When fossil fuel energy was discovered, the timing and intensity of the resulting climate impacts depended on what the natural CO₂ concentration in the atmosphere was at that time. The natural CO₂ concentration is thought to be controlled by complex, slow-acting natural feedback mechanisms, and could easily have been different than it turned out to be. If the natural concentration had been a factor of two or more lower, the climate impacts of fossil fuel CO₂ release would have occurred about 50 or more years sooner, making it much more challenging for the developing human society to scientifically understand the phenomenon of anthropogenic climate change in time to prevent it.