Glacier Evidence against the Medieval Warm Period In Western North America

Some researchers have argued for a one- to two-century period of warm climate during medieval time. This time of purportedly warm climate has become known as the Medieval Warm Period. Dating of the Medieval Warm Period is uncertain, but it is generally placed sometime between the 9th and 14th centuries. Other researchers, however, have questioned the global significance and synchroneity of the Medieval Warm Period, arguing that temperatures during that time were not as warm as today. Here, we present new evidence showing that several glaciers in western North America were advancing during the Medieval Warm Period. Some of the glaciers reached extents similar to their maximum Holocene extents at the peak of the Little Ice Age. For example, an advance of Llewellyn Glacier, which drains the northeast sector of the Juneau Icefield, at the time of the Medieval Warm Period, cannot be reconciled with temperatures similar to those of today. This evidence suggests that temperatures were significantly lower than at present during the Medieval Warm Period, calling into question the existence of prolonged warmth at that time. We conclude that the Medieval Warm Period is at best an ill-defined term that encompasses a number of possibly unrelated climate anomalies.