Were that process to continue or accelerate, many scientists say, the anticipated rise in sea levels over the next few decades may have to be revised upwards.

The National Oceanic and Atmospheric Administration, in its annual Arctic Report Card, published this week, said dramatic melting of the Greenland ice sheet had occurred in July, "covering about 97 percent of the ice sheet on a single day."

Martin Jeffries, co-author of the report, said on the NOAA website: "As the sea ice and snow cover retreat, we're losing bright, highly reflective surfaces, and increasing the area of darker surfaces -- both land and ocean -- exposed to sunlight. This increases the capacity to store heat within the Arctic system, which enables more melting -- a self-reinforcing cycle."

All the evidence says that what in effect is the world's source of air conditioning is getting weaker, with consequences that will be felt far below the 48th parallel.