mongst all the hubbub about CO2 concentrations at Mauna Loa reaching 400 parts per million, another important point is being overlooked—were it not for the fortuitous presence of natural carbon sinks, the CO2 concentration would already have been much higher. We are being buffered from the full impact of our CO2 emissions, but this may not continue indefinitely.

The historical CO2 rise is probably only about half of what it should have been if natural carbon sinks such as global vegetation did not exist.  Since the industrial revolution, fossil fuel burning has emitted nearly two trillion tonnes of CO2—but the amount of CO2 in the atmosphere has only increased by less than a trillion tonnes.  The rest has been taken up by being dissolved in ocean waters and by increased photosynthesis of global vegetation in response to the higher CO2 levels.  If it were not for these natural processes, last week’s Mauna Loa measurements would probably have been closer to 500ppm of CO2 not 400ppm.

The question is, can this partial offset of human emissions continue indefinitely? Probably not. Extensive experimental evidence shows a law of diminishing returns—the enhancement of photosynthesis by rising CO2 concentrations tends to tail off at higher levels, at which point a further increase in CO2 gives no further increase in photosynthesis.  In contrast, the release of CO2 by respiration in plants and soil microbes seems to continue to increase with warming. With photosynthesis flattening off and respiration increasing, we can therefore expect the net uptake of CO2 by plants to become weaker, reducing this free service we have received through the partial offset of our emissions.

Similarly in the ocean there are competing effects of CO2 and warming.  The overall effect is complex and uncertain, but when the state-of-the-art understanding of all these feedbacks is brought together in Earth System Models, it points towards an overall weakening of the natural carbon sinks in the future.  The rate of removal of our CO2 emissions from the atmosphere is expected to weaken, with CO2 concentrations therefore rising faster.

The impacts of this CO2 rise will continue to be complex, with both harmful and beneficial effects.  Increasing CO2 will continue to warm the planet as a greenhouse gas, thus increasing sea levels and modifying rainfall patterns, and it will also increase the acidity of the ocean, with many negative effects. The additional effects of CO2 on plants may indeed offer positive effects, at least at first.  Overall, though, it seems that in the long run, the negatives outweigh the positives.  The fact that CO2is long-lived in the atmosphere means that long-term effects are important to consider.  There are judgement calls to be made on how we weigh up consequences for future generations against costs and benefits to ourselves, and these judgements should be informed by a more complete view of the science.  The science suggests that we should not be complacent about the beneficial effects of rising CO2 on plants.  We’re being given a free service, but this free offer may be for a limited time only.