Today, there is more CO2 in our air than during the past three million years. In Greenland and Antarctica, ice sheets are losing 125 billion tons of their mass each year. The changes humans have inflicted upon Earth's atmosphere and climate over the millennia have been dramatic and often catastrophic. But new technological breakthroughs like carbon capture, global shifts in attitudes about clean energy, and even changes to dietary behavior have given scientists and environmentalists hope that can reverse some of the damage we have done.