Loops of Change: the Positive Feedback Loops that Drive Climate Change (Part IV)
Permafrost Thaw and Bogland Loss
You may or may not be familiar with the bog man named Tolland Man. Tolland Man lived during the 4th century BCE and is believed to have been ritualistically sacrificed and buried in a bog in what is now Tolland, Denmark. The relatively low temperature, acidic, and oxygen-poor conditions of the waterlogged bog inhibited decay of his body, allowing his skin and other soft tissues to be remarkably well preserved. So much so that the Danish woman who discovered him in 1950 while harvesting peat to burn in her stove assumed that she had come across a recent murder victim. Only after the local police hired an archaeologist to investigate the site was Tolland Man discovered to have lived more than 2,000 years prior, during the Iron Age. His body now sits in the Silkeborg Museum in Denmark.
Fascinating from an archaeological standpoint, the preservation capacity of bogs is also an important component of the global climate system. That is, the same chemical and physical properties that slowed decomposition of Tolland Man’s body also do so for dead plant material. Since decomposition releases greenhouse gases like carbon dioxide and methane, the limited decay rates in bogs allows them to serve as a significant terrestrial carbon sink.
For a terrestrial habitat to be a carbon sink, plants need to grow and photosynthesize (absorb carbon) at a faster rate than they respire or decay (release carbon). The waterlogged nature of bogs creates an anoxic, or oxygen-depleted, environment that greatly slows decomposition and allows carbon absorption to outpace release. Over time, accumulated layers of partially decayed plant matter, or peat, form a secure bank of sequestered carbon similar to the sedimentary deposits created by marine snow on the sea floor.
Climate change may drain bogs of their characteristic waterlogged conditions, thereby threatening their role as a global carbon sink. Bogs are susceptible to warming because they rely on permafrost, or frozen soil, to maintain their structure. Generally occurring in northern boreal forests, bogs form when seasonal snowmelt pools at the soil surface above a barrier of permafrost below. As permafrost thaws with climate change, this barrier weakens and bog water drains from the surface. Soils that were once waterlogged and anoxic become aerated, thereby increasing decomposition rates and carbon dioxide emissions. As permafrost continues to thaw in this positive feedback loop, drained bogs become carbon sources.
Still, while permafrost drives bog hydrology, other factors affect bog health and carbon sequestration capacity. For example, in a paper published last month in Nature Geoscience, University of Victoria professor Christopher Avis and his research team suggest that increased precipitation with climate change could help bogs maintain waterlogged conditions as permafrost degrades. On the other hand, they point out, warmer conditions will increase both aerobic and anaerobic decay rates. Anaerobic decomposition produces methane, a stronger greenhouse gas than carbon dioxide. Since bog decay occurs primarily anaerobically, methane emissions are expected to increase, thereby expediting further warming.
Tolland Man would likely be surprised to know of the global climatic implications of the bog conditions that preserved his skin and allowed him to gain fame more than 2,000 years after his death. Now imagine yourself waking up 2,000 years from today. What will the global climatic landscape look like then? Though we are in an era of rapid change now, the Earth system will eventually reclaim equilibrium over the course of geologic time. We can’t know if this will occur 200, 2,000 or more years from now.
As chaotic as 21st century climate change may seem, it at least provides us a chance to understand the Earth as a unified system in a way that Tolland Man could never have conceived. Perceiving the Earth as a unified system, and understanding the strengths and vulnerabilities of this system, may help ground us through a future of change.
Posted by Laura Poppick, Assistant Editor of Maine Climate News.
Loops of Change is a weekly series through July exploring the major positive feedback loops that drive climate change.
*Cai, S., and Yu, Z. 2011. Response of a warm temperate peatland to Holocene climate change in northeastern Pennsylvania. Quaternary Research: 75. 531 – 540.