There’s been a bit of debate as to the accuracy and validity of the CO2 Countdown clock here at Climate Shifts, so I thought i’d answer a few questions and set the record straight. The code for the original countdown was from the CO2 Clock website, which puts the current atmospheric CO2 concentration at about 397ppm.  A few people had emailed to suggest that the number of decimal places made the counter ‘unrealistic’. The author of the CO2 clock used the number of significant digits for “time lapse effect rather than empirical accuracy”, and hey, the continuing countdown made quite a few people sit up and take notice!

The CO2 clock methodology is based upon the following assumptions:

Continuously updated CO2 concentrations are derived from the montly data points provided by the in situ measurements, and currently are taken as a linear extrapolation of the previous two data points. Clock is recalibrated after the release of each new monthly data point. Future updates will take into account the seasonal trend variations in the forward interpolation.

Put simply, the source data, based upon in situ air measurements at Mauna Loa Hawaii (source here) has an inherent lag time, as the last monthly average is from May this year. The clock is recalibrated after each new month appears, but based upon the current algorithm, the data runs slightly higher than actual in the interim. We had discussed with Markin Eakin and the guys from NOAA in developing a projection based upon global synthesized air CO2 datasets (see the online Carbon Tracker for more), but in the meantime, one reader suggested that we use the CO2 Now monthly carbon tracker, an we agree.

Here are the monthly CO2 levels for August 1958-2009 based upon the Mauna Loa dataset, which should give a much more accurate reading of current atmospheric CO2 levels:

What the world needs to watch

Global warming is mainly the result of CO2 levels rising in the Earth’s atmosphere. Both atmospheric CO2 and climate change are accelerating. Climate scientists say we have years, not decades, to stabilize CO2 and other greenhouse gases.

To help the world succeed, CO2Now.org makes it easy to see the most current CO2 level and what it means. So, use this site and keep an eye on CO2.  Invite others to do the same. Then we can do more to send CO2 in the right direction.

We felt that approaching the 400ppm level was a significant milestone (both for science and policy), and we wanted to get our readings correct and not jump the gun! Thanks again to Peter Morris and other readers who have bought this to our attention.