Water isotopes have revealed early signs of drought in the Central Amazon, according to a study led by researchers from São Paulo State University (UNESP) in Brazil. Historic droughts in 2023 and 2024 significantly reduced river flows and changed rainfall patterns in the region. Using daily monitoring in Manaus, the capital of the Brazilian state of Amazonas, scientists observed these changes by analyzing water isotopes—essentially a "fingerprint" of water molecules. This method showed how climate phenomena like El Niño and warming in the North Atlantic affected the sources of moisture and rainfall. This study is one of the first in over 30 years to use isotopic analysis in Amazonian hydroclimatic research. Water isotopes are different forms of hydrogen and oxygen atoms found in water molecules. Most hydrogen atoms have a nucleus with just one proton, but some have an extra neutron, making them heavier. Similarly, most oxygen atoms have eight protons and eight neutrons, but some have more. These variations affect how easily water evaporates or condenses. Lighter isotopes evaporate more readily, while heavier ones tend to condense first. These differences in isotope ratios change throughout the water cycle, depending on evaporation and condensation processes. Research on rainfall isotopes in the Central Amazon began in the 1960s, led by the Center for Nuclear Energy in Agriculture at the University of São Paulo (CENA-USP) and the International Atomic Energy Agency (IAEA). Monthly rainwater samples were collected in Manaus between 1965 and 1990, but the program was later halted due to a lack of funding. In 2020, a new monitoring network was launched by researchers at the National Institute for Amazonian Research (INPA), the Geological Survey of Brazil, and other institutions, reviving this important line of study. For the new study, INPA researchers collected 207 daily rain samples in Manaus from March 2023 to February 2025. These samples were analyzed at multiple laboratories, including the Center for Stable Isotopes at UNESP and the Tracer Hydrology Group at the University of Texas at Arlington. Researchers compared isotopic data with local weather measurements and global meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.S. National Oceanic and Atmospheric Administration (NOAA). The study found that during the rainy months of March to May, the Intertropical Convergence Zone (ITCZ)—a band of clouds near the equator—controls the isotopic ratios, bringing moist air from the North Atlantic. In contrast, during the dry months of June to August, the South Atlantic Subtropical High (SASH) influences the region, bringing drier air and reducing rainfall. The study highlights how local meteorological conditions, such as rainfall intensity and humidity, affect isotope concentrations. While earlier studies from the 1970s and 1990s hinted at the influence of the ITCZ and local conditions on rain isotopes, they were limited by monthly sampling and technological constraints. The current study’s daily monitoring allowed researchers to distinguish the effects of both the ITCZ and the SASH, marking a significant advancement. Comparing recent data with historical samples from 1965 to 1990, researchers observed changes in isotopic composition linked to current warmer and drier conditions. The Amazon experienced record-breaking heat waves and prolonged droughts between 2023 and 2024, driven in part by El Niño—a natural climate phenomenon that warms Pacific Ocean surface waters and disrupts global weather patterns. El Niño, combined with warming in the North Atlantic, shifted the ITCZ northward, reducing moisture inflow into the region. Researchers noted an early appearance of dry-season isotopic signatures in 2024, which could serve as an early warning for prolonged droughts. As Gomes continues his research, he plans to expand his study to 19 rain collection stations across the Amazon and neighboring regions, aiming to better understand the origins of "flying rivers"—moist air currents that carry significant rainfall to other parts of Brazil.