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How do we track a changing climate? Among all the charts on our site, the one here reaches further back in time than almost any other: the concentration of CO₂ in Earth’s atmosphere over the last 800,000 years. For most of that time, CO₂ rose and fell between roughly 180 and 300 parts per million (ppm), driven by natural cycles in Earth’s orbit. We know this from cores of Antarctic ice, which trapped tiny bubbles of ancient air as it formed. By 2025, the concentration — now measured by NOAA’s Global Monitoring Laboratory — reached 426 ppm, higher than at any point in that record. Concentrations over the past decade have been rising at 2 to 3 ppm each year. This dramatic increase in CO₂ is mainly from human activity; this and other greenhouse gas emissions are the primary drivers of current climate change. CO₂ concentration is just one of the indicators we use to track a changing climate. Others include global temperatures, ocean heat content, and snow cover. Each of these is compiled by a dedicated scientific institution, from the UK’s Met Office Hadley Centre to NOAA in the US. Our colleague Pablo Rosado recently updated our climate charts with the latest data from these sources, something he does every month.