cover (leaving darker water and vegetation exposed), and more water vapor (a major greenhouse gas) in warmer air.
During the ice ages of the past 800,000 years and more, and further back to warmer times for 15 million years before then, Earth’s surface and oceans have warmed and cooled. The relationship between CO2 levels and temperature (with some variation) has been much the same in warmer and colder eras. With today’s CO2 levels in the air, Earth then (say, 14 million years ago) was 5°C hotter than the 20th century average. In the future, if existing trends continue, global temperatures are expected to rise 3-4°C by 2100. This would lead, among other things, to more extreme weather events, rising sea levels, less food and fresh water, and different ecological and socio-economic challenges. Beyond 2100, if CO2 and methane emissions phase out only slowly, due to feedbacks, temperatures are expected to increase to more than 9°C (above 20th century average) by 2400, with CO2 almost 3 times today’s, including feedback (such as thawing permafrost) CO2 and methane increases from warming. If we phase out fossil fuel emissions by 2050, CO2 levels are projected to stabilize at 7°C above the 20th century average. With a strong added effort to cool Earth - featuring CO2 removal and re-brightening Earth (using a mix of the many methods available) - soon enough - temperature rise can be held below 3°C and possibly below 2°C.
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