The “brown belt” between Africa and the Americas is the Great Atlantic Sargassum Belt (GASB), a recurring aggregation of floating brown algae in the tropical Atlantic. Satellite observations have tracked it since 2011. It can span a vast area, but it is not a solid, continent-sized sheet: it consists of scattered patches and rafts. The real warning is that abundance varies sharply, and when mats wash ashore, they can damage coastal ecosystems, disrupt infrastructure and tourism, and release irritating gases as they decay.
What is the Great Atlantic Sargassum Belt?
Sargassum is a brown algae, often called seaweed. The GASB is a recurrent concentration of this floating algae across the tropical Atlantic, with satellite-observed extent from waters off West Africa toward the Caribbean and Gulf of Mexico. NOAA notes that individual patches can range from a few centimeters to hundreds of meters across: “belt” describes a broad pattern of abundance, not an unbroken carpet covering the ocean. NOAA’s Sargassum FAQ and NASA’s satellite overview explain the distinction.
That distinction matters for wildlife and for interpreting satellite maps. A map highlights areas with more Sargassum at the time of observation; it does not mean every point inside the mapped region is covered. Offshore, floating mats can provide valuable habitat. Near shore, excessive accumulations can have harmful local effects.
How large was it in 2026?
NASA Earth Observatory’s July 29, 2026 account, based on June observations, said that 2026 was the second-highest year in the satellite record at that point, slightly behind 2025. That is a midyear assessment, not a final ranking for the whole of 2026. NASA’s account of estimates by University of South Florida scientists reported record-high amounts in each of three regions for June:
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| Region | Estimated Sargassum in June 2026 |
|---|---|
| Western Caribbean | 3.6 million metric tons |
| Eastern Caribbean | 9 million metric tons |
| Gulf of Mexico | 5 million metric tons; nearly twice the region’s previous record, also set in 2025 |
These are regional estimates for June 2026, as reported by NASA; they should not be added together or presented as a single global belt total. NASA explains that satellites detect reflected sunlight, including a near-infrared signal, to identify Sargassum and estimate its density. The June map averages observations for each pixel over that month, so it represents a monthly view rather than a direct count of every floating mat. See NASA Earth Observatory’s June 2026 report.
A frequently quoted larger figure is historical, not a measurement of current conditions. A 2019 paper by Wang and colleagues estimated that the belt in June 2018 stretched 8,850 kilometers and contained more than 20 million metric tons of biomass. The figure describes that period and should not be confused with the June 2026 regional estimates. The paper, published in Science in 2019, provides that historical estimate.
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Why did the belt expand, and what makes it grow?
No single cause is established by the sources cited here. Researchers describe interacting influences: a source population in the Atlantic, nutrient availability, temperature and salinity, and winds and ocean circulation. The U.S. Environmental Protection Agency lists potential nutrient inputs from rivers such as the Congo, Amazon and Mississippi; African and equatorial upwelling; vertical mixing; Saharan dust; biomass burning in central and southern Africa; and runoff from urbanized coasts. Its assessment says the belt does not appear to be fueled by just one nutrient source. EPA’s Sargassum overview summarizes these interacting drivers.
Ocean circulation is especially important to where Sargassum gathers and travels. Wang and colleagues’ 2019 study concluded that “the spatial distribution of the GASB is mostly driven by ocean circulation.” The authors proposed that the 2011 bloom may have followed Amazon River discharge in earlier years, while subsequent year-to-year increases appeared linked to West African upwelling in boreal winter and Amazon discharge in spring and summer. These are proposed influences, not proof of a single trigger or a settled explanation for every year.
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NOAA’s 2023 FAQ also records hypotheses about the original 2011 expansion, including an unusual wind shift in winter 2009–2010 and combinations of river outflow, ocean nutrients, dust, temperature and runoff. NOAA said researchers were still assessing those explanations. NOAA’s FAQ sets out that uncertainty.
Why can Sargassum be both useful and harmful?
Offshore mats provide habitat
In moderate amounts offshore, floating Sargassum offers food and shelter for turtles, invertebrates, fish and birds. Its photosynthesis also adds oxygen to the water. The belt is therefore not harmful everywhere it appears: location, quantity and what happens to the mats determine their effects. NASA describes both the habitat value offshore and the risks when excessive amounts move toward shore in its June 2026 report.
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Nearshore accumulations can harm ecosystems and communities
Large mats close to shore can tangle or suffocate marine life. When they sink, accumulations can smother coral and seagrass. On beaches, decaying Sargassum releases hydrogen sulfide, a gas with a rotten-egg odor that can irritate the respiratory system. NOAA also warns that Sargassum can contain arsenic, other heavy metals, organic contaminants and marine debris; that warning does not mean every batch has the same contamination or that the entire belt is uniformly toxic.
Beachings can affect coastal ecosystems, tourism and public health. Accumulations can also clog water-intake pipes, including those used by desalination plants. NOAA says cleanup options are limited and costly. For information about impacts and response, see NOAA’s coastal Sargassum facts.
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What does “serious red flag” mean?
It is a warning about recurring abundance and the possibility of major local impacts, not a scientific classification or a claim that every part of the Atlantic belt is dangerous. Satellite monitoring helps show the basin-scale pattern while coastal effects vary from place to place. Brian Barnes, a marine scientist at the University of South Florida’s Optical Oceanography Laboratory, told NASA: “The belt is a basin-scale phenomenon that can have devastating local-scale impacts throughout the Caribbean and Gulf, and satellite observations are the only method that captures both scales on a daily basis.” NASA published the quotation in its July 29, 2026 account.
Can it be harvested or cleared safely?
Harvesting is not automatically harmless. NOAA says federal agencies that authorize, fund or carry out harvesting that may affect species listed under the Endangered Species Act must consult under Section 7. It also encourages private entities to consider potential interactions with listed species and to seek an Incidental Take Permit under Section 10 if impacts cannot be avoided. Requirements depend on the proposed action and affected species; consult current agency guidance and local rules before planning removal. NOAA’s guidance is available in its coastal Sargassum facts.
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