Scientists say this year’s record declines in Arctic sea ice extent and volume are powerful evidence that the giant cap of ice at the top of the planet is on a trajectory to largely disappear in summer within a decade or two, with profound global consequences. As the northern summer draws to a close, two milestones have been reached in the Arctic Ocean — record-low sea ice extent, and an even more dramatic new low in Arctic sea ice volume. This extreme melting offers dramatic evidence, many scientists say, that the region’s sea ice has passed a tipping point and that sometime in the next decade or two the North Pole will be largely ice-free in summer. NASA and U.S. ice experts announced earlier this week that the extent of Arctic sea ice has dropped to 4.1 million square kilometers (1.58 million square miles) — breaking the previous record set in 2007 — and will likely continue to fall even farther until mid-September. As the summer melt season ends, the Arctic Ocean will be covered with 45 percent less ice than the average from 1979 to 2000.