Two posts related to the Carbon Cycle

Carbon debt and the role of wetlands

[This is the last midweek post for the UK summer holidays – just one post a week for the next few weeks.

Firstly, I recently came across the work of Zeke Hausfather, a writer on climate issues for the New York Times. Here is a summary of his views on the concept of ‘carbon debt’. I think they are useful to students of the carbon cycle.

Secondly, a brief overview on the role of wetlands in the carbon cycle.]

1.      The concept of carbon debt

Many people state it is critical for society to get to net-zero CO₂ emissions to stop the world from continuing to warm. However, by the time we get to that point it is highly likely we will have gone well past the 1.5C target of the Paris Agreement. Are we now passing down our ‘carbon debt’ to future generations that will have to be paid if they ever want to recover the climate of the past that shaped both the natural world and the development of human civilization? And increasingly, are we normalizing this carbon debt into our mitigation strategies and climate models?

For example, nearly all integrated assessment models (IAMs) that limit warming to 1.5C or 2C by the end of the century exceed the remaining ‘carbon budget’ by about 600Gt of CO₂. We are relying on carbon dioxide removal (CDR) technologies to remove vast amounts of CO₂ from the atmosphere later in the century. In effect, the Paris Agreement’s aspirational 1.5C target has become defined as an overshoot scenario.

Such carbon debt will be expensive to pay off. It is estimated that for every 0.1C we want to cool the climate after we get to zero emissions, we will have to pay around $22 trillion – assuming that we can get the cost of permanent carbon removal down to $100 per tonne. Where will this money come from?

We may end up in a world where future generations are vastly more prosperous and technically advanced than today, analogous to the difference between the 1920s and 2020s, and these future generations may be able to clean up our mess and restore the global climate to more optimal conditions. But they also may not be this wealthy – it is an assumption that future generations will inevitably be richer. There have long been concerns around inter-generational financial equity, with the fundamental unfairness of maximizing benefits for people alive today at the expense of damaging the prospects of future generations.

Future generations have no say in our decisions today despite living with their consequences. While some carbon debt is likely inevitable, should we try at least to minimize the burden we pass down to our children and future generations? As a society we generally seek to avoid passing the debts of parents down to our children. Does such a rule also apply as we consider our growing carbon debt?

We can minimize our carbon debt by rapidly reducing emissions now. We can only pay down our carbon debt once lower emissions have occurred by permanently removing carbon from the atmosphere. There is a value to temporary carbon removal in the biosphere – such as planting trees or putting more carbon in soils – but it does not actually reduce the carbon debt unless we can ensure that the carbon remains in those reservoirs indefinitely.

Similarly, since the warming from our CO₂ emissions will persist for millennia, geoengineering approaches such as solar radiation management do not actually pay off the carbon debt. Instead, they postpone when the debt is due. While there is a case to be made that they could buy us time to develop technologies to pay off the debt, the problem is that there are also real risks that governments will ‘kick the can down the road’ to reduce the impetus to either reduce emissions or deploy permanent carbon removal.

Our climate debt ultimately leaves us with two options: either pay the future price of adapting to the ravages of a hotter world – and acknowledge that large parts of the natural world will be lost in the process – or permanently remove enough carbon from the atmosphere to pay down the debt. And the more we emit before we get emissions down to zero, the more costly the carbon debt becomes.

2.      Wetlands and the carbon cycle

Wetland soils (such as peatland and fenland) sequester and store large amounts of carbon. Wetlands are areas of land that are saturated or flooded, either permanently or seasonally. They include:

·       freshwater wetlands (ponds, marshes, swamps, floodplains, fens, and bogs). The photograph below shows part of Potteric Carr, Doncaster, a large wetland owned by the Yorkshire Wildlife Trust.

·       coastal wetlands (saltmarshes, estuaries, lagoons, mangroves, coral reefs)

·       wetlands made by humans (rice paddies, fishponds, salt pans).

Between 1970 and 2020, global wetland area declined by 35% due to:

·       changes in land use, such as drainage and conversion to agricultural or grazing land

·       water diversion through dams and canals

·       construction of roads and buildings, especially in river valleys and coastal areas.

Wetlands have a significant role in the global carbon cycle:

·       they contribute an estimated 20–25% of global methane emissions to the atmosphere, which makes them the largest natural source of this greenhouse gas

·       peatlands store 20–30% of total soil carbon, roughly the same amount of carbon as is currently held in the atmosphere.

Processes

Plants take up carbon dioxide from the atmosphere and use energy from the sun in the process of photosynthesis to turn the carbon plus water into carbohydrates, releasing oxygen as a waste product. Some of the carbohydrates are used in aerobic respiration by a plant to release energy for its day-to-day activity. This process takes up oxygen and releases carbon dioxide. The rest of the carbohydrate is converted into biomass (peat), storing carbon in the plant’s structure. When plants die or shed leaves this plant litter is deposited on the peat surface.

Invertebrates and microorganisms in peat feed on the plant litter, using oxygen and respiring carbon dioxide. Due to this activity, oxygen in the waterlogged soil of fens is used up more quickly than it can be replenished. Under the low-oxygen conditions that develop, organic matter is broken down very slowly, and so the carbon-rich organic matter accumulates as peat.

Over thousands of years, peat deposits can grow to more than 10m thick. Peatlands are among the most carbon-dense ecosystems, holding more carbon per square metre than the trees in a tropical rainforest.

Under waterlogged and low-oxygen conditions, bacteria cannot use aerobic respiration to release chemical energy. Instead, they break down organic matter by fermentation, producing alcohols. Another group of microorganisms break down the alcohols, releasing methane as a by-product. If this methane travels upward into more oxygen-rich zones of the soil, another group of microorganisms break it down by a process of methane oxidation, producing carbon dioxide as a by-product.

The balance of these processes determines the carbon and greenhouse-gas balance of wetlands.

Pathways

The greenhouse gases produced in peat move out of the fen by several different pathways.

The first pathway is diffusion, which occurs when a gas moves in response to a concentration gradient, e.g. if the concentration of the gas is higher in the peat soil than in the atmosphere the gas will diffuse out of the soil to the air.

The second pathway is plant-mediated transport - movement through plants that contain networks of tubes in their stems called aerenchyma. Some reeds have this characteristic. For plants living in waterlogged soils, aerenchyma also provide a means of transporting oxygen from the atmosphere down to the plant roots. Carbon dioxide and methane dissolved in the soil water can travel in the opposite direction, from the roots, through the aerenchyma and out into the atmosphere.

The third pathway involves the bubbling of methane. When methane reaches high concentrations in waterlogged soils, it can form gas bubbles. Pressure changes can enable these gas bubbles to travel rapidly upward through the water to the atmosphere.

The effect on the carbon budget

Measuring the carbon budget of fen systems is difficult. Some floodplain fens are carbon sinks and others are carbon sources. Healthy fens accumulate carbon (through the accumulation of peat) but changes in land management or drainage may impact on their effectiveness as a carbon sink.

The situation is further complicated because a site that is releasing lots of methane may still be classed as a carbon sink but is still a net contributor to climate change because methane is a more powerful greenhouse gas (methane is 20 times more potent as a greenhouse gas than carbon dioxide).