Answers

1. What percentage of glaciers in the world are retreating?
100%! All the glaciers in the world are "retreating". "Retreating" and "advancing" refers only to the terminus or bottom end of a glacier. The retreating of glaciers is caused by the warming of the globe. Overwhelming evidence shows this warming is being caused or at least accelerated by the unnatural release of greenhouses gasses by humans burning fossil fuels.

2. Can a glacier move forward and retreat at the same time?
Yes, amazing as that may seem. Retreating glaciers in fact do move forward as ice moves downhill like it's on a very giant and very slow moving conveyer belt. As snow falls on the top of a glacier, the weight of the snow causes the glacier to move forward. Further down the glacier is the "ablation area", where the snowfall is equal to the snowmelt. Here the glacier is in equilibrium. Whenever this area is disturbed by increased snowfall or warmer temperatures, the glacier begins to advance or retreat respectively.

3. What is a crack in a glacier called?
A crevasse. Crevasses are one of the most dangerous aspects of a glacier. Crevasses are created as the awesome forces caused by a moving glacier deform and crack glaciated ice. These can be hundreds of feet deep and are easily made invisible by being covered by snowfall on the surface of a glacier. The inexperienced should never try to traverse a glacier. They are very very dangerous.

4. What is moraine?
Moraine is the name given to rock and other debris pushed into piles by a glacier. In Homer we landed on a lake that was formed by a giant end moraine left by a retreating glacier. Falling rain and glacier run-off was trapped by this moraine forming a lake.

5. What part of a glacier moves the slowest?
A glacier is slowest moving where it comes in contact with the ground. This is actually a pervasive physical phenomena that is also true about other flowing mediums like air moving over an airplane wing or water flowing down a river. This is referred to as a "boundary layer" in engineering.

Glaciers move incredibly slowly or quite quickly depending on how you look at it. One of my friends at the United States Geological Survey threw a bear transmitter into a crevasse on the Exit Glacier (we visit this glacier on the "Seward" episode). Mapping the transmitter's movement over the past few years has shown that the middle of this glacier is moving forward at about 2 feet per day! Even though the middle of the glacier is moving forward, the Exit Glacier is still "retreating" at its bottom end.