It's not entirely news that the Earth's ice budget is under threat from a warming climate, but a new study has looked into just how fast that's happening. Using recently declassified spy satellite photos, researchers found that compared to the last quarter of the 20th century, Himalayan glaciers have been melting twice as fast this century. In a rare piece of good news, a separate study has shown that a Greenland glacier has gained ice for the third year in a row.

While the Arctic and Antarctic are home to the majority of our planet's ice, the Himalayan region is hoarding some 600 billion tons of its own, giving it the occasional moniker of Earth's "Third Pole." Of course, that makes it quite vulnerable in our warming world, and plenty of studies have tried to measure just how much ice is lost every year.

To get a clearer picture of the changes in ice over the last few decades, a research team led by Columbia University has now analyzed satellite data gathered over 40 years, of some 650 glaciers spanning India, China, Nepal and Bhutan.

The older images were actually taken by US spy satellites in the 1970s and 80s, and have only been recently declassified. The researchers developed a way to build 3D models of the mountains in the images by comparing overlapping photos of the same areas and mapping the slight differences between them.

With those 3D models the researchers could measure the changes in elevation of the ice, while other images can help highlight changes in the area of the glaciers. This data was then compared to newer shots from more sophisticated satellites, which are able to directly measure elevation changes. Using both sets of data, the team compared the periods of 1975 to 2000, and 2000 to now.

The difference is stark. Since 2000, Himalayan ice appears to have been melting at twice the average rate of the previous 25 years. Between 1975 and 2000, the average loss of glacial ice was about 25 cm (10 in) per year, but this doubled to 50 cm (20 in) in the 21st century. These are average figures, spread out across the region, and in the worst-hit areas, that ice loss is as much as 5 m (16 ft) a year.

The main driver of this accelerated ice loss seems to be warming air temperatures. The researchers investigated this by collecting data from ground stations, then calculating how much melting would be expected at different times based on those figures. Sure enough, the numbers matched what was observed. That said, other factors play their part, like changes in snowfall and increased fossil fuel burning in Asia, which leaves more soot lining the glaciers. That in turn absorbs more heat from sunlight and speeds up the melting process.

"This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why," says Joshua Maurer, lead author of the study.

It's not all bad news though – in a separate study, a NASA team has found a glacier in Greenland that's growing. As part of the Oceans Melting Greenland (OMG) mission, the researchers studied radar images of Jakobshavn Glacier taken over the last few years.

The study showed that this glacier, which has been shrinking for most of the last 20-odd years, has now added extra ice for the third year in a row. Between 2016 and 2019, Jakobshavn has grown by between 20 and 30 m (65.6 and 98.4 ft) each year.

But before people start declaring the climate change emergency over, there are a few caveats to consider. Unfortunately, this isn't expected to last very long – the team says this is part of a known climate pattern that's due to flip again soon. On top of that, it's not even really offsetting its own melting. Although it's growing on the surface, NASA says the glacier is still melting from below at a faster rate than it's adding new ice.

The Himalayas research was published in the journal Science Advances.

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