The impact of natural and anthropogenic forcings on past and present global climate change

Understanding and quantifying natural climate variability is necessary to detect and attribute anthropogenic warming and to predict future climate change. It is important to extend the evaluation of models used for climate projections through the pre-industrial period when natural variations were pronounced while anthropogenic influence was small. The four main relevant forcings (greenhouse gases, solar variability, volcanism, land-use change) have different time-dependence over long periods, so can be separated more effectively than for the shorter instrumental period. The potential role of solar variations in modulating recent climate has been addressed in his paper. We used a climate model to analyze past climatic responses to solar and volcanic forcing, using a solar irradiance history partially based on a recent 10Be findings from Antarctica. Our results suggest that, while solar irradiance changes and volcanism were the dominant forcings in preindustrial times, their combined role has been changing over the past century. Although these natural forcing factors could be responsible for some modification of the decadal structure over the 20th century, they only played a minor role in the most recent warming. Therefore, the 20th century warming is largely caused by anthropogenic forcing.