CO2, but from where?
Geologists have many records that tell the story of the last glacial maximum, the time between about 20,000 and 15,000 years ago when the glaciers of the last ice age reached their peak size and started to retreat. Ice cores, sediment cores, records of plants, soil, wind-blown loess deposits, ice-rafted debris in the ocean, etc. One story told over and over is that CO2 in the atmosphere went up significantly, from about 180 ppm to 280 ppm (for comparison, we’re currently very close to 400 ppm). That CO2 pulse into the atmosphere warmed the planet and created a runaway process that melted the glaciers.
One big question has always remained though; where did this CO2 come from? We know where the CO2 pulse today is coming from; fossil fuels, but 15,000 years ago there were no coal plants.