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

Understanding and quantifying natural climate variability is a prerequisite to detect and attribute anthropogenic warming and to project 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. In anticipating future climate change, there are three main sources of uncertainty. 1) We do not know the future anthropogenic emissions and resulting atmospheric concentrations of greenhouse gases and aerosols. 2) The response to greenhouse gas and aerosol forcing differs between various models, simulated regional climate changes being particularly model-dependent. 3) In addition to anthropogenic forcing, climate changes are induced by natural forcing (e.g., volcanoes and variations in solar activity) as well as by unforced internal variability in the climate system. The tools most commonly adopted for projecting future climate are coupled atmosphere-ocean general circulation models (AOGCMs). These numerical models provide a comprehensive three-dimensional representation of the climate system, describing the main dynamical and physical processes, their interactions and feedbacks. They can generate regional estimates of climate in response to given changes in greenhouse gas and aerosol concentrations. The four main relevant forcing (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 debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Century-scale solar irradiance variations have been proposed as cause for past climatic changes. However, recently, astronomical evidence has been used to suggest that low-frequency variability of solar irradiance might be very low, possibly restricted to the range of the observed high-frequency variability. 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 forcing 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 not a reflection of a rebound from the last Little Ice Age cool period, but it is largely caused by anthropogenic forcing. A small role of solar forcing for late 20th century climate change is additionally supported by the absence of a trend in the satellite-based irradiance record covering the past 30 years. In conclusion, our model results indicate that the range of Northern Hemisphere temperature reconstructions and natural forcing histories (cosmogenic isotope record as a proxy for solar forcing, and volcanic forcing) constrain the natural contribution to 20th century warming to be +0.2°C. Anthropogenic forcing must account for the difference between the small natural forcing and the observed warming in the late 20th century.