Biogeochemical Feedbacks as Important as CO2
According to a new report in Nature Geoscience, scientists are beginning to realize that previously ignored aspects of the terrestrial biosphere can act as key regulators of atmospheric chemistry and climate. Not only that, changes in the biosphere can happen quickly—in the course of a few decades. “Although interactions between the carbon cycle and climate have been a central focus, other biogeochemical feedbacks could be as important in modulating future climate change,” states the report. Because a number of these feedbacks can have a cooling effect, the impact on global warming predictions could be earthshaking. The problem is, these feedbacks are only poorly understood and they are so interrelated that modeling them will be difficult, if not impossible.
The efforts of climate scientists, particularly the ones who blame human CO2 emissions for global warming, have concentrated on the carbon-cycle and the impact of greenhouse gasses on climate. In a new review paper, entitled “Terrestrial biogeochemical feedbacks in the climate system,” A. Arneth et al. delve into other, less studied factors involved in climate regulation. The paper surveys recent progress in understanding terrestrial biogeochemical feedbacks and their linkages, and provides an estimate of the potential magnitude of those feedbacks. Motivation for the review was stated this way:
Research into land–atmosphere exchange processes in climate science has traditionally focused on the surface radiation budget and its effects on sensible and latent heat fluxes, and more recently on carbon-cycle–climate interactions1. But many more bidirectional land–atmosphere fluxes modulate atmospheric composition and climate. Biogeochemical feedbacks are intrinsic to the climate system, owing to the nonlinear stimulation of all biological processes by increasing temperatures. Many biogeochemical processes also respond to changes in atmospheric composition and precipitation. Biogeochemical cycles are therefore strongly affected by anthropogenically forced climate change and other human activities.
The interesting point here is that most of the feedbacks listed have been pretty much ignored by climate modelers, due to the ill defined nature of the mechanisms. “During past periods of climate change, vegetation cover and interactions between the terrestrial biosphere and atmosphere changed within decades.” state Arneth et al.. “The overall magnitude of the biogeochemical feedbacks could potentially be similar to that of feedbacks in the physical climate system, but there are large uncertainties in the magnitude of individual estimates and in accounting for synergies between these effects.” These feedbacks are summarized in the figure below, taken from the report.
Radiative forcing from terrestrial biogeochemistry feedbacks.
The bars indicate approximate minimum-to-maximum feedback range over the twenty-first century. The carbon cycle CO2 and climate feedbacks include average and standard deviations (white). For calculations and data sources see the report's Supplementary Information. Totals include top-end estimates, without and with (black rectangle) carbon–nitrogen interactions. Notice how CO2 fertilization has the potential to cool the climate, though the values given here are mostly guesswork. In the words of the authors': “Estimates are based on a number of assumptions that had to be made and clearly point to the need for more interdisciplinary studies.” Confidence in scientific understanding is at best low (L), in most cases very low (VL).
The report lists three “principal pathways” through which biogeochemical cycles interact with the atmosphere and climate. These principle mechanisms of change are:
- Climate change alters the biogeochemical cycling of atmospherically well-mixed greenhouse gases, which act directly as radiative forcing agents.
- Changes in atmospheric composition influence the biogeochemistry of radiatively active compounds.
- Climate change alters the biogeochemistry of substances that are not radiatively active in themselves, but that affect the atmospheric concentration of other climatically active compounds.
These three factors interact to form a complex web of feedbacks between atmospheric gasses, terrestrial chemicals and biology. One of the most important factors is the impact on photosynthesis by rising CO2 levels. Many plants thrive on higher levels of CO2, for whom it acts like plant food. But increased plant growth is only a temporary sequestration of carbon. Eventually the plants die and decompose, releasing what ever carbon they have absorbed back into the environment. The balance between net primary productivity (NPP) and losses from soil carbon decomposition could have a significant impact on climate change.
“The stimulation of photosynthesis by increasing atmospheric CO2 concentrations and lengthening growing seasons exerts a key influence on the future trajectory of the land-carbon sink, but regional reforestation and ecosystem responses to nitrogen deposition also play a role,” states the review. Scientists think that the effect is large, but whether it represents a net gain or loss is unclear. “As yet, there is no consensus on the relative climate and CO2 trajectories that either NPP or decomposition will follow.”
Chemical substances that can rapidly react with other chemical in the environment also play a role. “As well as influencing atmospheric CO2 and CH4 concentrations, ecosystems exchange substances with the atmosphere that are readily reactive and climatically relevant,” claim the authors. Other substances, including nitrogen oxides (NOx), are directly connected to carbon–nitrogen interactions through plant-litter and the use of fertilizer. These reactions may be linked to observed changes in CO2 absorption by soil.
Aerosol particles also join in the fray. According to the report, “indirect aerosol effects on cloud microphysics, lifetime and precipitation rate are associated with large, poorly constrained climate uncertainties.” Uncertainty seems to be the unifying factor. Pronounced significant, yet poorly quantified and uncertain, these biogeochemical factors illustrate a fundamental lack of scientific understanding regarding how climate really works.
The food web is part of terrestrial biogeochemistry.
So, what are we to make of this new report, revealing a plethora of missing or previously ignored factors involved in climate regulation? From the levels of understanding shown in the figure above, it is clear that these factors are not well explained. When we stated in The Reslilient Earth that our knowledge of how climate works was sorely limited, the revelations made public in this report were only hinted at. Yet the Nature Geoscience report intimates that these processes and feedbacks are too important to ignore. In fact, the authors state that, “although interactions between the carbon cycle and climate have been a central focus, other biogeochemical feedbacks could be as important in modulating future climate change.”
“There is growing evidence that the impact of biogeochemical feedbacks must not be ignored in climate change studies,” the report summarizes. “Given the limited number of quantitative experiments available, our estimates of the total radiative forcing from biogeochemical feedbacks that operate in the climate–chemistry system can only serve as a very rough guide. Nonetheless, we conclude that the climate effect of changes in element cycles and atmospheric chemistry is comparable in magnitude to that of physical feedbacks such as water vapour or clouds.” You may recall that clouds have been giving climate modelers fits for years.
This means that all of those IPCC predictions, and all of those old climate models, are missing a number of climate regulating factors—factors that could be as important as all the ones they did model. Evidence of climate sciences' ignorance mounts daily. With each new revelation, the inadequacy of our scientific understanding becomes undeniable. Yet there are those who still stubbornly insist that the models are trustworthy, that the predictions of a calamitous future are accurate. No one with slightest amount of scientific understanding can honestly say that the IPCC projections are correct.
Be safe, enjoy the interglacial and stay skeptical.