Climatic changes on time scales of years to centuries have the most direct impact on society and humanity. Yet changes on these time scales are hard to predict because we have yet to isolate climate forcing factors in this frequency range. Project ARRCC aims to evaluate past variations in climate on annual to century time scales in order to develop a better understanding of the types and magnitude of the changes observed as well as to evaluate two potential causes: changes in solar activity and the effects of volcanic aerosol input into the atmosphere. Project ARRCC will produce detailed time series of climate variations for the last 500 years, an interval which includes the Little Ice Age. During this interval there was also a significant reduction in the number of sunspots (the Maunder Minimum) leading many researchers to suggest some causal relationship. The PIs in Project ARRCC will synthesize the quantitative paleoclimate data for this time interval and compare the results to model simulations which vary the intensity of incoming solar radiation and the reflectivity of the atmosphere because of changes in volcanic aerosol concentration. This research is important for three reasons: 1) it will be the first synthesis of this important paleoclimate event; 2) it will produce a rigorous, global data base of past climate for use by other researchers; and 3) it will be the first rigorous attempt to quantify the effects of solar radiation and volcanic activity on climate change. This research is a high-priorty for the US Global Change Research Program.//
