No time to waste: Tropical forests become source of global warming

Findings put even most dystopian climate change models into perspective
Tree measuring, Salonga National Park, Democratic Republic of Congo. Credit: Simon Lewis, University of Leeds

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Terry Sunderland, a collaborating author of the Nature article “Asynchronous carbon sink saturation in African and Amazonian tropical forests,” is a senior associate with the Center for International Forestry Research (CIFOR) and professor in the Faculty of Forestry at Canada’s University of British Columbia.

According to findings of research published last week in Nature, Tropical forests are losing the capacity to remove carbon from the atmosphere, which could have dramatic implications for their potential to mitigate the impact of global warming.

Intact tropical forests are a vital global carbon sink, slowing climate change through carbon sequestration, a process through carbon is removed naturally from the atmosphere and stored in trees.

Climate models upon which targets in most international agreements – including the U.N. Paris Agreement — are based, typically predict that tropical forests will act as a carbon sink for decades.

However, during investigations led by Wannes Hubau at Belgium’s Royal Museum for Central Africa and Simon Lewis at Britain’s University of Leeds, we have learned that their storage capacity is decreasing.

With contributions by researchers at almost 100 institutions, Hubau and Lewis oversaw a major project, tracking 300,000 trees over 30 years in 565 tropical forests throughout Africa and the Amazon.

Much of the data originate from individual research teams and projects, often primarily focused on local aspects of forest dynamics and diversity. What the team lead by Professor Lewis and others has done is bring these disparate data sets into a “big data” network.

Their findings demonstrate that undisturbed tropical forests are gradually becoming weaker sinks, and tentative projections suggest that in the Amazon — but not Africa — they might eventually start to release some of the extra carbon they have absorbed in the latter part of the 2030s.

The transformation is caused mainly by carbon losses from tree mortality, mostly due to rising temperatures and accompanying droughts, which have started to catch up with the enhanced growth caused by CO2 fertilization effects.

The overall uptake of carbon into the planet’s intact tropical forests actually peaked in the 1990s. By the 2010s, on average, the ability of a tropical forest to absorb carbon had dropped by one-third.

The fundamental message of these findings is that due to drought and overall temperature increases in both the Congo and Amazon basins, large-scale tree mortality is occurring.

Coupled with ongoing deforestation and degradation, this means that the carbon storage potential of these systems is being compromised. This is the fundamental message of the paper that has caught the attention of international news media — and rightly so.

However, there are wider implications from the study. The research presents dramatic evidence that makes even the most dystopian of climate models look better.

The potential consequences are enormous.

Given the widely accepted notion that large blocks of tropical forests are carbon sinks, we have probably been too complacent in terms of arresting emissions elsewhere because they are regarded as being “offset” by forest conservation.

Clearly this needs to be reconfigured if we consider the purpose of the 2015 U.N. Paris Agreement on climate change under which the aim is to hold average temperatures in check and stop them from rising more than 1.5 to 2 degrees Celsius above pre-industrial times.

We cannot achieve that target if our forest sinks are unable to sequester the continual carbon emissions emanating mainly from the Global North.

If these tropical forest blocks turn from sinks to sources of carbon, this will not only have major policy implications — yet another failed commitment and missed targets on global warming — but will mean we have wasted yet another opportunity to mitigate the impacts of climate change.

The 10 goals established under the 2014 New York Declaration on Forests signaled a commitment to halve the rate of deforestation, particularly related to agricultural commodity production, by 2020, to end deforestation by 2030 and restore millions of hectares of degraded landscapes – again, another failed target.

According to the five-year assessment, despite large commitments from countries, the world is not on track to halve deforestation and restore 150 million hectares of forests, putting the achievement of climate targets at risk.

In addition, it is worthwhile noting that although tropical forests contain more than 50 percent of the world’s terrestrial biodiversity; their degradation and loss has a much more profound impact on extant biodiversity upon which we rely upon in a myriad of ways, as detailed in The State of the World’s Biodiversity for Food and Agriculture, produced by the U.N. Food and Agriculture Organization.

The huge network of permanent sample tree plots the research in Nature is based on were established and re-measured over time – in some cases over 30 years. These research sites are expensive to establish, maintain and re-census, yet are woefully underfunded.

If the global community is serious about ensuring we have an early warning system such as these plot networks in place to warn of potential climate impacts, then long-term funding for such networks must be made available.

Indeed, as Hubau said: “The lost sink capacity in the 2010s compared to the 1990s is 21 billion tonnes carbon dioxide, equivalent to a decade of fossil fuel emissions from the UK, Germany, France and Canada combined.”

This translates as follows: intact tropical forests removed 17 percent of human-made carbon dioxide emissions in the 1990s, an amount reduced to only 6 percent in the teens.

He and Lewis emphasize the need for policies to stabilize the earth’s climate before they are no longer able to sequester carbon.

I wholeheartedly agree and support this because, as they say, the future of humanity is at risk if “carbon-cycle feedbacks really kick in, with nature switching from slowing climate change to accelerating it.”

Next we need to measure the impact of this shift on biodiversity.

The paper was funded by over 50 grants over two decades, including a European Research Council grant, Tropical forests and the changing Earth system, T-FORCES, to Oliver Phillips and Simon Lewis.

[This story was updated on March 16, 2020 to clarify how tropical forests are gradually becoming weaker sinks and the potential consequences]

For more information on this topic, please contact Terry Sunderland at
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