Atmospheric ducts are anomalous stratification layers in the lower troposphere in which the vertical gradient of what meteorologists call modified refractivity turns negative. In ordinary conditions, the index of refraction of air decreases gently with height, causing radio waves to curve slightly downward and follow the Earth’s curvature over limited distances. When a duct forms, however, the refractivity drops so steeply within a thin layer that electromagnetic energy becomes trapped inside it, bouncing between the layer and the sea surface much like light in a fiber-optic cable. The effect can extend the detection range of coastal radars dramatically, but it also creates shadow zones where targets vanish from screens and radar clutter overwhelms displays — rendering standard propagation models unreliable precisely when they matter most.

The operational stakes of these hidden waveguides are not academic. Analysts investigating the April 2022 sinking of the Russian cruiser Moskva have suggested that anomalous ducting conditions may have facilitated the unexpected beyond-line-of-sight engagement by Neptune missiles, a reminder that refractivity structure can shape modern naval encounters. Military meteorologists have long tracked ducts for exactly this reason, and the ability to predict these blind zones or extended ranges is often described as decisive in electronic warfare. Yet most of what science knows about ducting comes from short field campaigns rather than long-term climate-scale analysis.

That gap is what motivated a team led by Xiaofeng Zhao and Chunshan Wei of the Army Engineering University in Nanjing, together with colleagues from the National University of Defense Technology, Sun Yat-sen University, the Southern Marine Science and Engineering Guangdong Laboratory and the Beijing Institute of Technology. Since the early 2000s, major experiments such as Wallops-2000, the Rough Evaporative Duct Experiment, the Tropical Air–Sea Propagation Study and the Coupled Air–Sea Processes and Electromagnetic Ducting Research have refined the understanding of how air–sea interactions build refractivity structures. But those campaigns captured weeks or seasons, not decades, and regional climatological studies — of Arctic sea ice influence or West African monsoon dynamics, for instance — left the global picture unexplored.

To close it, the team turned to ERA5, the European Centre for Medium-Range Weather Forecasts reanalysis that reconstructs the global atmosphere hourly from 1940 to the present on a quarter-degree grid. Using model-level profiles, they identified the lowest trapping layer below 2,500 meters at every location and time, extracting three key quantities: how often ducts occur, how strong they are, and how high their tops sit. Duct strength is measured in M-units, a dimensionless convention that expresses the modified refractive index used throughout propagation research. At each grid cell, annual values were regressed against year, with significance judged by a two-tailed t-test at the 95 percent confidence level, and the results were cross-checked with corrections for temporal autocorrelation and spatial multiple comparisons.

The findings are striking. Over the oceans between roughly 60 degrees south and 60 degrees north, duct occurrence probability, strength and top height all show widespread, statistically significant increases. The enhancement accelerated alongside the rapid global warming that began in the late 1970s and noticeably slowed during the so-called warming hiatus from 1998 to 2012 — a temporal fingerprint that immediately suggested a warming-driven cause. Over land, the changes are milder, with the largest positive shifts in duct frequency and strength appearing over South America. The Southern Ocean and Antarctica break the pattern entirely: there, ducting characteristics decline markedly, a regional exception that turns out to reinforce the physical explanation.

That explanation rests on a well-understood asymmetry in how warming changes atmospheric stability. The rate at which temperature falls with height — the lapse rate — is central to duct formation, and the dry-adiabatic lapse rate is essentially temperature-independent. Warming therefore changes the environmental lapse rate mainly by reducing the moist-adiabatic lapse rate, which governs how air cools as it rises when condensation is involved. Over humid oceans, where vertical mixing follows moist-adiabatic processes closely, this reduction increases static stability in the marine boundary layer, favoring the formation of temperature inversions that trap moisture near the surface and steepen vertical humidity gradients — the recipe for more frequent and more intense ducts. Over dry continents, mixing stays closer to dry-adiabatic and stability changes little, which is why land sees only modest shifts.

Antarctica behaves differently for its own reasons. The cold, dry polar atmosphere is already so stably stratified that convection and turbulent mixing are suppressed, limiting vertical heat transport. Preferential near-surface warming then weakens the existing temperature inversion and reduces lower-tropospheric stability. On top of that, warming-driven sea ice loss weakens surface radiative cooling, further damping stability and cutting duct occurrence. The team also notes that rising duct top heights, which a stability-only argument would predict to fall, are consistent with a weakening of large-scale tropical subsidence under global warming: since ducts preferentially form in subsidence regimes near the top of the boundary layer, reduced subsidence can lift the boundary layer and raise the ducts with it. Reconstruction of the leading variability mode tied to global temperature shows a roughly 3 percent increase in duct occurrence probability and nearly a 60-meter rise in duct top height over mid- to low-latitude oceans, with duct strength strengthening by about 0.8 M-units overall.

Separating a long-term climate signal from natural ups and downs required care, because interannual drivers such as the El Niño/Southern Oscillation clearly leave their mark. The team applied empirical orthogonal function analysis to seasonally averaged duct characteristics and then used partial correlations against a suite of climate indices, including ENSO, the Arctic and North Atlantic Oscillations, the Pacific Decadal Oscillation, the Indian Ocean Dipole and global mean surface temperature. When the long-term trend was retained, the leading mode of duct variability correlated significantly and positively with global temperature; once the trend was removed, that correlation largely vanished. ENSO, by contrast, dominated the second mode, with correlations against the Niño 3.4 index exceeding 0.9 and its strongest responses concentrated over the equatorial Pacific, where changes in convective rainfall alter stability and humidity gradients. But because ENSO swings on short timescales and over shifting regions, its net contribution to the global mean trend is limited. Warming, the authors conclude, is the dominant driver of the long-term evolution.

The study carries practical caveats — the analysis tracks only the lowest trapping layer, and with ERA5’s lowest model level at 10 meters, it cannot resolve shallow evaporation ducts — but its implications reach well beyond meteorology. Shipping, maritime radar, coastal surveillance, navigation and satellite-to-sea communications all depend on predictable radio propagation, and a background atmosphere that grows steadily more stratified means longer anomalous ranges, deeper shadow zones and noisier displays over much of the world’s ocean. If warming continues on its current path, the marine boundary layer will keep sliding toward the highly stratified regime the study describes, turning a once-marginal propagation quirk into a first-order consideration for anyone who designs, operates or defends the world’s wireless links at sea. Data and code from the study have been released via Zenodo, allowing forecasters and engineers to begin translating the finding into operational practice.

Subject of Research: Warming-driven intensification of atmospheric ducting over global oceans

Article Title: Warming-driven lower-tropospheric stratification intensifies ducting over global oceans

Article References: Zhao, X., Wei, C., Zhu, X., Wang, D., Lyu, M., Yang, P., & Liu, Y. (2026). Warming-driven lower-tropospheric stratification intensifies ducting over global oceans. Nature Climate Change. https://doi.org/10.1038/s41558-026-02774-y

Image Credits: AI Generated

DOI: 10.1038/s41558-026-02774-y

Keywords: atmospheric ducting, climate change, marine boundary layer, radio propagation, radar, ERA5 reanalysis, El Nino Southern Oscillation, atmospheric stability, refractivity, oceans, telecommunications, electromagnetic waveguides

The research, published in Discover Geoscience, analyzed 42 years of meteorological data from 1981 to 2022 at eight locations spanning Nigeria’s major eco-climatic zones, from the humid Mangrove swamps of Ikot Akpaden and the Freshwater zone of Yenagoa in the south, through the transitional Rainforest and Guinea Savannah sites of Ibadan, Abuja, and Jalingo, to the semi-arid Sudan and Sahel Savannah stations of Sokoto, Gombe, and Kukawa in the north. Using temperature, relative humidity, atmospheric pressure, and precipitation records drawn from NASA’s POWER data repository, the team calculated refractivity using the standard formula recommended by the International Telecommunication Union, which combines a dry component driven by pressure and temperature with a wet component governed by water vapour pressure.

The results reveal a striking north-south divide. Refractivity reached its highest values, around 380 N-units, at the coastal locations of Yenagoa and Ikot Akpaden, where relative humidity hovers near 90 percent, while the arid northern stations of Sokoto and Kukawa recorded the lowest values, around 310 N-units. This gradient matters because high refractivity in humid coastal air creates conditions favorable to atmospheric ducting, a phenomenon in which radio waves become trapped in layers of the atmosphere and travel in unexpected ways. In practical terms, network operators in the humid south may need to increase transmission power to maintain reliable connectivity, while northern sites face different propagation challenges altogether.

To detect long-term trends, the researchers deployed two complementary statistical tools. The Mann-Kendall trend test, a non-parametric method widely used in climate science, identified statistically significant monotonic changes in the time series. Five of the eight locations, including Yenagoa, Ikot Akpaden, Ibadan, Sokoto, and Kukawa, showed significant increasing trends in refractivity, with the coastal stations exhibiting Sen’s slope values of roughly 0.16 N-units per year. The second tool, Şen’s Innovative Trend Analysis, or ITA, goes further by splitting each record into two halves and plotting them against each other, allowing researchers to see whether trends are confined to low, medium, or high values rather than spread uniformly across the distribution.

The ITA approach proved crucial for uncovering behavior the Mann-Kendall test missed. At Kukawa in the Sahel, for example, the overall trend was increasing, but ITA showed the rise was concentrated only in the high-value range of the data. At Jalingo and Gombe, refractivity actually declined in the lower and medium value ranges even as other sites rose steadily. This range-specific sensitivity matters because localized climate effects can push different portions of a distribution in different directions, and a single summary statistic can obscure exactly the kind of variability that matters most for engineers designing radio links in a specific place.

The seasonal analysis added another layer of insight. Rather than treating the year as simply wet or dry, the team divided it into four phases: onset, peak rainy, cessation, and dry periods, defined by the movement of the Inter-Tropical Discontinuity, the boundary where moist maritime air from the Atlantic meets dry continental air from the Sahara. Positive refractivity trends were largest and most consistent during the wet and cessation periods, particularly over humid and coastal areas, reaching 0.13 to 0.15 N-units per year. In contrast, the dry and onset periods in the Sahel and Sudan Savannah showed weaker or negative trends, ranging from -0.236 to 0.224 N-units per year. During the onset months in the north, the sporadic northward migration of the Inter-Tropical Discontinuity produces alternating pulses of moist and dry air that suppress any persistent trend.

Correlation analysis confirmed what the physics predicts: relative humidity is the dominant driver of refractivity. At the northern stations the relationship was almost deterministic, with correlation coefficients exceeding 0.90 at Kukawa, Sokoto, and Gombe, peaking at 0.98 in Kukawa. Temperature showed a moderately positive relationship with refractivity but a strongly negative one with humidity, reaching -0.93 at Abuja, reflecting the well-known inverse coupling between heat and moisture in tropical environments. Atmospheric pressure, by contrast, correlated only weakly with refractivity, indicating that its role, while embedded in the governing equation, is secondary in practice.

To quantify these relationships, the team built a multilinear regression-based sensitivity framework, standardizing all variables so that coefficients could be compared fairly across locations. The models explained between 82 and 99 percent of refractivity variability, which is unsurprising given that refractivity is physically defined by the same meteorological inputs. The percentage contributions, however, told a fascinating story about climate. Relative humidity accounted for 65 to 77 percent of refractivity variation in the arid Sudan and Sahel zones, but only about 52 percent in the humid coastal south, where temperature played a proportionally larger role. The explanation lies in the saturation physics of the Clausius-Clapeyron relation: in dry climates with high saturation vapour pressure, even small changes in humidity produce large swings in the water vapour term of the refractivity equation, amplifying its influence.

The implications extend well beyond Nigerian borders. As climate change continues to warm the planet and alter moisture distributions, the propagation environment for radio waves is shifting in ways that most network planning models never anticipated. Rising refractivity in humid regions heightens the likelihood of super-refraction and ducting, which can cause signal blackouts, multipath interference, and unpredictable communication ranges. Declining refractivity in arid zones during dry seasons points toward sub-refraction and reduced signal range. The authors recommend that telecommunications operators incorporate real-time meteorological data into network performance monitoring, that regulators consider atmospheric variability in spectrum management, and that meteorological agencies collaborate directly with telecom providers. For a country of more than 200 million people whose digital economy depends on reliable wireless connectivity, the message is clear: the weather is not just something that interrupts your signal during a storm. It is slowly, measurably, and predictably rewriting the rules of how radio waves travel, and the networks of the future will need to be designed with the climate in mind.

Subject of Research: Long-term trends in meteorological parameters and radio refractivity across Nigeria's eco-climatic zones

Article Title: Spatiotemporal trends and seasonal variations in meteorological parameters and radio refractivity across Nigeria’s eco-climatic zones

Article References: Agbo, E. P., Nathaniel, E. U., Thomas, J. E., Offorson, G. C., Nkajoe, U., & Ndoma, E. G. (2026). Spatiotemporal trends and seasonal variations in meteorological parameters and radio refractivity across Nigeria’s eco-climatic zones. Discover Geoscience, 4(1), Article 336. https://doi.org/10.1007/s44288-026-00708-x

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00708-x

Keywords: radio refractivity, Nigeria, climate change, telecommunications, radio wave propagation, relative humidity, Mann-Kendall test, innovative trend analysis, eco-climatic zones, atmospheric ducting, NASA POWER, seasonal variability