In these recent hot summer days, as my colleague Xinnan Zhu was walking outside exposed to the outdoor temperature of nearly 100°F, she felt like she was going to melt like an ice cube under the sun. As the Arctic Climate Impacts Stanback researcher, it was far more hot and humid outside our UCS office than the work we were focusing on this summer, the opportunities for the Arctic Council and the first ever White House Arctic Science Ministerial.
This feeling involuntarily reminded Xinnan of what is happening in the Arctic under the effect of global warming. However, the sea ice melt is not the only environmental issues triggered by climate change. Here is what Xinnan has prepared to help convey the perils of ignoring Arctic permafrost.
The perils of ignoring Arctic permafrost
Starting from the late 2000s, thawing permafrost has become a growing Arctic issue being mentioned and investigated by the researchers and scientists as a side effect of the global warming. Yet, the majority of the public might not have even heard about permafrost.
So for this blog, we will introduce you to permafrost including its definition, global distribution, formation and the associated environmental risks. The reason why the thawing permafrost becomes an urgent issue that needs more attention will be explained at the end.
Permafrost distribution: Nearly a quarter of the exposed land area in the Northern Hemisphere
Permafrost is defined as ground (soil, rock, or organic materials) that remains at or below 0°C for at least two consecutive years. Most permafrost is located within or near the Arctic and some in the Antarctic region, however, the alpine permafrost is also found at high mountain region with lower latitude such as Qinghai-Tibetan Plateau in China.
Based on the lateral continuity of distribution, permafrost is generally classified as continuous permafrost, discontinuous permafrost, sporadic permafrost and isolated permafrost, typically in high latitude regions or isolated patches in mountainous regions. Another type of permafrost is found in the Arctic Ocean, called subsea permafrost. It was once part of the exposed continent in the glacial era and now is under the sea with the rising sea level after major ice sheets and glaciers melt.
Permafrost formation and soil layers
As leaves fall down onto the ground, neither carried away by wind or flushed away by rain, they remain and are eaten by microbes and decompose, are covered with dust, sediments or other organic materials and finally “disappear” as a part of the soil. This describes dominant processes for how soil is formed. However, in the case of permafrost, the process changes whenever the temperature is below zero.
Traced back to hundreds of thousands of years ago (during the Quaternary period), the organic matter settled and mixed with other dust and sediment loads from floodplains and other processes. Plant remains (leaves or roots) and animal bones were frozen before complete decomposition occurred when the temperature decreased below zero. Under repeated glacial periods, this frozen ground was formed and buried under the ground through geologic processes.
The vertical soil temperature varies, and it depends on the air and deep geothermal gradient temperature as well as the distance to the soil surface and Earth’s core. The vertical soil column is divided into three layers called active layer, permafrost, and talik. The active layer is the top soil that is seasonally frozen, since it is affected by the seasonal air temperature. The bottom talik layer is the unfrozen ground heated by the deep geothermal gradient. Permafrost lies between the active layer and deep talik, as a continuously frozen layer. The thickness of the permafrost ranges from less than a meter in the discontinuous permafrost to 1450m in the northern Siberia. In the continuous permafrost in the northern hemisphere, the typical thickness ranges from 350m to 650m.
We have to admit the fact of global warming not only because we ourselves are experiencing the consequences, but also it is confirmed by the data. The global mean surface temperature study by NASA showed a 0.5-1°C increase during the period of 1960-2011in most of the lower 48 states of the United States. However, the Arctic region has warmed twice or even more than the world average. The latest data of the global surface temperature in June, 2016 showed a peak of temperature increase at the 70ºN (within the Arctic Circle) about three times higher than the global mean.
As the Arctic is warmed by climate change, not only the sea ice melts much faster, but also the permafrost in the Arctic region starts to thaw. More importantly, the thawing permafrost will have an intensifying effect on global warming. As the global mean temperature rises, the ice melts and permafrost thaws. Those old “frozen” plant remains (e.g. leaves and roots) will begin to decompose when the soil temperature exceeds 0ºC, and release carbon in the form of carbon dioxide (CO2) and methane (CH4). As these carbon gases are increasingly released to the atmosphere, global warming will be accelerated since these additional greenhouse gases can absorb more long-wave radiation and heat up the air. Based on this chain reaction, the thawing permafrost will pose a self-reinforcing cycle on global warming.
Global carbon storage
As mentioned before, permafrost contains a high concentration of frozen organic materials. How does the carbon storage compare with the other terrestrial carbon pools of soil and vegetation? How does it compare with the carbon stored in the atmosphere and the ocean? Finally, how does it compare with fossil fuels that can be extracted from geologic sources located in either terrestrial or ocean areas?
From the global scale, carbon can be stored in the atmosphere, terrestrial and ocean, and cycle within these three pools over different time periods. According to the 2014 Intergovernmental Panel on Climate Change Fifth Accessment report (IPCC AR5), the stored carbon in permafrost is around 1,700 Pg C (Petagram Carbon = 1015 g), even more than the carbon stored in the discovered fossil fuel reserves, and more than twice as the atmosphere carbon storage (589 Pg C in 1750 and 829 Pg C in 2011).
Furthermore, permafrost is ranked as the second largest terrestrial carbon pool right after global soil (1950 Pg C). As the global temperature increases and permafrost starts to thaw, there is the potential for a large amount of carbon gases to be released to the air. This leads to a surprising fact that the permafrost starts to change from a carbon sink (storage pool) to a carbon source (production pool).
Since industrial development starting from 1750, we burn fossil fuel for energy power, cut or burn the forests and shrubs for construction, agriculture or other land uses. All these human activities have changed the ecosystem carbon cycle and carbon storage in the pools. Over the past 260 years, we burned around 25% of the total discovered fossil fuel reserves carbon (around 375 Pg C), and a net loss of carbon from vegetation by around 20 Pg C through deforestation and land use change. In total, we lost 395 Pg C through both of these pools. Consequently, on average over 60% of that loss in carbon flowed into the atmosphere, and the rest, nearly 40%, was stored in the deep ocean. As the permafrost studies only gained data on a scale relevant for carbon cycle models in this century, it is typically excluded in global carbon cycle models, the permafrost carbon loss for the last two hundred years is hard to estimate.
The 2014 IPCC fifth Assessment report indicated that we hold high confidence that if unabated, global warming can activate the permafrost to thaw. The thawed permafrost may trigger the release (in the form of CO2 and CH4 to the atmosphere) of the huge carbon buried underneath for thousands of years. The urgent problem we are facing is that we know the fact of thawing permafrost but we are uncertain about the magnitude of carbon loss due to the limited understanding of release rates at various scenarios (i.e. levels of human activities that contribute to climate change). If the nightmare truly starts, we don’t even know how far it will go and how severe it will react to an accelerated global warming. International collaboration among the world’s scientists can help resolve the remaining questions regarding the pace of change if allowed to thaw or what it takes to keep it mostly frozen this century. More about what we know about the future risks will be explored in an upcoming blog.
Correction (August 31, 2016): An earlier version of this post incorrectly listed the amount of the total discovered fossil fuel reserves carbon and the net loss of carbon from vegetation through deforestation and land use change. It is 375 Pg C and 20 PG C respectively, not 365 Pg C and 30 Pg C.