Scrutinizing the Carbon Cycle and CO2 residence time in the atmosphere: Hermann Harde asserts excess CO2 will resolve rapidly
In this paper, Hermann Harde claims that scientists’ understanding of how carbon moves through the atmosphere, plants, and oceans on Earth is incorrect. Furthermore, Harde claims that humans are not responsible for the increase in carbon dioxide in Earth’s atmosphere and that natural processes will absorb excess carbon in the space of about 50 years.
There are several problems with Harde’s approach that invalidate their conclusions. First, Harde only tracks the amount of carbon in one place: the atmosphere. There are many places where carbon can be on Earth besides the atmosphere, like the ocean, animals, plants, and soil. Ignoring all these other carbon reservoirs means that Harde cannot account for how reservoirs interact. For example, plants and the atmosphere interact through processes like photosynthesis, respiration, transpiration, and wildfire. Harde makes some effort to link the atmosphere with other reservoirs that consume carbon, but they apply a blanket assumption that uptake by other reservoirs will not slow down in the future. Such an assumption is not appropriately justified, especially when others in the literature go to great effort in order to account for changing carbon uptake rates.
Another important misconception in Harde’s approach is the difference between carbon residence time and carbon adjustment time. Harde’s calculations give an estimate of the residence time of carbon in the atmosphere. This value is the average amount of time a carbon atom will remain in the atmosphere before moving to another reservoir. Adjustment time, in contrast, is the amount of time it takes for a carbon reservoir to return to its original state following a perturbation. So, it is not accurate to say that Earth will naturally return to its original baseline within 50 years of stopped emissions.
It is easy to show that Harde’s representation of the climate system is inaccurate based on data presented in the paper itself. Harde estimates atmospheric carbon dioxide concentration from surface temperature for 1850 through 2012, and finds that the model reproduces “[carbon] concentrations at 1850 and 2012… as well as the slightly non-linear progression from 1960 to 2015.” However, when the model is applied to paleoclimate data, Harde finds that “CO2 variations of about 100 ppm between glacial and interglacial periods typically go along with temperature changes of about 8 °C, whereas our preceding estimates already gave a 110 ppm increase at a temperature boost of only 0.9 °C.” If Harde’s approach is incorrect by nearly an order of magnitude on past climate change, how can we consider it reliable when applied to future climate change?
Modern climate models undergo much more rigorous validation, and are required to accurately reconstruct past climate trends before applying them to the future. This poor performance, as well as the gross simplifications in model structure, means that Harde’s paper should not be trusted whatsoever. For a more detailed critique by professional climate modelers, see: https://www.sciencedirect.com/science/article/pii/S0921818117301364#s0015.
