Tropical Glacier-Climate-Interaction on Long Timescales

Glaciers throughout the world, including tropical glaciers, have been shrinking over most of the 20th century. Generally, this is attributed to global warming. While climate-glacier interactions in the mid and high latitudes are reasonably well studied, glaciers in the tropics are typically harder to access, situated in a region characterized through extremely sparse observations, and therefore less well understood. Recent glaciological studies have lead to the conclusion that tropical glaciers are often more sensitive to variability in moisture than temperature, and connections have been drawn between glacier mass balance and large-scale climate modes, e.g. variability of the Indian Ocean Dipole Mode influencing mass balance on Kilimanjaro, Tanzania, and low-frequency modulation of El Nino/Southern Oscillation influencing the glaciers of the Cordillera Blanca, Peru. However, the respective roles of anthropogenic (e. g. emission of greenhouse gases) or natural (e. g. volcanism) changes to the forcing of the climate system, internal variability, or interactions of these two, remain unclear. In order to understand the currently observed strong retreat of tropical glaciers around the world, and to assess their prospect over the next century, it is important to understand the climate dynamics governing atmospheric conditions at the site of these glaciers, i.e., in the tropical mid troposphere. The project will combine the best available glaciological observation from three continents, reconstructions of past glacier extent, and multi-century ensemble climate reconstructions from four different state-of-the-art, global coupled climate models in order to gain insight into the large scale climate dynamics governing the conditions at the glacier sites and acting on glacier timescales. The modes of multidecadal to centennial climate variability will be identified, and the past atmospheric variability and its effect on the glaciated sites will be reconstructed. These reconstructions will be validated using direct observations, and proxy-reconstructions of atmospheric state and the glacier extent. The relative importance of internal variability and anthropogenic influence during the 20th century will be assessed. This will lead to conclusions about long-term predictability, and - if deemed possible - to estimates of the most likely atmospheric conditions that the glaciers will experience during the 21st century.

Glaciers are important indicators of climate change. Their widespread and widely documented retreat is a response to slow changes in climate, while their yearly mass-balance (the sum of mass gain in form of snow and mass loss by melting) is an indicator of the year-to-year variations of weather and climate. In the Tropical Andes and the Mountains of Asia, climate observations are very scarce. In these regions, glaciers can be used as proxies: an indirect measurement of variations in climate, provided that we know how to read this information. This task is made difficult by the number of factors influencing glaciers, at very local (e.g. topography, shape) to large (e.g. global climate change, El Niño) scales.In this project we developed methods to convert these climate signals into glacier mass-balance information, in order to be able to unravel the importance of different factors on glacier response, in the Tropical Andes and at global scale. In particular, we were able to quantify the relative importance of man-made climate change in comparison to the natural variability of climate. The current retreat of glaciers (on global average) is a combination of a natural response (generalized warming since a colder period known as the Little Ice Age in the second half of the last millennium) and of anthropogenic climate change. The relative importance of the human induced climate change becomes detectable (i.e.: statistically significant) in the 1920s to 1930s, and increases towards the end of the century, reaching roughly 70 % of total mass loss during the most recent 20 years. This discovery has important implications past and future sea-level change estimations. In addition to this slow glacier retreat, the year to year fluctuations of glacier mass changes have important implications for the downstream populations. In the Cordillera Blanca of Peru, glaciers are of particular importance since they provide fresh melt water during the dry season when almost no precipitation occurs. There, it is of primary importance to be able to predict glacier mass-balance at a monthly scale. Therefore, we developed a tool, which uses past observations to statistically link the local glacier mass-balance to large-scale atmospheric variations. In particular, we addressed the question to which extent glaciers in the Tropical Andes will respond to one of the most famous climate phenomena: the El Niño Southern Oscillation (ENSO). It appears that ENSO has a very strong influence on glaciers in the Cordillera Blanca: El Niño events tend to be warmer and drier than average, meaning that the glaciers are going to melt more than average during these events. During a Niña year, the opposite occurs. It was shown for the first time that this relationship is robust over the last 34 years, which will help to predict past and future water availability in the region. More work is needed to see how this relationship varies in other regions, and how it can be used to study the past climate over hundreds or thousands of years.