The Arctic permafrost

Carbon feedback mechanisms

[A second post on the Arctic. The number of subscribers continues to grow - a special welcome to colleagues in Australia. My next post will be in a week’s time or so - I’m off to Edinburgh for a family birthday, just as a certain Taylor Swift is there too. Hotel rooms have increased six times in price and road closures have been announced. Perhaps it would provide an interesting context for a post or fieldwork activity on Place?]

Introduction

Despite experiencing cold, dark months in a long winter, the Arctic supports complex food chains, from algae to large mammals. When these organisms die, their remains become part of the detritus system. Organic detritus which does not fully decompose plays an important role in soil in such an environment. Incomplete decomposition leads to huge accumulations of organic matter (carbon) in soils and sediments. These areas of preserved organic matter act as significant carbon stores.

Much of the Arctic land surface, and much more land to the south, is underlain by permafrost. This is defined as ‘ground (soil or rock and included ice or organic material) that remains at or below 0°C for at least two consecutive years.’ Much of this permafrost formed during colder glacial periods but persists in locations where the mean annual temperatures are too low, and the thaw season too short, for thaw to penetrate much more than a metre or so below the ground surface.

Permafrost occurs beneath 25% (23 million km²) of the exposed land area in the northern hemisphere (Figure 1).

Figure 1. Permafrost areas in the northern hemisphere

Detritus deep-freeze

The decomposition of plant and animal remains in soils is mainly carried out by microorganisms and soil invertebrates. This releases carbon and other nutrients from organic remains and drives the growth of the soil organisms (the ‘decomposers’) themselves. Decomposition releases carbon dioxide back to the atmosphere and nutrients such as nitrogen and phosphorus into the soil. Here, organic matter tends to be produced faster than it decomposes, partly because soil organisms are more sensitive to temperature than photosynthesising plants. This leads to a build-up of organic matter so that over several centuries, an increasing amount of organic matter/carbon is stored in soils and sediments.

As this organic matter accumulates, it further restricts the depth to which the seasonal thaw can penetrate, and the deeper thaw layer starts to freeze, upwards, from the permanently frozen ground below. Hence, in permafrost soils, there is a huge amount of organic matter. Such soils are estimated to contain about 1,500 billion tonnes of carbon in the top three metres alone. Much of this currently remains frozen year-round, but that situation is now changing.

Climate change

The Arctic is warming two to three times the rate of the global mean, and for large areas the mean annual temperature has increased by 2–3°C in the last 30 years. The implications are:

·       reduced extent of Arctic sea-ice.

·       reduced depth, duration, and extent of seasonal snow cover on land.

·       increases in productivity and changes in vegetation composition of Arctic ecosystems.

·       an increase in the thaw of permafrost across large areas.

Permafrost thaw can expose organic matter that has been frozen for hundreds/thousands of years to temperatures above freezing. With such melting microorganisms can start to become metabolically active with access to oxygen. A by-product is the release of carbon dioxide.

If oxygen availability is limited (anaerobic conditions where the ground is waterlogged and oxygen is slow to diffuse) some bacteria, known as ‘methanogens’ (methane producers) metabolise using fermentation reactions which produce methane as a by-product. Therefore, permafrost thawing effectively releases ancient stores of organic matter which have been safe in the ‘deepfreeze’ for thousands of years. It could lead to substantial emissions of both carbon dioxide and methane.

Methane is an even more potent greenhouse gas than carbon dioxide. Such emissions can be seen most dramatically where thawed soil organic matter finds its way into lakes and ponds, settles to the bottom, and is fermented by bacteria beneath the seasonal ice cover. Bubbles of methane make their way through the sediment and water above and are trapped beneath the ice. So much methane can accumulate that if the ice surface is broken the gas can escape violently.

Feedback

As a context, fossil-fuel combustion, and other human activities since 1850, have released around 350 billion tonnes of carbon to the atmosphere (mainly as CO₂) – the equivalent to only 22% of the carbon currently found stored in permafrost soils. Scientists now estimate that, between now and the year 2100, some 120 billion tonnes of carbon may be emitted to the atmosphere as a direct consequence of permafrost thaw.

Scientists suggest that this ‘permafrost carbon feedback’ could be equivalent to around 6% of global anthropogenic carbon emissions to the atmosphere by 2100. This could increase global temperatures by around a third of a degree Celsius - equivalent to the current contribution that China makes to total anthropogenic greenhouse-gas emissions.

More warming will lead to more permafrost thaw, which will release more greenhouse gases, meaning more warming, accelerating permafrost thaw even further - in other words positive feedback (Figure 2). Such permafrost carbon feedback has significant implications for all of us.

Figure 2. Positive feedback mechanism