How we know the extra CO2 comes from technology.

In the last three posts, I've examined the evidence that the Earth is warming and that the warming is due to CO₂.  Continuing this series, today I'll look at the evidence that shows that humans are behind the increase in CO₂.


First, a brief overview of how CO₂ levels have changed.  Before 1850, CO₂ levels were around 280 ppmv, as shown by ice core records (i.e. Indermühle et al. 1999, Petit et al. 1999), and had not been above 300 ppmv in at least 800,000 years (Lüthi et al. 2008).  Continuous measurements of CO2 began in March 1958 at Mauna Loa Observatory, Hawaii and are shown here, along with the 12-month moving average (in red):

The monthly average for May 2013 was 399.77 ppmv, an increase of nearly 120 ppmv from the pre-industrial value.  The 12-month moving average, which eliminates the influence of the yearly cycle, is at 394.83 ppmv.  CO₂ levels so far this year are running an average of 2.4 ppmv above the previous year's levels.  With roughly 8 billion metric tons per 1 ppmv, that translates to an increase of 19.2 billion metric tons of CO₂ in the atmosphere in just one year.  Directly comparing Mauna Loa CO₂ levels to Epica Dome C levels for the last 800,000 years shows that the current rise is unprecedented.

Epica Dome C data downloaded from NOAA

Now for the evidence that shows that human technology, primarily fossil fuel using technology, is behind that increase.  Let's start with the global carbon dioxide budget.  We know that terrestrial and marine ecosystems both emit and absorb CO₂.  Recent data from 2011 shows the following amounts for emissions and absorption:

Source	CO2 emitted	CO2 absorbed	Net emissions	Total
Terrestrial	433.060 billion metric tons	-440.4 billion metric tons	-7.34 billion metric tons	+15.65 billion metric tons
Marine	355.990 billion metric tons	-367 billion metric tons	-11.01 billion metric tons
Human technology	34 billion metric tons	0 metric tons	+34 billion metric tons

Sources: NASA, Quéré et al. 2012, PBL

While human emissions each year are only 4% of total yearly emissions, they represent 100% of the net gain each year.  This conclusion is further supported by isotope analysis.

Carbon has three main isotopes: ¹²C, ¹³C, and ¹⁴C.  The ¹³C:¹²C ratio has been decreasing rapidly since , indicating that the amount of ¹²C has increased (Francey et al. 1999; Ghosh and Brand 2003).

¹³C:¹²C ratio since 1000 AD from Francey et al. 1999

Plants preferentially absorb ¹²C during photosynthesis, so an increase in ¹²C indicates that the carbon dioxide is coming from plants.  This rules out volcanoes, as they produce both ¹²C and ¹³C and would not alter the ¹³C:¹²C ratio.  Volcanic CO₂ emissions are estimated to be 0.26 billion metric tons per year (Gerlach 2011), 0.76% of annual emissions from fossil fuels.

Another piece of the puzzle is the ¹⁴C:¹²C ratio.  The ¹⁴C:¹²C ratio has decreased over time (Levin et al. 1980, Miller et al. 2012), indicating  that the source of the additional carbon dioxide is old enough that all of its ¹⁴C has decayed to ¹²C.  This rules out recently living plants as the source.  The final piece of the puzzle is that the amount of O₂ has decreased in proportion to the rise in CO₂, indicating that CO₂ is being generated by burning carbon with oxygen: C + O₂ + heat → CO₂.

Data from Scripps

What the data show is that the extra CO₂ is 1) coming from a) a source that was generated by plants, 2) has a source that is old enough for the ¹⁴C in it to decay away, 3) being generated by burning old carbon with oxygen, and 4) started in the late 18th and early 19th centuries.  The only sources of carbon that matches all those characteristics are fossil fuels.

Add up all the evidence, from recent carbon dioxide budgets to carbon isotope analyses to the decline in oxygen concentration and there's little doubt that human technology is behind the recent rise in carbon dioxide.