World’s Largest Tropical Forests Reaching Dangerous Climate Tipping Point
The Intergovernmental Panel on Climate Change (IPCC) recently released the first installment of its Sixth Assessment Report. Unlike past reports, this one details “tipping points,” thresholds past which natural processes become dramatically altered, sometimes irreversibly (like the melting of the Greenland ice sheet).
One such daunting tipping point would be the transformation of the world’s largest tropical forests from carbon sinks into sources. Right now, tropical forests still absorb more carbon dioxide than they release because despite massive carbon emissions from deforestation, forests are a net carbon sink thanks to existing forests and reforestation. Tropical forests, in particular, have played an outsized role in carbon uptake, to say nothing of their wealth of biodiversity. But new research shows some tropical forests may only have a short time left as carbon sinks, if we don’t act soon.
Despite growing ground observations and improved satellite data, scientific consensus is lacking on exactly how much carbon is being emitted and absorbed by tropical forests — referred to as carbon fluxes. Researchers had previously wondered if climate change would actually increase tropical forests’ long-term ability to absorb carbon, but a slew of recent research indicates otherwise.
Amazon closer to tipping point, but African forests will soon follow
A groundbreaking study published in Nature in 2020 by a slew of researchers led by Wannes Hubau and Simon Lewis attributed competing positive and negative drivers of carbon absorption in Amazonian and African tropical forests. In both regions, they found an increase in tropical forests’ absorption of carbon (3.7 percent) due to higher levels of carbon dioxide (CO2) in the atmosphere (referred to as CO2 fertilization) — but from there, the results diverged. Carbon uptake in African tropical forests was marginally hampered due to increasing droughts (-0.5 percent) and rising temperature (-0.1 percent). But in the Amazon, those losses were significantly more pronounced, with droughts accounting for 2.7 percent reductions in the forest’s carbon uptake and increased temperatures causing an additional 1.1 percent loss.
But what accounts for the difference? Hubau and colleagues point to differing conditions — African tropical forests are on average 1.1 degree Celsius (°C) cooler and grow at higher elevations than their Amazonian counterparts. African tropical forest tree species may also be better adapted to drought. Amazonian trees also retain carbon for shorter intervals (average of 56 years) than African trees (average of 69 years).
We can therefore expect to see greater carbon emissions from African tropical forests about 10 to 15 years after the Amazon, with a projected 14 percent decline in carbon absorption there by 2030 (Figure 1a). Even more sobering, the Amazon is projected to change from a carbon sink to a net source of carbon by around 2035 (Figure 1d).
The Amazon’s dwindling role as a carbon sink has been reinforced by research published in 2021 by Gatti and colleagues, as well as by Saatchi and colleagues. The eastern (and particularly southeastern) regions of the Amazon have seen a marked increase in deforestation, warming, drought, and fires over the last 40 years. These drivers are self-perpetuating — creating even drier conditions that further exacerbate tree mortality, diminished photosynthesis, and higher carbon emissions.
Models still counting on tropical forests as carbon sinks , but that’s wrong — and potentially dangerous
These studies send a strong signal that the world’s largest tropical forests have already reached or are nearing their “saturation points,” after which they can no longer absorb more carbon than they produce. In fact, Hubau’s study suggests tropical forests’ peak sequestration capacity occurred in the 1990s and has decreased by about 50 percent from the 1990s to 2000s. Between 1990 and 1999, tropical forests were responsible for absorbing 17 percent of total global anthropogenic CO2 emissions, but in the last decade that has dropped to only 6 percent.
And the world’s best climate models are not capturing this phenomenon. A publication by Koch and colleagues builds upon Hubau’s research, showing that the saturation and decline in carbon uptake has not been captured by Earth System Models (both CMIP5 and CMIP6). Even in the most pessimistic of model scenarios, tropical forests are represented as a modest net carbon sink for the next several decades.
Making the world’s tropical forests a global priority at COP26
So what can be done? Authors such as Harris and Xu reinforce the importance of curbing deforestation, which has risen precipitously in the last decade (Figure 2 a, d). This is especially important in carbon-rich tropical forests in the Amazon, Africa, and Southeast Asia. Xu also notes the impact of prioritizing growth in secondary forests in tropical ecosystems — for instance, through policies that incentivize regrowth of forests on abandoned farmlands as farmers migrate to cities, and then protecting those forests from being re-cleared at maturity.
As COP26 and the Global Stocktake approach, and country representatives are challenged with the task of how to meet existing and new commitments to lower national emissions, Harris and Xu encourage decision-makers to take advantage of new inventory frameworks to track and analyze data on international, subnational, and regional scales in order to strengthen the accountability of their forest-based mitigation targets.
Decision-makers can also now refer to a newly released tropical forest vulnerability index produced by Saatchi and colleagues. This first-of-its-kind index can highlight especially vulnerable subregions within the world’s tropical forests, based on a variety of ecologically important response variables (Figure 3). It can be used to signal early warnings for areas in danger of reaching their “tipping points,” transitioning into new and irreversible states.
Decision-makers must also consider natural climate solutions within the Earth system as a whole. For instance, another publication by Koch recently performed a series of idealized model experiments in which all tropical forest deforestation was instantly discontinued, allowing for restoration. This resulted in greater carbon sequestration in tropical biomass, but interestingly it didn’t translate into a notable reduction of global temperatures by 2100. The authors attribute this to greater sequestration by oceans and biomass in temperate and boreal regions — basically, without tropical forests absorbing atmospheric CO2, absorption rates in the oceans and non-tropical forests is higher and compensates.
But these findings do not mean that stopping deforestation in tropical forests is not a global climate imperative. Rather, the researchers point out that sequestration in forests does not serve as an alternative to rapidly eliminating fossil fuel emissions — we urgently need both. Plus while the model scenarios didn’t translate into temperature reductions, widespread restoration did reduce peak atmospheric CO2 and temperatures in the next few decades, buying valuable time for negative emissions technologies to scale up. And it reduced the carbon uptake required by the oceans (and associated impacts on marine systems). Furthermore, restoring tropical forests will be critical for species migration and survival over the coming decades.
Finally, multiple researchers emphasize that natural climate solutions in the tropics should be co-developed with local stakeholders. Without question, preserving and restoring the world’s most carbon-rich, biodiverse forests is a top priority for curbing the worst impacts of climate change. And thanks to advances in on-the-ground observations and satellite data, decision-makers are better equipped than ever to understand the role tropical forests can play.
