Escalating Drought Risk in the Amazon River Basin

11 November 2024

Spanning over 2.5 million square miles and nine countries, the Amazon rainforest is home to 10% of the planet’s known species and over 47 million people. It is also one of the world’s largest land-based carbon sinks, making it an essential component of the global carbon cycle.

The Amazon River Basin stores up to five times of the world’s annual greenhouse gas emissions each year. However, a study released in May 2024 found that over one third of the Amazon is struggling to recover from four supposedly “one-in-a-century” dry spells in less than 20 years, and that the rainforest is in danger of a “critical slowing down.”

The trend continues into this year as vital rivers in the region report unprecedented low levels. The drought, which began in 2023, has had a devastating impact on the basin's biodiversity and communities. On September 17th, the Madeira river, a major tributary of the Amazon, dropped to 48 centimeters in Porto Velho – a significant decrease compared to its normal level of 332 centimeters. This resulted in a state of emergency declaration in six cities within the state of Amazonas due to drought.

In Manaus, the drought has reduced water levels of the Rio Negro river to 21 meters deep in September – 3 meters below its recorded depth at the same time in 2023. The decline in level has continued, dropping to 12.66 meters when measured on October 4th – the lowest the levels recorded since measurements began 122 years ago. As these levels decrease, communities have become isolated due to declines in inland shipping as navigability of the rivers becomes more difficult. The Brazilian Geological Service (SGB) warned that water levels have been decreasing since June, and that all rivers in the Amazon basin are expected to continue to drop below their historical lows.

ISciences last examined the state of the Amazon in 2016 using our Water Security Indicator Model (WSIM), which monitors worldwide surface water anomalies. While the Amazon occasionally experiences drought, our findings suggested that the Amazon’s droughts from 2005 to 2016 were unusual in both frequency and intensity. With the release of WSIM v3 and 8 years of additional data, it is time to reevaluate drought risk in the Amazon.

Recent Droughts vs. Historical Drought Risk

According to the World Research Institute’s Aqueduct Water Risk Atlas, the Amazon Basin has low to low-medium drought risk. This assessment is based on data from 2000–2014, prior to the droughts we highlighted in our previous analysis. Furthermore, it considers the product of three independent determinants: hazard, exposure, and vulnerability, and the exposure component is based on “gridded indicators of population and livestock densities, crop cover and water stress.” This formulation reduces the assessed risk because the Amazon is typically water abundant and has relatively low population, livestock, and crop densities. However, that is not much comfort to the people who do happen to live and have assets in the Amazon.

Changing Climates and Deforestation

In contrast to the World Resources Institute’s risk assessment, portions or all the Amazon basin have experienced significant droughts in 2005, 2010, 2015/16, and 2020 through present. The figure below depicts annual ISciences composite water anomalies from 1985 through 2024 as computed by WSIM v3. The darkest red regions depict exceptional droughts expected to occur, on average, less than once every 40 years using data from ERA5 and CFSv2 and a 1981-2020 baseline period. Note how exceptional drought has been persistent and widespread in portions of the Amazon basin from 2020-present and how the entire region has experienced exceptional drought in 2023 and 2024.

Exceptional drought spreads from 2020 through present. This figure depicts annual composite water anomalies as calculated by WSIM v3 from 1985 through 2024. The darkest red regions depict exceptional droughts expected to occur once every 40 years or more on average using a 1981-2020 baseline period. Note how exceptional drought has been persistent and widespread in the Amazon basin from 2020-present. The map panel for 2024 uses forecasts for November and December.

How can this be? The answer to this question is that drought risk is changing due to both a changing climate and deforestation. Much of the Amazon continues to be deforested in favor of agricultural development, specifically for beef production. This process, which typically is in the form of illegal burning of the forest, is one of the main causes of widespread fires in the area, which further worsens the region’s existing deforestation.

A record amount of Amazonian land was deforested in 2022, totalling at 4.89 million acres, making it the worst year for deforestation since 2004. This year in Brazil, an area equivalent to the size of Italy has burned. As of September 20th, over 191,000 fires have been recorded in Brazil in 2024, which is more than double the 94,829 recorded for the same period in 2023.

Deforestation reduces regional rainfall in the Amazon, but deforestation itself also exacerbates the severity of droughts. A recent study concluded that global climate change is the main driver of recent drying in the Amazon. However, a feedback loop between drought and deforestation implies that increases in either of them will impede efforts to curb both. As climate change and deforestation continues, recent studies suggest the area affected by mild and severe meteorological drought will nearly double and triple, respectively, by 2100.” The same study suggests that “continued emissions of greenhouse gasses will increase the likelihood of extreme events that have been shown to alter and degrade Amazonian forests.”

Increasing Temperatures Amplify Drought Severity

The figure below depicts annual temperature anomalies as calculated by WSIM v3 for the Amazon from 1985 through 2024. Relatively cool temperatures are depicted in shades of blue and relatively warm temperatures are depicted in shares of red. The darker the color, the rarer the anomaly. Note that in the Amazon Basin, temperatures from 1985 to 1996 were consistently cooler than expected, but that temperatures from 2009 to the present have been considerably warmer. The figure also illustrates that temperatures in 2023 and 2024 were exceptionally hot throughout the entire basin. High temperatures amplify drought severity. The World Weather Attribution Project estimates that the likelihood of drought has increased by a factor of 10-30 due to increasing temperatures and climate change.

The warming Amazon. This figure depicts annual temperature anomalies from 1985 through 2024 as calculated by WSIM v3, which steadily increase from 2015 into the present. The darkest red regions depict exceptionally warm temperatures expected to occur once every 40 years or more on average using a 1981-2020 baseline period. The map panel for 2024 uses forecasts for November and December.

Conclusion

The Amazon basin is a complex and important ecosystem for planetary health. Both the frequency and intensity of widespread drought in the basin has increased substantially since 2005 due to factors including climate change and deforestation. Even as rain currently approaches the Amazon, experts are unconvinced that years of unprecedented drought can be remedied with one rainy season. “We thought 2023 was bad, but 2024 has been far worse,” says Renato Senna, a climatologist at Brazil’s National Institute for Amazon Research.

Similarly, more people reside in the basin and are dependent on crops, livestock, and other water dependent assets for their livelihoods. We can no longer rely on drought risk assessments that assume a stationary climate, discount human driven land use/land cover change, and static population and asset counts. In the field of Bayesian statistical analysis, probabilities are continuously updated in light of new evidence. As global climates shift, ISciences uses tools such as WSIM v3 to strengthen our predictive abilities as conditions change and new information becomes available.

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