Climate variations are not unusual. The Earth has swung between ice ages and warm periods for hundreds of thousands of years. But the rate and the extent of current warming go beyond anything in recent geologic time. The overwhelming consensus of scientists is that changes in climate since the start of the industrial revolution 150 years ago cannot be attributable to natural causes. Industrial civilization—driven by burning fossil fuels—then becomes the common denominator. Burning coal, oil and gas releases heat-trapping carbon dioxide. Precise scientific measurements reveal CO2 levels in the atmosphere have been steadily climbing as more of these fuels are burned. When fossil fuels are burned, they release CO2 and other greenhouse gases that trap heat in the atmosphere, making them the primary contributors to global warming and climate change.