xCO2

{{DISPLAYTITLE:xCO₂}}

XCO₂ is the column-averaged{{Explain|date=January 2022}} of carbon dioxide in the atmosphere, represented in parts per million (ppm). Rather than taking a single observation at the surface, an integration of atmospheric CO₂ above a specific location is observed. The 'X' refers to the observation taking place from a satellite platform. CO₂ observing satellites cannot observe green house gasses directly, but instead average the entire atmospheric column of CO₂. These estimates from satellites need ground truthing to ensure that XCO₂ retrievals are accurate, with an average accuracy from OCO 2 and GOSAT of 0.267 ± 1.56 ppm between September 2014 to December 2016.{{cite journal |last1=Liang |first1=Ailin |last2=Gong |first2=Wei |last3=Han |first3=Ge |last4=Xiang |first4=Chengzhi |title=Comparison of Satellite-Observed XCO₂ from GOSAT, OCO-2, and Ground-Based TCCON |journal=Remote Sensing |date=10 October 2017 |volume=9 |issue=10 |pages=1033 |doi=10.3390/rs9101033|bibcode=2017RemS....9.1033L |doi-access=free }}

The largest recorded value XCO₂ was recorded during May 2018 over the Northern Hemisphere, with a value of approximately 410 ppm.{{Cite web|url=https://climate.copernicus.eu/greenhouse-gases|title = Greenhouse Gases | Copernicus}} These values have been increasing steadily over recent years. Space-based CO₂ measurements are used for climate-level scientific studies such as a further understanding of the El Niño–Southern Oscillation{{cite journal |last1=Chatterjee |first1=A. |last2=Gierach |first2=M. M. |last3=Sutton |first3=A. J. |last4=Feely |first4=R. A. |last5=Crisp |first5=D. |last6=Eldering |first6=A. |last7=Gunson |first7=M. R. |last8=O’Dell |first8=C. W. |last9=Stephens |first9=B. B. |last10=Schimel |first10=D. S. |title=Influence of El Niño on atmospheric CO₂ over the tropical Pacific Ocean: Findings from NASA's OCO-2 mission |journal=Science |date=13 October 2017 |volume=358 |issue=6360 |pages=eaam5776 |doi=10.1126/science.aam5776|pmid=29026014 |pmc=5668685 }}