By Earl J. Ritchie, Lecturer, Department of Construction Management
Many recent articles, such as this New York Times piece, have described the flattening in estimated carbon dioxide emissions from fossil fuels as a possible turning point in anthropogenic greenhouse gas emissions. In the face of this flattening, the unabated rise of atmospheric CO2 has been declared a mystery. A number of possible explanations, such as saturation of carbon sinks and El Niño, have been given for the apparent disconnect.
There are two simple explanations: The first is that CO2 emissions from human sources have not actually decreased. The flattening in fossil fuel emissions has been offset by increases due to land use changes, such as logging of forests and increased development. The second is that even if CO2 emissions had flattened, the total amount of CO2 pumped into the atmosphere would not be greatly reduced. The atmospheric CO2 growth rate would slow, but the difference would likely be undetectable in the short term.
The “mystery” as portrayed
The quandary is illustrated by the graphs below. Fossil fuel CO2 emissions have noticeably flattened over the past three years, while atmospheric CO2 concentration has continued upward.
Anthropogenic CO2 is still rising
Because fossils fuels are by far the largest source of man-made CO2, there is a tendency to compare atmospheric concentrations to those alone. Carbon dioxide emissions are estimates rather than facts, however, the bigger issue is that fossil fuel emissions are not the only anthropogenic sources of CO2. The graph below shows that when land use is added, emissions steadily increase.
Emissions from land-use changes are recognized as the least certain of carbon budget components. However, their absolute uncertainty is no greater than uncertainty in fossil fuel emissions. The effects from land-use changes cannot be ignored.
Flattening is not enough
According to models from the Intergovernmental Panel on Climate Change, short-term carbon sinks – a forest, ocean or other reservoir that can absorb carbon dioxide – could not keep up even if CO2 emissions were significantly reduced. Therefore, atmospheric CO2 continues to increase even if emissions were to be stabilized. This can be seen by the black line in the graph below. The decrease in growth of atmospheric CO2 after emissions are stabilized is gradual and would be difficult to detect over a few years. Actual decrease in concentration requires emissions reductions on the order of 80%.
Other factors which may contribute to the discrepancy between fossil fuel emissions and CO2 concentration include:
- El Niño
- Saturation of carbon sinks
- Uncertainty in emissions estimates
- Other feedbacks
- Natural CO2 sources
El Niño, a warming of the Pacific area, is known to increase atmospheric CO2. There was a strong El Niño in 2015 and 2016, which contributed to the record 3 ppm increases of those years. This is a short-term fluctuation but did temporarily mask longer term effects. The United Kingdom’s Met Office forecast for 2017, a year without an El Niño, is 2.5 ppm, slightly above the recent average.