Variability and Long-Term Trends of CO₂ & CH₄ in European Countries, Using CAMS Global Reanalysis Data ^†

Abstract

In this study, Copernicus Atmosphere Monitoring Service (CAMS) reanalysis data (EAC4 & EGG4) are used. To capture short-term variations and analyze long-term changes in CO₂ and CH₄, this study focuses on two specific regions of interest in each of three European countries: Greece, Italy, and France. Both CO₂ and CH₄ exhibit a positive trend with seasonally averaged increases of over 6% and 2%, respectively, compared to the reference period 2003–2013. Enhanced CH₄ concentrations in Greece are observed during winter, primarily linked to anthropogenic sources such as fossil fuel combustion, heating, industrial activities, and gas distribution. Additionally, positive CH₄ residuals exceeding 0.6% were detected in autumn, likely due to regional agricultural activities in N. Greece and/or wildfires in Athens. Winter, spring, and autumn are the seasons during which CH₄ concentrations are typically highest in the Basilicata and Po Valley regions of Italy, primarily due to agricultural activities, waste management processes, and natural gas extraction, particularly in the Val d’Agri region. Higher CH₄ variability was found during winter in France. Regarding CO₂, all countries show a large diurnal variability (approximately ± 2 ppm), that of a typical mid-northern-hemisphere site, largely associated with the biospheric cycle of photosynthesis and enhanced by anthropogenic emissions and wildfire episodes.

1. Introduction

The increasing concentrations of greenhouse gases (GHGs), particularly methane (CH₄) and carbon dioxide (CO₂), are driving global warming and climate change. In this work, reanalysis data from the Copernicus Atmosphere Monitoring Service (CAMS), specifically EAC4 & EGG4 [1], are analyzed to investigate these greenhouse gases. Industrialization, urbanization, transportation, and residential heating continuously escalate the demand for fossil fuel combustion, which is the primary source of CO₂ and a major contributor to the anthropogenic carbon cycle in the Northern Hemisphere (NH). Additionally, activities such as agriculture, coal mining, waste management, and natural gas use significantly contribute to the rising concentrations of GHGs, such as CH₄, in the atmosphere. The increasing anthropogenic emissions in many European countries account for the human-induced global warming and GHG long-term positive trends. Therefore, it is of the utmost importance to consistently monitor the concentrations of these gases by various remote sensing techniques, ground-based measurements [2,3], satellite observations [4] or model observations [5]. In the Northern Hemisphere—where the majority of anthropogenic emissions originate—ground-based measurements, satellite observations, and model forecasts consistently reflect this annual increase. Annual mean CO₂ and CH₄ growth rates, based on NOAA’s globally averaged marine surface data [6], show a clear upward trend, while it is worth mentioning that 2024 has recorded the highest annual growth rate in atmospheric CO₂ concentrations in decades [7].

2. Methodology

In this study, seasonal CO₂ anomalies and short-term variations were analyzed over France, Italy, and Greece using reanalysis data from the CAMS (Copernicus Atmosphere Monitoring Service). The dataset was spatially limited to two regions per country of interest and temporally to the years 2020 and 2023 for CO₂ and CH₄, respectively. Seasonal anomalies were computed by subtracting the multi-annual seasonal climatology from the corresponding seasonal means and expressing the difference as a percentage relative to the climatology, defined from 2003 to 2013. To assess diurnal variability and short-term variations, the data were resampled to 3-hourly means, and each data point was categorized by a specific meteorological season. For each season, mean diurnal cycles were calculated, and increased concentrations were derived by subtracting the corresponding seasonal 3-hourly mean from the original values. The resulting anomalies were further detrended using a centered 15-day moving average to isolate short-term variations. The analysis focused on identifying regional patterns of enhancement and variability, particularly over key sub-regions such as the Po Valley and Basilicata region in Italy, while for France, Paris and the Occitanie region were selected as areas of special interest. Regarding Greece, the northern part and the capital city, Athens, were analyzed accordingly, enabling detailed spatial and temporal analysis.

3. Results

In this section, seasonal-averaged anomalies of CH₄ and CO₂ in terms of percentage difference, compared to the reference period, are illustrated in Figure 1, Figure 2 and Figure 3 for Greece, Italy and France, while Figure 4, Figure 5 and Figure 6 demonstrate the short-term increased and decreased concentrations. Seasonal CH₄ anomaly patterns reveal a consistent positive trend across Greece, Italy, and France relative to the reference period 2003–2013, with the most pronounced increases occurring during winter. While all countries also exhibit positive trends in autumn, the magnitudes are notably lower. Italy shows the highest wintertime CH₄ increase among the three, largely influenced by anthropogenic emissions in the Po Valley region in northwestern Italy and along the border with France, which plays a key role in driving this trend. In contrast to CH₄, CO₂ seasonal trends across Greece, Italy, and France show the highest increases during summer (over 7.2–7.4%) and autumn, with comparatively smaller rises observed in winter (around 6.8–7.1%). These patterns are primarily driven by anthropogenic emissions—such as those from traffic, heating, and industrial activities—which remain significant throughout Europe. However, the enhanced CO₂ trends during summer and autumn may also reflect the broader impacts of climate change. Specifically, a reduction in the efficiency of natural carbon sinks—such as photosynthetic uptake during the growing season—combined with persistently high anthropogenic emissions, could be contributing to the limited seasonal drawdown of CO₂, thus amplifying concentrations during periods when lower levels would typically be expected.

In Greece (Figure 4), ΔCH₄ shows higher variability during winter and autumn, primarily due to increased anthropogenic emissions that peak during this period. Increased CH₄ residuals are also observed in summer, attributed to wildfires that occurred in the final weeks of August and whose emissions were transported across Athens and the southwestern regions from northeastern Greece. Regarding ΔCO₂, the 2023 data indicate increased variability during the summer and autumn months. This pattern is consistent with the seasonal cycle of CO₂, which typically peaks in late spring and declines during summer, reflecting enhanced photosynthetic uptake by vegetation. Notably, short-term positive ΔCO₂ residuals are observed in early August, suggesting the presence of localized emissions such as increased energy demand, or even biomass burning events that can temporarily elevate CO₂ levels. Stagnant air masses, low wind speeds, or temperature inversions can trap CO₂ near the surface, causing temporary concentration increases [8]. France and Italy exhibit similar seasonal patterns to those observed in Greece for both CH₄ and CO₂. These patterns reflect common regional influences such as agricultural practices, energy consumption trends, and biospheric activity across Southern and Western Europe. In particular, Paris shows the most pronounced ΔCH₄ increased concentrations during late autumn and early December. This increase likely reflects elevated anthropogenic emissions associated with residential heating and transport. Regarding ΔCO₂, increased concentrations were found in mid-summer and autumn in the region of Paris (Figure 6). In Italy, the Po Valley region exhibits significantly elevated CH₄ concentrations during autumn and winter (Figure 5). This is likely due to the region’s dense agricultural activity—particularly livestock farming and manure management—combined with stagnant meteorological conditions and frequent temperature inversions. Conversely, the Basilicata region shows pronounced CO₂ variability during mid-summer. This could be associated with potential reductions in photosynthetic uptake due to high temperatures or drought stress, which can temporarily reduce vegetation efficiency in sequestering CO₂ or wildfire episodes occurring this time of the year in Europe.

Figure 7 presents the spatial and temporal variability of greenhouse gases, highlighting regional increases and sinks over France, Greece, and Italy during periods of elevated residuals in 2020 and 2023 within the study’s focus areas.

4. Conclusions

CH₄ shows a positive seasonal trend in Greece, France, and Italy, relative to the reference period 2003–2013. Enhanced short-term CH₄ variations were observed in the Po Valley, Basilicata, and the Occitanie region (over 1% increase), suggesting localized sources likely linked to agricultural and anthropogenic activities. In Northern Greece, ΔCH₄ peaks during winter and autumn further support the influence of agricultural practices, such as manure management and biomass burning, alongside increased domestic emissions during the colder months, as observed in urban areas like Athens. These findings highlight the importance of region-specific emission inventories and seasonal mitigation strategies in accurately assessing CH₄ dynamics across Southwestern Europe.

ΔCO₂ exhibits greater variability during the summer months in Greece and Italy, whereas France shows pronounced variability in late spring and early autumn (±2%), likely driven by enhanced boundary layer dynamics, which influence the accumulation and dispersion of surface emissions, as well as by the seasonal patterns in the NH. The observed weakening of the seasonal CO₂ drawdown may indicate a decline in the biosphere’s carbon uptake capacity, possibly due to increasing heat stress, drought frequency, and shifts in vegetation dynamics associated with global warming. As a result, even during periods traditionally characterized by CO₂ sequestration, net atmospheric CO₂ continues to rise. These findings highlight how the connection between anthropogenic emissions and climate-driven biospheric changes can modulate seasonal CO₂ behavior, with important implications for carbon cycle feedbacks and long-term climate trajectories in S and W Europe.