(Repeating for wider distribution)

* Tough old ice melts, replaced by frail young ice

* Arctic ice influences global weather patterns

By Deborah Zabarenko, Environment Correspondent

WASHINGTON, July 7 (Reuters) - Arctic sea ice has thinned dramatically since 2004, with the older, thicker ice giving way to a younger, thinner kind that melts in the northern summer, NASA scientists reported on Tuesday.

Researchers have known for years that ice covering in the Arctic Sea has been shrinking in area, but new satellite data that measure the thickness of ice show that the volume of sea ice is declining as well.

That is important because thicker ice is more resilient and can last from summer to summer. Without ice cover, the Arctic Sea's dark waters absorb the sun's heat more readily instead of reflecting it as the light-colored ice does, accelerating the heating effect.

Using NASA's ICESat spacecraft, scientists figured that overall Arctic sea ice thinned about 7 inches (17.78 cm) a year since 2004, for a total of 2.2 feet (0.67 metres) over four winters. Their findings were reported in the Journal of Geophysical Research-Oceans.

The total area covered by thicker, older ice that has survived at least one summer shrank by 42 percent.