Carbon dioxide in Earth’s atmosphere provides an important regulator of climate. Without it, or with too little, Earth would be completely frozen. But the rapid rise in carbon dioxide in recent times due to fossil fuel consumption and changes in land use has caused unprecedented warming with consequences to human civilization.

Understanding how Earth’s climate responds to atmospheric carbon dioxide is an important problem not only for anthropogenic climate change today but also for understanding Earth’s distant past (when the sun was fainter than today) and distant future (when the sun becomes brighter than today). In a paper by Eric Wolf, Brian Toon, and myself titled “Evaluating climate Sensitivity to CO2 across Earth’s history” and published in Journal of Geophysical Research – Atmospheres, we calculate the expected warming for early-, modern-, and future-Earth scenarios across a much wider range of carbon dioxide levels than typically considered for present-day climate change. We show that a doubling of atmospheric carbon dioxide would have caused a greater amount of warming on early Earth (when the carbon dioxide fraction of the atmosphere was high) compared to today (when carbon dioxide is a trace constituent). In general the amount of warming to be expected from such a carbon dioxide doubling (known as the “climate sensitivity”) depends upon the amount of solar energy received, the starting carbon dioxide budget, and the mean temperature of the planet.