Global warming leads to drier and hotter Amazon: reducing uncertainty in future rainforest carbon loss

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 (Figure 1). While the Amazon rainforest is typically warm and humid all year round, continued climate change poses the threat of more frequent and severe droughts and heat extremes. Our understanding of how the Amazon may respond to future climate change remains limited, with significant uncertainties still present in the projections of Earth system models ^{note 1}. 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 ^{note 2}.

Researchers use the latest generation of Earth system models from the Coupled Model Intercomparison Project which contributed to the IPCC's Sixth Assessment Report. Some of these models predict greater global warming in response to increasing atmospheric carbon dioxide. However, a previous study ^{ref. 1} noted that such models overestimate both recent global warming trends during recent decades and future projections of global temperature and rainfall increases. This increases uncertainty in future predictions of global and regional climate change and its impact on the carbon cycle. The authors of this study employed an advanced technique known as Emergent Constraints ^{note 3} that allows reducing uncertainties in future predictions using past observations.

The model analysis suggests 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 (Figure 2). 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 the Amazon rainforest drier and warmer, reducing the rainforest's ability to absorb carbon dioxide through photosynthesis while increasing the 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 (Figures 3a and 3b). Likewise, the researchers identified a significant negative correlation between past global warming trends and future climate change-driven ^{note 2} Amazon carbon sink, as simulated by the models (Figure 3c). 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," the authors state.

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. Lead author Dr. Irina Melnikova, cautions, “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.”

Notes
¹ Earth system models are state-of-the-art complex models that integrate the interactions of atmosphere, ocean, land, and biosphere. These models help us understand how human activities affect the planet, track environmental changes from the past to the present, and predict future global warming.
² 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.
³ Emergent Constraint (EC) is an approach to reduce uncertainties in future climate projections by leveraging statistical relationships between past and future climate simulations from Earth system models (ESMs) and observational data.
References
¹ Shiogama, H., Watanabe, M., Kim, H. & Hirota, N. Emergent constraints on future precipitation changes. Nature 602, 612–616 (2022). https://www.nature.com/articles/s41586-021-04310-8

Program for the Advanced Studies of Climate Change Projection (SENTAN, grant number JPMXD0722681344) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Environment Research and Technology Development Fund (JPMEERF23S21130) of the Environmental Restoration and Conservation Agency and the Ministry of Environment of Japan, KAKENHI (JP21H01161 and 21H05318) of the Japan Society for the Promotion of Science

【Article Title】Emergent constraints on future Amazon climate change-induced carbon loss using past global warming trends
【Authors】Melnikova I., Yokohata T., Ito A., Nishina K., Tachiiri K. & Shiogama H.
【Journal】Nature Communications
【DOI】10.1038/s41467-024-51474-8

National Institute for Environmental Studies
　Earth System Risk Analysis Section, Earth System Division
　　Irina Melnikova
　　Yokohata Tokuta
　　Shiogama Hideo
　Biogeochemical Cycle Modeling and Analysis Section, Earth System Division
　　Nishina Kazuya

The University of Tokyo
　Graduate School of Agricultural and Life Sciences
　　Ito Akihiko

Japan Agency for Marine-Earth Science and Technology
　Research Institute for Global Change
　　Tachiiri Kaoru

【Contact for this research】
National Institute for Environmental Studies
Earth System Risk Analysis Section, Earth System Division
　Irina Melnikova
　Yokohata Tokuta

【Contact for this research】
National Institute for Environmental Studies
Public Relations Office, Planning Division
E-mail: kouhou0 (please append ‘@nies.go.jp’ to complete the e-mail address)