LATE-19TH CENTURY TO EARLY 21ST CENTURY BEHAVIOR OF SOUTHERN ALASKAN GLACIERS

One of the most significant indicators that southern Alaska regional climate has been changing during the period from the late-19th to early-21st century has been a comprehensive photographic record documenting variations in the distribution and extent of the region's glaciers. Historical photography of many of these glaciers began in the mid-1880s, resulting in a visual record that depicts characteristics of many Coast, St. Elias, Chugach, and Kenai Mountains glaciers for at least half of the post-Little-Ice-Age period. By 1920, photographs of more than 200 southern Alaskan glaciers existed. By the late 1940s, all had been photographed.

Weather station observations have documented that mean annual temperatures throughout Alaska have increased an average of ~2.0^o C during the past six decades. However, the photographic record shows that long before an instrumental record was available, nearly all of the larger valley glaciers throughout this region were thinning and retreating. Their response, with loss of length, area, and volume began in some regions as early as the mid-18th century. These observations suggest that a significant temperature increase began to impact glacier health much earlier. Other data that reinforce this observation include early maps depicting glacier extent, historical naratives, and vegetation analysis.

In most areas analyzed, every glacier that descends below an elevation of ~1,500 m is currently thinning and/or retreating. Many glaciers have an uninterrupted history of continuous post-Little-Ice-Age retreat and thinning that spans more than 250 years. Other glaciers are characterized by multiple late-19th to early-21st century fluctuations. Today, more than 98% of the glaciers examined are thinning and/or retreating.

In southern Alaska, only about a dozen lower elevation glaciers are currently advancing and thickening. Many of these advancing glaciers are or formerly were tidewater glaciers. Several of these glaciers have been advancing and thickening for more than two centuries. This presentation documents the post-Little-Ice-Age behavior and variability of the response of many southern Alaskan glaciers to changing regional climate.