Defining 'pre-industrial climate': Past global changes during the most recent two millennia

The past two thousand years is a key interval for climate science because the period encompasses both the era of human-induced global warming and a much longer interval where Earth's climate was governed principally by natural forcings. This earlier 'pre-industrial' period is particularly important for two reasons. We now have a rich archive of proxy data that span the past two millennia and that are well-dated, have high temporal resolution, and show sensitivity to climate. Further, an improved understanding of pre-industrial climate provides a point of comparison for present-day climate change, sets real-world targets to evaluate the performance of general circulation models, and allows us to address many other questions that cannot be answered using only a century and a half of observational data. Here we highlight the activities of the PAGES 2k Network, an international collaborative effort focused on global climate change during the past two thousand years. These projects span climate variability from the tropics to the poles, investigate the multidecadal signals contained in the records, integrate climate reconstructions with archaeological evidence, and combine climate data with models. We also discuss those aspects of pre-industrial conditions (including both past climate changes and past climate forcings) that are not yet well constrained, and suggest potential areas for research on this period that would be relevant to the arc of Earth's future climate.