The two primary natural causes of climate change and variations over the past century are changes in the sun’s energy and the emission of sulphate aerosols into the stratosphere through explosive volcanic eruptions. The intensity of solar irradiance reaching the earth’s surface has been reconstructed through isotopic analysis of tree rings. These indicate that the solar intensity has increased during the last few centuries until about 1950. Since then, it has varied on decadal time scales (in keeping with phases of the sun-spot cycle), but the mean value has not changed significantly. The net radiative forcing of global climate due to increased solar activity since 1750 is estimated at about 0.3 W/m2, with an uncertainty range of +/- 0.2 W/m2. Hence, there is still very low confidence in this value. Volcanic eruptions tend to cool surface climates because the increased stratospheric concentrations of sulphate aerosols that they cause increase the reflection back to space of incoming sunlight. This negative radiative forcing is relatively short-lived (up to 5 years after eruption) but can cause periods of sustained cooling if such eruptions occur regularly. During the past century, the frequency of such eruptions declined between 1900 and 1950 (this reducing the average cooling effect), then began to increase again in the second half of the century (causing a radiative forcing towards cooler surface temperatures). The net effect of natural climate forcings during the past century, therefore, appears to have had a net surface warming effect up to 1950, and a net cooling effect during the past 50 years.
• from: Environnement Canada • in detail XlnkS5F5 XlnkC1797