While deepening droughts have made headlines in recent years, extreme floods have been steadily increasing in frequency and severity in parts of the Amazon basin, putting lives and livelihoods at risk. According to a new study, longer and more extreme floods are becoming increasingly common due to a combination of fluctuations in atmospheric circulation systems and human-driven climate change.
Publishing in the journal Science Advances, Jonathan Barichivich at the Universidad Austral de Chile and colleagues collated daily water level records from the Port of Manaus on the Rio Negro from 1903 to 2015, and from Óbidos on the Amazon mainstem from 1970 to 2015. They found that the frequency of severe floods in both locales has increased steadily since 1970. Together these stations measure flow for the whole of the upstream Amazon.
"In the whole 20th century, there hasn't been floods as severe or as frequent as in the last decade," Barichivich says. Meanwhile, droughts have intensified in parts of the Amazon. An increase in intense flooding, alternating with deeper droughts, is a finding in keeping with current global climate change models.
River records are useful windows into the dynamics of large regions, as their levels are influenced by the water levels in all tributaries upstream and downstream of the measurement site. They are also among the longest ecological records available.
The team identified 14 severe droughts and 14 severe floods in the Amazon since 1903. They used the geological survey of Brazil's official critical water levels to define a severe flood in Manaus as one with levels higher than 95 feet, and a severe drought as lower than 51 feet.
The researchers also looked at the severity and duration of floods and found that extreme floods have tended to be higher and longer-lasting, with water levels over 97.5 feet for more than 70 days occurring once every three years, compared to once every 50 years in the 1900s.
Average water levels at Manaus port have increased by 3.3 feet during the 113-year record, a rise caused by increased precipitation during the wet season.
Extreme flood events can have disastrous effects. Traditionally, as the Amazon basin wet season ends, the nutrient rich floodplains offer the perfect conditions for agricultural plantings and grazing livestock. But prolonged, severe flooding destroys these crops and pastures, contaminates water supplies, and cuts people off from their homes for weeks or months at a time. In May of 2012, after months of torrential rain, water levels at Manaus port reached 97.5 feet, their highest level on record, and remained there for over 70 days. Large scale floods could potentially pose future threats to the structural integrity of Amazon dams.
Flooding also alters sediment transport—an important process for downstream ecosystems, fisheries, and farms—and carbon storage in plants, soils, and peatlands.
Using atmospheric circulation models coupled with real-world data, the team was able to link the increased Amazon precipitation driving these extreme flood events to an increase in the strength of the Walker circulation—an ocean-driven pattern of air circulation that carries warm, moist air from the tropical Atlantic across South America toward the Pacific.
Since the early 1970s, the Atlantic Ocean has been in a warming phase of a natural cycle called the Atlantic Multidecadal Oscillation, which has been accelerated by human-caused climate change. However, in 1998 the Pacific Ocean entered a cooling phase of the Pacific Interdecadal Oscillation, increasing the temperature disparity between the two oceans. This temperature gradient is what drives the Walker Circulation, which supplies rain clouds to the central and northern Amazon basin.
"The wet season was getting wetter in this period, while the Pacific was cool and the Atlantic was warming very fast," Barichivich says. This pattern is mirrored in the Manuas port river records, he says, with more frequent extreme floods during the wet season and deeper droughts during the dry season. "The amplitude of this natural water level fluctuation started to increase ... due to a combination of both anthropogenic climate change and natural variability."
Zed Zulkafli, a hydrologist at Universiti Putra Malaysia in Selangor, who was not involved in the study, praised the rigor of the data analysis based on the length of the river records, saying, "the novelty [of the study] is in ... using long-term records at Manaus that reveal the severity of recent floods ... as well as in linking the Walker circulation to the increasing flood severity."
In addition to the AMOC and PIO, the Amazon flood cycle is subject to natural fluctuations caused by the oscillation between El Niño and La Niña extremes in the Pacific—linked to Amazon droughts and floods respectively, which explains why the devastating drought of 2010, which began as an El Niño year, happened despite a general trend toward higher precipitation.
"The perception for most people is that droughts are the most important thing in the Amazon," says Barichivich, but the century-long records at Manuas port show that "floods are outstanding in meteorological change" the Amazon has experienced.
"Until the 1960s we had one [extreme] flood every 20 years more or less," says Barichivich, but the average number has increased to one every four years since the early 2000s. Interestingly, the team failed to find a long-term trend in the frequency of droughts in the Amazon, with their average occurrence once ever five to 12 years remaining relatively static throughout the record.
As the Atlantic continues to warm faster than the Pacific, extreme floods look set to be a recurring feature of the Amazon basin. A 2016 study led by Zulkafli predicted a larger increase in wet-season rainfall compared to dry-season drought for the Peruvian Amazon basin by the end of the 21st century. However, "the degree to which extreme flooding will become more frequent is more challenging to quantify," she says.
Within a decade or so, the study authors predict that the Pacific will move into a warming phase of the PIO, reducing the disparity in sea temperature between the Atlantic and Pacific and thus weakening the Walker circulation that carries rainfall to the Amazon basin. This, Barichivich says, could slow the acceleration of extreme weather events over the Amazon. However, this effect will not be enough to reduce the frequency and intensity of extreme flooding to pre-1990 levels. "We think that [extreme] floods should still be a problem in the next coming decades," Zulkafli says.
"The biodiversity as well as the riverine communities along the Amazon and its tributaries have developed as a result of [the] Amazon flooding regime. And so they will be impacted by more frequent and prolonged flooding," Zulkafli concludes.
The authors hope their models will allow for more accurate predictions of annual Amazon flooding, allowing seasonal forecasts to take Walker circulation strength, and Atlantic/Pacific ocean conditions into consideration. This forecasting ability could save lives and property, aiding farmers and ranchers. It remains to be determined how more frequent, extreme flooding will impact plants and wildlife.
This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.