In this article we try to quantify the Amazon rainforest climate tipping point, based on available scientific literature. We conclude there’s no real basin-wide threshold temperature to activate the forest-killing biome switch. Rather it seems to be a sliding process, that we are already largely committed to under current CO2 concentrations.

The most rapid warming-induced die-back of the Amazon rainforest probably occurs at a global average temperature rise from 1 to 3 degrees Celsius above the pre-industrial climate. The vegetation effect is delayed, initially masking part of the damage. Yes, that’s sadly yet more climate inertia…

As humanity is slowly trying to come to terms with climate urgency, we long for a compromise. Atmospheric ‘safe limits’, emission budgets, overshoot scenarios and remaining years of ‘acceptable fossil fuel use’. The sought compromise is based on human psychology (symptoms of addiction), not on physical science. Earth’s climate system is far too intricate to offer large-scale binary switches. In case of the Amazon rainforest, the climate tipping point is real, but there’s no such thing as a threshold temperature to it. Image credit: Caroline Bennett, Amazonwatch.org

The Amazon climate tipping point – icon of the Anthropocene

The Amazon rainforest is seriously threatened by climate change. In our previous articles we’ve looked at mechanisms to understand why that is – increased evaporation, increasing seasonal droughts, and an important vegetation-climate feedback. But now we feel it’s time to try to quantify the actual urgency:

The Amazon is referred to as a climate tipping point because research shows following a 21^st century global average temperature rise most of the Amazon basin may dry out, leading to a massive biome shift – accompanied by many gigatonnes of extra CO2 emissions and almost unimaginable biodiversity loss, placing the cascading Anthropocene Extinction in top gear.

There is however no clear-cut threshold temperature to this tipping point (“above this temperature the rainforest dies, below we’re fine”). Instead it slides along the temperature scale, following an S-shaped curve: ‘some warming’ kills part of the Amazon, ‘a lot of warming’ will almost wipe the rainforest completely off the map – and ‘somewhere in between’ the climatic deforestation progresses the fastest.

Based on available literature we can try to quantify this scale. Serious ecological damage is probably already committed between a warming of 1 to 2 degrees Celsius above preindustrial climate. Just above 2 degrees warming more than half the Amazon basin can no longer support rainforest, while at 3 degrees already 75 percent of the rainforest could dry out and die.

Warming-induced droughts are already witnessed right now, in a 1 degrees warmer world. ‘Once in a century droughts’ were observed in the Amazon in 2005, in 2010 and again in 2015.

‘Committed terrestrial ecosystem changes due to climate change’

When trying to position the Amazon tipping point on the scale of the global temperature rise, one of the most-often cited studies is one from the year 2009, performed by a team of researchers of the UK Met Office Hadley Centre, led by Chris Jones and published in Nature Geoscience.

It’s not even about the Amazon specifically, but rather the entire world, titled ‘Committed terrestrial ecosystem changes due to climate change’, but it’s this study that led to headlines, shortly before the big UN climate conference in Copenhagen that same year, of how unabated climate change could wipe out most of the world’s largest remaining rainforest.

The researchers used a climate-vegetation model that showed (like several similar studies) a clear increase in Amazonian drought following a global average temperature rise – leading to a large-scale die-back of rainforest, switching to grassland and savanna climate suitability.

4 degrees: 85 percent of the rainforest dies – but it’s sliding process:

The British team calculated that under a 3 degrees warming scenario, 75 percent of the Amazon basin could no longer support rainforest. This seems more or less in line with the 2008 publication in Climatic Change that we covered in part 21 of this series – that linked a global CO2 concentration of just over 750 ppm to a 70 percent die-back of Amazon rainforest, starting from the South.

The Nature Geoscience publication shows however this Amazon tipping point is no binary switch, but rather slides along the temperature scale. While at +3 degrees 75 percent of the rainforest dies, increasing temperatures to +4 degrees leads to an 85 percent die-off.

Two degrees is ecologically speaking no safe limit the British state, as it would still lead to the disappearance of between 20-40 percent of the rainforest – and even below that, around 1 degrees Celsius above the preindustrial global climate, the Amazon rainforest would decrease in size, their model showed.

Ecological climate inertia – temperature threshold to irreversible damage

Their model also showed climate impact inertia, with full vegetation impact some time after temperature stabilisation:

“We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 °C.

“We have introduced the implications of the hitherto unconsidered application of the concept of committed changes to the terrestrial ecosystem. Although these results are from a single model and hence subject to quantitative uncertainty, we believe the concept of committed changes in the terrestrial biosphere is likely to be robust. The terrestrial biosphere can respond slowly to large, regional-scale forcing, but may not always be in equilibrium with that forcing at any point in time, leading to subsequent commitments to significant future change for decades or centuries following stabilization of forcing.”

“There is a threshold beyond which some die-back is committed and this commitment rises markedly for greater global temperature rise. In our model this threshold is below 2 °C, a threshold often used by policy makers in their definition of dangerous climate change27, although the quantitative nature of our results carries significant uncertainty. Any subsequent recovery is on such a long timescale as to make the die-back effectively irreversible on human timescales of the next 1–2 centuries.”

So although we’ve witnessed the first clearly climate change-related Amazonian droughts and the world just reached the +1 degrees warmer state, we probably still haven’t seen the true local effects of this new temperature – and by the time we do see it, it will be warmer still.

Ecological climate inertia: the difference between committed and realised die-back of the Amazon rainforest. In this Hadley Centre model study Forest cover decreases most rapidly from +1 to +3 degrees Celsius of global average warming, suggesting the Amazon tipping point slides along the temperature scale following an S-shaped curve. If you were to define a temperature threshold, it should be below +2 degrees, these researchers say.

Other studies of the Amazon climate tipping point

The existence of an Amazonian climate tipping point is confirmed by other model studies, including the above-mentioned Climatic Change publication from 2008 – that suggests a large-scale die-back (70 percent) from about 3 degrees onwards, starting in the South of the basin.

In that same year yet another research team, led by Yadvinder Malhi of the University of Oxford, published a similar study in the journal PNAS, titled ‘Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest’.

This team compared several general circulation models and concluded 21^st century warming would likely lead the ecosystem beyond a biome-switch tipping point, starting in the eastern part of the Amazon basin, either from rainforest to savanna or from rainforest to seasonal forest, a situation with increased forest fire incidence and decreased biomass.

This PNAS publication from 2008 also confirms the existence of a climate tipping point for the Amazon rainforest. As global temperatures rise, seasonal droughts increase, and part of the rainforest could switch – either to a seasonal forest or savanna state.

The Oxford team also offers an interesting elaboration on the mechanism. We’ve seen before how climate change (due to relatively strong northern hemisphere warming) could lead to a northward migration of the Intertropical Convergence Zone (ITCZ) – promoting seasonal droughts in the (southern) Amazon basin. Accompanying this process (due to intensification of the general circulation) the ITCZ could also become narrower, the British team writes – while suppressing convection outside this zone – further increasing the likelihood and magnitude of seasonal Amazonian droughts:

“The 21st-century intensification of dry seasons suggested by our analysis is probably partially driven by the general intensification of tropical circulation caused by increased temperatures and tropospheric moisture content. This intensification increases precipitation within the convective zone but also causes narrowing of the convective zone and suppression of convection in the neighboring air subsidence zones. Hence, wet seasons intensify, but dry seasons also lengthen and intensify.”

Two sides to ecological climate inertia – and the importance of stopping deforestation

These researchers also offer an interesting additional perspective on the above-mentioned ecological climate inertia – stating that trees can be such long-lived organisms that perhaps they can also increase forest resilience. They write that once a deep-rooted closed-canopy forest is in place, it might be able to withstand a temporary overshoot to savanna climatic conditions, as long as the forest remains intact – illustrating the importance of halting active deforestation:

“Forest trees can be long-lived organisms, and it is possible that “demographic inertia” (inertia caused by long lifespans and slow community turnover) in the forest system may delay any forest dieback. Moreover, the presence of forest may modify local microclimate (evapotranspiration, rainfall generation, exclusion of invading grasses, shading of soil surface, and seedlings) sufficiently to favor the persistence of forest (“microclimatic inertia”). Once established, closed-canopy, deep-rooted forest may persist even if the local climate has shifted to savanna conditions.”

“A useful analogy may be drawn from physics. In a phase transition from liquid to solid, a liquid may persist in a supercooled state beyond the equilibrium threshold for solidification if there are insufficient nucleation points (e.g., impurities) to induce solidification. By analogy, a forest may persist in the drying-induced transition to savanna if there are insufficient nucleation points (most likely fire ignition points, see The Role of Fire) to break open the forest and trigger the transformation.”

Tropical rainforests can be resilient to climate change

Thus far the studies reviewed in this article are all 8 or 9 years old. We’ve not been able to find more recent attempts specifically trying to quantify the Amazon climate tipping point. Two later studies, one from 2012, the other from 2013, do offer interesting additional insights.

One is relatively optimistic: ‘Simulated resilience of tropical rainforests to CO2-induced climate change’, performed by a research team led by Chris Huntingford of the UK Centre for Ecology and Hydrology and published in Nature Geoscience in 2013.

According to these researchers remaining uncertainties are more in the realm of biology: “we find that the largest uncertainties are associated with plant physiological responses, and then with future emissions scenarios. Uncertainties from differences in the climate projections are significantly smaller.”

Tropical rainforests in all three ecoregions (South America, Africa and Asia) show some level of resilience to climate change, although the Amazon basin is relatively the most vulnerable. Comparing different general circulation climate models these researchers find it is actually only the (often-used) Hadley Centre model that forces vegetation models to a biome switch:

“When forced by general circulation models (GCMs) other than HadCM3, vegetation models have usually simulated lower or even no losses of Amazonian forest cover. There are far fewer assessments of possible climate-change impacts on tropical regions outside Amazonia. Two existing studiessuggest that significant parts of tropical Africa and Asia may be less sensitive to climate change.”

One vegetation model (MOSES-TRIFFID) forced by 22 different climate models. This study shows a steady increase in biomass in all scenarios, to slowly decrease in the second half of the 21^st century. Here only under the UK Met Office’s HadCM3 immediate deterioration occurs.

Again more sobering is ‘Development of regional future climate change scenarios in South America using the Eta CPTEC/HadCM3 climate change projections: climatology and regional analyses for the Amazon, São Francisco and the Paraná River basins’ – a mouthful-titled publication in Climate Dynamics from 2012 that (indeed) uses the Hadley Centre climate model to conclude that droughts in the Amazon basin could increase rather dramatically.

This team, led by Jose Marengo of the Brazilian National Institute for Space Research (INPE), assesses the local impacts of the global SRES A1B emissions scenario, an old IPCC scenario for (A1) a world with rapid economic growth, decreasing population after 2050 and rapid implementation of efficient technologies with (B) a ‘balanced mix of energy sources’. In other words it’s a rather friendly-world emissions scenario, that the researchers then differentiated into four separate members according to different climate sensitivity assumptions – and translating a global temperature rise to a local climatic response:

“[…] all of South America is very likely to warm during this century. The projected warming is generally largest in continental tropical South America and immediately off the Pacific coast of South America between 5 and 35°S. […] The warming reaches up to 7°C in eastern and southern Amazonia by 2100, in both summer and winter.”

This relatively strong local temperature rise then contributes to increasing drought – most notably in the East Amazon and Northeast Brazil:

“It is the combination of higher temperatures and rainfall reductions over tropical South America that results in P < E [evaporation being larger than precipitation – ed.] and brings drier conditions, drought and aridization in parts of Northeast Brazil (the semiarid land of Northeast Brazil). This results in less soil moisture and lower river runoff, which may lead to soil degradation and potentially onwards to desertification.”

Projected climate change over the Amazon and Brazil. As temperatures go up, on average the ratio between precipitation and evaporation goes down, increasing droughts.

“Rainfall tends to decrease by 2100 across all of Brazil, including the three basins, with intense reductions in the Amazon region (−1 mm/day, varying between −0.7 and −1.2 mm/day) and the São Francisco River basin (−1 mm/day, varying between −0.5 and −1.5 mm/day). In summer, the rainfall reductions reach 2 mm/day in Amazonia and 3.5 mm/day in the São Francisco Basin by 2100.”

That’s odd. We would expect the rainfall reduction to be larger during southern hemisphere winter – the dry season. Never expect science to be fully settled.

We’ll be awaiting new Amazon climate impact studies. Meanwhile we are safe to conclude it’s a very threatened region – and one of the most notable hotspots of 21^st century climate change.

© Rolf Schuttenhelm | www.bitsofscience.org