A number of different and complex factors influence changes in the global climate over long periods of time. A. In the absence of instrumental records, proxy data allow scientists to infer information about past climates. B. Scientists see a consistent pattern in the global temperature variations that have occurred in the past. C. Computer models are used to estimate how the different causes of climate variability combine to account for the climate variability that occurs. D. Scientists have successfully separated natural climate variation from changes related to human activities. E. Scientists believe that activities outside the global climate system, such as volcanoes and solar activity may have significant effects on the system. F. Scientists have concluded that human activity accounts for the rapid global warming in recent decades.