Carbon cycling

Doesn't hide the ugly truth ...

My friend Jerry Burke asked me to comment on some rubbish by Climate Skeptic Keith White where he claimed - among other half-truths and lies - that man-made emissions of CO2 were so small they couldn’t possibly be significant. If he hasn’t been duped by the Merchants of Doubt, then perhaps he’s on their payroll.

Jerry reckoned I should make my reply more widely available. Given recent evidence, I can’t believe we still need to have this conversation. But, at the risk of giving air time to the deniers, here it is …

The human input is indeed small compared with natural sources. Prior to the start of the industrial revolution, those sources and sinks had been in balance, leading to stable atmospheric CO2 levels around 300 ppm or less.

Our problem is caused by the rapid release of CO2 by burning fossil fuels that had been building up over thousands of years.

The figure below, from an old IPCC Report, gives a nice illustration of what’s going on.

It shows that the input from fossil fuels to the atmosphere in the 1990s was 6.4 gigatons per year, which is only ~5 percent of the naturally cycling CO2. So, if you measure CO2 over land - as we do at Lauder - you see all sorts of variabilities, depending on the history of air motions and the height of any inversion layers. There’s also a strong day-night contrast, with lower concentrations during the day when photosynthesis by plants draws CO2 from the atmosphere. Concentrations also tend to be lower in summer when plant growth (through photosynthesis) is more active.

That exchange between atmosphere and plants is the largest of all the fluxes (about 120 gigatons per year). But by far the largest CO2 reservoir is the oceans, and exchanges between the atmosphere and the ocean are also large (about 70 gigatons per year). That large oceanic sink is a big concern. It will only continue to be a sink if the CO2 levels in sea water remain below saturation levels. As the concentration in sea water approaches saturation, the amount of CO2 absorbed will slow down. There’s already evidence of this happening because saturation levels are lower at high temperatures.

If you look at the amount of CO2 in well-mixed air from over the ocean at clean-air sites like Mauna Loa Observatory, in Hawaii, or Baring Head on the southern tip of new Zealand’s North Island (during clean southerly winds), a clear signal of man-made effects emerges.

Contrary to what White says, the man-made increases in CO2 are important. In fact, they’re very important. Some might say existentially so. The data from Mauna Loa show CO2 has increased from 310 ppm to 420 ppm in my lifetime. That’s an increase of 35 percent since 1950!!

The time series at Baring Head is much shorter, but the message is the same. Concentrations there have increased by 5.6 percent just in the last ten years. They are a few parts per million less than at Mauna Loa because of the population imbalance between hemispheres, which causes a lag of about 3 years.

Those seasonal fluctuations (most markedly visible at Mauna Loa) show the importance of the natural sources and sinks. But the substantial long term increase is because of fossil fuel consumption.

As I showed a couple of weeks back, even Exxon Mobil agrees on that!