It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Friday, September 20, 2024
Global warming leads to drier and hotter Amazon: reducing uncertainty in future rainforest carbon loss
A new study narrows the uncertainty in future Amazon carbon cycle predictions and reveals that future climate change may reduce the Amazon rainforest’s ability to act as a carbon sink by making it drier and hotter
National Institute for Environmental Studies
image:
In the Eastern Amazon flooded forest, trees stay partly covered by water due to the rainy season that typically lasts from February to June. Photo is taken in early August when the water levels start going down.
The Amazon, often called the "lungs of the planet", is the world’s largest tropical forest, playing a crucial role in the global climate system due to its vast carbon storage. While it is typically warm and humid all year round, continued climate change poses the threat of more frequent and severe droughts and heat extremes. A new study, published in Nature Communications delves into future projections of the Amazon carbon cycle, focusing specifically on the impacts driven by climate change.
Scientists use the latest generation of Earth system models from the Coupled Model Intercomparison Project which contributed to the IPCC's Sixth Assessment Report. In the study framework, the climate change impacts are isolated from other factors such as land-use changes, including deforestation, and the CO2 fertilization effect on photosynthesis. An advanced technique known as Emergent Constraints that allows reducing uncertainties in future predictions using past observations is employed.
The study shows that future climate change may lead to hotter and drier conditions in the Amazon rainforest that reduces the Amazon carbon sink, in other words carbon dioxide absorption by plants. The lead author Dr. Irina Melnikova, a research associate at NIES, said that “this happens because global warming is accompanied by a phenomenon known as polar amplification—greater warming in polar regions compared to others”. It causes the intertropical convergence zone, a tropical rain belt crucial for the Amazon's climate, to shift northward. Such a shift would make Amazon drier and warmer, reducing the rainforest's ability to absorb carbon dioxide through photosynthesis while increasing carbon dioxide emissions through plants and soil respiration. Combined with the increased risk of droughts and fires in the hotter and drier conditions, this results in a net loss of carbon from the rainforest.
The new study also reveals that Earth system models, which estimate higher past global temperature trends, are more likely to predict a hotter, drier Amazon under a high-emission scenario compared to other models. The study concludes that the models that can reproduce past observational global warming trends have higher reliability in predicting the future Amazon climate change-driven carbon sink. "By refining our projections with emergent constraints, we can provide more accurate predictions of future climate impacts, which are essential for informed policymaking," stated Melnikova.
This research successfully reduces uncertainties in predicting the Amazon's response to climate change, enhancing our understanding and highlighting the critical role of accurate climate models in shaping future conservation strategies and global climate policies. The findings also reveal the potential for further warming to trigger large-scale atmospheric circulation changes, leading to a drier and hotter Amazon climate and increased carbon emissions from the rainforest. The authors caution that “While our study provides a more nuanced understanding of the Amazon's future, it also underscores the urgency of mitigating climate change to prevent the worst-case scenarios. The Amazon's fate is not just a regional concern but a global one.”
Schematic representation of the physical mechanisms of the climate-driven changes in carbon cycle in the Amazon forest region.
The horizontal axes represent past global trends in surface air temperature (1980–2014) in °C per year. The vertical axes show future projections for the Amazon: (a) surface air temperature change (°C), (b) precipitation change (%), and (c) climate change-driven carbon sink (GtC per year) as estimated by the CMIP6 models. Pearson’s correlation coefficients and p-values are indicated at the bottom of each panel. Horizontal box plots display the mean (white line), 17–83% range (box), and 5–95% range (horizontal bar) of observed global temperature trends from HadCRUT4 (light blue). Vertical box plots provide the same information but for raw CMIP6 models (black) and the constrained ranges using observations (teal). The emergent constraint is estimated for 120–139 year means under the 1pctCO2 scenario and for 2072–2091 under the SSP5-8.5 scenario, both corresponding to an intermodel mean of 4.4 °C warming relative to preindustrial levels. The figure is adapted from Figs. 2 and S11 of Melnikova et al. (2024).
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