Glaciers are bodies of ice that move. They shape the landscapes when the rocks embedded in the ice carve out areas where they flow and by leaving moraines (piles of rocks carried by the glaciers) at both the edge of the ice and below it.
Glaciers form when snowfall remains on the ground over many years and then builds up to a sufficient thickness that it compacts, turns to ice, and then begins to move (deform). Precipitation is thus the central glacier-making ingredient. But temperature also influences glaciers because it determines the melt rate, altitude, and affects precipitation.
In fact, a host of climatic variables from temperature, precipitation, humidity, wind, etc. all affect glacier behavior. What's more, glacier size is also determined by slope, glacier physics, debris cover, and other local conditions. There is a relationship between climatic conditions and glacier size; but every glacier responds differently. This is why some glaciers today are advancing even as the vast majority of them are shrinking.
Although we usually hear about glacier size and the position of glacier tongues, it is actually mass balance that it most important for determining a glacier's health. Mass balance is the relationship between the amount of ice that is added (accumulation) and subtracted (ablation, or melting). A negative mass balance means the glacier is shrinking, but it may only be getting thinner rather than exhibiting a retreating glacier tongue.
It is the complex interactions of these local forces and the immensely complicated global and regional climate systems that make it so difficult to identify the specific mechanisms driving glacier behavior and affecting mass balance, even when there are very clear century and decadal-scale correlations between climatic warming and glacier retreat.
This correlation between climate and glacier size has led scientists for more than a century to study glacier tongue positions as a way of understanding long-term climatic conditions. Today the World Glacier Monitoring Service at the University of Zurich, the World Glacier Inventory at the US National Snow and Ice Data Center, the Glacier Mass Balance Data set (GMBAL), and Global Land Ice Measurements from Space (GLIMS) keep records on glaciers globally.
For additional technical information visit the National Snow and Ice Data Center: How do glaciers reflect climate change?