The anthropogenic global warming hypothesis has its roots in climate reconstructions based on ice core records from Antarctica and Greenland. However, there are some periods in the recent geological past when results from field surveys elsewhere do not match the isotope-derived temperatures of the ice cores. We consider the Younger Dryas, 12.9 - 11.5 thousand years ago, with substantial anomalies, the so-called Mystery Interval, 14.5-17.5 years ago, and a longer period around 50 thousand years ago, known as "Maritime Isotope Stage 3 (MIS-3)". The large fluctuations of isotopes seem to correspond much better to fluctuations between large droughts and wet periods. This is at odds with the isotope thermometer of ice cores. We examine the hydrographic process of the water cycle and find support for a greater reliance on the isotope signature for humidity. However, when past temperature is no longer consistent with isotopes, it is also inconsistent with greenhouse gas concentrations, and thus the correlation between concentrations of carbon dioxide and methane with temperature expires. Finally, we link a warm MIS3 period with a modified version of Milankovitch cycles on Earth's orbit.

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