Why CORSIA Exists: Origins, the EU ETS Dispute and the 2016 Agreement

International aviation occupies a genuine gap in climate governance. A flight from Delhi to Frankfurt emits over international waters and multiple national airspaces, and the resulting tonnes belong cleanly to neither country's inventory. CORSIA is the eventual answer to a question that went unanswered for two decades.

The Gap in the Kyoto Protocol

The 1997 Kyoto Protocol assigned emissions to countries. Domestic aviation fell inside national totals without difficulty. International aviation did not, because attributing a flight's emissions to the departure State, the arrival State, the airline's State of registry or the fuel's point of sale each produces a different and equally arguable answer.

Rather than resolve it, Article 2.2 of the Kyoto Protocol handed the problem to ICAO, asking developed countries to pursue limitation of aviation emissions working through that organisation.

ICAO worked on it for the better part of twenty years without producing a binding measure. Emissions from international aviation grew throughout.

The EU Loses Patience

By the late 2000s the European Union had concluded that waiting was not producing results. In 2008 it legislated to bring all flights arriving at or departing from EU airports into the EU Emissions Trading System from 2012 — including the full length of a flight from, say, Shanghai or Chicago, not merely the portion in European airspace.

The reaction was severe.

• China and India instructed their carriers not to comply.
• The United States passed legislation shielding its airlines from the scheme.
• A coalition of States signed the Moscow Declaration opposing the measure.
• Retaliation against European aircraft manufacturers was openly discussed.

The legal position was arguable in the EU's favour — the Court of Justice of the European Union upheld the measure in 2011 — but the diplomatic position was untenable.

In 2012 the EU adopted "stop the clock", suspending application to flights to and from third countries, and subsequently narrowed the scheme to flights within the European Economic Area. The explicit condition attached was that ICAO deliver a global scheme.

That pressure is the direct cause of CORSIA. It is difficult to argue the scheme would exist in its current form without it.

The 2016 Agreement

ICAO had already adopted an aspirational goal of carbon neutral growth from 2020 at its 2010 Assembly. The 2013 Assembly agreed to develop a global market-based measure.

At the 39th ICAO Assembly in 2016, Resolution A39-3 adopted CORSIA.

The design reflects the diplomacy that produced it:

The exemptions in that fifth row implement the UNFCCC principle of common but differentiated responsibilities inside a sectoral scheme designed around the Chicago Convention principle of non-discrimination between operators. Reconciling those two ideas was the central negotiating difficulty, and the phased design is the compromise.

The Pandemic Forces a Change

The baseline was originally defined as the average of 2019 and 2020 emissions.

Then international traffic collapsed. Averaging a normal year with 2020 would have produced a baseline far below any plausible future activity level, inflating every subsequent obligation dramatically — a windfall cost imposed on an industry already in crisis, for reasons unrelated to climate policy.

In 2020 the ICAO Council reset the baseline to 2019 alone.

The baseline change matters beyond its own arithmetic. It is direct evidence that CORSIA's parameters respond to circumstances. Any plan built on the assumption that today's settings hold unchanged to 2035 is assuming something the scheme's own history contradicts.

Criticism of the Scheme

An honest reference has to record the objections, which come from several directions.

"Offsetting is not decarbonisation"

Environmental groups argue that buying reductions elsewhere allows aviation to defer changing its own emissions profile, and that the sector needs fuel and technology change rather than a payment mechanism. CORSIA's defenders reply that it was designed as a bridge to cover growth while those technologies mature, not as a substitute for them.

"The baseline is too generous"

Setting the reference at 2019 — a record year for aviation — means the scheme addresses only emissions above an already high level. Critics note that emissions at or below the baseline, which are the large majority, are untouched.

"Credit quality has been questioned"

Independent research has challenged the real-world integrity of several widely used credit categories, particularly around additionality and baseline construction. The ICAO Emissions Unit Criteria are stricter than the general voluntary market, especially on double counting, but they inherit the underlying methodological debates.

"Coverage is limited"

Domestic aviation is outside the scheme entirely, and route-pair coverage during the voluntary phases left significant traffic uncovered. Coverage widens substantially in the second phase.

"Non-CO2 effects are excluded"

Aviation's climate impact includes contrails, nitrogen oxides and other non-CO2 effects, which several assessments consider to roughly double the warming from CO2 alone. CORSIA addresses CO2 only.

None of these criticisms reduce the compliance obligation. They are worth understanding because they shape the periodic review process, and reviews have already changed the scheme once.

Timeline of Key Decisions

Where to Go Next

• What is CORSIA — the plain definition
• CORSIA phases and timeline — what each period requires
• CORSIA governance — who decides what
• CORSIA vs EU ETS — how the two now coexist

Primary sources: ICAO CORSIA and the UNFCCC Kyoto Protocol text.