The Interior Department made the images public on Wednesday afternoon. The academy's report urging the release of the pictures was issued at 11am EDT (1500 GMT) on Wednesday.

Some 700 images show swatches of sea ice from six sites around the Arctic Ocean, with an additional 500 images of 22 sites in the United States. The images can be seen online at http://gfl.usgs.gov/.

Changes in the Arctic affect global climate, since the Arctic region acts as an "air conditioner" for the planet.

The Arctic images have a resolution of about 1 yard (1 metre), a vast improvement on previously available pictures of sea ice, said Thorsten Markus of NASA's Goddard Space Flight Center.

"These are one-meter-resolution images, which give you a big picture of the summertime Arctic," Markus said on Thursday. "This is the main reason why we are so thrilled about it. One meter resolution is the dimension that's missing."

The next-best resolution for images of Arctic sea ice is 15 to 30 meters, Markus said by telephone. This risks missing small features that can have a big impact on warming in the area.

SMALL PUDDLES, BIG IMPACT For example, during the summer months, pools of melted water form on top of Arctic ice floes, and these puddles can stretch across 30 meters. The water in the puddles is dark and absorbs heat, as opposed to the white ice all around them, which reflects heat.

Knowing about these melt pools is valuable to producing models of what might happen in the Arctic in the future, but with images that have a resolution of 30 meters or so, these pools might well be missed. While individual puddles are small, collectively they cover about 30 percent of the Arctic.

"The (forecasting) models do well at capturing the overall sea ice cover in the Arctic," Markus said. "But there are certain processes that we cannot adequately model yet, mainly ... because we don't have enough data."

Markus said the public release of these images was "a huge surprise -- I expected after the report, months could go by until somebody moved."

"That doesn't happen every day," said a person familiar with the government's decision. "This is a great example of good government cooperation between the intelligence community and academia. In the science community, we call it a no-brainer."

The images were derived from classified images made as part of the Medea program, which lets scientists request spy pictures from environmentally sensitive locations around the globe.

Medea scientists asked for intelligence images of Arctic sea ice during the summer melting season, but these were considered unsuitable for public release. Images suitable for release were made, but were not made public until now.