The earth has experienced only five mass extinctions in the past 600 million years, and all of these are believed to have been initiated from outside the earth by meteor impact. We are now in the sixth extinction crisis, which differs from these because the cause is on the earth (humans) and we have the power to stop it. Present extinction rates are 1,000-10,000 times higher than the average background rate without humans, on the order of 20,000 species per year. We could lose 10% of all species by 2025 and 50% by 2100. Some have likened this loss of life to burning a library without reading the books.