Solve Climate reports on a massive electronic billboard displaying the real-time stock of greenhouse gases in the atmosphere, unveiled on 18 June outside New York City’s Penn Station.

The world’s first “Carbon Counter”, launched by Deutsche Bank, will be seen daily by half a million people and millions more can do so online at know-the-number.com.

The basis for the number displayed on the Carbon Counter – over 3.6 trillion tons and rising by 800 tons per second – is not immediately clear. Deutsche Bank explains the calculation of the figure on its website:

Greenhouse gas concentrations are frequently expressed as an equivalent amount of Carbon Dioxide (CO₂). This CO₂-equivalent concentration in parts per million (ppm) can then be expressed in terms of metric ton of CO₂, a standard of measurement, which as a stock of gases in the atmosphere is readily understood.

According to the IPCC AR4 Synthesis Report, atmospheric CO₂ concentrations were 379 ppm in 2005. The estimate of total CO₂-eq concentration in 2005 for all long-lived GHGs is about 455ppm.

On June 18th as the counter started, long-lived GHGs in the atmosphere were estimated to be 3.64 trillion metric tons, growing at 2 billion metric tons per month, or 467 ppm, of which CO₂ was 385 ppm.

The Carbon Counter, therefore, displays in metric tons the absolute amount of all greenhouse gases in the atmosphere (as opposed to the concentration) but excludes the cooling effect of aerosols.

The use of the absolute amount of greenhouse gases in the atmosphere yields a big number that is rapidly increasing, but it is questionable whether this muddies the already confusing array of units used to explain the rising pressure of greenhouse gases on the atmosphere.

Atmospheric concentrations of carbon dioxide is a simpler and much more widely used unit used to explain the rising pressure of greenhouse gases on the atmosphere, though less dramatic for a real-time billboard aiming to capture the attention of passing commuters.

CO2 Now suggests that atmospheric concentrations of carbon dioxide reached 390.18 ppm in May 2009, up nearly 2ppm from 388.50 ppm in May 2008, the highest level in at least the past 800,000 years.

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