The dramatic nature of global warming captured world attention in 2001, when the IPCC published a graph that my co-authors and I devised, which became known as the “hockey stick.” The shaft of the stick, horizontal and sloping gently downward from left to right, indicated only modest changes in Northern Hemisphere temperature for almost 1,000 years—as far back as our data went. The upturned blade of the stick, at the right, indicated an abrupt and unprecedented rise since the mid-1800s. The graph became a lightning rod in the climate change debate, and I, as a result, reluctantly became a public figure. In its September 2013 report, the IPCC extended the stick back in time, concluding that the recent warming was likely unprecedented for at least 1,400 years.
Most scientists concur that two degrees C of warming above the temperature during preindustrial time would harm all sectors of civilization—food, water, health, land, national security, energy and economic prosperity. ECS is a guide to when that will happen if we continue emitting CO2 at our business-as-usual pace.
These findings have implications for what we all must do to prevent disaster. An ECS of three degrees C means that if we are to limit global warming to below two degrees C forever, we need to keep CO2 concentrations far below twice preindustrial levels, closer to 450 ppm. Ironically, if the world burns significantly less coal, that would lessen CO2 emissions but also reduce aerosols in the atmosphere that block the sun (such as sulfate particulates), so we would have to limit CO2 to below roughly 405 ppm.