Cap Trajectories and Ambition Ratchets
An ETS cap is not static. It must decline over time to drive the emissions reductions needed for climate goals. How fast should the cap fall? How should the trajectory be structured? And how do you increase ambition as climate science and policy evolve? This lesson explores cap trajectories and the mechanisms for strengthening them.
Why Caps Must Decline
A fixed cap only prevents emissions from growing. To actually reduce emissions, the cap must decrease over time.
Simple illustration:
- Year 1 cap: 100 million tons
- Year 1 emissions: 100 million tons (right at the cap)
- Year 5 cap (still 100 million): Emissions could still be 100 million
If the cap does not decline, the system merely constrains growth, not drives reductions. For net-zero by 2050, caps need to approach zero.
Linear vs Exponential Decline
Caps can decline in different patterns:
Linear decline
The cap falls by a fixed amount each year.
Example: Starting at 100 million tons, declining by 2 million tons per year.
- Year 1: 100M → Year 5: 92M → Year 10: 82M
Linear reduction factor
The cap falls by a fixed percentage of the original baseline each year.
Example: EU ETS Phase 4 uses a 4.2% linear reduction factor.
Exponential decline
The cap falls by a fixed percentage of the current cap each year.
Example: 5% annual reduction from the previous year's cap.
- Year 1: 100M → Year 5: 77.4M → Year 10: 59.9M
| Approach | Pattern | Pros | Cons |
|---|---|---|---|
| Linear | Steady absolute cuts | Predictable, simple | May be too slow initially |
| Exponential | Accelerating % cuts | Faster long-term decline | Complex, harder to communicate |
| Mixed | Linear transitioning to tighter | Balances early flexibility with long-term ambition | More complex |
Setting the Trajectory
The cap trajectory should align with climate targets:
Step 1: Define the endpoint
Where does the cap need to be in 2030? In 2050? For net-zero, the 2050 cap should be zero or near-zero for covered sectors.
Step 2: Work backwards
From the endpoint, calculate what annual reduction rate achieves the goal.
Step 3: Front-load or back-load?
Should most reductions happen early (front-loaded) or later (back-loaded)?
- Front-loaded: Faster near-term cuts, easier later
- Back-loaded: Easier now, harder reductions deferred
- Climate science argues for front-loading due to cumulative emissions
Think of a cap trajectory like a flight descent path. The pilot (policymaker) knows the destination (net-zero by 2050) and current altitude (current emissions). The trajectory is the glide path. Too steep and you risk a hard landing; too shallow and you miss the runway. The optimal path gets you down safely on time.
The EU ETS Trajectory
The EU ETS provides a clear example of trajectory design:
| Phase | Years | Linear reduction factor | Trajectory |
|---|---|---|---|
| Phase 1 | 2005-2007 | None | Pilot phase |
| Phase 2 | 2008-2012 | ~1.7%/year | First decline |
| Phase 3 | 2013-2020 | 1.74%/year | 21% below 2005 by 2020 |
| Phase 4 | 2021-2030 | 4.2%/year (increased from 2.2%) | 62% below 2005 by 2030 |
The EU increased its reduction factor from 2.2% to 4.2% in 2023 as part of its "Fit for 55" package. This shows how trajectories can be tightened through "ratchet" mechanisms when ambition increases.
Ambition Ratchets
Climate goals evolve. What seemed ambitious in 2010 may be inadequate by 2025. Systems need mechanisms to increase ambition.
Paris Agreement ratchet
Countries submit new climate commitments (NDCs) every five years. Each round should be more ambitious than the last. ETS trajectories should align with these updated commitments.
Built-in review provisions
Many ETS laws include scheduled reviews where cap trajectories can be adjusted.
Market signals
If allowance prices are very low, it suggests the cap is not stringent enough. Some systems have provisions to tighten caps in response to market signals.
The EU ratchet mechanism:
The EU's climate law requires emissions reductions of at least 55% by 2030 and net-zero by 2050. When the 2030 target was increased from 40% to 55%, the ETS cap trajectory was automatically tightened.
The law also requires the Commission to propose updates to the trajectory if climate science or international commitments warrant.
This creates a one-way ratchet: ambition can increase but cannot legally decrease below the floor set by the climate law.
Handling Surplus and Banking
A key complication in trajectory design is allowance surplus.
What is surplus?
If actual emissions fall below the cap (due to recession, technology, or loose cap-setting), unused allowances accumulate.
Banking
Most systems allow allowances to be saved (banked) for future use. This creates flexibility but can lead to a large stock of saved allowances.
The problem:
Banked allowances can satisfy future caps, meaning actual emissions reductions may not match the cap trajectory. A declining cap does not guarantee declining emissions if entities use banked allowances.
The EU ETS accumulated a massive surplus during the 2008 financial crisis:
The problem:
- 2009: Recession crashed demand, emissions fell below cap
- 2010-2013: Surplus grew to over 2 billion allowances
- Prices collapsed from €30 to under €5
- Investment signals disappeared
The solution: Market Stability Reserve (MSR)
In 2015, the EU created the MSR:
- If surplus exceeds 833 million allowances, a portion is held in reserve (not auctioned)
- If surplus falls below 400 million, allowances are released from the reserve
- Since 2023, allowances in reserve above the previous year's auction volume are cancelled
Impact: The MSR has helped drain surplus and restore price signals. EU ETS prices rose from €5 in 2017 to over €80 by 2023.
Lesson: Trajectory design must account for banking and surplus. Otherwise, the cap may not bite for years due to accumulated allowances.
Communicating Trajectories
Clear communication of the cap trajectory is essential:
Publish long-term trajectories
Do not just announce next year's cap. Publish the trajectory through 2030 or beyond.
Explain the logic
Connect the trajectory to climate targets. Help stakeholders understand why this path was chosen.
Provide forward guidance
Signal likely tightening in advance. If ratchets are expected, say so.
Be credible
If the trajectory changes frequently or unpredictably, market confidence erodes.
A well-communicated trajectory is as important as the numbers themselves. Investors and businesses need confidence that the path is credible and durable to make long-term low-carbon investments.
Trajectory Design Choices
Several design choices affect how trajectories work:
Review frequency
How often is the trajectory revisited? Too frequent, and uncertainty increases. Too infrequent, and the system cannot adapt.
Adjustment triggers
What events trigger trajectory reassessment? New climate commitments, sustained price levels, economic shocks?
Grandfathering protection
Are existing allowances protected if the trajectory tightens, or can they be cancelled?
Sunset provisions
When does the system end? Or is it designed to continue indefinitely toward net-zero?
Aligning with Net-Zero
For 2050 net-zero goals, ETS trajectories must eventually reach zero emissions from covered sectors. This raises questions:
What is the end point?
Zero allowances? Or some residual for hard-to-abate sectors with offset provision?
When does decline accelerate?
Linear decline to zero by 2050 means steady annual cuts. Is this realistic, or should decline be back-loaded with steeper cuts after 2040?
What about residual emissions?
Some sectors may not reach zero. Should the ETS incorporate removals or offsets for the last 10-20%?
Pathways to 2050 net-zero:
| Scenario | 2030 cap | 2040 cap | 2050 cap |
|---|---|---|---|
| Linear decline | 60% of baseline | 30% of baseline | 0% |
| Back-loaded | 70% of baseline | 40% of baseline | 0% |
| Net-zero with offsets | 50% of baseline | 20% of baseline | 10% (offset) |
Each path has different implications for near-term action, technology needs, and economic adjustment.
Looking Ahead
With the cap and trajectory set, the next major question is how to monitor and verify that emissions stay within limits. The next lesson explores MRV systems for emissions trading.