The dramatic melting of the world’s mountain glaciers – from the Alps to the Himalayas – is mostly the result of man-made global warming rather than natural variability in the climate, a study has found.
Scientists have laid to rest the idea that glaciers as far apart as Patagonia and Indonesia are melting primarily because of natural changes to the climate caused by such things as solar variability and volcanic eruptions.
An assessment of about 200,000 glaciers in the world, some of which have been monitored since the mid 19th century, has found that about two thirds of the current rate of glacial melting is due to human influences on the climate.
Scientists found that while much of the melting a century or more ago was most probably due to natural variability in the climate, it is now primarily caused by anthropogenic global warming resulting from industrial greenhouse gases.
“We can clearly detect an anthropogenic effect on glaciers and it’s been steadily rising over the last 100 years,” said Ben Marzeion, a climate scientist at the University of Innsbruck in Austria.
“In the 19th century and first half of the 20th century we observed that glacier mass loss attributable to human activity is hardly noticeable but since then it has steadily increased,” said Dr Marzeion, the lead author of the study published in the journal Science.
In comparison to anthropogenic warming caused carbon dioxide emissions, variations in solar radiation and the effects of volcanic eruptions, which can lower temperatures by blocking solar radiation, now play only a minor role in influencing whether a glacier recedes or expands, the scientists found.
The study found that about 25 per cent of the global loss of glacier ice that occurred between 1851 and 2010 could be attributable to human influences, but that this rose to about 69 per cent between 1991 and 2010, based on computer simulations of the climate that included natural and man-made effects.
“The influence of anthropogenic warming becomes robust around the middle of the 20th century. It seems obvious that if it gets warmer, glaciers will melt but no-one has actually shown that before – and it’s possible to say that now this is primarily caused by humans,” Dr Marzeion said.
“While we keep factors such as solar variability and volcanic eruptions unchanged, we are able to modify land use changes and greenhouse gas emissions in our models,” he said.
“In our data we find unambiguous evidence of anthropogenic contribution to glacier mass loss. What is happening in recent decades is not explicable by natural climate effects such as variations in solar radiation or volcanic activity,” he added.
It widely accepted that the global trend in glacier melting began in the mid 19th century, at the end of the “little ice age” when temperatures in some parts of the world were below average for many decades.
It can take decades or even centuries for glaciers to adjust to climate change and the real effects of the more recent human influences on the climate have not been clear until this study, Dr Marzeion said.
“There is a lag response of several decades which means that we are only seeing a fraction of the total anthropogenic effects on melting glaciers that we will eventually see as a result of the carbon dioxide in the atmosphere now,” he said.
Although mountain glaciers store less than one per cent of the total ice on earth, they are a major cause of global sea-level rise over the 20th century because they have melted so rapidly. The smaller glaciers of the Alps and Rocky Mountains are among the fastest disappearing masses of non-polar ice.
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