Carbon Budget Optimization

If the carbon budget is finite and shrinking, the question is not just "how do we reduce emissions?" but "where does each tonne of avoided CO2 buy the most?" This reframes decarbonization from a moral imperative into an optimization problem with measurable tradeoffs.

The carbon budget for 1.5C (50% probability) was roughly 500 Gt CO2 as of 2020, shrinking by about 40 Gt per year. Every policy dollar spent on high-cost abatement (say, $300/tonne to decarbonize steel) is a dollar not spent on low-cost abatement (say, $20/tonne for coal-to-gas switching). If the budget is hard-capped, sequencing matters enormously.

Marginal abatement cost comparison. Replacing coal with natural gas in power generation abates at roughly $20-50/tonne CO2. Solar displacing gas abates at roughly $0-30/tonne (depending on integration costs). Direct air capture currently costs $400-600/tonne. Green hydrogen for steel: $100-200/tonne.

Should policymakers always fund the cheapest abatement first?

Not always, because learning curves matter. If green hydrogen costs $200/tonne today but could fall to $50/tonne at scale, early investment (while expensive per tonne) may unlock cheaper future abatement. Pure cost optimization ignores the dynamic effects of deployment on technology costs. The practical challenge is balancing immediate budget efficiency against long-term cost reduction, a problem that neither "cheapest first" nor "invest in everything" solves cleanly.

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