High-resolution records of greenhouse gas concentrations from the Law Dome Antarctic ice core show small fluctuations in atmospheric CO2 and CH4, including a 10 ppmv decrease in CO2 and a 50 ppbv increase in CH4 during the Little Ice Age. Explanations for the biosphere-atmosphere carbon exchanges that caused these fluctuations, and their impacts on global climate, are lacking. We produce here a wetland index derived from the North American Pollen Database and show that for most of the past two millennia, this index is negatively correlated (antiphase) with atmospheric CO2 levels measured in the Law Dome ice core. Our analysis indicates the potential for global wetlands to act as carbon sinks, contributing to small fluctuations in atmospheric greenhouse gas concentrations, and thereby influencing submillennial-scale climatic change. Although our wetland index–CO2 relationship is statistically significant, the relationship between our wetland index–CH4 curve is not, possibly due to the confounding influence of wildfires on CH4.