evidence: The NSF-ICF reports that ice cores preserve evidence of much lower levels of atmospheric carbon dioxide than today. Since the start of the Pleistocene Epoch, roughly 2 million years ago, some periods in which glaciers retreated (called glaciations and interglacials) caused massive swings in carbon dioxide. For a period of at least 800,000 years, CO2 concentrations ranged from 180 to 300 parts per million, according to a high-resolution ice-core record from Antarctica. Since the start of the Industrial Age, however, human-caused greenhouse-gas emissions have steadily raised CO2 concentrations. The National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory reports that, as of the week beginning February 26, 2023, CO2 emissions stood at 421.91 parts per million. [...] Paleoclimatology relies on ice cores but also tree rings, cores drilled from lake and ocean sediments, and fossils. Piecing together all the evidence produces a picture of a planet that was often much hotter than today, but was cooler than the present climate for most of the last 11,000 years. Ice cores also show that current carbon dioxide concentrations are now higher than they have been for hundreds of thousands of years, and the higher concentrations are driven by human burning of fossil fuels.