How do rivers flowing from the world’s largest mountain range play a role in moderating climate?

Rivers are the veins of the Earth, transporting the water and nutrients needed to support the planet’s ecosystems, including human life. In Asia, some of the world’s largest rivers flow from the highest mountain ranges to vast floodplains below and finally into the ocean. Like veins these rivers connect the rocks in the mountains to the sediment at the bottom of the ocean and give life to everything in between. While many nutrients are essential to the survival of life, there is one element transported by water in rivers that holds the key to life and to the future of our planet - carbon.

Dr Edward Tipper of Cambridge University works on Asia’s largest rivers to understand how and where carbon is transported.

“Large river systems in tropical regions like South Asia are hotspots of weathering due to the monsoonal climate and high mountain ranges and so they are zones of major carbon transfer from the atmosphere and continents to the oceans. Recent work has shown that this carbon transfer is not as simple as previously thought, and multiple competing processes could actually release carbon dioxide back into the atmosphere,” says Tipper.

Tipper and his colleagues are investigating carbon transport in Asia’s rivers by collecting samples from the largest rivers, including the Ganges, Salween, Irrawaddy and Mekong.

“At peak flow the Mekong River transports as much carbon every single second as would be released by driving 10,000km in an average sized family car. Understanding carbon transfer processes is of utmost importance to understanding future climate.”

Carbon is everywhere and understanding the way it moves and is either emitted or stored by the Earth system is a complex science in itself. Carbon starts its journey downstream when natural acid rain (which contains dissolved carbon dioxide from the atmosphere) dissolves minerals in rocks. This neutralises the acid and transforms carbon dioxide to bicarbonate in the water that then flows in our rivers. Bicarbonate can remain in water for thousands of years - it’s actually in every bottle of mineral water, you’ll see on the back listed in the ingredients label.

This process of chemical weathering is one of the main ways carbon dioxide is removed from the atmosphere on what Earth scientists call long time-scales. Carbon is transported by rivers to the oceans and once that carbon reaches the ocean it is stored naturally in deep sea sediments for millions of years. To mitigate the impact on climate we need to know how much carbon reaches the ocean and so we have to understand what happens to it in rivers.

As carbon travels down a river, different processes may impact whether it continues to flow downstream or whether it is released into the atmosphere. Tipper says the natural system in these rivers is at a tipping point.

“Extensive dam construction is taking place across the region, which will result in dramatic changes to how water and sediment travel down the river. This in turn will have profound impacts on the people living downstream. It is our last chance to capture a snapshot of these rivers in their more or less natural state, before they are strongly altered by human engineering.”

Professor Robert Hilton from Durham University works to better understand the feedbacks between carbon transfers and the climate system, and thus how Earth surface processes may damp (or amplify) climate change.

“Through research projects like this, we can use rivers to better understand how landscapes are cycling carbon emitted by humans. It is very important that we work out whether changes in climate and river management could result in rivers being emitters of carbon dioxide, as this would force more pronounced warming.”

Earth’s climate is changing due to large variations in the carbon cycle induced by increasing emissions of carbon dioxide from burning fossil fuels. We all know about the uptake of carbon dioxide by plants photosynthesising, but do we all know about rivers? Rivers not only provide the nutrients for our agriculture and the water for us to drink, but they also provide one of the main ways of moderating climate. Altering the chemistry and the course of rivers may have profound impacts on how they transport carbon. Wherever we live, we all live downstream.

Vivien Cumming travelled to Asia as part of a European Geosciences Union Science Journalism Fellowship

By Vivien Cumming
Featured image by Vivien Cumming