For his part, Hansen reached his conclusion through climate model calculation. In their climate model, the chief effect of the particles is to change the properties of clouds so that the clouds reflect more sunlight thereby compensating for the warming due to the buildup of the carbon dioxide. These results led Hansen to note that at least half, if not all, of the almost 1.5 degrees F warming that the Earth has experienced since 1850 is due to non-fossil fuel greenhouse gases, principally, methane, nitrous oxide, and chlorofluorocarbons. Consequently, Hansen proposed that since the growth rate of these non-fossil fuel greenhouse gases has slowed during the past two decades, and furthermore, since technology appears to make reductions in the emissions of these gases feasible, more emphasis should be placed on reducing the emissions of these gases rather than in reducing the emissions of the fossil fuel greenhouse gases, primarily carbon dioxide. The problem with this scenario is that it relies on the response of clouds to a buildup of haze particles and on how the clouds will change as the climate changes.
Clouds are complex.
Their properties are difficult to measure with certainty. These properties are even more difficult to predict with any reliability. The resulting differences in the outcomes from the different climate models simply reflect the large uncertainties that remain in assessing climate change. The 1.5 degree F rise in global temperature since 1850 is larger that any that has occurred during the past 1,000 years, to the extent that we can reliably reconstruct theses past climate records. In other words, people born today will live through a rise in temperature that rivals any during the past 1,000 years, or for that matter, any that have occurred since biblical times, again, to the extent that we can reliably reconstruct these past climate records.