26 January 2022

Have tipping points already been passed for critical climate systems? (4) Forests and the Amazon: A faltering carbon sink

by David Spratt

Fourth in a series.    Read 1  |  2  |  3  |  4  |  5  |  6  |  7

Deforestation in the Brazilian Amazon.
Image: Chatham House

The plant-based terrestrial biosphere may be understood as including the world’s land-based plants, soils, derived dead organic matter, such as litter, and soil organic matter. 

Plants photosynthesis uses carbon dioxide and water to produce sugar (glucose) and oxygen. Around 30% of the additional carbon dioxide (CO₂) produced by human actions has been drawn down from the atmosphere (mitigated) by increased plant photosynthesis. This adds to the land-based sink of stored carbon. But plants also respire, for example at night and in winter, by converting oxygen and stored glucose back into water and carbon dioxide. 

As the planet continues to warm, a point of warming is reached — the “thermal maximum for photosynthesis” — after which combination of the rate of photosynthesis decreasing and the rate of respiration increasing results in the net flux of CO₂ from the atmosphere decreasing.

Together with more severe droughts and wildfires that also add to plant-based CO₂ emissions, the total amount of carbon stored in the terrestrial biosphere (the land sink) then starts to fall. This may be understood as a tipping point, a threshold beyond which large change is initiated in the terrestrial biosphere.

In ground-breaking research published in January 2021, Katharyn Duffy and colleagues mapped the relationship between increasing temperatures and carbon uptake by analyzing more than 20 years of data from 250 sites that measure the transfer of CO₂ between plants, land and the atmosphere. They found that in recent hot periods the thermal maximum for photosynthesis had been exceeded. The land sink is now approaching a tipping point, and the sink could halve in as soon as two decades:

We show that the mean temperature of the warmest quarter (3-month period) passed the thermal maximum for photosynthesis during the past decade. At higher temperatures, respiration rates continue to rise in contrast to sharply declining rates of photosynthesis. Under business-as-usual emissions, this divergence elicits a near halving of the land sink strength by as early as 2040.

When those hot periods become the norm — as they will within a decade or two, because further warming of half a degree or more is already in the system — a tipping point will have been reached (with just the current level of greenhouse gases enough to trigger that event).

Christopher Schwalm, an ecologist and earth system modeler at the Woodwell Climate Research Center, says the findings mark a tipping point at which “the land system will act to accelerate climate change rather than slow it down”. He said that “For my money, the results are conservative, because forest die-offs are not factored into this”, but he was “surprised that this tipping point would happen so soon, maybe in 15 to 25 years, and not at the end of the century.”

Some ecosystems are more vulnerable than others, especially rainforests that are also under pressure from land-clearing for agriculture. A particular concern is the Amazon, which currently stores more than 120 billion tonnes of carbon. Lower precipitation, deforestation, drought and increasing wildfires could turn the Amazon rainforest to savanna. The Amazon carbon sink declined by a third between 2005 to 2015, when compared to the 1990s, mostly a result of increased tree mortality.

In 2018 scientists concluded that “severity of the droughts of 2005, 2010 and 2015-16 could well represent the first flickers of [an] ecological tipping point” and that “the whole system is oscillating” to non-forest ecosystems in eastern, southern and central Amazonia”. Another study, in 2020, found the recent climatic instability in both Amazonia and the Andes and the drought-induced and fire-induced tree mortality “are aspects of critical slowing down; both possibly portending an imminent tipping point”.

In fact, Brazil's Amazon basin forest emitted more CO₂ than it has absorbed between 2010 and 2019: it gave off 16.6 billion tonnes of CO₂, while drawing down only 13.9 billion tonnes. "We half-expected it, but it is the first time that we have figures showing that the Brazilian Amazon has flipped, and is now a net emitter," said study co-author Jean-Pierre Wigneron. Another study concluded that southeastern Amazonia, in particular, is acting as a net carbon source (total carbon flux minus fire emissions) to the atmosphere. 

As well, a 2021 study concluded that current warming from non-CO₂ agents (especially methane and nitrous oxide) in the Amazon Basin largely offsets — and most likely exceeds — the climate service provided by atmospheric CO₂ uptake.

How cascades connect: Arctic sea ice, Greenland, the Gulf Stream and the Amazon

Tipping points and potential cascade effects
Contributing to the Amazon problem are changes in another major climate system, the Atlantic Meridional Overturning Circulation (AMOC) — often colloquially known as the “Gulf Stream” — which has been weakening for several centuries, and has slowed 15% since the mid-20th century.  The rate of change is accelerating, with climate models projecting further slow down. 

The near-term loss of summer Arctic sea ice is driving an accelerating rate of ice mass loss from Greenland, with the resultant increasing freshwater injection from Greenland contributing to the AMOC slowdown. An AMOC slowdown would reduce regional warming a little, especially in Europe, but would also lead to a reduction of ocean CO₂ uptake, and thus an acceleration of global-scale warming.

AMOC weakening can have a significant impact on the terrestrial primary productivity and carbon storage of the American Tropics,  including changes in  the seasonal cycle of precipitation over Amazonia.

Niklas Boers of the Potsdam Institute for Climate Impact Research reported in 2021 that he had found “an almost complete loss of stability over the last century” of AMOC  currents, and detected “significant early-warning signals” of an AMOC collapse that would have “severe impacts on the global climate system and further multi-stable Earth system components”. 

Some scientists say the future impact of changes in Greenland on AMOC and ocean circulation more generally are being underestimated. Former NASA climate chief James Hansen, wrote in December 2021 that:  

On the scientific front, several colleagues and I assert that IPCC has underestimated the sensitivity of climate to growing freshwater injection from melting ice. One potential consequence – if we continue with business-as-usual emissions – is shutdown of the overturning North Atlantic and Southern Ocean overturning ocean circulations by mid-century, each of which will contribute to acceleration of mass loss from the Antarctic ice sheet, with the likelihood of sea level rise of several meters within the lifetime of children born today.