The table summarizes annual increases in atmospheric CH 4 based on globally averaged marine surface data.

The annual increase in atmospheric CH 4 in a given year is the increase in its abundance (mole fraction) from January 1 in that year to January 1 of the next year, after the seasonal cycle has been removed (as shown by the black lines in the figure above). It represents the sum of all CH 4 added to, and removed from, the atmosphere during the year by human activities and natural processes. Our first preliminary estimate for the annual increase of a particular year is produced during April of the following year, using available data from the previous year. It is important to recognize that the initial, April estimate of the annual increase is likely to change significantly as more data are added to the analysis. That estimate will be updated in subsequent months as more samples are measured for CH 4 and included in the analysis. By autumn of the following year the annual increase will typically converge toward a “final” value.

Estimates of the globally-averaged CH 4 abundance (monthly- and annually-averaged means), and the annual increase, are updated every month as new samples are returned to Boulder, measured for CH 4 , and added to the analysis. Adding new, more recent data improves the accuracy of the initial estimate by increasing the spatial density of data and eliminating “end effects” of the curve fitting procedures used. We’ve investigated the impacts of adding new data to the parameters reported here, and a summary of the results follows:

Initial estimates of the CH 4 annual increase made in April for the previous year are biased compared to those that follow using additional data. The average bias in the initial estimate is +1±0.8 ppb yr-1 (1 standard deviation shown). Over the next few months, the average bias slowly decreases until it is negligible by July or August. In any given year though, bias in the initial estimate of the annual increase can be much larger than the average, with bias up to ±3 ppb yr-1; that is, it can be positive or negative. In other words, until late in a year, bias in the annual increase can be much larger than the uncertainty reported based on the bootstrap method described below.

Behavior of initial annually-averaged means and monthly-averaged means are similar (see links to files below). For monthly mean CH 4 , the initial value is typically too high, by up to 7.6 ppb.