Signal Atmospheric CO2 at 426 ppm against a 350 ppm boundary

Summary

Atmospheric carbon dioxide, the first control variable of the climate boundary, stands at 426 ppm in the 2026 assessment, up from 417 ppm in the 2023 assessment. That is roughly 50 percent above the pre-industrial Holocene level of about 280 ppm, far beyond the proposed safe boundary of 350 ppm and approaching the high-risk line of 450 ppm. The 350 ppm boundary rests on paleoclimate evidence and modeling, beyond which risks of irreversible change such as large-scale polar ice sheet melt rise significantly. It also aligns with the Paris Agreement goal of limiting warming to 1.5°C. Carbon dioxide from fossil fuels, deforestation and cement production keeps rising and remains the dominant driver of climate change. The report presents the 426 ppm figure as a measure of how strongly the climate boundary has been transgressed.