Glacier variability in the Alps during the Little Ice Age - overview on course, evidences and causes

The Little Ice Age (LIA) was originally understood as a period of increased glaciation in the late Holocene. Today, the term is used to describe the multi-centennial glacier advance and maximum level period in the last millennium, but it is also used to refer to the contemporaneous cooler climatic conditions beyond glaciated areas. Glacier dynamics in the Alps during the last centuries of the LIA are especially known from historical documents, i.e., written and pictorial sources, which essentially date from around 1600 CE and cover some well-known glaciers. Today, these data are enhanced in particular by tree-ring analyses on remnants of trees buried during glacial advances, which can provide calendar dates for advances and glacier maxima, also for the early centuries of the LIA. Moreover, our knowledge of the LIA period is increasingly enhanced by regional climate reconstructions and analyses on climate forcings. The LIA in the Alps can be defined as the period between the onset of climate cooling, which led to a first LIA-type maximum of glaciers, and the last LIA maximum level generally observed around the middle of the 1800s, i.e., between 1260 and 1860 CE. The first LIA-type maxima are demonstrable for the 1300s, around 1320 and 1380 CE, and then further, often seven maxima for the period ca. 1600-1860 CE. Accordingly, and taking into account the climatic variability, the LIA can be divided into an early (ca. 1260-1380), intermediate (ca. 1380-1575) and main phase (ca. 1575-1860 CE). Compared to the preceding period of the Medieval Climate Anomaly, reconstructions demonstrate increased climatic variability for the LIA, marked by repeated and pronounced cooling phases that finally triggered the glacier advances. These climatic disturbances correlate remarkably directly with significant volcanic eruptions or phases of increased volcanic activity and, albeit less clearly, with periods of reduced solar insolation, which can be derived from reduced solar activity. Distinctive and historically documented glacier advance phases are often correlated with climatic disturbances following major volcanic eruptions, e.g., the advance period around 1820 CE is following the preceding volcanic events of 1809 and 1815 CE. Today, the LIA is not only the coolest multi-centennial period of the last 10,000 years but also the reference period for assessing the changes from a system of climate and glacier variability largely determined by natural factors to an environmental system clearly shaped by human activities.