Generated Summary
This document, originating from the NOAA Global Monitoring Laboratory, presents data on atmospheric methane (CH4) trends. It describes the methods used to measure and analyze methane concentrations, focusing on globally averaged monthly means derived from marine surface sites. The study involves a network of air sampling sites that have been measuring methane since 1983. The document details how global averages are constructed, smoothing the data for each site over time and plotting smoothed values by latitude. Annual increases in atmospheric CH4 are also analyzed, with a focus on the increase from January 1st to January 1st of the following year. The document highlights the preliminary nature of the latest year’s data due to ongoing recalibrations and quality control steps, and also details the methods for estimating uncertainties in the measurements, including bootstrap and Monte Carlo methods. The document provides data on both monthly and annual means, and growth rates.
Key Findings & Statistics
- February 2021: 1888.9 ppb of CH4
- February 2020: 1873.1 ppb of CH4
- Methane is reported as a “dry air mole fraction”, defined as the number of molecules of methane divided by the total number of molecules in the sample, after water vapor has been removed.
- The mole fraction is expressed as nmol mol ¹, abbreviated “ppb” (for parts per billion; 1 ppb indicates that one out of every billion molecules in an air sample is CH4).
- 1984: 12.97 ppb annual increase
- 1985: 12.36 ppb annual increase
- 1986: 13.02 ppb annual increase
- 1987: 11.36 ppb annual increase
- 1988: 10.59 ppb annual increase
- 1989: 11.31 ppb annual increase
- 1990: 8.79 ppb annual increase
- 1991: 13.92 ppb annual increase
- 1992: 2.43 ppb annual increase
- 1993: 3.89 ppb annual increase
- 1994: 7.35 ppb annual increase
- 1995: 3.74 ppb annual increase
- 1996: 2.46 ppb annual increase
- 1997: 6.37 ppb annual increase
- 1998: 12.29 ppb annual increase
- 1999: 2.11 ppb annual increase
- 2000: -1.77 ppb annual increase
- 2001: -0.38 ppb annual increase
- 2002: 3.33 ppb annual increase
- 2003: 4.74 ppb annual increase
- 2004: -4.72 ppb annual increase
- 2005: 0.03 ppb annual increase
- The average bias in the initial estimate is +1±0.8 ppb yr ¹ (1 standard deviation shown).
- For monthly mean CH4, the initial value is typically too high, by up to 7.6 ppb.
- In the figures, the red lines and circles are globally averaged monthly mean values centered on the middle of each month. The black line and squares show the long-term trend (in principle, similar to a 12-month running mean) where the average seasonal cycle has been removed.
Other Important Findings
- The data is gathered from a globally distributed network of air sampling sites.
- The annual increase in atmospheric CH4 is measured from January 1 to January 1 of the next year, after the seasonal cycle is removed.
- The initial estimate for the annual increase is produced in April of the following year and is updated in subsequent months.
- Initial estimates of the CH4 annual increase made in April for the previous year are biased compared to those that follow using additional data.
- The estimated uncertainty in the global annual CH4 increase varies by year and is estimated using bootstrap and Monte Carlo methods.
Limitations Noted in the Document
- Values for the last year are preliminary, pending recalibrations of standard gases and other quality control steps.
- The initial estimate of the annual increase is likely to change significantly as more data are added to the analysis.
- The initial estimates of the CH4 annual increase made in April for the previous year are biased.
- Bias in the initial estimate of the annual increase can be much larger than the average, with bias up to ±3 ppb yr ¹.
Conclusion
This document from the NOAA Global Monitoring Laboratory offers a comprehensive overview of atmospheric methane trends, derived from globally averaged data. The document highlights the dynamic nature of methane concentrations and the methods employed to analyze them. The use of data from marine surface sites provides a crucial perspective on global methane levels. The analysis of annual increases in CH4 underscores the ongoing changes in atmospheric composition, influenced by both human activities and natural processes. The document’s emphasis on preliminary data and the use of bootstrap and Monte Carlo methods underscores the importance of acknowledging the complexities and uncertainties involved in these measurements. The study’s detailed methodology, including smoothing techniques and the consideration of seasonal cycles, provides a robust framework for understanding long-term trends. The document presents a detailed look at the variability in methane levels, while also acknowledging the inherent limitations in the initial estimates. The data reveals both the complexity and the importance of accurately assessing global methane levels. The continuous monitoring and refinement of measurement techniques are crucial for understanding the full extent of atmospheric methane changes. The annual increase in atmospheric CH4 represents the net effect of methane sources and sinks during a given year, emphasizing the need for ongoing research to fully understand these processes.