This lecture provides evidence for why climate change is a planetary boundary, based on temperature changes since the Industrial Revolution. We have already seen 1 degree Celsius of warming over the last 100 years, and are moving rapidly towards three or four degrees. Sea level rise is also evidence for a climate boundary. Professor Rockström makes use of IPCC data to show the risk of human-induced catastrophic tipping points with water, oceans, and biodiversity that result from climate change. The uncertainties of the implications of climate change are so great that applying a precautionary principle is very important. We are quickly approaching levels of 450 ppm of carbon dioxide in the atmosphere, much higher than historical amounts, that lead to climate forcing. The appropriate boundary is therefore set at 350 ppm of carbon dioxide in the atmosphere.