Editing Annual trend in Atmospheric CO2 mole fraction (declared baseline convention)

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{{SignalObject}}

The {{SignalObject}} Annual trend in Atmospheric CO2 mole fraction (declared baseline convention) represents the year-over-year change in the concentration of carbon dioxide molecules in Earth's atmosphere. This trend is a key indicator of changes in the global carbon cycle and is closely linked to anthropogenic emissions, natural carbon sinks, and atmospheric processes. Monitoring this trend provides insight into the progression of atmospheric greenhouse gas accumulation and its potential impacts on climate systems.

Atmospheric CO2 mole fraction is expressed as the number of CO2 molecules per million molecules of dry air. The annual trend quantifies the net increase or decrease in this ratio over a calendar year, reflecting the balance between sources and sinks of CO2 on a global scale. This signal is relevant for understanding long-term climate variability, informing climate models, and contextualizing environmental changes observed across multiple scales.

The global nature of atmospheric mixing means that the annual trend in CO2 mole fraction integrates contributions from diverse geographic regions and processes. It is therefore considered a fundamental environmental signal within global monitoring frameworks, providing a standardized metric for assessing changes in atmospheric composition over time.

== Geographic / System Context ==
The {{SignalObject}} Annual trend in Atmospheric CO2 mole fraction is inherently global in scope, encompassing the entire Earth’s atmosphere. Due to the well-mixed nature of atmospheric gases on annual timescales, CO2 concentrations measured at various monitoring stations around the world can be aggregated to represent a global average trend. This global perspective includes contributions from terrestrial and oceanic carbon sources and sinks distributed across continents, oceans, and atmospheric layers.

Geographically, variations in CO2 mole fraction can occur due to regional emission patterns, seasonal biological activity, and atmospheric transport processes. However, the annual trend as defined here smooths over these spatial heterogeneities to provide a coherent global signal. This approach supports integration with climate models and facilitates comparison across different environmental monitoring systems.

== Monitoring and Measurement ==
The annual trend in atmospheric CO2 mole fraction is derived from continuous and discrete observations collected at a network of atmospheric monitoring stations worldwide. These stations employ high-precision gas analyzers, often based on infrared absorption spectroscopy or cavity ring-down spectroscopy, to measure CO2 concentrations in ambient air samples.

Data from remote sites, including oceanic buoys, mountain observatories, and polar stations, complement measurements from continental locations to capture spatial variability. Observations are quality-controlled and calibrated against international standards to ensure consistency. Time series analysis is applied to these data to extract the annual trend, accounting for seasonal cycles and short-term fluctuations.

Satellite remote sensing also contributes to spatial coverage, providing complementary data on atmospheric CO2 distributions, though ground-based measurements remain the primary source for trend determination due to their higher precision and continuity.

Within the SIGNAL system, this phenomenon is treated as a defined environmental signal whose boundaries and measurement conventions are described below.

== Signal Definition ==
The {{SignalObject}} Annual trend in Atmospheric CO2 mole fraction (declared baseline convention) is defined as the change in the mole fraction of carbon dioxide (CO2) in dry air, expressed in parts per million (ppm), averaged globally over a calendar year relative to a declared baseline period. This signal quantifies the net annual increase or decrease in atmospheric CO2 concentration, reflecting the integrated effect of all sources and sinks influencing atmospheric CO2 on a global scale.

== Boundary Conditions ==
Boundary inclusions for this signal encompass all atmospheric CO2 molecules present in the global troposphere and lower stratosphere, as measured by standardized observational networks. The signal includes data from all geographic regions, seasons, and atmospheric layers relevant to the annual averaging period.

Boundary exclusions involve localized CO2 concentration anomalies that do not represent background atmospheric conditions, such as emissions plumes near point sources or urban areas, which are filtered out during data processing. Additionally, measurements affected by instrument errors or non-atmospheric contamination are excluded to maintain data integrity.

== Aggregation Semantics ==
Geographically, the annual trend aggregates data from a globally distributed network of observation sites to produce a single global average value, thereby smoothing regional variability and emphasizing large-scale atmospheric changes. Temporally, the signal aggregates continuous and discrete measurements over a one-year period, aligning with calendar years to facilitate interannual comparisons.

Cross-signal aggregation involves integrating this CO2 trend with other atmospheric composition signals, such as methane or nitrous oxide trends, to assess combined greenhouse gas dynamics. The declared baseline convention provides a consistent reference period against which annual changes are calculated, ensuring comparability across temporal datasets and supporting longitudinal analyses.

== Observational Status ==
Monitoring of atmospheric CO2 mole fraction and its annual trend is well-established, with several decades of high-quality observational data available from global networks such as the Global Atmosphere Watch and the NOAA Earth System Research Laboratories. These datasets provide robust context for understanding long-term trends and interannual variability.

Current observational efforts continue to improve spatial coverage, temporal resolution, and measurement precision. Future SIGNAL releases may incorporate enhanced integration of satellite-derived data, expanded monitoring networks, and refined baseline conventions to improve the accuracy and applicability of the annual trend signal within broader environmental assessments.

== Related Signals ==
* None specified

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== Key Associated People ==
* '''Charles David Keeling''' — Steward-candidate (Scripps Institution of Oceanography) [Lead author]
* '''Corinne Le Quéré''' — Advisor (University of East Anglia) [Domain expert]
* '''Pierre Friedlingstein''' — Steward-candidate (University of Exeter) [Assessment author]
* '''Pieter Tans''' — Contributor (NOAA Global Monitoring Laboratory) [Monitoring lead]
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== Sources ==
* NOAA CO2 Monitoring
* Friedlingstein et al. Global Carbon Budget
* IPCC AR6 WG1 Ch5
* Keeling 1960 Tellus
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