The greenhouse effect provides a natural shield that helps make the Earth habitable. Without it, the average temperature of the planet would be about minus 18°C. Since the Industrial Revolution however, carbon dioxide, methane, nitrous oxide and other greenhouse gases have been accumulating in the atmosphere and causing excessive warming. Over the last 130 years, the Earth has warmed by 0.85°C, and in the absence of global measures to reduce CO2 and other emissions from human activity, the temperature could rise by 4°C by 2100. An increase of this magnitude could have a serious impact on the world's populations by causing droughts, floods, rising sea levels and the extinction of plant and animal species.