By Tim Radford
Just as Scottish scientists deliver dramatic visual evidence of the retreat of Europe’s most famous glacier over the course of a century because of global warming, German scientists have mapped an even more devastating retreat of Andean glaciers in just 16 years.
In another demonstration of the impact of warming on what had always been considered the cryosphere, the world of ice and snow, Swedish scientists have shown that the chemistry of the northern forests has begun to change in ways that could even accelerate rising temperatures.
And in the US, researchers have shown that winter is on the wane and the snows in retreat – with dramatic consequences for wildlife, water supplies and human wealth and health.
All four studies are further confirmation of what climate scientists have already shown – that the high latitudes and high altitudes are warming faster than almost anywhere else on the planet, with ominous consequences.
In August 1919, pioneer aviator Walter Mittelholzer flew near the summit of Mont Blanc in Europe in a biplane to photograph Europe’s highest peak and the Mer de Glace glacier, one of the great tourist attractions and celebrated by artists and poets for two centuries
Exactly one century later, researchers from the University of Dundee in Scotland used global positioning satellite guidance and digital help to take a helicopter to exactly the same position and altitude of 4,700 metres to repeat the 1919 aerial study.
“It was both a breathtaking and heartbreaking experience, particularly knowing that the melt has accelerated massively in the last few decades”
Kieran Baxter, aerial photographer, digital media practitioner and researcher at the University of Dundee, says: “The scale of ice loss was immediately evident as we reached altitude, but it was only by comparing the images side by side that the last 100 years of change were made visible.
“It was both a breathtaking and heartbreaking experience, particularly knowing that the melt has accelerated massively in the last few decades.”
Glaciers store rainy season ice and snow, and release it as meltwater in the hot dry summers. They keep the rivers flowing, the crops growing, and the hydroelectric turbines turning.
Scientists from the University of Erlangen-Nuremberg in Germany report in the The Cryosphere journal that they used satellite imagery to calculate glacier loss in Peru this century.
Almost three-quarters of all tropical glaciers and 90% of their area of ice are concentrated in the Peruvian Andes, at around 4000 metres or more.
At the beginning of this century, there had been a count of 1,973 rivers of ice in the region. Of these, 170 have vanished altogether, while the others have retreated uphill, and their accumulated area has dwindled by around 550 square kilometres.
Loss of ice mass
Eight billion tonnes of ice have melted away, and at an ever-faster rate. The loss of ice mass between 2013 and 2016 was around four times higher than in the previous 12 years, perhaps because of local climate changes triggered by a periodic climate phenomenon known worldwide as an El Niño.
Glaciers are an important part of the climate machine, but the great forests that flourish in the snows below them are even more important.
They add up to 14% of the planet’s vegetation coverage, they absorb atmospheric carbon to cool the climate in one way, and counter the greenhouse gases that fuel global warming in another, subtle way. The conifers exude terpene aerosols – the pine-fresh fragrance from their resins – that have a cooling effect on the air over the forests.
But scientists from Sweden report in Nature Communications journal that, thanks to atmospheric pollution driven by global agriculture and industrialisation, the terpene particles from the forests are getting smaller in diameter – some smaller than a wavelength of optical light.
That means that the same particles are now less effective at reflecting solar radiation back into space. Ammonia and sulphur dioxide discharged by humankind have changed the chemistry of the forests: there are now more aerosols, but their diameter is dwindling.
Study leader Pontus Rodin, a researcher at Lund University, Sweden, says: “The heavily-oxidized organic molecules have a cooling effect on the climate. With a warmer climate, it is expected that forests will release more terpenes, and thus create more cooling organic aerosols.
“However, the extent of that effect also depends on the emission volumes of sulphur dioxide and ammonia in the future. It’s very clear, though, that this increase in organic aerosols cannot by any means compensate for the warming of the climate caused by our emissions of greenhouse gases.”
Weather station data
And while European scientists examined the detail of loss, scientists in the US looked at 100 years of weather station data from the forests of the US and Canada.
They report in Ecological Applications journal that they found a significant decline in the number of “frost days” when the temperature dropped below freezing, and “ice days” in which the thermometer stayed below freezing.
Snow and ice sustain ecosystems by preventing disease spread and reducing beetle and aphid numbers. Deep snow insulates tree roots, provides wildlife habitat, and promotes soil nutrient recycling.
“Winter conditions are changing more rapidly than any other season, and it could have serious implications,” says Alexandra Contosta, assistant professor in the Earth Systems Research Centre at the University of New Hampshire.
“Whether precipitation falls as snow or rain makes a big difference, whether you are talking about a forest stream, a snowshoe hare, or even a skier.”