A point of no return as alarming as the tundra feedback
A new study in Nature Geoscience ($ub. req’d, abstract below) projects that “a warming of 4 degrees C causes a 40 percent loss of soil organic carbon from the shallow peat and 86 percent from the deep peat” of Northern peatlands. And that amplifying carbon cycle feedback is dangerous for three reasons:
- The northern peatlands are believed to store some 320 (+/- 140) billion metric tons of carbon, roughly half of what the atmosphere contains.
- Peatlands tend to emit much of their carbon in the form of methane, which is more than 20 times as powerful a greenhouse gas as carbon dioxide.
- A warming of 4 degrees C this century is all but inevitable if we don’t sharply reverse emissions trends quickly (see here).
This study provides yet more evidence that the carbon cycle has a point of no return beyond which it becomes all but impossible to stop catastrophic global warming — the point at which we start to lose the northern peatlands and the permafrost.
Most of the world’s wetlands are peat, which are better known as bogs, moors, mires, and swamp forests. Wikipedia notes, “Under the right conditions, peat is the earliest stage in the formation of coal.” The Reuters article on the study explains why peatlands contain so much carbon:
Peat is the accumulation of partially decayed vegetation in very wet places and it covers about two percent of global land mass. Peatlands store large amounts of carbon owing to the low rates of carbon breakdown in cold, waterlogged soils.
The carbon cycle feedback begins as human-caused global warming dries out the peatlands:
“This will cause carbon loss from the soil which means an increase in carbon dioxide concentration in the atmosphere, which will further worsen global warming,” said Takeshi Ise from the Japan Agency for Marine-Earth Science and Technology. “So we have to do something to mitigate global warming,” he told Reuters.
This, of course, is very similar to the carbon cycle feedbacks from the melting of the tundra or permafrost (see here).
Again, if we don’t keep total warming substantially below 4 degrees C, then we risk triggering vast releases of methane and carbon dioxide from the permafrost and the northern peatlands at rates that are much faster than humanity can plausibly reduce our own emissions.
Here is the full abstract of “High sensitivity of peat decomposition to climate change through water-table feedback:”
Historically, northern peatlands have functioned as a carbon sink, sequestering large amounts of soil organic carbon, mainly due to low decomposition in cold, largely waterlogged soils. The water table, an essential determinant of soil-organic-carbon dynamics interacts with soil organic carbon. Because of the high water-holding capacity of peat and its low hydraulic conductivity, accumulation of soil organic carbon raises the water table, which lowers decomposition rates of soil organic carbon in a positive feedback loop. This two-way interaction between hydrology and biogeochemistry has been noted but is not reproduced in process-based simulations. Here we present simulations with a coupled physical-biogeochemical soil model with peat depths that are continuously updated from the dynamic balance of soil organic carbon. Our model reproduces dynamics of shallow and deep peatlands in northern Manitoba, Canada, on both short and longer timescales.
We find that the feedback between the water table and peat depth increases the sensitivity of peat decomposition to temperature, and intensifies the loss of soil organic carbon in a changing climate. In our long-term simulation, an experimental warming of 4 degrees C causes a 40 percent loss of soil organic carbon from the shallow peat and 86 percent from the deep peat. We conclude that peatlands will quickly respond to the expected warming in this century by losing labile soil organic carbon during dry periods.
Finally, global peatlands contain over twice the carbon as northern peatlands — and while tropical peatlands are unlikely to dry out from human-caused global warming, they can and are being destroyed directly by humans (see here).
We have indeed met the enemy, and he is us.