November 2020   |   Volume 22 No. 1

Cover Story

Carbon Sinks Losing Ground

Mangroves are amongst the most valuable of natural ecosystems, supporting coastal fisheries and biodiversity.
Tropical forests and mangroves play an important role in absorbing greenhouse gas emissions, but rising temperatures are putting them at high risk of dying off.

Intact tropical forests that are untouched by human activity absorbed 17 per cent of human-made carbon dioxin emissions in the 1990s, or about 46 billion tonnes. But two decades later that has fallen to six per cent, or 25 billion tonnes. 

The compromised capacity of these forests to literally take some of the heat from rising emissions was the subject of a major international study published on the cover of Nature this year that tracked more than 300,000 trees in the Amazon and Africa over 30 years. 

The scientists, including earth-system modeller Dr Alexander Koch, a Post-doctoral Fellow in HKU’s Department of Earth Sciences, provided hard evidence that these forests are at the brink of a path of diminishing returns. 

“The tropics are currently a carbon sink because growing trees absorb more carbon than they emit when they die off and decay. But future warming will increase mortality. More trees will die, or they will grow faster and die faster,” he said. 

The threat to the trees comes from higher temperatures, faster temperature increases and more frequent droughts. While these factors have been known to be affecting forests for some time, they had not been quantified to the same extent as the current study. 

Scientists from 94 institutions were involved in taking measurements of trees at 565 forest patches every few years over three decades, including diameter and estimated height. They also calculated the carbon stored in both the trees that survived and those that died, to track carbon storage over time. 

Measuring trees

Measuring trees in Lope National Park, Gabon. (Courtesy of Simon Lewis, University of Leeds)

A large tree in a forest

A large tree in Esuboni Forest Reserve, Ghana. (Courtesy of Sophie Fauset, University of Plymouth)

Trees dying off

This information was fed into a statistical model that incorporated past and projected emissions of carbon dioxide, temperature and rainfall up to 2040. The data showed the forests’ overall carbon uptake peaked in the 1990s and that the Amazon sink began to weaken first, starting in the mid-1990s. Africa followed about 15 years later. 

Dr Koch and colleagues have also provided an answer as to when forests will reach a tipping point, in another recent study published in Science that focussed on temperature in tropical forests. 

They found when the monthly mean rose above 32.2 degrees Celsius, there was a substantial drop in a forest’s carbon uptake. “Above this temperature, especially under dry conditions, tree mortality exceeds tree growth and reduces biomass,” he said. 

The forests in their study had monthly means ranging from 28 to 34 degrees Celsius, with many approaching 31 degrees. “If temperatures increase by two degrees [in the coming decades], we would lose a substantial amount of tropical forest and potential carbon uptake,” he said. 

Yet two degrees may be a best-case scenario. Under the 2015 Paris Agreement on reducing greenhouse gas emissions, parties pledged to keep temperature increases ‘well below’ two degrees by 2100 and to aim for 1.5 degrees, but this will require substantial investment in emission reduction. Some projections suggest the world could heat up by more than four degrees Celsius this century if nothing is done to curb emissions. 

Researchers studying the growth of mangroves

Dr Nicole Khan (right) and her colleague investigating the growth of mangroves.

Sea-level rises threaten mangroves 

Tropical forests are not the only carbon sinks threatened by global warming. A study published in Science by another international group of scientists, including Dr Nicole Khan, Assistant Professor of Earth Sciences, found faster rates of sea-level rise associated with higher temperatures also threaten mangroves. Mangroves soak up greenhouse gas emissions at greater densities than other forests, provide protection from storm surges and accommodate nursing grounds for young fish. 

The researchers used sedimentary archives to examine how mangroves responded to sea-level fluctuations over the past 10,000 years and to estimate the probability of mangrove survival under rates of sea-level rise projected for low and high emissions scenarios. They found mangroves would be unlikely to keep pace with rates projected for 2050 if emissions remain high. 

A factor in this is that much of their hinterland has been developed. Mangroves typically keep pace with sea-level rises by building up their substrate vertically. When sea levels rise faster than their ability to build vertically, they encroach inland. 

“If we continue on a high-emissions trajectory, mangroves will face a high risk of loss. We need to adopt coastal management and adaptation measures to give them room to grow,” Dr Khan said. (A separate HKU study has demonstrated that such mitigation measures can help – see Reasons to Feel Less Helpless

Dr Khan was also involved in another study that showed global mean sea levels could rise by more than one metre by 2100 if emissions are not reduced. By comparison, the level has risen by 0.2 metres since the late 19th century. 

“Hong Kong and the Greater Bay Area have a large population and extensive infrastructure located in vulnerable, low-lying areas. These will be increasingly exposed to the impacts of sea-level rise and coastal flooding if emissions targets are not met,” she said.

If temperatures increase by two degrees [in the coming decades], we would lose a substantial amount of tropical forest and potential carbon uptake.