Glaciers around the world are melting at an unprecedented rate due to climate change, according to the most extensive scientific study conducted so far.

Mountain glaciers, which are essentially frozen rivers of ice, serve as crucial freshwater sources for millions of people and contain enough water to raise global sea levels by 32 centimetres (13 inches) if they were to melt completely.

However, since the beginning of the 21st century, these glaciers have lost over 6,500 billion tonnes of ice, equating to approximately 5% of their total mass.

The rate of glacial melting has also accelerated significantly. Over the last decade, ice loss has increased by more than a third compared to the period between 2000 and 2011.

This alarming trend is revealed in a study that compiles more than 230 regional estimates from 35 research teams worldwide, reinforcing scientific confidence in both the accuracy of the findings and projections for the future.

Glaciers are widely regarded as reliable indicators of climate change. In a stable climate, they maintain their size, gaining ice through snowfall at a rate roughly equal to the amount they lose through melting. However, over the past two decades, glaciers have been shrinking nearly everywhere due to rising global temperatures caused primarily by human activities such as burning fossil fuels.

Between 2000 and 2023, glaciers outside of the Greenland and Antarctic ice sheets lost an average of 270 billion tonnes of ice annually.

The 270 billion tonnes of ice lost in a single year “corresponds to the [water] consumption of the entire global population in 30 years, assuming 3 litres per person and day”, according to Michael Zemp, director of the World Glacier Monitoring Service and lead author of the study.

Some regions have been affected more drastically than others. In Central Europe, for instance, glaciers have lost an astonishing 39% of their ice in just over two decades.

The study, published in the journal Nature, does not simply confirm that glaciers are melting at an accelerating pace—something already well established—but rather, it highlights the importance of synthesising research from various scientific sources to improve the accuracy of assessments.

There are multiple methods for measuring glacial change, including direct field measurements and different types of satellite observations, each with its own strengths and limitations. Field measurements provide highly detailed information but cover only a small fraction of the more than 200,000 glaciers worldwide. By integrating various measurement techniques, scientists can develop a far more precise understanding of the extent and rate of glacial melting.

Community-wide scientific estimates like this are essential because they allow researchers, policymakers, and the public to have greater confidence in the findings.

These community estimates “are vital as they give people confidence to make use of their findings”, said Andy Shepherd, head of the Department of Geography and Environment at Northumbria University, who was not an author of the recent study.

“That includes other climate scientists, governments, and industry, plus of course anyone who is concerned about the impacts of global warming.”

Glaciers do not respond to climate change instantly. Depending on their size, it can take years to decades for them to fully react to shifting environmental conditions. This means that the melting trend will continue into the foreseeable future.

However, the total volume of ice lost by the end of the century will depend significantly on how much humans continue to warm the planet by emitting carbon dioxide and other greenhouse gases. The study suggests that if global climate targets are met, about a quarter of the world’s glacier ice could be lost.

However, if emissions remain unchecked, nearly half of all glacier ice could disappear.

“Every tenth of a degree of warming that we can avoid will save some glaciers, and will save us from a lot of damage,” Prof Zemp explained.

The implications of glacier loss extend far beyond their immediate surroundings. As Professor Zemp puts it, “what happens on the glacier doesn’t stay there”. Hundreds of millions of people worldwide depend on seasonal glacier melt for their water supply. Glaciers act as natural reservoirs, buffering against drought conditions. Once they vanish, the water supply they provide will also be gone.

The global impact of melting glaciers is profound. Even minor increases in sea levels—driven by melting mountain glaciers, the Greenland and Antarctic ice sheets, and thermal expansion of the oceans—can dramatically heighten the frequency of coastal flooding.

“Every centimetre of sea-level rise exposes another 2 million people to annual flooding somewhere on our planet,” said Prof Shepherd.

Since 1900, global sea levels have risen by more than 20 centimetres (8 inches), with nearly half of that increase occurring since the early 1990s. The rate of rise is expected to accelerate in the coming decades, posing significant risks to coastal communities worldwide. Without urgent action to mitigate climate change, the consequences of glacial melt will only become more severe.