Assuming we follow a “business as usual” approach and don’t make dramatic changes to our energy mix, conservative projections are that we’ll reach that first doubling to 560 ppm by the year 2050, increasing to 900 ppm by the year 2100, and even more beyond that. The first doubling of CO2, concentration would have taken about two centuries, and the second one only about seventy years. In other words, our business-as-usual scenario has us pumping so much carbon dioxide into the atmosphere that the temperature rise would accelerate (be faster than linear) despite the exponential/linear relation between CO2 and temperature.
What’s more, CO2 –based greenhouse warming is just one forcing. As sea ice melts, more of the sun’s energy will be absorbed by the planet. As Arctic permafrost peat bogs thaw, vast quantities of methane will be released into the atmosphere—and they’ll be joined by further releases from melting methane clathrates* in the ocean. (Remember that methane is a greenhouse gas more than twenty times stronger than carbon dioxide!) Rainforests in some regions will release their stored carbon as they dry out. Same goes for broad swaths of land that will experience desertification. More water vapor will enter the atmosphere due to increased evaporation, and remember water vapor is a potent greenhouse gas itself, thereby warming things further. These are examples of positive feedback cycles that will amplify warming, accelerating it beyond that already accelerating warming from our own emissions.
Yes, Brad, me worry. – Excerpt from book: “How to Change Minds about Our Changing Climate” by Darling Sisterson