 The word glacier is familiar to most people, but do you really know what a glacier is? You might think of it as a big rigid piece of ice, however under pressure glaciers act like a soft plastic. They can bend and flow downhill like slow motion rivers of ice. Most people also picture glaciers on mountains, however ice caps, ice sheets, and really any masses of ice which remain year round are also considered glaciers. Even the most basic fact that warmer temperatures will melt glaciers has an interesting exception. We'll come back to that later. Overall glaciers are shrinking in a warming climate. This has been measured by satellites that feel gravity getting weaker over glaciers as they melt. Overall, excluding the polar ice sheets, glaciers are losing about 150 billion tons of ice each year. In some areas this is concerning because glaciers act like water towers for cities downriver. They store water in winter and release it during the summer months. Most glaciers expanded until the late 19th century and began retreating soon afterwards. The retreat has accelerated over the past two decades, particularly in the polar regions. In some areas rapid warming has completely disintegrated some small glaciers. Many of the smallest mountain glaciers in the European Alps and other mid-latitude locations will eventually disappear as well. In the Canadian Arctic, for instance, sample cores taken from ice show recent melt is the greatest in 4,000 years. Researchers have also found that ancient vegetation preserved under ice is now thawing out. Dating these frozen plants shows that they have been covered by ice for thousands of years. This tells us that some glaciers are now smaller than during any time in the past several thousand years. The same thing has been found in western Canada and on Baffin Island. Over the past decade the rate of glacier loss in the Arctic was similar to thousands of years ago when the Earth received more sunlight in the summer because of a different orbit. Current global warming is melting most glaciers and will continue to do so. But one myth says the opposite. It gives a few examples of growing glaciers. There are over 100,000 glaciers in the world, so even though most are shrinking it's possible to find a few examples that aren't. This myth relies on a cherry pick. The observation that a small fraction of glaciers are growing is actually an interesting scientific question. In order to explain it fully, let's take a bit of time to understand how glaciers form. For a typical glacier, snowfall builds up on its surface. Over time all the layers of snow press down on the layers beneath, compacting the snow crystals into ice. This ice forms the main body of the glacier. Glaciers gain ice from snowfall and they lose ice through surface melting, melting from beneath, and in some cases by gradually flowing into lakes or oceans and breaking off into icebergs. In winter, new snow weighs down on the glacier, pushing it downwards towards an ocean, lake, or the end of a valley. In summer, ice melts and can cause the glacier to recede. The difference between the total gains and losses of an ice mass measured over a year is called its annual mass balance. Because of the seasonal back and forth between advancing and retreating ice, the size of a glacier is typically measured at the end of summer when ice covers its smallest extent. As glaciers advance, they often push up ridges of sediment, called moraines. You can see these ridges with your own eyes and they remain long after a glacier has retreated up a valley. Moraines are useful to estimate how big a glacier used to be. We are currently studying the rate of change in many glaciers, and what have we learned? Glaciers are very sensitive to climate. They can only form in regions with low summer air temperatures and high winter snowfall. Changes in summer air temperatures or winter snowfall can therefore change the yearly mass balance of a glacier. So how will a warmer climate affect glaciers? It seems obvious that warmer air will cause them to melt faster, but it's a bit more complicated than that. Warmer winter air can also increase snowfall. This is because warmer air holds more moisture. If temperatures warm from very low below freezing to a little below freezing, then you can get more snow. This means that each individual glacier will react differently to changes in climate, depending on whether its particular mass balance is more controlled by temperature or by snowfall. Usually the warmer summers are enough to melt the extra winter snowfall. It is possible for increases in snowfall to balance out or even beat out the effect of warming summers. So even though it seems strange, some glaciers will actually get larger in a warming climate. If you hear someone use the argument that seeing a glacier grow disproves global warming, now you know why that is false. Hand picking a few growing glaciers is cherry picking the data because it ignores the bigger picture. A slightly warmer regional climate might cause some glaciers to temporarily grow, but overall almost all glaciers worldwide are now shrinking. This will continue as it warms over the next century.