Estimation of methane emissions using TROPOMI and divergence analysis: A top-down assessment of Ghana’s national-scale methane budget

Methane is a potent greenhouse gas, invisible to the eye but powerful enough to trap heat in the atmosphere and drive climate change. While carbon dioxide often dominates the conversation, methane is far more efficient at warming the planet in the short term, making its control a critical priority for global climate goals. In many parts of the world, especially developing nations, tracking where this gas comes from is a difficult puzzle. Traditional methods rely on counting activities—like tallying the number of cows or measuring fuel consumption—and multiplying those numbers by estimated emission rates. This approach, known as a bottom-up inventory, often misses the mark because it depends on incomplete data and assumptions that may not reflect reality on the ground. To get a clearer picture, scientists are turning to a top-down approach: looking at the atmosphere itself. By measuring the actual concentration of methane in the air from space, researchers can work backward to figure out how much gas is being released from the surface below, offering a direct check on what is happening in the real world.

In a new study focused on Ghana, researchers Nadine Osei-Essah and Yuichi S. Hayakawa applied this satellite-based perspective to map the country's methane emissions for the first time on a national scale. They utilized data from the Sentinel-5P satellite, which carries an instrument called TROPOMI capable of detecting methane molecules in the atmosphere. The team did not simply look at where the gas was most concentrated; they analyzed how the wind moved that gas. By combining the satellite's measurements of methane levels with detailed wind data, they used a mathematical technique called divergence analysis. This method essentially tracks how the gas spreads out as it travels with the wind. If the wind carries a patch of air with high methane levels away from a source, the change in concentration across that area reveals the strength of the emission underneath. This approach allowed the scientists to bypass the need for ground-based sensors, which are scarce in the region, and instead let the atmosphere itself tell the story of where the gas is coming from.

The results painted a distinct picture of Ghana's methane landscape, revealing a clear divide between the north and the south. The satellite data showed that methane concentrations were significantly higher in the southern part of the country, particularly along the coastal regions and in the Volta River basin. In contrast, the northern areas showed much lower levels. The researchers found that the wind patterns in the region, driven by the West African monsoon, generally pushed the air from the north toward the south and from east to west. This movement helped transport emissions from their sources, creating a flow that the scientists could measure. Most of the country exhibited low to moderate emission rates, with fluxes generally staying below 35 kilograms per square kilometer per hour. These levels suggest that the primary sources are likely related to agriculture, such as rice cultivation and livestock, as well as mixed human activities, rather than massive industrial leaks or super-emitters from fossil fuel infrastructure. However, the study did identify a few distinct hotspots with much higher emissions, reaching up to nearly 88 kilograms per square kilometer per hour, concentrated in the Greater Accra, Western, and Volta regions.

When the researchers compared their satellite-derived numbers with the official national inventory, which is based on the bottom-up counting method, a significant discrepancy emerged. The satellite analysis suggested that the existing inventory was underestimating the total amount of methane being released. The satellite data captured a wider range of emission intensities and showed that the actual emissions were higher than what the traditional reports indicated. This gap is not surprising given that bottom-up inventories often struggle to account for the complex, diffuse nature of emissions in tropical environments, where factors like wetlands and scattered agricultural practices can release gas in ways that are hard to predict with standard formulas. The study also highlighted that the satellite method is not perfect; cloud cover and atmospheric particles in the southern coastal areas sometimes blocked the satellite's view, meaning some high-emission zones might have been missed or underestimated. Despite these limitations, the study confirms that satellite observations, when paired with wind analysis, provide a powerful and necessary tool for monitoring greenhouse gases in data-scarce regions.

The findings offer a new foundation for understanding Ghana's contribution to global methane levels and support the country's efforts to meet its climate commitments. By demonstrating that satellite technology can successfully map emissions across a tropical nation, the study provides a replicable framework for other countries in Sub-Saharan Africa that lack extensive ground monitoring networks. The research underscores that while traditional inventories provide a baseline, they may not tell the whole story. The atmosphere, when read correctly, reveals a more complex and often larger picture of emissions, driven by a mix of natural wetlands and human activity. As the world seeks to reduce methane to slow climate change, these top-down insights are becoming essential for verifying progress and guiding effective policy, ensuring that the efforts to curb emissions are based on what is actually happening in the air, not just what is recorded on paper.