation Foundation
Senior Fellow of UC Berkeley’s Institute of Transportation Studies (ITS)

The following Q&A addresses key questions and offers in-depth details for building Autonomous Aerial Firefighting (AAF)

Why is the Sustainable Aviation Foundation focused on wildfires, instead of other aviation initiatives?

The terrible wildfires that devastated California from 2017 to 2022 destroyed the homes and lives of many who were personally connected with us at the Sustainable Aviation Foundation (SAF) and turned our attention to how advances in autonomous, electrically-powered aircraft could end wildfires. The COVID pandemic interrupted SAF’s annual in-person symposia and afforded it time to deeply research aerial fire-fighting. That research distilled a large body of data on the amount of global burn area and its potential cumulative impact on global CO2 emissions. The result was SAF’s alarming discovery that wildfires now account for the largest annual increase in atmospheric CO2, amounting to 101 Gigatonnes of CO2 in 2020 (indexed to the year 2000).

This 101 Gigatonnes of CO2 caused annually by wildfires is 2.5 times larger than the roughly 40 Gigatonnes emitted annually from the global use of fossil fuels, meaning that the global warming effect of wildfires is now far worse than and additive to that from burning fossil fuels. To SAF, this alarming discovery demands immediate attention if we are to reach the net zero in CO2 emissions deemed necessary to save our planet. The good news is that SAF research shows that a mass-scale system of Autonomous Aerial Fire-fighting (AAF) to end wildfires can be built with existing technology and that it could be the most effective remedy in curbing the climate crisis. That remedy can only occur if we raise the awareness of the public that ending wildfires is now JOB ONE and convince large organizations and agencies to take action. To do that will require innovation to prevail over preserving the status quo and a bold, concerted moonshot program to prevail over cautious incrementalism. 

Fortunately, awareness of the enormous importance of nature’s carbon sink and its potential to mitigate the climate crisis has spread among leading climate agencies and experts, who now share the realization that “there is no net zero without nature.”

Often the first question that many ask SAF about an AAF system is “can we afford such a moonshot program?” The economic analysis says we cannot afford not to. The US EPA now estimates that each Gigatonne of CO2 emitted into the atmosphere costs our planet $190 billion dollars. By that metric, the 101 Gigatonnes of CO2 found attributable to wildfires in 2020 cost our planet over $19 trillion dollars in one year.

How do wildfires cause CO2 emissions and affect the climate crisis?

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Wildfires emit CO2 directly as smoke, but also eliminate the natural photosynthetic sequestration of CO2 by the forests and grasslands that have been burned. This lost sequestration increases every year as wildfires burn new areas. Burned forests take more than 20 years to regrow to 50% of their pre-fire photosynthetic capacity, while burned grasslands take 4 years to do so.

The Sustainable Aviation Foundation (SAF) sought to better understand the net CO2 impacts of wildfires, given newly available data on global wildfires and net ecosystem exchange values for lost forests, grasslands, savannahs and other land-cover types.

Sequestration loss calculations

To calculate these sequestration losses we applied the latest, most accurate data from Chen et al on mean global wildfire burn areas over the 20-year period from 2000 to 2020 to the most recent compilation by Zhuravlev et al of the respective net ecosystem exchange values (NEE) for the main land-cover types of those burn areas (forest, grasslands, savannah, etc.) to obtain a first-order calculation of annual global atmospheric CO2 attributable to wildfires. 

We chose the period from 2000 to 2020 for our calculations to align with the data available in the studies linked above, and because it forms a representative and conservative sample of the modern era in which global wildfires have rapidly increased in size and frequency.

Our findings are stark: When the losses of global CO2 sequestration are accumulated for the 20 years ending in 2020, they sum to over 95 Gigatonnes of CO2, and when combined with the 6 Gigatonnes of CO2 directly resulting from fire smoke in 2020, they comprise an annual CO2 emission of 101 Gigatonnes, compared to a world in which we had prevented wildfires. This astounding figure is more than 2.5 times the 40 Gigatonnes of CO2 annually emitted by global use of fossil fuels from all human activities.

In each subsequent year of increasing loss of vegetation due to wildfire this loss will continue to accumulate beyond 101 Gigatonnes. While this value is shocking, it is also conservative for at least three reasons: 1) It only includes the most recent 20 year period during which we have accurate data for burn areas and NEE; 2) it only accumulates sequestration losses for the first 50% of the burned area’s regrowth period; 3) our calculation models CO2 sequestration loss on a linear rate of regrowth rather than an exponential one. 

Cumulative global annual CO2 sequestration lost, in gigatonnes, due to fire damage. Includes linear regrowth to 50% of original photosynthetic capacity. Green bars are smoke CO2.

Sources: IPCC data with consensus to NASA, FAO, GCW, and GFED; Sustainable Aviation Foundation

Various sources of CO2 emissions, comparing commonly discussed emission types to the loss of CO2 sequestration due to wildfires.

Source: Sustainable Aviation Foundation

Wildfire damage accumulation for years 2020-2022 (NASA data)
Source: Copernicus Atmosphere Monitoring Service (CAMS)

The huge CO2 emissions caused by wildfire smoke and its damage to vegetation drive a feedback loop that further warms the planet and causes more extreme weather that increases the incidence of future megafires. This vicious cycle deepens net CO2 emissions even further because the annual amount of forfeiture of CO2 sequestration is cumulative. Adding to that forfeiture is the uncertain amount of lost photosynthetic capacity in the incalculably larger areas of forest and grass vegetation that, though not directly burned by wildfire, are injured by smoke and being covered with ash.. Some estimate that we will see more than a doubling of global wildfires in the next 30 years.

The Dire Climate Impacts Of Unchecked CO2 Emissions

How bad can this get? The positive feedback loop driven by wildfires is already causing the number and severity of extreme weather events to increase faster than generally predicted. This is likely related to under-accounting for the escalating impacts of wildfire. Carolyn Kormann in the New Yorker of September 6, 2023 writes “Phoenix is the fifth-largest city in the U.S., and the hottest large city, with an average summer temperature of 93.7 degrees — an average that has increased by 3.8 degrees since 1970. Nighttime summer temperatures, largely owing to urbanization, now average a low of eighty-three degrees, an increase of 5.4 degrees since 1970…  there was a stretch of sixteen days [in July 2023] when the nighttime low was ninety degrees or above, including one night when the low was ninety-seven degrees.” The hottest temperature ever recorded in Sonoma County, California is 115 degrees Fahrenheit. This temperature was recorded at Santa Rosa on September 6, 2020. Additionally, Atlantic Ocean temperature recorded at a buoy in Manatee Bay near Miami reached 101°F on July 25, 2023, a likely new world record.

The Paris Climate Agreement set a goal to limit global temperature rise to 1.5°C, but recent emissions control efforts appear insufficient to achieve that goal. The extreme weather events that can occur from even a 1.5° to 2.0° C rise in global temperature can be horrible and can kill millions. These events include heatwaves, floods, hurricanes, tornadoes and wildfires that could wipe-out vital infrastructure.

Without decisive action to curb the climate crisis, we will live with the potential for dangerous cascades of events caused by extreme weather events. Recent history gives us examples of: 

• Widespread power blackouts
• Air conditioning outages during soaring temperatures
• Gridlocked highways due to evacuating populations
• Gas shortages and pump failures leaving people stranded in vehicles
• Failed water pumps and other key infrastructure outages
• Hospitals overflowing with heat-stroke and respiratory emergencies
• Internet and cellular phone service outages 
• Wildfire smoke causing severe respiratory damage
• Hurricane flooding blocking escape routes and ruining drinking water
• Tornadoes leveling community shelters and fire stations

Recent assessments by a team of experts in the U.K. from the Institute and Faculty of Actuaries and University of Exeter have revealed further indications that the climate crisis is already worse than we’ve been told.

Why have Climate Summits not prioritized ending wildfires?

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Climate Summits have generally agreed to dismiss wildfires as untouchable nature, a part of the planet’s ecosystem whose adverse effects on atmospheric CO2 had to be accepted as “baked-in” and unalterable. 

Estimates made by Copernicus at section 2.7 admit that “the loss of additional sink capacity from reduced forest cover is missing in the combination of approaches used here to estimate both land fluxes (Emissions and Sequestration)”. This omission is a policy that reflects awareness of both the restorative effects of wildfires on ecosystems and the untenable cost and difficulty of ending all wildfires globally using present-day equipment and systems. Up to now, Climate Summits have agreed that reducing CO2 emissions by curbing the use of fossil fuels is the highest priority in curbing the climate crisis. In light of SAF’s discovery of the enormous annual amount of atmospheric CO2 attributable to wildfires, and the feasibility of controlling it with aviation, we call upon future Climate Summits to acknowledge the judicious ending of wildfires as a crucial priority in curbing the climate crisis.

If we end wildfires globally, how much will it help curb the climate crisis?

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SAF calculations outlined above account for loss of CO2 sequestration for the 2000-2020 period. Our calculations and figures below attempt to project those findings forward.

Figure 1 shows the diagonally sloping downward dotted lines that depict the steep pathways we must follow for reducing future atmospheric CO2 if we are to limit global temperature rise to 1.5, 1.7 and 2.0°C, respectively. 

Figure 2 is a duplicate of that information but with an overlay of three sets of solid vertical downward arrows (green, then blue, then purple, then orange) that depict the size and the sequence of annual reductions in atmospheric CO2 that could occur over a 4-year period after we end wildfires. For purposes of illustrating how fast ending wildfires could curb the climate crisis, each of these three 4-year sequences (as shown by its solid green vertical arrow) is depicted to begin in a different year (2026, 2031 and 2036), as the year in which we hypothetically first achieve an end to wildfires. The smaller downward solid arrows below the green arrows (blue then purple then orange) are scaled to show the relative amounts of CO2 reductions that could occur in each of the subsequent 3 years. The graph’s main messages are 1) that the sooner that wildfires are ended, the greater the chance of limiting global temperature rise to 1.5°C, and 2) that the CO2 reductions from ending wildfires are much larger and more rapid than those expected from efforts to reduce the global use of fossil fuels. Together, the two graphs below show that ending wildfires globally with Autonomous Aerial Fire-fighting (AAF) would likely be the most effective way to reduce global CO2 emissions within the short time-scale necessary to limit global warming to 1.5°C. 

The lower of the two graphs below actually underestimates the maximum rate at which global CO2 reductions could occur because it does not yet depict the as yet indeterminate trajectory of reductions in the global use of fossil fuels that would augment the large reductions obtained by AAF. Similarly, the lower graph does not show the additional CO2 reductions that could be achieved with concurrent aggressive increases in reforestation and cessation of deforestation. This is not to say that the large gains in CO2 reduction from AAF should cause us to slacken efforts to drastically reduce the use of fossil fuels and efforts to develop the best types of renewable energy. Rather, we should continue those and other efforts to curb the already dangerous climate crisis with an ‘all-hands’ approach whose goal is to keep global warming as far below 1.5°C as possible.

Figure 1

Via Global Carbon Project

Source: Friedlingstein et al 2022; Global Carbon Project 2022

Figure 2

Global temp rise depends on when wildfires are ended. Sequences of 5 green vertical down-arrows show the relative maximal amounts of CO2 that could be saved in each of the 5 years after fires are ended. Ending fires in 2031 is shown to potentially limit global warming to less than 1.5°C.

Via Global Carbon Project

Source: Friedlingstein et al 2022; Global Carbon Project 2022

I’ve heard that we are not allowing enough wildfires – what about controlled burns and sustainable forestry?

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In a world that has successfully curbed the climate crisis, we should employ sustainable forestry, which includes controlled burns, proactive fire breaks, potential operational delineations (PODS) and understory management of forests and their cycles of regrowth.

However, we are not in an equilibrium, but instead in a state of emergency. The urgency of the present – the already crisis-level of CO2 emissions – demands that conventional prescribed burn methods be deferred while wildfires are ended by implementing effective wildfire suppression. Once the rise in global CO2 ppm is under control, conventional methods of sustainable forestry can be reinstated. For now, the prime task is to stop burning massive biomass substrates.

When conventional methods are reinstated a functioning AAF system will continue to make them safer and more controlled, with a lower chance of escape into wildfire.

What challenges currently limit our ability to end wildfires?

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In September, 2020 SAF undertook a discovery meeting with Cal Fire Aviation leaders to learn about the main challenges to the current aerial firefighting regime. Those and others discovered with further research are as follows:

• Difficulty in timely reaching remote wildfires with ground crews, fire engines and bulldozers
• A shortage of trained air-tanker pilots
• A shortage of air attack bases due partly to the very long runways required by VLATs
• Inability of air tankers to fly at night or in heavy smoke
• Remoteness of large air attack bases from wildfire outbreaks (long times to arrive on-scene)
• High cost of VLATs ($24M each, on average)
• A need for a global early detection network that can issue prompt dispatch of aerial assets
• A need for aerial attack that can make precise, low-level drops of fire suppressing liquid with minimal wind scatter (VLATs drop retardant from too high up, suffering wind scatter)
• A need to avoid contamination of waterways with eco-toxic fire retardants
• 53% of Wildfires of > 10,000 acres happen outside the 10AM-6PM pilot’s flying window.

Mega-fires grow in severity each year, and public agencies responsible for controlling them can only act within the limits of budget, policy, labor and political will. The climate crisis has caused wildfires to surpass these limits.

Other challenges: The annual Cal Fire budget is now more than $1B. With more than $44B in property loss and damage in California from megafires in just 4 years (2017-2021), this means that, on average, wildfire loss and damages are directly costing $11B per year in California, not to mention their incalculable damages to climate, evacuation costs, lung damage, death, etc. The costs of those fire losses are now being passed on to California voters as either cancellation of their homeowners insurance coverage or quadrupling of their annual insurance premiums.

Editor’s note: To be clear, SAF has deep admiration for CAL FIRE and other firefighting agencies who have bravely and tirelessly served the public in the fight against wildfire devastation, and our goal is to augment their efforts.

Business as usual is not an option for addressing these challenges: The present-day, piloted, daytime-only, fossil-fueled aerial fire-fighting methodologies from huge, remote air attack centers cannot overcome the challenges listed above and are manifestly insufficient for preventing megafires, as shown in the bar-graph below. The strategy of using fire retardant drops by VLATs at the perimeter of large wildfires to slow their spread is helpful but has likewise proven insufficient to prevent megafires. Conventional aerial fire-fighting is undertaken by heroic public servants at great individual risk, and their efforts should be lauded. At the same time, from a public policy and future climate outlook perspective, we need to acknowledge that this approach is proving insufficient.

Source: https://www.reuters.com/graphics/CALIFORNIA-WILDFIRES/gdpzyjxmovw/

Apart from Autonomous Aerial Fire-fighting (AAF), what are the alternative programs to effectively end wildfires?

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Simply put, none. Instead, incremental efforts to expand present methods to control (not end) wildfires are being made by numerous agencies and with NASA getting involved as a coordinator of those agencies. However, in light of the magnitude of the newly discovered annual atmospheric CO2 attributable to wildfire damage, these incremental efforts will likely be too little, too late. None of the current agency initiatives aim to do what the AAF system is proposed to do: i.e., comprehensively end wildfires. The partial solutions that are being considered appear to be either prohibitively expensive, too time consuming, environmentally unsound or incapable of reaching all fires. Wildfires are proving to be too numerous, remote and fast-spreading to be consistently extinguished within the first hour after outbreak by the positioning of ground crew, bulldozers and firetruck assets alone, even with massive spending. This is particularly true when the fire ignites at night in a remote and inaccessible forest. Initial attack at such fires can only be safely and promptly achieved by aerial attack using autonomous aircraft.

With AAF aircraft, we can effectively create our own very localized type of rain. The amount of water needed depends upon the size of the fire which in turn depends upon how long the fire has been burning since it ignited. Evidence suggests that the only way to effectively end wildfires and megafires is to extinguish them wherever they begin and before they can grow – at night and in smoke in remote areas, before they can spread. This requires the very prompt dispatch of a squadron of autonomous aerial fire-fighting aircraft that can provide a bucket brigade that delivers a near-continuous spray of fire-dousing water. That squadron must be available 24/7 from a widely distributed network of small airparks that blanket fire-prone areas with adequate proximity to any fire’s location. As such, AAF would comprise an aerial fire-dousing water delivery system that would be able to extinguish any fire within the first hour after its outbreak.

The graph below shows how effectively the proposed AAF system could address wildfires, relative to data for the conventional aerial attack system that was employed on the actual LNU Complex Fire in California. The graph shows calculated burn area for the untreated fire relative to the slightly reduced burn areas that would occur with the infrequent drops made by each of the VLATs. The electrically-powered, uncrewed aircraft listed in the graph’s legend are called eTanker150 and eTanker 300. These are the proposed autonomous eTankers that each carry 150 gallons and 300 gallons of fire-dousing liquid, respectively. The “d/min” shown in the legend indicates the number of drops per minute that the various eTankers are performing.

Notes: Wind 10 fps, flame spread at 10% of wind speed with 24° spread angle on each flank. eTankers with either 6 drops/minute (d/min) or 10 d/min. Minutes from take-off to 1st drop: 35.4, 26.4, 4.5 for VLATs, S-2T and eTankers, respectively. VLATs are DC-10 and B-747. Drop intervals after 1st drop are: 20, 20, 18 for DC-10, B-747 and S-2T, respectively. B-747 data is hypothetical for 17,964 gallons per drop. NOTE: Does not account for slowing of flame spread by drops. Source: Sustainable Aviation Foundation

How will AAF operate safely in US airspace?

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Simulations of high density air traffic have shown it to be a manageable challenge when optimized technology is implemented. A 2019 study by the MIT Technology Review found that the use of drones [i.e. driverless aircraft] could potentially increase the number of small aircraft safely operating in low-altitude airspace by up to 100 times [i.e., up from 1000 to 100,000 aircraft]. However, the study also found that this would require significant changes to the airspace, including new separation standards, the creation of new air corridors and the implementation of new safety procedures.

Before taking off, each AAF vehicle will be provided an integrated precisions de-conflicted 4D flight path from airpark to airpark. “4D” refers to a three dimensional (3D) flight path that also includes an exact time for arriving at each point along that path. That flight path would effectively be a unique pathway in the sky, automatically de-conflicted with all other air traffic. Any intrusion into such assigned airspace by a rogue piloted or unpiloted aircraft would be automatically detected, interrogated, tracked and announced to all other relevant traffic. The relevant traffic would all use their on-board sentient sense and avoid guidance systems to avoid any conflicts and proceed on appropriately revised, re-assigned 4D flight paths. As is commonly done in today’s controlled airspace, private aviation and airline traffic could be directed to both fixed and resilient high volume flight corridors to keep their aircraft away from geo-fenced AAF operations.

Why are eVTOLs not suited for AAF?

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As of 2024 numerous well-funded startups propose to build electric vertical takeoff and landing aircraft to serve a variety of markets. Much attention and investment is being applied to these startups and vehicle programs, and understandably SAF is often asked why the AAF model does not leverage eVTOL systems.

The limited energy density of batteries is the main reason that essentially disqualifies eVTOLs from offering competitive range and payload capabilities in aerial fire-fighting. With a given battery pack, the aircraft that optimize lift and minimize drag and weight (in order to reduce the power required and noise) will thereby optimize the aircraft’s range and payload. This is conventionally accomplished by having a low span loading and long, slender wings of high aspect ratio, such as are seen on the Boeing 777, B29 and modern sailplanes. These design features provide a high lift-to-drag ratio (L/D), the key metric for efficient, long-range flight according to the Hepperle modification of the Breguet Range Equation.

A fixed-wing aircraft with a cruise L/D ratio of 20:1 not only enjoys longer range but also lower power required that gives ultra-low noise emissions and lowers its cost of operation. Long experience with vertical take-off and landing aircraft, by contrast, shows them to typically have a poor L/D of about 5:1. Such a low L/D translates to requiring at least 3.3 times more power than the fixed-wing eTanker in order to lift the same payload weight on take-off and landing. That excessive amount of power required worsens both the noise and the range of the eVTOL aircraft. The excessive power required for eVTOLs ruins their suitability for carrying a heavy load of fire retardant and reaching and fighting distant wildfires.  

Another important reason that eVTOLs are not workable for AAF is that the noise emissions from their rotors cannot be made quiet enough to fulfill the FAA and W.H.O. requirements when operating in close proximity to residential areas. This is evidenced by the quietest of today’s eVTOLs having take-off noise emissions being 25 dBA in excess of those requirements. Future noise requirements for effective AAF must respect not only the 2021 FAA National Curve of Noise Tolerance, but also the even more stringent limits on continuous noise events as the guiding metric, because in effective AAF the take-offs occur almost continuously (every 10 seconds).

All eVTOL designs inherently share the same requirement to surround their cabin payload with several lift rotors in order to position their thrust axes to balance and control the aircraft’s attitude and keep it right-side-up. And eVTOL aircraft suffer increased airframe interference drag due to their need for multiple large rotors (whether shrouded or open-rotored), each of which must be mounted on a strut or structural support with an intersection that joins it to either the fuselage or wing of the aircraft. These multiple intersections on the airframe produce numerous interactive turbulent wakes with separated airflow and increased noise, as shown in this video of VTOL turbulence revealed by computational fluid dynamics.  

These rotor geometry drawbacks further penalize the performance and capacity of eVTOLs compared to those of fixed-wing ESTOL AAF vehicles. In addition, it is generally conceded that eVTOLs will cost more and take longer for the FAA to certificate than more conventional and simpler ESTOL fixed-wing AAF aircraft. Current progress in hybrid electrically-powered ESTOL aircraft affirms its capability for using extremely short runways.

What are the cost estimates to launch AAF statewide in California?

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California is chosen as the ideal birthplace of AAF due to its resources, weather, fire history and aerospace industries. These following cost figures depend very much upon the resolve and urgency applied to the project, including competitive bidding, streamlined permitting, concurrent development, prioritized inter-agency cooperation and leadership. They also depend upon developing public-private partnerships after the initial proof of concept prototyping. The most current 2024 cost and timeline estimates for a rapidly deployed moonshot program for a fully built AAF system statewide in California are:

• $100M for a prototype fixed-wing ESTOL AAF system flight demo. Year 1
• $200M for 3 eTankers to demo AAF autonomous fire suppression Year 2
• $1500M Concurrent AAF FAA Certification. Year 3
• $12B Concurrent build of 1200 three-acre AAF air attack airparks at $10M each. Years 3-5
• $10B Concurrent mass-produce 33,000 AAF eTankers at $303K each. Years 3-5
• Total: estimated at about $23.8B cost spread over the first 5 years = $4.76B per year

That amounts to $1.59B per year when amortized over 15 years of service life for aircraft and airparks. We should remember that that $1.59B per year is a very small amount relative to the $19 trillion dollars per year in losses attributable to wildfires. These estimated costs could be substantially reduced if the AAF fleet of aircraft were multi-purpose Sky Taxi aircraft that could earn revenue in serving other markets.

I need a summary: what are the overall benefits of AAF?

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The benefits of Autonomous Aerial Fire-fighting (AAF) for ending wildfires are:

1. Effectively and rapidly reduce global CO2 emissions, potentially by tens of Gigatonnes annually
2. Thereby substantially curb the climate crisis and avert extreme weather disasters
3. A safe and nearly-continuous aerial firefighting that can drop 20X more fire-suppressing liquid
4. Safe aerial attack by emission-free aircraft without flammable fuel on-board
5. Capability to make precise, low-level drops of retardant with minimal wind scatter
6. Prompt detection, geo-location, dispatch & on-scene 3-4X faster than present air attack system
7. A ubiquitous network of AAF small ESTOL airparks puts fire suppression much closer to fires
8. 24/7 operation, even in heavy smoke or at night
9. 20X lower vehicle costs with multi-purpose aircraft replacing $24M air tankers
10. Uncrewed aircraft avoids operational limits due to pilot shortages
11. Eliminate toxic smoke clouds that force mass regional evacuations and cause respiratory damage
12. Use fire-dousing water instead of retardant to avoid contaminating ecosystems and waterways
13. An organizing stimulus for progress in EVs, battery and charging standards and renewable energy
14. Substantial job growth and advances in aerospace and tech industries
15. Potential modification of eTankers for use in other forms of civil aviation (RST LINK)

How can we get AAF launched?

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We need your help to get this plan enacted, with a commitment to scale that would meaningfully address the challenge and opportunity. Whether you are an individual or member of the public sector, government agency or international organization, we would like to engage with you on these topics to quickly take action. 

Steps to get involved

1. Spread the word by sharing this article with your network
2. Join our mailing list to receive periodic updates on SAF and AAF work
3. Let us know if you’re interested in collaborating on advancing AAF. We are currently seeking advocates in the public sector who can advance the AAF project within local, state and national agencies.