Air travel and freight play a growing role around the world. Passenger bookings nearly doubled from 2004 to 2015, to 3.5 billion, and are expected to rise to 6.63 billion by 2032. Cargo flights are forecast to nearly triple, to 4.4 million in 2040. And with this growth, greenhouse gas emissions are rising as well.

International aviation emitted about 490.4 million tonnes (Mt) of carbon dioxide in 2013, 1.5% of global CO2 emissions from fossil fuel combustion that year. The UN agency that oversees the industry, the International Civil Aviation Organization (ICAO), expects CO2 emissions to rise to 682–755 Mt by 2020 and to 1223–1376 Mt by 2035, even with projected efficiency improvements in aircrafts and operations.

Amid growing pressure to curb aviation emissions, ICAO set a global aspirational goal that growth from 2020 onward be “carbon neutral” in terms of net CO2 emissions. Improvements to aircraft technology and operations would narrow the gap. The rest would be closed by using alternative fuels and a global Market-Based Measure (MBM) that would allow carriers to offset their emissions by buying permits and credits from other sectors, starting in 2020. Final approval of the MBM is expected at the 39th ICAO Assembly, scheduled for September 27 to October 7.

In a study commissioned by WWF-UK, SEI examined the potential supply of carbon offsets and of jet fuel alternatives, considering both climate benefits and potential sustainable development impacts.

The offset analysis found that in 2020–2035, carbon offsets from project types for which there is high confidence in environmental integrity, and which also advance sustainable development goals, could yield cumulative emission reductions of around 3.0 Gt CO2e, or 70–90% of ICAO’s projected demand for emission reductions of 3.3–4.5 Gt CO2e.

Including medium-confidence project types would expand the potential supply to 4.6 Gt CO2e. Further expanding eligibility to project types with neutral development impacts would increase supply to 5.1 Gt CO2e. Jurisdiction-scale REDD+ (Reducing Emissions from Deforestation and forest Degradation) programmes could potentially add another 2.4 Gt CO2e to the overall supply of offsets over that period.

Total cumulative supply of offsets, 2020–2035, by screening category. Click to enlarge and see a detailed caption.

“Our study shows that over the next 15–20 years, airlines can achieve ICAO’s carbon neutral-growth goal even if they set a high bar for the types of carbon offsets they use,” says Derik Broekhoff, a senior scientist at SEI-US in Seattle and author of the offset analysis.

“By focusing on high-confidence mitigation actions with strong sustainable development potential, ICAO can ensure that airlines make a lasting contribution towards avoiding serious climate change,” Broekhoff adds. “Realizing this potential will require strong standards and third-party certification, but there is no need to shy away from these due to supply concerns – robust offsetting can be a viable short-term solution to global aviation emissions growth.”

Aviation biofuels’ potential is less certain

The biofuels analysis, conducted by Rob Bailis, a senior scientist at SEI-US in Boston, found considerable uncertainties – particularly because the alternative jet fuels industry is in its infancy. In examining alternative jet fuels, SEI classified fuels by their potential emissions reductions relative to conventional jet fuel, impacts on land use, and likelihood of helping or hindering achievement of Sustainable Development Goals.

ICAO’s “most likely” scenario foresees alternative fuels meeting about 3% of global jet fuel demand in 2020 – about 6.5–7.2 Mt. But considering facilities that are currently operating, under construction or planned, this seems unlikely. If all are online by 2020, SEI estimates production capacity that year could reach 1.8 Mt. From that baseline, production could reach 14 Mt per year by 2035 if capacity grows by 14%, the average annual growth rate that U.S. bioethanol achieved between 2000 and 2015.

The emission reductions that could be achieved by using that growing supply of alternative fuels depend on the production method (“pathway”) and feedstock used. Bailis estimated the savings if the fuels reduced emissions by 25% or 75% relative to conventional jet fuel – in both cases, assuming they were produced with little or no impact on land use. From 2020 to 2035, the savings would range from 0.1 to 0.3 Gt CO2e.

The biofuels analysis stresses that strong safeguards are needed – preferably through reliance on credible certification schemes – to ensure that the production of alternative fuels does not have land use change impacts (direct or indirect), and that it does not harm social welfare or the environment.

“Recent experience with biofuels for ground transport shows us that, while the industry is capable of rapid growth in response to strong demand, it can come at an environmental and social cost,” Bailis says. “Those risks can be reduced by using low-impact feedstocks such as algae and cellulose, but those feedstocks and related production pathways are just getting started. It’s going to take tremendous effort to get these pathways mobilized and close oversight to ensure that social and environmental standards are not compromised.”

Estimated GHG emissions from alternative feedstocks and pathways, without considering land use change impacts. Click to enlarge.

Carrie M. Lee, an SEI-US scientist in Seattle and coordinator of the project, adds that while ICAO’s plan to ensure carbon-neutral growth through offsets and alternative fuels is feasible, putting aviation on a path consistent with keeping global warming below 2°C will require actually reducing emissions.

“It is encouraging to see ICAO and airlines taking action, but the current proposals are just a start,” Lee says. “While the global MBM can help reduce emissions over the short term, the real question is: how do we get where we need to be in 2050 and beyond? Offsets can’t be a long-term solution to decarbonizing the international aviation sector.”

Read the SEI study »

Read a WWF briefing based on the study (external link to PDF)