Tuesday, January 22, 2013
Anthropogenic and Natural Forcing and Slowing Temperature Increase
The figure shows the total anthropogenic and natural radiative forcing and their sum. Anthropogenic forcing includes greenhouse gases and black carbon aerosols that have a warming effect and sulfate aerosols that have a cooling effect. Natural forcing is solar irradiance and sulfur aerosols emitted by large volcanic eruptions into the stratosphere. It is assumed that carbon dioxide, methane etc. are totally anthropogenic. This is of course not the case as atmospheric concentrations depend on the rate of removal of these gases and our results will show that the concentrations are endogenous. Based on these data anthropogenic forcing was very low until around 1970 after which it has increased monotonically. The forcing due to sulfur and black carbon aerosols is very uncertain. The estimated effect of sulfur is quite large here and that of black carbon is low compared to the most recent estimates. If sulfur has less cooling effect and black carbon more warming effect the increase in anthropogenic radiative forcing would be greater, especially in earlier years.
Natural forcings are dominated by large volcanic eruptions. In the last decade total forcing has been fairly constant due to a decline in natural forcing and something of slowing in the anthropogenic forcing. This might explain the relative hiatus in global temperature increase in this period (Hansen et al., 2013; Kaufmann et al., 2011). This chart shows global temperature and ocean heat content (upper 700m) over the same period:
But as the system has long lags, attributing changes in this way is probably more tempting than true.