Chart of the Day, 10 March 2015: The Stability of Carbon Sinks and Sources

About time we revisited the Big Number which sits on the right side of my blog: the atmospheric concentration of CO2. I dub this “the most important risk indicator in the world” since it will have a greater impact on humanity than anything else I can think of (barring the earth getting hit by a stray astroid or such).

The monthly average is back over 400 parts per million (ppm) as of February. As a reminder, the cyclicality is a result of the northern hemisphere (which accounts for 65% of global land mass) plant growth and decay cycle. Source for the two charts below: NOAA (click for larger images).

The annual average year-on-year continues to grind up despite the fact that the first United Nations Climate Conference of Parties (COP) took place back in 1995.

COP 21 will take place in Paris this December, yet the above chart demonstrates that little progress has been made in mitigating carbon emissions.

In a blog post I wrote three years ago called “A Fraction for Your Thoughts” I highlighted a hidden risk contained in the above chart: the stability of the carbon sink and source relationship. As you can see from the chart below taken from the Global Carbon Budget 2014 (click for larger image), only a portion of emissions remain in the atmosphere.

Moreover, if the land and ocean sinks did not exist, CO2 concentrations would already be close to double pre-industrial revolution levels.

Over the last decade, roughly 44% of CO2 emissions have remained in the atmosphere, while 29% have been absorbed by the land (biosphere) and 26% by the oceans.

Will these ratios remain stable in the future (they have been reasonably well-behaved to date)? If not, and the land and ocean sinks lessen, we could see a step change higher in atmospheric CO2 concentrations. With respect to the oceans, Andrew Dickson, a marine chemist with the Scripps Institution of Oceanography thinks there are numerous reasons to be concerned.

First, as a given amount of CO2 is dissolved into sea water, “there is a growing resistance to adding still more CO2 (source: Robert Monroe of Scripps here). Second, warmer water absorbs less CO2. Third, warmer surface water mixes less with deeper water, interrupting the transfer mechanism that takes CO2 down into the deep ocean. Finally, the changing chemical composition of sea water, together with an altering of the transport mechanism taking nutrients from the deep ocean to the surface, reduces the amount of carbon that gets locked up by marine organisms. Robert Monroe of Scripps puts it this way (here):

All this adds up to what scientists expect to be a gradual slowing of ocean CO2 uptake if human fossil fuel use continues to accelerate. As a smaller fraction of the excess CO2 goes into the oceans, a larger fraction may remain in the atmosphere, and the chemical changes in seawater that can affect organisms will continue to grow in lockstep with the relentless increases in the excess CO2 in the overlying atmosphere caused by human activities.

The terrestrial sink seems even more uncertain going forward. Up to now, the sink has expanded along with the emissions of CO2. But as Pieter Tans of NOAA points out, the science in this area is very underdeveloped. In sum, while the terrestrial absorption fraction has been stable for the last 50 years, we don’t really know why this has happened. Tans states (source: NOAA here):

Since we don’t know why or where this process is happening, we cannot count on it.

We need to identify what’s going on here, so that we can improve our projections of future CO2 levels and how climate change will progress in the future.

Overall, the atmospheric carbon fraction is one level of uncertainty within a pyramid of risk. We have the uncertainty of carbon emission trajectories, the uncertainty of how much of that carbon stays in the atmosphere, the uncertainty of how much the world will warm given the level of Co2 and the uncertainty of how this warming will impact on the planet and its people.

Together, this adds up to a whole lot of risk. For me, all these stacked uncertainties make me more nervous, not less. They open up the potential for catastrophic outcomes. All the more reason to attack the base of this pyramid of risk through getting carbon emissions down.