The UK Met Office's annual atmospheric CO2 forecast puts the May 2026 peak at the Mauna Loa Observatory at 432.2 ± 0.6 parts per million, the largest single-year increment in the forecast's history and a number tracking in line with NOAA's daily readings through the first half of the month. [1][2] The number is the scale bar climate coverage routinely skips.

The May peak matters because the seasonal cycle of atmospheric CO2 — the Keeling Curve's annual sawtooth — produces its yearly maximum each May before the Northern Hemisphere growing season pulls carbon back into the biosphere. The May reading is therefore the floor against which next year's growth is measured. A 432.2 reading sets the floor at the highest May figure on record by about 3 ppm.

The El Niño coefficient is the variable that drove the forecast. The 2026 forecast was issued in January under the assumption that the 2025-26 El Niño would suppress tropical land carbon uptake; the Saturday running average of the daily Mauna Loa readings is consistent with the central forecast, not the lower bound. The Met Office's previous-year forecast was within 0.3 ppm of the observed peak.

The reader's takeaway is the number. 432.2 ± 0.6 ppm. The pre-industrial baseline was 280. The "safe" threshold the climate community talked about in the 1990s was 350. The current observation is 432. The threshold has not moved; the number has.

The paper will carry the NOAA confirmation when it prints the May monthly mean in the first week of June.

-- DARA OSEI, London