by Guido van der Werf
The 200% increase in fossil fuel emissions Murry Salby claims is about 20% in reality, and the constant CO2 growth rate he found actually increased by roughly 20% as well over the same time period.
The broad scientific consensus is that the increase in atmospheric CO2 concentration from about 280 parts per million (ppm) before the industrial revolution to about 400 ppm today is due to the combustion of fossil fuel and deforestation. Many lines of evidence support that statement, from bookkeeping to sophisticated measurements of oxygen and carbon isotopes.
Murry Salby disagrees though and in a recent talk stated that “The premise of the IPCC that increased atmospheric CO2 results principally from fossil fuels emissions is impossible”. This received a lot of attention in especially the more skeptical community and at very first glance Salby seems to base his message on an interesting finding. When viewed more carefully, I think the conclusion is wrong though and below I try to show why. Let’s start with understanding his reasoning.
Figure 1, replicate of the figure Murry shows after 8 minutes and where he plots fossil fuel emissions. The key message is that the growth in fossil emissions after 2002 was three times as large as before that year, 3% per year after 2002 and 1% before. He mentions that fossil fuel emissions have increased by a factor 3 and that this should be seen in the atmosphere so let’s look at the atmospheric CO2 measurements.
Figure 2 replicating Murry’s figure about 9 minutes into his talk. This is monthly atmospheric CO2 data, and d1 and d2 seem virtually identical. In other words, there seems to be no increase in the CO2 growth rate which apparently led Salby to reject the general consensus that increased atmospheric CO2 concentrations result principally from fossil fuels emissions.
So what is wrong? Basically two things. Most importantly, fossil fuel emissions have fortunately not increased by a factor 3 over the past two decades. Let’s plot Figure 1 again but now a) add CO2 emissions from deforestation because that is another anthropogenic source of CO2, b) have the y-axis start at zero, and c) focus on actual emissions instead of the growth of emissions because that is what matters. The figure then looks like this:
Figure 3, please keep in mind this is based on exactly the same data as Figure 1 except for the deforestation part. Deforestation emissions have decreased somewhat over time but are in general a minor contribution, roughly 10% of total anthropogenic emissions. The uptick in 1997 is due to peat fires in Indonesia during that year, these estimates are based on the work my lab carries out in collaboration with US colleagues at NASA and UCI.
The increase suddenly looks less impressive and is about 20% instead of Salby’s 200%. However, it is still strange that this is not seen in the atmosphere. Or is it? I actually think it is and d2 was 10% larger than d1 in Figure 2 but this depends on exactly which start or endpoint one uses. However, it is difficult to see these details when you plot it as in Figure 2 and that is the second thing that went wrong in his analysis.
You could see it though; if one looks carefully the linear trend line in Figure 2 (the green line) does not fully capture the signal. But let’s not do this visually but have a proper look at the data instead. By taking out the seasonal cycle and focusing on the actual annual growth rate the patterns become much more apparent (and interesting!):
Figure 4 which shows the same data as Figure 2 but now the focus is on where it should be: the annual growth rate. This varies much more from year to year than the anthropogenic emissions and since the 1970’s we know that the natural carbon cycle is slightly out of balance during El Nino and La Nina periods and after volcanic eruptions: low growth after the eruption of Mt. Pinatubo in 1991 and after La Nina’s, high growth rates after El Nino’s, see the peak in 1998 and also 2016 is on track to be record setting. The exact mechanisms are still not fully understood though.
On topic: the growth rate actually does increase, see Figure 4. Exactly how much is difficult to say over such a short time period, I only added the green and yellow line to compare them with Salby’s results but by slightly modifying the years over which one averages the increase can be roughly between 10 and 30%.
In summary, the 200% increase in fossil fuel emissions Murry claims is about 20% in reality, and the constant CO2 growth rate he found actually increased by roughly 20% as well over the same time period. In other words, nothing that challenges our current understanding of the global carbon cycle where most research is geared towards understanding whether it will keep up with growing fossil fuel emissions or become a positive feedback on climate change.
Biosketch: Prof. dr. Guido van der Werf is a faculty member of Earth and Life Sciences at Vrije Universities Amsterdam. His research focuses on the global carbon cycle and interactions with the climate system. Funded by several national and international grants including a prestigious European Research Council (ERC) grant he is specifically interested in forest fires and deforestation. Combining biogeochemical modeling, satellite data, and atmospheric modeling enables him and his group to quantify fire and deforestation carbon emissions, and these are the basis for exploring their response to climatic, demographic, and socio-economical changes. In addition, satellite data is used to test ecological hypotheses over large scales and drones are used to investigate fire plume chemical composition. He is a contributing author to the IPCC AR5 and member of the Global Carbon Project, GOFC-GOLD fire implementation team, and Carbon Community of Practice.
JC note: Guido sent this to me via email in response to my post on Murry Salby’s talk. This is exactly the sort of thing that I hoped my post would elicit (but didn’t expect). Pls make an extra effort to keep your comments civil and relevant.