Over the past 200 million years, there have been significant changes in the CO₂ concentration of the atmosphere. However, more recently over the past several million years, atmospheric CO₂ is thought to have been relatively low. We know from ice-core records that over the last 400,000+ years, atmospheric CO₂ concentration varied between 180 and 280 ppm. Since the dawn of the Industrial Revolution, atmospheric CO₂ levels have increased. The most dramatic and significant increases during this period have occurred since about 1960.