The Northern Hemisphere’s largest expanses of ice have thawed faster and more extensively this year than scientists have previously recorded. And the summer isn’t over.

Studies suggest that more of the massive Greenland ice cap has melted than at any time since satellite monitoring began 33 years ago, while the Arctic sea’s ice is shrinking to its smallest size in modern times.

“This year’s melting season is a Goliath,” said geophysicist Marco Tedesco, director of the Cryospheric Processes Laboratory at City University of New York. “The ice is being lost at a very strong pace.”

Scientists monitor the annual thaw closely because changes in the ice of the far North can raise sea levels and affect weather throughout the hemisphere by altering wind currents, heat distribution and precipitation.

Shrinkage of the Arctic sea ice since 2006, for instance, helped lead to seasons of severe snow across Europe, China and North America, researchers at Columbia University, the Georgia Institute of Technology and the Chinese Academy of Sciences reported earlier this year.

As the seasonal ice abates more each year, new polar shipping lanes also open up, as do opportunities for mineral exploration. By some estimates, as much as 25% of the world’s oil and natural-gas reserves are under the Arctic seafloor. Russia, Denmark, Norway and Canada are vying to control these assets.

The giant ice cap at the top of the world partly melts every summer and refreezes every winter. In recent years, the thaw has become progressively more extensive, NASA and European satellite observations suggest. At the same, the refreeze has been smaller—adding up to long-term shrinkage in the ice cover.

This year’s unusual summer thaw was spurred partly by natural variations in weather, but also reflected rising levels of heat-trapping carbon dioxide and methane in the air, amplified by carbon soot from widespread wildfires and the burning of fuels, said scientists at Stanford University and the National Snow and Ice Data Center.

Carried north across the Arctic by winds, soot not only darkens snow and ice, making it absorb more heat from sunlight, but also interferes with the formation of clouds that might otherwise providing cooling shade.

“They all cause enhanced warming in the Arctic,” said Stanford University atmospheric scientist Mark Jacobson, who advocates for renewable energy. “Soot can double the warming.”

In many ways, the Arctic ice pack and Greenland ice cap are mirror opposites. The ice pack is a vast layer of frozen salt water, a few yards thick at most, floating atop an open sea, like ice cubes in a highball. Changes in the size of the Arctic ice can alter weather patterns globally, though the melting doesn’t raise sea levels since the ice displaces the same amount of ocean water when frozen as when liquid.

The Greenland ice sheet is a land-based formation of frozen fresh water up to two miles thick. The water runoff from Greenland ice dilutes the salinity of ocean water, changing its density and altering currents. The runoff that doesn’t refreeze adds to rising ocean levels.

Despite their differences, their fates are linked. “There is little doubt that in terms of warming, things are coming together in the Arctic,” said glaciologist Paul Mayewski, director of the Climate Change Institute at the University of Maine. “Without a doubt, warming in the Arctic is very, very strong,”

In fact, more melting occurred across the Greenland ice cap—the world’s second-largest ice sheet after Antarctica—in June and July than in any year since at least 1979, when satellite monitoring of the island’s ice began, Dr. Tedesco and his colleagues reported earlier this month. The Greenland thaw began in May, a month earlier than usual.

On average, about half of the surface of Greenland’s ice sheet naturally melts during the summer, and then mostly refreezes with the approach of winter. This year, nearly the entire ice cover, from its thin, low-lying coastal edges to its two-mile-thick center, experienced some melting at its surface, according to measurements from three independent satellites analyzed by NASA and university scientists.

“This summer, we have seen melting at the very highest elevations of the Greenland ice sheet, which we have not seen before in the satellite record,” said climatologist Thomas Mote of the University of Georgia, who studies snow cover. Researchers expect much of it to refreeze.

By Wednesday, the Arctic sea ice had shrunk to 1.54 million square miles, about 70,000 square miles smaller than the previous modern low set in September 2007, according to the satellite readings compiled by NASA and the National Snow and Ice Data Center in Boulder, Colo. By that measure, the six lowest Arctic sea ice levels on record all occurred in the past six years.

Even when the Arctic ice refreezes, the new ice is often thinner, making it more vulnerable to storms and seasonal temperature variations, said climate scientist Julienne Stroeve at the Snow and Ice Data Center.

About a week remains in the melt season. Researchers won’t know the full extent of this year’s melting until the end of September.

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