More than a decade after writing The Jungle Book, British novelist Rudyard Kipling penned a poem in honor of the major waterway running through London: the Thames River. “The River’s Tale” speaks of modern bridges stretching across the river, young and naive in comparison to the age-old waters of the Thames.
Centuries later, scientists continue to plumb the depths of the river’s wisdom.
It is well known that widespread industrialization, particularly during the mid-1800s, drastically altered landscapes all over the globe. These land use changes are thought to have increased the amount of carbon being transported by rivers to the world’s oceans. This phenomenon, known as “global browning,” affects aquatic life, water resources, and climate.
Research suggests that much of the additional carbon entering waterways through global browning comes from soil. However, scientific knowledge of this and other possible driving factors is limited, as past studies have used mainly short-term observations from the past 50 or so years.
Rather than muddy the waters with more short-term studies, Noacco et al. reconstructed concentrations of carbon in the Thames River Basin over 130 years using more than 20,000 samples of water color and dissolved organic carbon concentration measured at the outlet of the basin. This region, with its lowland landscape, temperate climate, mineral soil, and a mix of urban and rural land uses, also provides more diversity than in past studies, which have focused mainly on rural areas.
The researchers adapted a carbon export model covering the entire United Kingdom, linking environmental properties of the Thames River Basin to the release of carbon into rivers. They combined this with another model that showed the effects of land use changes on the release of carbon from soils. In concert, these models allowed them to compare the influence of various factors related to land use, climate, and population on changes in carbon levels. Then, armed with knowledge of land use and land use change history, climate, and population changes to the basin, they estimated carbon levels throughout the basin since 1884.
The team found that urbanization is responsible for close to 90% of the observed long-term rise in carbon. Urbanization led to a boom in population growth, which (by causing a major increase in the amount of liquid sewage discharged to waterways) released much more carbon than previously believed. This urbanization was responsible for around 671,000 metric tons of carbon feeding into the basin over the 130-year period.
Shorter-term effects on carbon concentrations in the Thames, the researchers found, came from soil disturbed by land use changes. The biggest disturbance by far was World War II, during which nearly half of the grasslands in the Thames River Basin were plowed to create farmland. As a result, in just a few years, 45,000 metric tons of carbon that had built up in the soil over decades were released into nearby rivers.
The basin’s population, which has grown fourfold since the 1880s, continues to increase. This puts pressure on drinking water resources (the Thames currently provides two thirds of Londoners’ drinking water), as well as on wastewater treatment, water quality, and river ecology. Although the researchers did not find that warming temperatures have increased carbon levels in the Thames River Basin thus far, they expect rising temperatures and shrinking river flows due to climate change to further amplify these pressures.
The study develops a framework for modeling major drivers of carbon—not just in the Thames, but in rivers around the world—and highlights the importance of urban areas in the global transport of carbon to rivers and oceans. Given that more than half of the world’s population currently lives in urban areas and nearly 70% is expected to do so by 2050, the authors speculate that it may be necessary to actively protect sensitive basins from urban development because it is likely to trigger the release of even more carbon into rivers. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2016JG003614, 2017)
—Sarah Witman, Freelance Writer