Melting glaciers set to release more organic carbon as temperatures climb

Melting ice may affect more than sea levels, according to new research. As the earth warms, more of the carbon locked up in glaciers and ice sheets will be released into surrounding rivers and oceans.

This means that, as well as pushing up sea levels, melting ice could have unknown impacts on marine life.

Carbon release to increase by half

Glaciers and ice sheets cover around 11 per cent of Earth’s land surface and hold around 70 per cent of its freshwater. These giant stores of ice also hold organic carbon. Carbon accumulates in new snow and ice, and is released as the glacier melts.

A new study, published today in Nature Geoscience, finds the release of this carbon will speed up as the Earth warms due to climate change.

Around 15 million tonnes of extra organic carbon will be lost from melting glaciers over the next 35 years in the form of tiny dissolved particles, the researchers say. Ice also contains larger ‘particulate’ carbon, which are like bits of sediment that a river carries.

This extra organic carbon is 47 per cent more than we could expect without climate change, and equivalent to around half of what the Amazon river carries each year, the researchers say.

Organic carbon provides food for tiny organisms at the bottom of the food web. So the extra carbon flowing into rivers and oceans may affect the plants and animals that live around the ice sheets, the researchers say. Adding organic carbon can also affect the chemistry of water, by making it more acidic, for example.

A research first

The research is the first to estimate the total amount of organic carbon held in ice across the world. Researchers collected measurements of organic carbon concentrations from more than 300 samples of glacier and ice sheets in four continents, as the map below shows.

Scientists collected organic carbon samples across four continents. Photos show examples of a) Alaska, b) Tibet, c) Dry Valley glaciers in Antarctica, and d) the Greenland Ice Sheet. Source: Hood et al. (2015)

Worldwide, glaciers and ice sheets contain about six billion tonnes of organic carbon. But most of it is locked up in the vast Antarctic ice sheet. As the ice melts or large chunks break off, known as calving, the carbon gets released, the paper explains.

Around three million tonnes of organic carbon are released from glaciers and ice sheets each year, the researchers say, with around 13 per cent caused by melting from rising global temperatures.

Although the Antarctic Ice Sheet holds the majority of organic carbon in ice, it accumulates and loses ice more slowly than mountain glaciers and the Greenland ice sheet. This means the release of organic carbon is relatively small considering how much Antarctica holds, and less than half when compared to mountain glaciers. You can see this in the graph below, which shows the amount of organic carbon that ice sheets and glaciers store and release each year.

Storage (top) and release (bottom) of glacier dissolved organic carbon (DOC) in million tonnes (teragrams) per year for the Antarctic Ice Sheet (AIS), Greenland Ice Sheet (GIS) and Mountain Glaciers (MGL). Source: Hood et al. (2015)

Marine food webs

The amount of carbon released from land to ocean around the Earth’s poles is currently fairly limited, so more carbon flowing out from melting ice could affect the balance of marine life. As co-author, Prof Robert Spencer puts it:

“It could change the whole food web. We do not know how different ecological systems will react to a new influx of carbon.”

Melting glaciers and ice sheets may cause more problems than rising sea levels, Spencer concludes:

“The thing people have to think about is what this means for the Earth. We know we’re losing glaciers, but what does that mean for marine life, fisheries, things downstream that we care about? There’s a whole host of issues besides the water issue.”

Main image: Perito Moreno glacier, Argentina.