Our Sun is a rather quiescent star

 

Special Code (PLPL 7 XX KJH 547 MNBV 8 UUYU )

Climate sensitivity is a measure of the effect of rising levels of greenhouse gases on Earth’s temperature. It is usually defined as the eventual increase of global average temperature after a doubling of CO2 concentration in the atmosphere compared to pre-industrial levels.

In this paper, we conclude that the estimate of aerosol climate forcing () by the United Nation’s scientific advisory body (the Intergovernmental Panel on Climate Change, IPCC) is an underestimate, and thus the Faustian bargain is worse than expected. We also show that IPCC’s emphasis on global climate models led to a marriage of aerosol forcing and climate sensitivity, such that underestimate of aerosol forcing led to underestimate of climate sensitivity. The result is a double whammy that helps explain global warming acceleration and alters projections of future climate, magnifying the danger of intergenerational injustice. The delayed response of climate still allows a potential bright future for today’s young people, but that happy result requires understanding of the factors driving climate change. These issues can be readily understood via the most basic concepts, beginning with climate forcings.

Climate Forcings

Climate forcings are imposed changes of Earth’s energy balance. If the Sun suddenly became 1% brighter, for example, that would be a forcing of +2.4 W/m2 (W/m2 is watts, a measure of energy transfer over time, per square meter) because Earth normally absorbs 240 W/m2 of solar energy averaged over Earth’s surface. Solar variability is one of the two natural climate forcings that are important on time scales of years to centuries, the other being large volcanic eruptions that inject gases and aerosols into Earth’s stratosphere (at about 15-50 kilometers, 10-30 miles). It is helpful to compare these well-understood natural forcings with human-made climate forcings.

Our Sun is a rather quiescent star, in the family of all stars, yet the variability of dark areas (sunspots) on the solar surface has long caused suspicion that the Sun may drive climate change on Earth. Fortunately, NASA has done a good job of monitoring the solar energy received at Earth since the late 1970s. Variations of the Sun’s brightness during the solar sunspot cycle are about 0.1% (), a forcing change of about 0.25 W/m2 between solar minimum and solar maximum outputs. This solar forcing, we will show, is much smaller than human-made forcings, but large enough to be a partial cause of present climate extremes.