Glaciers provide excellent records of climate change. Samples of particulates and gases can be extracted from ice cores, from which the past composition of the atmosphere can be reconstructed. Ice cores also record temperature and moisture, and all of this information is stored in dateable, consecutive layers. The position of end moraines, past and present, can be used to discern past glacier positions, and these deposits can be dated with radiogenic isotopes to reveal the timing of past shifts in climate. The response of a glacier to climate change may be fairly immediate for alpine cirque glaciers, but a response time is usually involved, ranging from decades for valley glaciers to thousands of years for ice sheets. Long-narrow glaciers will have a longer response time than will compact ice caps of the same area. A negative mass balance may cause an immediate response in the position of the glacier snout, but a positive regime may not be detected at the snout for many years. Weekly, monthly, and seasonal variations in glaciers are especially sensitive to ablation rates and meltwater lubrication. Glacier response over years and decades is tied to mass balance, which reflects a range of climate conditions, especially as influenced by ocean temperatures and position of the jet stream. Glacier responsealso depends on the distribution of glacier area with elevation. As climate warms, the elevationof the snow line will progressively experience upstream migration, shifting precipitation from solid to liquid on low-altitude glaciers. In this context, a small mountain glacier with most of its area at lower elevations will experience retreat and thinning due to rain. However, a valley glacier with an important accumulation area at higher elevations will experience stability or even thickening if the maximum snowfall is also migrating upstream. Thus, glaciers with bigger and higher elevation accumulation areas may advance during periods of climate warming, whilst small glaciers without high-altitude areas will more likely tend to disappear if ongoing climate trends persist over time.

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