Methane, GWP-20 vs GWP-100, and Why Climate Targets Keep Tightening

Two different gases warm the planet in two very different ways, and greenhouse gas accounting collapses that difference into a single number called CO₂-equivalent. It is a necessary simplification. It is also the source of persistent confusion, and occasionally of genuinely misleading corporate claims.

What GWP actually measures

Global Warming Potential answers a specific question: if I release one kilogram of this gas, how much more heat does it trap over the next N years than one kilogram of CO₂ would?

The time horizon is not a technical detail — it is the whole argument. Carbon dioxide, once emitted, persists for centuries; a meaningful fraction is still influencing the climate a thousand years later. Methane is destroyed in the atmosphere with a lifetime of roughly 12 years, but while present it is a dramatically more effective absorber of infrared radiation.

So the comparison depends entirely on how long you watch:

Horizon	Methane GWP (IPCC AR6, fossil)	What it captures
20 years	~82.5	Near-term warming while the methane is still present
100 years	~29.8	Long-run average, most of it after the methane is gone
500 years	~10	Almost entirely dilution of a pulse that ended centuries earlier

Why the choice can move a footprint by 3×

For most companies this is academic — methane is a rounding error next to CO₂ from energy use. For a subset it is decisive.

Consider a company with 100,000 tCO₂e of CO₂ emissions and 1,000 tonnes of methane:

• At GWP-100 (~30): 100,000 + 30,000 = 130,000 tCO₂e. Methane is 23% of the footprint.
• At GWP-20 (~83): 100,000 + 83,000 = 183,000 tCO₂e. Methane is 45% of the footprint.

Same physical emissions, same year, a 41% difference in the reported total — and a complete change in where abatement capital should go. Under GWP-100 this is an energy company with a methane side-issue. Under GWP-20 methane is nearly half the problem.

In India, the businesses where this bites are specific: landfill and waste operators, wastewater treatment, coal mining, gas distribution, dairy and livestock supply chains, and rice-based food companies. If you are in one of those, the horizon choice is a live commercial question, not a footnote.

What to actually do

Report GWP-100 as your headline. The GHG Protocol specifies it, BRSR and CDP expect it, and SBTi validates against it. Deviating makes your figures incomparable and invites the suspicion that you chose the flattering number.

Use current AR6 values. Many inventories still run on AR4 (GWP-100 of 25) or AR5 (28) because that is what the template said years ago. AR6 is the current science. State explicitly which assessment report your factors come from — this is a common and easily fixed audit finding.

Disclose the GWP-20 view if methane is material. For a landfill operator or dairy company, a supplementary GWP-20 figure demonstrates that you understand your near-term impact. Presented as additional context it reads as rigour. Presented instead of GWP-100, it reads as manipulation.

Never mix horizons within one number. Every tonne in a single reported figure must use the same horizon and the same assessment report. Mixed-basis totals are meaningless and will not survive assurance.

The carbon budget, and why targets only tighten

The second half of this story is about why any of it is enforced.

The remaining carbon budget is the total additional CO₂ that can be emitted while keeping warming below a threshold with a stated probability. It follows from a robust physical result: peak warming is close to proportional to cumulative CO₂ emissions. Not the rate — the total.

That makes the budget a finite, depleting quantity, and it sits underneath everything:

• National NDCs are, implicitly, claims on shares of it.
• India's CCTS emission-intensity targets are calibrated against a national trajectory derived from it.
• Science-based targets are explicitly defined as a company's fair share of it.
• CBAM exists because a carbon price without border adjustment leaks budget consumption offshore.

Where natural feedbacks come in

Here is the connection back to the science. The budget is defined by total warming, not warming from human sources alone. Emissions from natural systems that increase in response to warming consume the same finite headroom — and they answer to no regulator.

This is why the 2026 Science study on Siberian methane is more than a curiosity. It documents Siberian methane emissions rising by roughly 10.4 Tg per year per decade over 2010–2023, driven by warming-induced wildfire and wetland methanogenesis rather than industry. Its projections — explicitly conditional sensitivity estimates, not forecasts — put eastern Siberian emissions at 97.5 ± 32.0 Tg per year by 2081–2100 under SSP3-7.0, and 169.2 ± 54.1 Tg per year under SSP5-8.5.

The implication for planning is straightforward and does not require accepting any particular projection: if natural sources take a larger share of the budget than models assumed, the share left for everyone else is smaller. That translates into faster tightening — of national targets, of CCTS benchmarks, of what a "science-based" corporate trajectory requires.

What this means for planning

Assume tightening, not stability. A decarbonisation plan built on today's obligation levels is planning for a world that will not exist. Build headroom.

Front-load abatement. Because warming responds to cumulative emissions, a tonne cut in 2027 is worth more than the same tonne cut in 2037 — it removes warming for the intervening decade. Delay has a physical cost, not only a regulatory one.

Treat methane as a near-term lever. Its short lifetime cuts both ways: it means GWP-100 flatters methane emissions, and it means methane cuts deliver benefit unusually fast. Reduce methane today and the atmospheric concentration responds within roughly a decade, unlike CO₂. For companies with material methane, it is often the cheapest near-term climate return available.

Set targets against the science, not the current rulebook. A target that merely meets today's compliance threshold will need revising. One built on a science-based trajectory is durable — which is what makes SBTi validation worth the effort rather than a badge.

Know your methane exposure before you are asked. If methane is material and you have never examined it closely, that is the gap most likely to produce an unpleasant surprise — from an assurance provider, a buyer, or a satellite.

Carbon Credit Consulting builds greenhouse gas inventories on current AR6 factors, and helps companies set costed, science-aligned targets that survive tightening compliance. Explore our ESG & BRSR reporting and GHG accounting services, or talk to us.