Don't drain that swamp

Wetlands deserve more love (7 minute read)

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Wetlands are not friendly places for humans. In the course of my time spent in the outdoors, I’ve spent some time in wetlands and I’ve learned this first hand. I’ve trudged through mangrove swamps with my feet getting heavier as more and more dark, sulphurous mud stuck to my boots. I’ve stepped on what looked like solid ground near a mountain tarn and found myself up to my ankles in icy water the colour of tea. I’ve stepped off a track in a bog and sunk until I was almost waist deep in sodden sphagnum moss.

My experiences showing the hazards of wetlands are trivial – I suffered some brief discomfort and indignity, and nothing more. But wetlands can be more than an inconvenience. People have long recognised an association between wetlands and malaria, in fact, old names for the disease include “swamp fever” and “marsh fever”. Long before the association between mosquitoes and malaria was widely understood, swamps were drained to control the disease. It worked too, although not, as people thought, because of bad air emanating from swamps. Draining swamps reduced the habitat of malaria-spreading mosquitoes, which is part, but only part of the reason malaria was eradicated from countries like the United States, Italy and England.

Malaria isn’t the only reason that we drain wetlands. The main reason is, at first glance, wetlands don’t seem to be useful to us. They’re not much good for agriculture, unless you’re growing rice. They’re not great places to build towns and cities. They’re difficult to move around in. As a result, the loss of wetlands globally is staggering – 87% have been destroyed in the last 300 years. In New Zealand, where we like to think we do a good job at protecting out natural environment, it is even worse. We’ve lost around 85% of the wetlands in the South Island, and 95% in the North Island. But the South Island may be catching up. In the last 25 years, nearly half of New Zealand’s total wetland loss, around 2600 hectares, was in Southland.

Wetlands are found on every continent, from the coast to high in the mountains, and even in deserts, although in deserts they are usually wet only some of the time. One of the most remarkable desert wetlands is Lake Eyre, or Kati Thanda, in the centre of Australia. Most of the time, it’s as dry as dust and covered with a salty crust. But around every eight years, flood waters fill the area with water and it springs to life. When wet, the lake and surrounding wetlands support plankton, invertebrates, fish and waterbirds as well as numerous plants.

Although people have often dismissed wetlands as waste areas, wetlands are much more important for us than they at first appear. Coastal wetlands, for example, are an important buffer against storms. In the USA, areas where coastal wetlands are more intact suffer less severe damage from hurricanes. One study found that during Hurricane Sandy, flood protection from coastal wetlands prevented US$650 million in damage. On the other hand, the loss of coastal wetlands in Florida resulted in around US$430 million of additional damage from Hurricane Irma. As well as reducing flooding caused by storm surges, wetlands also reduce downstream flooding as they trap water and release it slowly. One study estimated that for every dollar spent on wetland conservation in Saskatchewan, the return on investment was $7.70 in terms of reduced flood damage.

Wetlands also help us by improving water quality. They act as filters, not only physical filters which trap sediment, but also chemical filters, where wetland plants absorb nutrients from the water. Nitrogen and phosphorus from agricultural chemicals are a problem in lakes and rivers. However, if the water moves through a wetland, these chemicals can be absorbed by plants.

But the main reason that I wanted to talk about wetlands is that they have an important, but not well-known, role in climate change. Like forests, wetlands are important carbon sinks. However, unlike forests, we don’t hear much about them.

In November, I talked about the carbon cycle when I wrote about deforestation. Very briefly – plants absorb carbon dioxide from the air when they grow, store it while they are alive and then release it back into the atmosphere when they die and decompose. As well as storing carbon while the plants are living, ecosystems such as forests also store carbon in the soil, in the form of partially decomposed plant (and animal) matter. There’s a huge amount of carbon stored in soil – around 1550 gigatons. In comparison, only around 400 gigatons is stored in the living plants of forests.

But the carbon stored in soil isn’t distributed evenly, and that’s where wetlands come into the story. Although they cover only about 5-8% of the land’s surface, wetlands hold 20-30% of the soil carbon. But why do wetland soils hold so much carbon?

To understand why, it helps to understand that not all wetlands are the same. There are many different words for wetland – swamp, bog, mire, marsh and fen for example – and the words actually refer to different types of wetland. A bog refers to an area where peat has accumulated and the water is usually highly acidic. Bogs are fed only by rainwater and have very low fertility. Mires and fens are also peat-dominated wetlands, but differ in age, fertility and water sources. A swamp, on the other hand, is a fertile wetland which receives nutrients from surface and ground water. Marshes are wetlands with a fluctuating water level – the most obvious example is a salt marsh, inundated by the tide twice a day.

Salt marsh, estuary of the River Nevern, Wales (Image credit: Michael Roberts/ Getty Images)

The carbon stored in wetlands is mostly stored in the form of peat in bogs, mires and fens. Peat forms when plant material breaks down in conditions of waterlogged soil, with low oxygen, low nutrients and high acidity. As the broken down plant material builds up, it compacts, and becomes the material known as peat. Most of today’s peat wetlands started forming at the end of the last ice age, around 12,000 years ago, and have been storing carbon ever since.

But the link between wetlands and climate change is more complicated than at first appears. Because wetland soils are waterlogged, oxygen from the air can’t move through the soil. Because there’s no oxygen, the breakdown of plant material doesn’t form carbon dioxide, as it does in a typical garden soil. Instead, most of the carbon in organic matter breaks down to form methane. Methane is a much more potent greenhouse gas than carbon dioxide – weight for weight, methane contributes 25 times more to climate change than carbon dioxide. And wetlands are responsible for 20-25% of methane emissions.

So, while wetland soils are storing a lot of carbon due to their slower decomposition, they are also contributing to climate change by releasing methane.

What does this mean for the role of wetlands in climate change? Are they helping us, or making the situation worse? The answer, it turns out, isn’t simple. It depends on the type of wetland and its condition, as well as how you do the calculations. One large study from 2013, which measured and modelled carbon in a number of different wetlands from tropical to temperate, found that most wetlands were overall carbon sinks, meaning that they absorbed more than 25 times the carbon that they emitted. This finding applied to wetlands that had been restored or artificially created, as well as intact natural wetlands. When it comes to limiting climate change, wetlands are our friends.

But there was one notable exception in the study I quoted above. The exception was the Russian tundra, a vast area of peat wetland around the Arctic Circle. These wetlands are emitting more carbon than they absorb. And it isn’t just Russia. A similar picture is seen in the north of Canada, which also has vast areas of tundra.

Tundra, Alaska (image credit: Patrick J. Endres/ Getty Images)

When I read that fact, I stopped, went back and read it again. Because, of all the types of wetlands that there are, the type that is most important for carbon storage is tundra. The very wetland that is most important in terms of carbon storage is the one type of wetland which emits more than it absorbs. What’s going on? The answer to that question is complicated, so I will return to it in two weeks, when I will be taking a closer look at how climate change is affecting the Arctic.

The Turnstone comes out once a week. Every two weeks, I publish an original article, like this one. On one of the alternate weeks I send out “Talking about vaccines” - resources to help you have better conversations about vaccines. On the other week I share something about climate change, called “Talking about climate change”. The focus is conversations we can have and actions to take on climate change.

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