Larsen-B collapsed very quickly, what is the difference [between] that and the Greenland and Antarctic ice sheets?

NASA: Larsen-B was a floating ice shelf in the Southern Ocean. Greenland and Antarctica are much thicker and based on land. Monitoring of these ice shelves by ICESAT and plane based altimeters is ongoing.

NSIDC: NSIDC has a Web site that discusses the Larsen Ice Shelf breakup events, and the climate changes leading up to these events.

Floating ice does not displace more seawater when it breaks apart or melts; however, some ice shelves may act as a ‘braking system’ for glaciers that flow into them. When the ice shelves are removed, ice on land may begin to flow faster. This acceleration would contribute to sea level rise.

Warmer air alone may also cause glaciers to speed up. As summers get warmer on the large ice sheets, snow melt at the surface can penetrate through the cracks in the glacier and lower the friction between rock and ice. This process is already underway in some parts of Greenland.

You may also be interested in:

• Scambos, T., C. Hulbe, M. Fahnestock, and J. Bohlander. 2000. The link between climate warming and break-up of ice shelves in the Antarctica Peninsula. Journal of Glaciology 46(154): 516-530.
  
  
• Zwally, H. Jay, W. Abdalati, T. Herring, K. Larson, J. Saba, and K. Steffen. Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow. Science 12 July 2002; 297: 218-222