9.3 Understanding Pre-Industrial Climate Change
9.3.1 Why Consider Pre-Industrial Climate Change?
The Earth system has experienced large-scale climate changes in the past (Chapter 6) that hold important lessons for the understanding of present and future climate change. These changes resulted from natural external forcings that, in some instances, triggered strong feedbacks as in the case of the LGM (see Chapter 6). Past periods offer the potential to provide information not available from the instrumental record, which is affected by anthropogenic as well as natural external forcings and is too short to fully understand climate variability and major climate system feedbacks on inter-decadal and longer time scales. Indirect indicators (‘proxy data’ such as tree ring width and density) must be used to infer climate variations (Chapter 6) prior to the instrumental era (Chapter 3). A complete description of these data and of their uncertainties can be found in Chapter 6.
The discussion here is restricted to several periods in the past for which modelling and observational evidence can be compared to test understanding of the climate response to external forcings. One such period is the last millennium, which places the recent instrumental record in a broader context (e.g., Mitchell et al., 2001). The analysis of the past 1 kyr focuses mainly on the climate response to natural forcings (changes in solar radiation and volcanism) and on the role of anthropogenic forcing during the most recent part of the record. Two time periods analysed in the Paleoclimate Modelling Intercomparison Project (PMIP, Joussaume and Taylor, 1995; PMIP2, Harrison et al., 2002) are also considered, the mid-Holocene (6 ka) and the LGM (21 ka). Both periods had a substantially different climate compared to the present, and there is relatively good information from data synthesis and model simulation experiments (Braconnot et al., 2004; Cane et al., 2006). An increased number of simulations using EMICs or Atmosphere-Ocean General Circulation Models (AOGCMs) that are the same as, or related to, the models used in simulations of the climates of the 20th and 21st centuries are available for these periods.