8 in 10,000 — Net Zero Without an Audit

The Number

3 jumps · click an arrow

CO₂ in air today

~420–430 ppm

about 4 in 10,000 molecules

Doubled CO₂

~560 ppm

about 8 in 10,000 molecules

Scenario shortcut

SSP5-8.5

high-emissions pathway, often used as a worst-case frame

Policy leap

Net Zero

economy-wide target, not a measurement

Jump 1 · The doubling benchmark

From a measured concentration to a hypothetical doubling.

Today's ~425 ppm is observed. ~560 ppm is a modelled benchmark used because it allows ECS calculations — the doubling itself isn't a measurement, it's a reference point for sensitivity studies.

Jump 2 · The scenario shortcut

From a benchmark to one of many scenarios — treated as the scenario.

SSP5-8.5 is the highest-emissions pathway among many SSPs. It's a stress test, not a forecast — but it routinely appears in framing as if it were the expected future.

Jump 3 · The policy leap

From a physics-based scenario to an economy-wide commitment.

Net Zero is a policy target, not a derivation from any number above. It encodes specific assumptions about which policies, which costs, and which trade-offs — and skips the audit chain that would make those assumptions visible (see Section 3).

Consequences if the audit is missing

8 risks · filter by type

Type

The missing quality chain

7 steps · toggle compression

Often compressed or skipped

View

CO₂
number

Mechanism

Forcing

Temperature
response

Impact
pathway

Policy
options

Net Zero
decision

5 audit steps skipped

A concentration number is not self-explanatory. It does not by itself justify a full economy-wide policy program.

Core Audit Questions

1

What is measured?

Direct atmospheric measurement (ppm at Mauna Loa et al) is the only fully-measured term in this chain. Everything downstream is derived, modelled, or chosen.
2

What is modelled or scenario-based?

Forcing, ECS, TCR, SSP5-8.5, and "8.5 W/m² by 2100" are all modelled or scenario-based. Each carries assumptions and uncertainty that the headline number doesn't.
3

What assumptions connect CO₂ to Net Zero?

A long chain: which feedbacks, which scenario, which damage function, which discount rate, which policy mix, which distributional choice. Each link is a separate audit.
4

What trade-offs and omitted costs matter?

The 8 consequences in Section 2 are the omitted costs. They don't appear in the headline argument but they show up in implementation.
5

What is the claim actually entitled to support?

The framework's signature question. The number licenses concern, not specific policy. Concern is the start of the audit, not its conclusion.

Net Zero without an audit can convert uncertainty into irreversible commitments.