Link between Dust from Sahara and Amazon

 Dust from the Sahara keeps the Amazon rainforest alive

Scientists have just uncovered an incredible link between the world’s largest desert (the Sahara) and its largest rainforest (the Amazon). New research says that the Sahara Desert replenishes phosphorus in the Amazon rainforest via vast plumes of desert dust blowing over the Atlantic Ocean. Cold air pushing into the US may seem far removed from the soils of the Brazilian Amazon. Yet the atmosphere does not care about borders. This is a small world, and we’re all connected together. Air moves heat, water, smoke, dust and nutrients across oceans. Large weather systems can shape whether the Amazon receives air packed with African particles or air that has been scrubbed clean by rain over the Atlantic. The Amazon looks endlessly rich. Its trees grow thick, its rivers run wide and its wildlife is famous worldwide. But much of its soil is surprisingly poor in nutrients. Heavy rain washes minerals out of the upper soil layers, a process called leaching. Phosphorus is the biggest missing nutrient. Calcium, potassium and magnesium also run low. Part of the fix comes from far across the ocean: mineral dust from the Sahara Desert and smoke aerosols from biomass burning in Africa.

Although the richest spectacle of life on the planet, the Amazon rainforest is famous for its nutrient poor soils. Indeed, around 90% of the forest’s soils are low in phosphorous, which has long made intensive farming next-to-impossible in the region. Moreover, tens of thousand of tons of nitrogen wash away through river systems in the Amazon every year. So, how does the rainforest replenish its lost phosphorous? The researchers found that rainy days in the tropical Atlantic often lined up with cold air incursions into the US. High-pressure systems dominated the eastern US, while pressure also rose over the central and southern Atlantic. This setup strengthened low-level wind convergence near the equator. Moisture moved more strongly toward the Amazon. Rain increased. The air got cleaner before it reached the forest. We know that dust is very important in many ways. It is an essential component of the Earth system. Dust will affect climate and, at the same time, climate change will affect dust. Using data from a NASA satellite, it was estimated for the first time just how much dust reaches the Amazon from the Sahara Desert. According to the research, 27.7 million tons of dust on average ends up in the Amazon from the Sahara in what is described as the world’s largest transfer of dust.

Luiz Augusto Toledo Machado is a professor in the Physics Institute at the University of São Paulo and a collaborator with the Department of Chemistry at the Max Planck Institute in Germany. “The results demonstrate that there’s an interconnection, a symbiosis of life on the planet. Climate change affects this pattern, causing a disruption whose outcome and consequences for future ecosystems are still unknown,” said Machado. Researchers used daily black carbon measurements from the Amazon Tall Tower Observatory, a 1,066-foot tower in the Uatumã Sustainable Development Reserve in Brazil. The tower tracks meteorological, chemical and biological data, including greenhouse gases. Black carbon is soot from burning fuel and biomass. During the rainy season, it helps scientists track long-distance particle transport. According to Machado, approximately 60% of the black carbon that reaches the Amazon during the rainy season originates in Africa. This is important because a tiny percentage of that dust, 0.08%, is phosphorous, but just enough to make a big difference. Overall, the scientists estimate that the amount of phosphorus reaching the Amazon annually from the Sahara, 22,000 tons, equals about how much the rainforest loses to rivers.

The team studied daily black carbon levels in January and February from 2015 to 2022, the start of the Amazon rainy season. Some days carried strong African influence. Other days were unusually clean. The key surprise was rain over the tropical Atlantic. Clean days over the Amazon came after peak rainfall over the ocean. Earlier thinking leaned toward wind direction as the main reason for these changes. This study points to a larger weather pattern. It adds a sharp detail to a bigger story: forests, deserts, oceans and weather systems are linked in ways that are easy to miss. A cold spell in one place can help change the mix of particles reaching another place thousands of miles away. Machado also pointed to earlier research showing that low phosphorus can limit Amazon growth even when the air has more CO2. It matters because faster plant growth can help pull carbon from the atmosphere. Later maps made with artificial intelligence confirmed low phosphorus levels across the region.

Contrary to what one might imagine, this region is very important for the health of the planet. Its dust contains crucial minerals not only for fertilizing the Amazon, but also for sustaining aquatic life. Among them are iron and phosphorus, which are fundamental for forest productivity and life in the oceans. This suggests that African dust may have important implication for maintaining the health of Amazon rainforests over the long term. Without the phosphorus input from African dust, the hydrological loss would greatly deplete the soil phosphorus reservoir over a time scale of decades or centuries and affect the health and productivity of the Amazon rainforest, but researchers also caution that they still don’t know the amount of dust needed to provide adequate phosphorus for maintaining the productivity of the Amazon rainforest. Particles and gases usually travel from Africa above the marine boundary layer, the lower part of the atmosphere that touches the ocean. The Amazon’s low-level jet stream then helps move them into the basin. Changes in these low-level jets could shift how much dust and smoke reach the forest. This could affect the Amazon’s long-term strength, especially as climate change alters rainfall, pressure systems, and wind patterns.

The scientists believe the most important source of phosphorous for the Amazon is dust whipped up from the Bodélé Depression in Chad. The frequent dust storms from this ancient lake bed contain massive amounts of dead microorganisms and, as such, are super-rich in phosphorus. The researchers also found that the amount of dust reaching the Amazon from the Sahara was hugely variable during the years of research. They theorize that rainfall in the Sahel, a vast region of drylands just south of the Sahara, may be responsible for the variation, though they note that more long term research is needed to have some conclusive theory.