Is methane responsible for nearly one-third of all global warming to date?

Question

A recent BBC article says that the latest IPCC report says methane is responsible for 0.3°C out of 1.1°C of all cumulative warming since pre-industrial times.

According to the IPCC, around 0.3C of the 1.1C that the world has already warmed by comes from methane.

I searched the report for "0.3" and "methane" and didn't find that.

Is this claim true?

UPD: This is THE report, actually. I didn't find it there too, though (I searched for "0.3°C")

Answer 1

Yes, this seems to be (approximately) true. According to this paper from 2016. The total radiative forcing due to changes in the carbon dioxide concentration is roughly 1.6 watts per square meter (see figure 3), the same figure for methane is about 0.65 watts per square meter (allow for some error margin due to reading from a graph by eye). This corresponds to methane being responsible for just over 1/3 of the global warming effect, which correlates with the 0.3 °C of 1.1 °C mentioned by the BBC.

I have not (yet) been able to find an explicit mention of this particular statement in the IPCC report. The report does however extensively discuss the role of (reduction of) methane emissions in managing future temperature rises (and cites the above paper). Of particular importance is the fact the methane stays in the atmosphere much shorter than carbon dioxide. A consequence of this is that effectively global warming due to CO₂ is proportional to the cumulative emission of CO₂ over human history, while the effect of methane is pretty much proportion to the current rate of emission. This means that we can obtain much more immediate (and relatively short term) effects on the global temperature by limiting methane emissions, while limiting CO₂ emissions gives a much slower (but long term effect). Methane can thus play an important role managing global warming while we are waiting on the effects of policy changes in CO₂ emissions.

You were looking at the wrong IPCC report. The linked report is the special report from 2019, while the BBC article is about the first part of AR6 released yesterday, https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/.

The BBC is most likely referring to (and slightly misquoting) the data in Figure SPM.2. This figure says that 0.5 °C of the observed rise in temperature can be attributed to methane, which is one third of the total 1.5 °C of temperature rise that can be attributed to greenhouse gasses. This is slightly higher than the sum of 1.1 °C temperature rise due to human influences, which also includes some other net negative effects. The BBC likely took the 1.1 °C total change, and one third attributable to methane and multiplied then (erroneously) to get the 0.3 °C figure.

Answer 2

I think so, but only if you count total warming as overall warming + cooling effects.

The critical numbers, from pages 98 - 100 of the report (units = Watts per square metre of earth's surface):

• The accumulation of energy is driven by a positive total anthropogenic effective radiative forcing (ERF) relative to 1750. The best estimate ERF of 2.72 W m^-2
• The ERF due to methane emissions is 1.21 [0.90 to 1.51] W m^-2
• Aerosols contributed an ERF of –1.3 [–2.0 to –0.6] W m^-2

Note aerosols have a cooling effect, so the sum of warming effects could be said to be 2.72 + 1.3 = 4.02. 1.21/4.02 = 0.3, which is close enough to the BBC figure to make sense.