Air travel is a major source of carbon pollution, and currently, there's no real way to stop it (short of grounding flights). Jet fuel made from oil is extremely energy-dense, and while many attempts have been made to blend jet fuel with biofuel, that solution is often prohibitively expensive and hardly carbon-neutral, to boot.

The Paris Agreement didn't set limits for carbon emissions from the aviation industry, but the International Civil Aviation Organization (ICAO)—an agency of the United Nations that works with the aviation industry in 144 countries—attempted to take up that mantle. The ICAO agreed in 2016 that airlines would be required to buy carbon offsets for every ton of carbon that they emit over and above 2020 emissions projections.

A letter published in Nature Climate Change today suggests that this requirement from the ICAO is not sufficient to mitigate the effects of the aviation industry's increased carbon dioxide in the atmosphere. Instead, the authors of the letter say the member states of the ICAO need to agree to mandate that the credits that airlines buy meet a specific set of criteria, to ensure that the aviation industry is actually reducing greenhouse gas emissions.

Problem: Some green projects are profitable on their own

The crux of the problem here is with the carbon-offset certification process. Airlines can choose to buy carbon offsets from a number of organizations, but the UN's Clean Development Mechanism (CDM) is a prominent certifier of offsets. CDM is a program run by the UN to help large companies buy into emissions reduction projects in developing countries. If a developer wants to implement such a project, it can register with the CDM to find potential "investors" via offsets. For every ton of carbon dioxide avoided, a project earns a certified emissions reduction (CER) credit. States and companies can buy and trade these to offset their own pollution and subsidize a green energy project across the globe.

The authors of the letter say that—if airlines buy credits issued by existing projects or projects that would make a profit on their own without the help of credits—airlines really haven't done anything to "offset" their considerable carbon footprint.

That means that a lot of projects, especially wind or hydro projects, would be off limits, generally because losing funding from selling credits to airlines won't change whether the project goes forward or not.

The letter in Nature notes that there is currently a glut of credits in the system due to low demand for offsets and falling costs of renewable energy development. This drives down the price of credits to around €1 per credit. In such a system, "offsetting" is artificially cheap.

The letter's authors did a survey of 1,300 projects under the CDM and found that 90 percent of projects registered with the CDM "continue the operation of their GHG [greenhouse gas] abatement activities, despite limited or no financial support from CER revenues." If airlines buy these offsets, they aren't really reducing emissions that wouldn't otherwise be reduced.

Fugitive emissions

The letter suggests that worthy carbon emissions credits come from projects to stop fugitive emissions of natural gas, bagasse projects, or other biomass fuel projects. "Some biomass projects are at high risk of discontinuing GHG abatement because the sourcing of biomass is only economically viable with ongoing financial support," the letter recommends.

Ultimately, the letter asks the ICAO to restrict what kinds of carbon offsets airlines can buy. These offets would only fund "Projects that are newly developed" in response to the airline's need to offset carbon, or they would be "implemented projects that are vulnerable to discontinuing GHG abatement."

Update: In a statement to Ars, ICAO Communications Officer William Raillant-Clark said that the Organization's role is to "assist and facilitate sovereign nation states in developing consensus on preferred solutions," that pertain to buying carbon offsets. He added that ICAO member states "are currently negotiating eligibility criteria" for the kinds of offsetting credits reviewed in the Nature Climate Change paper.

The ICAO Office of the Secretary General declined to speculate on how those member states would define the criteria for offsets, but said that "any advocacy in this regard would therefore have a much better chance of achieving the desired results" if it were aimed at member governments.

Correction: This article was clarified to reflect that the ICAO's decisions on emissions offset criteria are negotiated and decided by ICAO member states.

DOI: Nature Climate Change, 2019. https://doi.org/10.1038/s41558-019-0415-y (About DOIs).