One of the biggest predictors of global warming is the retreat of sea ice in the high northern latitudes. As oceans warm, the ice will take longer to form in the winter, and retreat faster in the spring. Scientists, therefore, have been watching the ice north of Canada very carefully. What they're seeing isn't very hopeful.

This picture, from the Terra Earth-observing satellite, shows the state of sea ice as it was on August 17. This region is the so-called Northwest Passage -- a waterway through the Canadian archipelago connecting the Atlantic and Pacific oceans. Usually the sea ice prevents regular trade routes from being utilized there. But over the past few years -- <sarcasm>coincidentally</sarcasm> the time when scientists say global warming is accelerating -- the sea ice has thinned considerably. In 2007 the sea ice underwent a record thinning and the passage opened enough for navigation. This year, the ice has thinned even faster, though overall the ice is not as thin as it was in 2007; in some places it is thicker. What this means is that it's very difficult to read what's going on in detail. But we knew that already: global warming also predicts changing currents, changing salinity, changing everything. It's a complex system that interacts with itself, and those are notoriously difficult to model. Positive feedback systems are exquisitely sensitive to small changes. That's precisely why images like these are so disturbing: they are showing that the system is changing, and it's the change itself that indicates trouble is brewing. Image credit: NASA's MODIS Rapid Response Team