Chinese researchers have created a new model to convert satellite surface elevation data for the Greenland Ice Sheet into a detailed estimate of how much ice it has lost in recent decades.

Using satellite altimetry data covering 2002 to 2023, they found that the ice sheet lost an average of about 120 gigatonnes (264.5 trillion lbs) of mass per year.

This figure is considerably lower than the widely cited figure reached by Nasa, which previously estimated Greenland’s annual loss to be nearly 270 gigatonnes using satellite gravity data.

The new figure is also lower than the Chinese team’s own estimate derived from the same data used by the US space agency.

The lower number does not suggest Greenland’s melting is less severe than previously thought. Rather, the researchers said their study sought to refine elevation monitoring to provide a high-resolution map of exactly where the loss was happening.

“A comparison between different satellite technologies reveals that satellite altimetry offers a significant advantage in spatial resolution over satellite gravimetry, enabling the identification of ice-flow channels in local glaciers,” the team said in a paper published in the Chinese-language Journal of Marine Information Technology and Application on October 19.

Gravity-based data can offer a full picture of how much mass has been lost. Altimetry, or elevation data, can provide a more zoomed-in 1.9km (1.2-mile) resolution, but it needs to be converted into mass using density.

The researchers from China’s Jiangsu Ocean University developed a dynamic conversion model that was able to identify glaciers experiencing the most rapid loss. This could help reduce uncertainties in estimates of ice sheet dynamics across space and time.

The Greenland Ice Sheet covers about 80 per cent of the territory’s total land area and is the second-largest ice sheet in the world.

The sheet stretches across 1.7 million sq km (656,300 square miles) and contains about 2.9 million cubic km (696,000 cubic miles) of ice, according to the US National Snow and Ice Data Centre (NSIDC).

Since the mid-1990s, the sheet has been losing mass “in earnest”, according to the US National Oceanic and Atmospheric Administration (NOAA).

This affects not only sea levels but also ocean circulation and properties, nutrient and sediment flux, as well as local ecosystems, contributing as feedback to future melting, NOAA said.

The sheet’s maximum elevation reaches 3,673 metres (12,050 feet), and the sheet’s average thickness is about 1,800 metres. The sheet’s ablation or melting period typically occurs from May to August, according to the Chinese team.

“Accurately estimating the spatiotemporal characteristics of the mass change of the Greenland Ice Sheet is of great scientific significance for understanding the complex feedback relationship between the polar ice sheet and the global climate system,” the team wrote.

Precisely determining ice mass changes and where they are happening is also important for predicting future evolution and impact on sea level rise, they added.

One of the primary methods to monitor long-term ice mass changes in the polar ice sheets is satellite gravimetry, which maps changes in the Earth’s gravitational field and can be used to determine mass loss.

“Satellite gravimetry allows for the direct monitoring of relative changes in the total mass of an ice sheet, yet its spatial resolution is insufficient to capture fine-scale, localised variations,” the team said.

Another method is satellite altimetry, which uses lasers or radar to track changes in the Earth’s surface elevation. This method offers the best resolution, but converting the data into mass can lead to estimation errors.

Over the past few decades, satellites equipped with radar altimeters have been launched to track ice sheet elevation and monitor the ice sheets.

In 2002, Nasa and the German Aerospace Centre launched two satellites as part of the Gravity Recovery and Climate Experiment (Grace), which orbited until 2017, collecting data on the Earth’s gravity field. Its successor, Grace-Fo, was launched in 2018.

Using data from Grace, Nasa estimated that Greenland lost 264 gigatonnes of ice per year between 2002 and 2025.

According to the Chinese team, although there had been previous studies using altimetry data, these often focused on using the data to downscale or correct gravimetric results. There was a lack of systematic comparison between mass changes obtained from both methods, they said.

In their paper, the team proposed a way to estimate mass changes using satellite altimetry data with the help of a climate model called Mar, allowing them to build a dynamic snow and ice density model.

They evaluated mass changes from 2002 to 2023 using Mar to process surface elevation measurements.

These measurements were derived from NSIDC’s multi-source satellite altimetry data, which incorporates measurements from Nasa’s ICESat.

The researchers then compared this to the results from Grace-derived satellite-based mass concentration gravity data released by the University of Texas at Austin.

The Chinese team found that the Greenland Ice Sheet “was in a state of continuous mass loss” between 2002 and 2023, with an annual loss of nearly 120 gigatonnes based on altimetry data. The melting rate of the ice sheet accelerated significantly after 2010.

This meant a total loss of about 2,248 gigatonnes, contributing about 6.26mm (0.25 inch) to global sea level rise.

Using the gravity-based data, the team estimated a total mass loss of about 2,830 gigatonnes over the same 22-year period. Thinning was most pronounced in the northwestern and southeastern parts of the ice sheet where the ice transitions into the ocean.

They found that significant mass loss signals were seen in the coastal Jakobshavn, Helheim, Kangerlussuaq and 79N glaciers.

In some basins with severe loss, the mass change they derived from altimetry data captured substantially less loss than the gravity results, suggesting melting could be out of view in the ice sheet base or an underestimation of ice and snow density.

According to the Chinese team, future research would seek to further refine density estimates by incorporating on-site density observations.

“This integrated approach is intended to improve the accuracy of mass change estimates derived from satellite altimetry, provide more reliable data for analysing long-term trends in ice sheet mass and reduce uncertainties regarding the contribution of polar ice sheets to future sea-level rise projections,” they said.