Tracking atmospheric methane (CH4) concentrations often gets summarized in media and blog conversations as a parts per billion (ppb) number representing a reading at a point in time, range of time, or across a set of locations.
At times I have used the publicized numbers without much thought. However I discovered that while CH4 numbers were, at times, impressive (or sensational), often they do not reflect a broader reality. I had assumed I knew what they measured - and didn't.
There are advantages to daily (real-time) reporting of something like the Keeling Curve for CO2, which continue to capture media attention for good reasons. But there is no ground-based equivalent of the Scripps CO2 reporting for methane, that is compiled an a real time basis, (one possibility is the GHG Earth Networks data, but it fluctuates and does not have a daily mean or average reading. See: http://ghg.earthnetworks.com/ )
So I start with a basic question: How do we measure CH4 trends and what is it we are really measuring? Is what we are measuring "accurate" as a global or regional proxy?
There are fundamental and important questions, whether tracking CH4 over the short, (days, weeks or months), recent (annually over the last few decades), or long term (centuries or over thousands of years).
Even more complexity becomes apparent when we consider the collection site location, whether in the ocean, on land, or the tropics vs the poles, winds and altitudes.
I have been grappling with this issue for a while, and decided to explore it. I decided to start with the key CH4 indices, such as Mauna Loa (MLO) CH4 trend data.
While the graph depicts the daily/monthly readings for the entire data set, once you begin to examine the data, it gets more complex.
For example, the graph depicts the flask data collected on site and re-verified by a QA process. This gives a strong sense of reliability for readings from May, 1983 up to December, 2012 - the end of the blue dots in the image above.
Looking at the historical data from May, 1983 until March 1987, there is no question over the data, we only have the flask CH4 available. However, beginning in April, 1987, the in situ readings commence, and throughout the data set to December, 2012 they are usually at variance with the flask data. The variability in any given month can range as much as +/- 9.6 ppb.
For example in October, 2011, the flask readings are 9.68 ppb less than the in situ reading. In November 1992, it was the opposite, the flask reading was 9.15 ppb above the in situ monthly average.
The data became more mixed, when in May, 1991, the CSIRO CH4 collection commenced at MLO.
With this, the ESRL flask and CSIRO readings revealed even greater variance, with the MLO flask readings being as much as 32 ppb below (June, 1991) or 26 ppb above (November, 1992) the CSIRO monthly averages.
So what advantages and disadvantages do we have in the MLO CH4 data? I suggest the following:
It seems to me that the advantages for Muana Loa CH4 flask data use are:
1) It represents a stream of readings collected at the same location over an extended time period, allowing the trends to be apparent in relation to seasonal variability.
2) Its trend serves as a global proxy for creating awareness of the impacts of GHG's on global climate change.
3) Its location places it farther away from local sources or sinks, that might "bias" the data.
As helpful as this information is, there are shortcomings.
1) In reality, we are only seeing the CH4 trend at one location, and at one altitude. It is a single point of data collection through time.
2) It does not provide the changes in methane globally, based upon seasonality, altitude, or sources.
3) CH4 trends at the poles are not reflected in the data.
4) In reality, depending on the device or organization, variant readings may mean that the actual needs to be some type and average of the different collection and recording methods.
5) There is no "provisional" data for near real time comparison or evaluation by the public. This means that others who have an interest in data have to wait as long as a year for access, despite the provisional being available for charting as above.
6) It is not global in a real sense, more collection points would have to be included.
A potential suggestion:
1) Since we do have three data sources for the same location, not including the Scripps readings mentioned above, perhaps all three monthly readings need to be averaged as a composite reporting number, unless there is a real issue with data validity in a given period and instrument.
2) Having more provisional "real time" readings would be a real plus for monitoring MLO methane readings.