Structured Abstract
INTRODUCTION
Anthropogenic carbon dioxide (CO2) emissions have driven an increase in the global atmospheric CO2 concentration from 280 parts per million (ppm) before industrialization to an annual average of 419 ppm in 2022, corresponding to an increase in global mean surface temperature (GMST) of 1.1°C over the same period. If global CO2 emissions continue to rise, atmospheric CO2 could exceed 800 ppm by the year 2100. This begs the question of where our climate is headed. The geologic record is replete with both brief and extended intervals of CO2 concentration higher than today and thus provides opportunities to project the response of the future climate system to increasing CO2. For example, it has been estimated that global surface temperature 50 million years ago (Ma) was ~12°C higher than today, in tandem with atmospheric CO2 concentrations some 500 ppm higher (i.e., more than doubled) than present-day values. Consistent with these estimates, Antarctica and Greenland were free of ice at that time. However, reconstructing these values prior to direct instrumental measurements requires the use of paleoproxies—measurable properties of geological archives that are closely, but only indirectly, related to the parameter in question (e.g., temperature, CO2). To date, at least eight different proxies from both terrestrial and marine archives have been developed and applied to reconstruct paleo-CO2, but their underlying assumptions have been revised over time, and published reconstructions are not always consistent. This uncertainty complicates quantification of the climate responses to the ongoing rise of atmospheric CO2 concentrations.