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| Wed, 24th Mar 2021 13:49:00 |
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The rivers that 'breathe' greenhouse gases |
At first glance you would assume the New Territories were one of Hong Kong's greenest areas – the region that borders the Chinese mainland and makes up the bulk of Hong Kong's territory seems a world removed from the bustling streets and dense cluster of skyscrapers that tower over much of the city centre. By contrast, the New Territories are mostly rural and home to large swathes of farmland, rolling greenery, wetlands, mountains, parks and rivers.
On the surface, the New Territories appear to be Hong Kong's green lung, but the reality is rather more disconcerting. The rivers that snake through this lush landscape are in fact breathing out large quantities of greenhouse gases, according to a study of 15 of the area's waterways.
"All the river waters were supersaturated with the three main greenhouse gases, namely carbon dioxide, methane and nitrous oxide," says Derrick Yuk Fo Lai, a professor in the department of geography and resource management at the Chinese University of Hong Kong. Lai found that the concentrations of these gases were sometimes 4.5 times higher than the atmospheric concentrations.
The study, which assessed the impact of water pollution on greenhouse gas emissions in Hong Kong, indicated that rivers in the area are persistent sources of atmospheric greenhouse gases and could contribute to climatic warming. "We found that all our studied rivers would contribute to greenhouse gas emissions," says Lai. The team found that the more polluted the river was, the greater its emissions.
Discharge from livestock farms, misconnections in old buildings and unsewered premises were the main reasons for the pollution. In fact, the mean saturation levels of CO2, methane and nitrous oxide (N2O) in the more polluted rivers were about 2.2, 1.5 and 4.0 times, respectively, than those in the less polluted rivers.
"While the magnitude of carbon emissions from our rivers is small relative to that arising from fossil fuel combustion in our daily activities, their contribution to the total greenhouse gas budget of Hong Kong should not be ignored and should be minimised as far as practicable to mitigate future climate change," says Lai.
The rivers in Hong Kong's New Territories region are not unusual in this regard. Surprisingly, rivers are a significant source of greenhouse gases globally.
It's estimated that rivers and streams release up to 3.9 billion tonnes of carbon each year (around four times the amount of carbon emitted annually by the global aviation industry). When you take into account the relatively small area taken up by rivers on the planet, that figure is remarkably large. In addition, it's estimated that aquatic systems like rivers and lakes contribute more than 50% to atmospheric methane, and global river N2O emissions have come to exceed 10% of human emissions.
The reason being, “rivers receive large inputs of carbon and nitrogen from the landscapes they drain”, says Sophie Comer-Warner, a biogeochemist and research fellow at University of Birmingham. “It used to be thought that rivers just transported these elements to the ocean, but we now know that they have high rates of biogeochemical reactivity.”
In other words, the various forms of carbon and nitrogen the microbes receive are broken down into other forms, usually through aerobic or anaerobic respiration, which release CO2 and may also release methane and N2O.
"To some extent rivers acting as a source of CO2 and other greenhouse emissions to the atmosphere is a natural part of the ecosystem," says Comer-Warner. "However, emissions are likely to become higher due to the condition or health of rivers."
For urban rivers in particular, higher emissions are becoming a growing problem. In some cases, urban rivers have been found to emit four times more than of the amount of greenhouse gases than rivers in natural sites.
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