Amazon basin is in transition to an anthropogenic disturbance-dominated regime, mainly driven by globalisation

The Amazon basin is facing its worst drought for the last 122 years. Extreme high air and water temperatures, large-scale fires, extreme low water levels in rivers and floodplains and strongly reduced rainfall cause severe impacts with complex environmental, social and economic dimensions. This challenges the society and public policies in the largest hydrobasin on Earth, which is undergoing an intensification of the hydrological cycle during the last decades, mainly driven by climate change.

The Amazon basin represents the largest hydrobasin on Earth (about 6,87 million sq km) with 16-18 per cent of the global freshwater discharge to the oceans. It is the largest and most intense terrestrial convective centre in the Earth system, coupled to global atmospheric circulations. About 150-200 billion tonnes of carbon is stored in biomass and soils and Amazonia is home to about 10 per cent of the known global biodiversity which is intrinsically related to the hydrological and biogeochemical cycles providing essential ecosystem services. These numbers underly the global importance of the largest tropical rainforest on Earth, composed by different ecosystems, among them the large river floodplains (about 750,000 sq km), regionally called várzea (nutrient-rich) and igapó (nutrient-poor), which are connected to the large rivers by the annual, regular and predictable flood pulse. This phenomenon is the main driver of geomorphological processes and biogeochemical cycles in the floodplain ecosystems, triggering life cycles and growth rhythms of the adapted biota, but also economic activities of the indigenous and traditional populations, such as fishery, livestock production, agriculture and the use of timber and non-timber forest products. Since Pre-Columbian times, the Amazonian floodplains are occupied by a dense population due to the easy accessibility, richness in natural resources and, especially in the várzea, the fertile alluvial soils.

In the Central Amazon region, the flood pulse regime is characterised by one regular and predictable high-water (May-July) and low-water period (October/November) with a mean amplitude (difference of the annual minimum and maximum water level) of about 10 metres. Since September 1902, the water level is daily monitored at the Port of Manaus, reflecting the rainfall conditions of the Negro and Solimões river catchments with an area of about 3 million sq km. Several extreme flood and drought events have been recorded on this 122-year-long instrumental record. The hydrological regime in the Central Amazon region is undergoing an intensification in this century, especially driven by the increasing magnitude and frequency of extreme flood events. Nine severe flood events have been recorded in this century, the same number as in the last century. However, the emergency situation caused by the recent extreme floods during the period 2009-2022 is already 29 per cent higher compared to the entire 20th century. Over the last 120 years, the mean water level in the Central Amazon region increased by about 1 metre, much more than the rise of the sea level during this period. After the record flood in June 2021, reaching the gauge of 30.02 metres at the Port of Manaus, nobody expected the occurrence of a historical drought, which reached on October 26, 2023 the lowest water level on record with 12.70 metres in Manaus; a difference of more the 17.3 metres, corresponding to the height of a building with five floors. This raises the question as to what caused this severe drought.     

Causes of the drought 

Several mechanisms with synergies and feedback loops are causing the severe drought condition affecting almost the entire Amazon basin. One factor is El Niño (the warm phase of the El Niño-Southern Oscillation — ENSO), the largest interannual climate variation in the Earth system. This climate phenomenon is characterised by anomalous warm surface waters in the east-central portion of the Equatorial Pacific, determining the upward branch of the Walker Cell. This results in descending branches of dry air masses localised over northern South America and the Indo-Pacific region, which suppress cloud convection and generate dry and hot weather conditions. El Niños which have their magnitude of anomalous warm sea surface waters in the eastern Equatorial Pacific (EP), such as the current El Niño event, have stronger impacts on the Amazonian climate in time and space compared to those events which have their magnitude in the central region of the Equatorial Pacific. In general, EP El Niños reduce the amount of rainfall in the trimestral period of June-August onwards, affecting the northeastern part of the Amazon basin, extending toward the southeastern, central and northwestern sections along the El Niño evolution, reaching its peak in the period December-February. As a result of the subsidence of dry air masses over the Amazon basin, the Hadley Cell inverts and causes an increase in rainfall and flooding in the subtropical region of South America (parts of south-eastern and southern Brazil, Uruguay and northern Argentina).

What differentiates this severe drought from former ones, is the simultaneous heating of the Tropical North Atlantic (TNA), characterising the warm phase of the Atlantic Meridional Mode (AMM). This leads to a position of the Intertropical Convergence Zone (ITZC) at higher latitudes in the northern hemisphere, compared to normal conditions. The water vapour generated by the warm surface of the TNA is no longer imported by the trade winds into the Amazon basin, causing severe meteorological droughts especially in the southern and southwestern section, intensifying the dry season (July-September). A third mechanism is the warm phase of the Atlantic Multidecadal Oscillation (AMO). This low frequency oscillation is characterised by a cyclical variation of the large-scale oceanic and atmospheric conditions in the TNA that increases (warm phase) and decreases (cold phase) the sea surface temperature (SST) with cycles of 60 to 80 years as a result of variability in the thermohaline circulation.

The majority (80 per cent) of the historical severe hydrological droughts in the Amazon basin coincides with the warm phase of AMO which is dominating since the mid 1990s.. The warm phases of AMM and AMO, further favour the formation of cyclones and hurricanes by the warm surface waters of the TNA, transporting moisture from the ITCZ to higher latitudes in the northern hemisphere (Caribbean and Gulf of Mexico). High deforestation rates and/or large-scale fires locally and regionally affect the hydrological cycle in the Amazon basin in addition creating feedback loops. Deforestation reduces the latent heat loss (evapotranspiration) and increases the sensible heat loss, contributing to the heating of the lower atmosphere and reducing the amount of water vapour in the atmosphere. The hot and dry conditions favour the occurrence of anthropogenic large-scale fires. This leads to the emission of large amounts of aerosols, which function in the atmosphere as cloud condensation nuclei. However, in consequence of the reduced water recycling by the forest vegetation, especially in regions undergoing high deforestation and landscape fragmentation, and the meteorological drought conditions the relative air humidity is low, suppressing cloud formation in the atmospheric boundary layer. Cloud formation occurs at higher altitudes leading to heavy, strong and isolated precipitation events of cold rain or even hail, as observed during this extreme drought event, accompanied by strong winds and lightning.

Overall, the synergies and feedback loops of these mechanisms induced a severe hydroclimatic drought covering almost the entire Amazon basin leading to extremely low water levels of rivers and floodplains. In addition, global warming leads to continuously increasing temperatures in the Amazon and under the current severe drought conditions, temperatures are exceeding 40°C of the air and even of water in floodplain lakes, with strongly reduced water volumes.  

Impacts of severe drought

The severe drought conditions lead to the isolation of communities of traditional and indigenous populations and municipalities which historically have been implemented along rivers and floodplain lakes and channels. Besides the risk for fluvial navigation by the extreme low water levels and appearing sandbars and rocks, the isolation causes massive problems to supply the population with drinking water, food, medicines and to provide essential public services (health, education). Populations in isolated regions must face long walkways under extreme hot conditions to access essential goods and services. The limited fluvial navigations cause further huge economic losses for commerce and many industrial sectors, as goods and materials are not timely delivered. The hot climate and severe drought result in losses of agricultural crops and livestock production. The extreme warm water in the floodplains cause massive mortality of fishes, aquatic mammals (manatees and the iconic river dolphins) and other aquatic organisms. Health conditions of the affected populations is impacted by air pollution in urban and rural regions caused by large-scale fires, extreme hot weather conditions and, especially in the floodplains, by illness caused by the consumption of contaminated water and food. Further observed and registered impacts are landslides (regionally called ‘terra caída’) as the extreme low water levels destabilise the alluvial soils, leading to death, loss of houses, communities and large infrastructures of industrial ports. These massive and complex impacts challenge various socioeconomic sectors, public policies and the society in general.

The historical drought in the Amazon basin must be seen in the context of increasing extreme hydroclimatic events and global climate change scenarios. The two strongest hydrological droughts (2023, 2010) and the four highest flood events (2009, 2012, 2021, 2022) of the 122-year long record occurred all during the last 15 years (12.3 per cent of the instrumental record). The annual amplitude in the Central Amazon region increased by 1.5 metres during the last 30 years, compared to the period before. This period is also characterised by a continuously increasing temperature. Overall, these rapidly intensifying changes are result of decadal climate variability, increasing extreme hydroclimatic events, synergisms and feedback loops between land-use and climate change which are unprecedent for the Amazon region. This has massive impacts on biodiversity, ecosystem services, food webs and the livelihood and food security of hundreds of thousands of people in the floodplains and society in general. T

The Amazon basin is in transition to an anthropogenic disturbance-dominated regime, mainly driven by globalisation (climate change and economic interests). Therefore, international support is urgently needed to strengthen national efforts to increase conservation of biodiversity and ecosystems, for large-scale ecological restauration and to promote a sustainable development for the Amazon region. Strengthening science and education is of high relevance under this scenario to provide new innovative technologies to adapt management systems and the livelihood of vulnerable populations and to mitigate the impacts on biodiversity and the services and functioning of ecosystems which are approaching more and more to critical points of no return. Compared to other massive climate shifts along Earth’s history, the ongoing climate change is caused by a species and occurs much more fasten. But this species also has the knowledge to mitigate and even impede this process, but time is running out. It depends on us what legacy we want to leave for the next and future generations.  

Jochen Schöngart is affiliated with Working Groups Ecology, Monitoring and Sustainable Use of Wetlands (MAUA) at the National Institute for Amazon Research (INPA) in Manauas, Brazil

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth

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