Carbon dioxide (CO₂) is a major greenhouse gas that significantly influences the Earth’s climate. Long-term monitoring of atmospheric CO₂ is essential for tracking changes in atmospheric composition, the global carbon cycle, and climate variability. Consistent measurement over time enables the identification of long-term trends, seasonal cycles, interannual variability, and anomalies associated with environmental disturbances.

In 2024, the global average CO₂ concentration reached a record high of 422.7 ppm, about 50% higher than pre-industrial levels in the 1950s. Human activities emit more CO₂ than natural processes can absorb, causing a sustained rise in atmospheric CO₂. Long-term observations are crucial for quantifying this increase and understanding its impact on the Earth’s climate system. Regional-scale CO₂ measurements in India are important because they help reveal how natural and anthropogenic changes influence the national carbon cycle. Unlike global background observations, regional measurements can capture seasonal differences. In this regard, at Shadnagar, India, long-term atmospheric CO₂ observations are collected at the Climate Research Laboratory for Atmosphere (CRL-A) of NRSC-ISRO, located approximately 65 km from Hyderabad. Measurements are acquired at a high temporal frequency of 1 Hz using a high-precision greenhouse gas analyser instrument.

Continuous monitoring of surface-level atmospheric CO₂ concentration at Shadnagar: A graphical history

Fig.1 Monthly Variability of Near-Surface Atmospheric CO₂ (in ppm) at Shadnagar, India

Figure 1 illustrates the monthly variation in atmospheric CO₂ concentration at Shadnagar, India, over a 13-year period (July 2013 to August 2026). Each box depicts the range of CO₂ measurements for a given month, with the central line indicating the median and the box encompassing the interquartile range (middle 50% of observations). The vertical whiskers represent the spread of CO₂ values within each month after outlier removal, reflecting the variability in hourly concentrations. Box colors correspond to the monthly mean CO₂ concentration, with blue denoting lower values, red indicating higher values, and intermediate colors representing concentrations between these extremes. The black dashed line with circles denotes the monthly mean CO₂ concentration, with each point representing a specific month. The upward trend of this line demonstrates a general increase in atmospheric CO₂ over the observation period, despite short-term fluctuations. The overall mean atmospheric CO₂ concentration from July 2013 to August 2026 was 414.42 ± 15.96 ppm, exhibiting a pronounced seasonal cycle and a clear long-term increasing trend. The annual mean CO₂ concentration exhibited a statistically significant increasing trend of 2.56 ± 0.27 ppm yr⁻¹, indicating a persistent long-term rise in atmospheric CO₂ at this site.

Fig.2 Mean Monthly Cycle of near-surface atmospheric CO₂ concentration at Shadnagar, India (2013-2026).

Figure 2 presents the typical seasonal pattern of atmospheric CO₂ at Shadnagar, based on observations collected from July 2013 to August 2026. Data from all available years are grouped by month to illustrate the annual variation in CO₂ concentrations. The blue line with circles indicates the mean seasonal cycle, calculated from multi-year observations, while the shaded blue area represents the variation in CO₂ across years for each month. A wider shaded region signifies greater interannual variability, whereas a narrower region indicates more consistent CO₂ levels. The amplitude of 13.067 ± 2.021 ppm quantifies the typical seasonal variation in CO₂.

CO₂ concentrations are highest during spring, particularly in April and May, and reach their lowest levels between July and September. Elevated concentrations during the pre-monsoon period are likely associated with increased CO₂ release from respiration and other sources, as well as atmospheric conditions that promote accumulation. In contrast, lower concentrations during the monsoon period may result from enhanced photosynthetic uptake by vegetation and changes in atmospheric mixing and transport.