Forests, CO2 and H2O

Much has been done to vilify carbon dioxide in the media. Listening to the talking heads and on-air “experts” could lead one to believe that CO2 is an evil scourge that the world would be better off without. Nothing could be further than the truth. CO2 is necessary for life on Earth, forests in particular. It is not just plant food, the maligned gas also plays a role in regulating water use by the world's forests. New research has uncovered an unexpectedly strong decrease in H2O uptake caused by increasing CO2. Along with global increases in photosynthesis, forest growth rates, and carbon uptake, higher CO2 levels contribute to enhanced timber yields and improved water availability. Who says higher CO2 levels are a bad thing?

One possible future problem for humanity is a shortage of fresh water. Climate alarmists have tried mightily to tie rising CO2 levels to an increase in drought. While that link has shown to be tenuous at best, there is new evidence that rising levels of carbon dioxide does have an effect on water use by forests. “Our analysis suggests that rising atmospheric CO2 is having a direct and unexpectedly strong influence on ecosystem processes and biosphere–atmosphere interactions in temperate and boreal forests,” write Trevor F. Keenan et al. In the journal Nature.

The biochemistry of photosynthesis predicts that increasing atmospheric CO2 should cause an increase in both carbon uptake and water-use efficiency. In a letter titled “Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise,” a team of researchers take a look at they unexpectedly strong response of the world's forests to rising CO2 levels. The article's abstract captures the main aspects and findings of their research.

Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange. We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata—small pores on the leaf surface that regulate gas exchange—to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation–climate models.

Plants assimilate atmospheric CO2 through photosynthesis, which produces the complex organic molecules that ultimately support most life on Earth. Uptake of CO2 is accompanied by the loss of H2O (as water vapor) through stomata, the small pores on leaf surfaces that regulate the diffusion of these two gases between the leaf and the atmosphere. The rate of carbon uptake per unit of water loss is referred to as water-use efficiency, We. Among other things, We quantifies how much water an ecosystem uses relative to carbon gained. Unfortunately, measuring the impact of rising CO2 levels on the real world has proven difficult in the past.

“CO2-enrichment experiments, proxy reconstructions, and models provide support for an increase in plant resource-use efficiency as CO2 concentrations increase, but are often inconsistent and inconclusive1,” state the authors, adding: “The magnitude of any response to elevated CO2 in natural forest ecosystems is largely unknown, leading to debate on the effect of nutrient limitations. This is not surprising, given the difficulties in translating results from isotopic proxies and small-scale manipulative experiments to intact natural ecosystems.”

Using direct, continuous long-term measurements of CO2 and water vapor fluxes, Keenan et al. have analyzed recent trends in water-use efficiency of actual forest canopies. Regional analyses show that efficiency increased at each forest site studdied. Of the seven regional focus sites, the change was significant (P < 0.05) at three sites. Moreover, at all other sites the slope was positive but with lower P values. Technically speaking, “the mean trend (±1 s.e.) across all seven sites is 1.07 ± 0.3 grams of carbon per kilogram of water times a hectopascal per year (g C per kg H2O hPa yr−1) (P = 0.011, Student’s t-test), with a joint probability of P = 0.0016 (Fisher combined probability).”

To account for the effects of atmospheric evaporative demand, D, an ‘inherent’ ecosystem-scale water-use efficiency can be calculated as Wei = WeD (see the paper's methods section for details). The results for all the sites in the study are shown in the figure below.

Shown is the annual change in water use efficiency, ΔWei, calculated using daytime fluxes from summer months at all sites. ΔWei is normalized to mean Wei at each site. The red line represents the mean trend over all sites, extrapolated over the entire measurement period. The gray area highlights trends within one standard deviation about the mean trend. Individual site observations and trends are given as gray dots and black lines respectively. The inset shows the distribution of the rate of change in Wei over all sites. Dashed red lines represent bootstrapped 95% confidence intervals for the mean slope. The gray shaded area represents the mean and standard deviation of the slope of increases in Wei simulated by 13 terrestrial biosphere models at four of the US regional sites.

According to the authors, “although the trend is not statistically significant at all individual sites, it is highly significant when multiple sites are considered.” Bottom line, there is a greater increase in water use efficiency due to CO2 than previously thought. In fact, when a number of computer models were run, aside from underestimating the increase, they attributed the change to the wrong factors:

This analysis indicates that only a small fraction of trends in carbon uptake can be explained by changes in climate forcings (temperature, precipitation, humidity, solar radiation) at any site. Previous work attributes a proportion of the net annual increase to a lengthening of the growing season. Our model-data fusion approach, however, shows a large increase in net uptake during the summer months, independent of season length.

These new findings do not just expand our understanding of forest growth response to increased levels of CO2. They also indicate that the models currently in use to predict overall impacts from climate change are wrong. Change in forest growth can change precipitation patterns, carbon uptake and directly affect surface energy. This illustrates the problem when scientists try to make their global models more accurate by adding more detailed estimates for factors from specialized models—each additional model input may be erroneous in its own way, introducing new sources of error into the overall model predictions.

“Increases in Wei may account for reports of global increases in photosynthesis, forest growth rates, and carbon uptake,” the authors conclude. “Our analysis suggests that rising atmospheric CO2 is having a direct and unexpectedly strong influence on ecosystem processes and biosphere–atmosphere interactions in temperate and boreal forests.”

The world's boreal forests like rising carbon dioxide levels.

Once again the dire warnings of the climate catastrophists have been shown to be exaggerated at best, and often just plain wrong. Rising CO2 levels make forests grow faster, be more productive and actually use water more efficiently. Those are good things to rational people. This report also shows that when you actually measure what is happening in the environment, instead of using computer models, nature can surprise you. In other words, measure nature and real science happens.

Be safe, enjoy the interglacial and stay skeptical.