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Author: Teresa Silverthorn, Postdoctoral Research Associate, University of Liverpool
Original article: https://theconversation.com/ditches-and-canals-are-a-big-yet-overlooked-source-of-greenhouse-gas-emissions-new-study-250240
It’s a cold winter morning in the bleak and bare arable fields of the East Anglian fens. At the edge of a field, a scientist dips a long pole into a ditch. So, what is a climate researcher doing here?
We are measuring greenhouse gas emissions from ditches and canals by collecting samples of ditch water and analysing them in the laboratory. We also use floating chambers – a low-tech creation (sometimes coupled with high-tech sensors) made of a plastic bucket and noodle-shaped swimming floats that sit on top of the water and collect the gases emitted from it.
As freshwater biogeochemists, we investigate how elements like carbon and nitrogen are cycled through freshwater ecosystems such as rivers, lakes and ponds. We study how human-induced pressures including eutrophication – when excess nutrients cause algal blooms that deplete oxygen – and climate change affect these cycles.
Unlike many other scientists, we have a fondness for ditches and canals (we’ll call them all ditches from now on), which don’t tend to receive a lot of attention in the freshwater research world.
Researchers have previously calculated that ditches emit up to 3% of the total global methane emissions from human activities. In our new study, we find they also emit a lot of CO₂ and nitrous oxide.
In fact, when comparing the same surface area, ditches emit more CO₂ and nitrous oxide than ponds, lakes and reservoirs – probably due to the high nutrient inputs that go into ditches.
Mike Peacock, Author provided (no reuse)
Using a rough approximation of the global surface area of ditches, we estimate that including ditches would increase global freshwater CO₂ emissions by up to 1% and nitrous oxide emissions by up to 9%.
These percentages might seem small, but they add up. When accounting for all three greenhouse gases, the world’s ditches emit 333Tg CO₂e (teragrams of CO₂ equivalents – a common unit to express the total climate impact of all greenhouse gases). This is nearly equivalent to the UK’s total greenhouse gas emissions in 2023 (379Tg CO₂e).
For this study, we collaborated with ditch experts from the UK, Netherlands, Denmark, Australia and China. We collected existing data of greenhouse gas emissions from 119 ditches in 23 different countries, across all major climate zones.
We estimated that global ditches cover about 5,353,000 hectares – about 22% of the UK’s total land area, or the whole of Costa Rica. However, researchers still don’t definitively know the global extent of ditches – they may actually cover a much larger area.
Ditches are human-made, linear waterways built to serve a variety of purposes. By draining wetlands, they can help create productive soils for growing crops or trees.
They also transport water for irrigating crops. Some are built to create desirable waterfront properties. Bigger canals play a role in shipping and transportation, while roadside ditches serve to redistribute storm water runoff.
The global length of ditches is unknown but very large. In many European countries, the total ditch length rivals that of their streams and rivers. The Netherlands has 300,000km of ditches criss-crossing agricultural land. In Finland, networks of forestry drains total around 1 million km.
INGI/Google (2024)
Ditches can emit large amounts of greenhouse gases (CO₂, methane and nitrous oxide) that contribute to global warming and climate change. Ditches often contain stagnant water and are commonly found in agricultural and urban landscapes, which means they can receive high nutrient inputs from agricultural runoff containing manure and fertilisers, and from stormwater runoff containing lawn fertilisers, pet and yard waste.
This creates the low-oxygen, high-nutrient conditions ideal for the production of greenhouse gases – especially methane and nitrous oxide, whose global warming potentials are much higher than CO₂. Given their extent, ditches therefore make a notable contribution to freshwater greenhouse gas budgets in many countries throughout the world.
Mike Peacock
Fence, plant and dredge
By considering ditches when reporting their annual greenhouse gas emissions, nations can build a more accurate picture of the problem. Proper quantification can also help researchers target ways to reduce greenhouse gas emissions from ditches. For example, stronger legislation can limit the use of fertilisers and manure near ditches.
In Australia, installing fences to prevent cattle from entering farm dams has reduced methane emissions from dams by half. A similar strategy could be applied to ditches to minimise the amount of nutrient-rich manure flowing into them.
Planting more trees along ditch banks could help take up some of the nutrients and lower water temperature through shading, which also reduces greenhouse gas production. Dredging ditches can remove nutrient-rich sediments, while aerating ditch water can make conditions less ideal for the production of methane.
So, solutions do exist – but they’ll only be employed and scaled up once the significance of emissions from ditches is quantified and more widely recognised.
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