More than 90% of climate scientists attribute the increase in global temperature over the past 30-40 years to greenhouse gases that humans have been adding to the atmosphere since the beginning of the Industrial Revolution in the 1700s. The great majority of these scientists agree that if this warming continues, it presents significant risks to humankind and all life on Earth: to our cities and towns, our water and food supplies, and our health. How do we know that this warming is happening and that it’s caused by humans? How strong is the evidence? What risks can we expect and what can we do about them?