The news tells us that global carbon emissions are now more or less constant, as in the 2014-16 data points of Global carbon emissions flatline continues. On the other hand, the Keeling graph of mean global CO2 keeps climbing upwards. So clearly there is some feedback which continues to put CO2 into the atmosphere, for at least 3 years after the emissions plateau. What is the source of this feedback? Is it one dominant process or a multitude of lesser feedbacks?
1$\begingroup$ The emissions in your first graph are probably the total amount of CO2 added in each year. Note that the second graph has an upward inflection from the increasing rate at which CO2 was added. If the emissions were a constant X megatonnes per year, the graph would be a straight line. $\endgroup$– jamesqfDec 1, 2016 at 5:01
$\begingroup$ By way of analogy, suppose you eat and eat and eat, eventually reaching 6000 calories per day. You step on the scale and find you are 100 pounds overweight. You gradually cut your calorie intake to 5000 calories per day, but by then you are 200 pounds overweight. You cut your calorie intake even more, to 4000 calories per day -- and by then you're 300 pounds overweight. You might have cut down your ice cream intake from gallons of ice cream a day to just a few, but that's not enough. Even a small ice cream cone a day can make one get overweight. $\endgroup$– David HammenDec 1, 2016 at 6:00
$\begingroup$ Our atmosphere is being force-fed the CO2 equivalent of gallons of ice cream per person per day. $\endgroup$– David HammenDec 1, 2016 at 6:01
There's no discrepancy here. The first graph (total emissions from fossil fuels and industry; Fig. 4a of Le Quéré et al, 2016) is an input to the system, and the second graph is a proxy for the amount stored in one part of the system (the atmosphere). For the latter, it probably makes more sense to look at the total atmospheric growth rate (Fig. 4c of Le Quéré et al), which has remained positive for decades.
Note that when you add in emissions from land-use it's likely that the total emissions have continued to increase. But that doesn't detract from the main point here: if you keep putting stuff in the bucket, the amount in the bucket will continue to increase. You don't need to invoke any feedbacks for that to happen.
Atmospheric CO2 rises because more CO2 is being put into the atmosphere than is being taken out. The airborne fraction (the proportion of cumulative carbon dioxide emissions from anthropogenic sources that remains in the atmosphere) is about 50%, suggesting that total natural uptake currently exceeds total natural emissions by about half of annual anthropogenic emissions (i.e. about 5GTC per year). Thus if we keep emitting at a constant rate of 10GTC per year, then atmospheric CO2 will initially carry on rising by about 5GTC per year, which explains what we see in the second graph, because we will still be emitting more than the natural environment is able to sequester.
Unfortunately, the net natural sink depends on the disequilibrium of CO2 between the atmosphere and the surface ocean (vaguely similar arguments also apply to the terrestrial biosphere), so if we keep emissions constant, then the disequilibrium will be reduced slightly (as the surface ocean absorbs more CO2 than it emits) and so net uptake will start to fall and the airborne fraction will start to rise, however this will take time.
It is a bit like putting your foot on the accelerator (gas) in your car. If you put increasing pressure on the pedal, your car will accelerate. If you keep it in the same position, you will stop accelerating, but you won't stop moving.