First of all, the two researchers report that the CO2 concentration in the air of several Belgian caves "has risen very significantly" since their first measurements in the 1960s. As an example, they say that after about forty years of measuring cave CO2 concentrations, they "noticed that values had strongly increased between 1966 and today (Ek and Godissart, 2009)." Between 1966 and 1968, for example, they could find no value above 6000 ppm; but nowadays they say they "commonly observe contents above 20,000 ppm in summer."

The Belgian scientists also report that soil air-space CO2 measurements, which were made at various locations above the caves, "show clear seasonal variations, the maximum occurring in early summer, well before the maximum in the caves," which chronological ordering, in their words, "confirms the influence of vegetation as a source of carbon dioxide in the caves." And in this regard, they also say "it is well known that atmospheric CO2 concentration and temperature are two factors influencing the growth of vegetation," implying that as these two factors rose together over this period, they were responsible for the fact that the stock of vegetation had been concomitantly increasing in Belgium, where the volume of wood in tree trunks in the Walloon Region forests rose about 30% in the thirty years from 1984 to 2005," which was "correlative with the soil biological activity." And they say they "firmly believe that this explains a large part of the CO2 rise in cave air."

In further support of this conclusion, Ek and Godissart write that "the total volume of tree trunks in the Walloon Region rose about 30% since 1984 (Laurent and Lecomte, 2008)," while noting that "Heyninck and Jonard (2009) quote a growth acceleration of 25% in North-West France and 50% in North-East France between 1900 and 2000." And in Europe as a whole, they say that "Chmielewski and Rotzer (2001) have shown that, in thirty years, between 1969 and 1998, the spring warming up caused an eight day advance in the development of leaves," adding that "Menzel et al. (2006) reach the same conclusion for twenty-one European countries, using an enormous systematic data set of more than 125,000 series of 542 plants and 19 animal species," while further noting that "the advance of spring matches the measured warming in 19 European countries," with the implication that there was thus "an important strengthening of vegetation."

And this is the point we wanted to emphasize here. Even below ground - in caves - there is strong evidence for the significant increase in forest and other vegetative growth that has occurred in concert with the historical and still-ongoing increase in the atmosphere's CO2 concentration, as well as during the earlier portion of this period when earth's air temperatures were also rising.

Sherwood, Keith and Craig Idso

References
Chmielewski, F.M. and Rotzer, T. 2001. Response of tree phenology to climate change across Europe. Agricultural and Forest Meteorology 108: 101-112.

Ek, C. and Godissart, J. 2009. Extreme increase of CO2 in Belgian caves. In: Proceedings of the 15th International Congress of Speleology, Kerrville, Texas, USA, pp. 1467-1473.

Ek, C. and Godissart, J. 2014. Carbon dioxide in cave air and soil air in some karstic areas of Belgium. A prospective view. Geologica Belgica 17: 102-106.

Heyninck, C. and Jonard, M. 2009. Des hetraies qui poussent plus vite Foret wallonne, 103, 7p.

Laurent, C. and Lecomte, H. 2008. La composition, l'evolution et l'exploitation de la foret. Etat de l'environnement wallon. Rapport analytique 2006-2007. Service public de Wallonie, pp. 184-195.

Menzel, A., Sparks, T.H., Estrella, N., Koch, E., Aas, A., Aha, R., Alm-Ku, K., Bissolli, P., Braslavská, O., Briede, A., Chmielewski, F.M., Crepinsek, Z., Curnel, Y., Dahl, Ĺ., Defila, C., Donnelly, A., Filella, Y., Jatczak, K., Mĺge, F., Mestre, A., Nordli, Ř., Peńuelas, J., Pirinen, P., Remišová, V., Scheifinger, H., Striz, M., Susnik, A., Van Vliet, A.J.H., Wielgolaski, F.-E., Zach, S. and Zust, A. 2006. European phenological response to climate change matches the warning pattern. Global Change Biology 12: 1969-1976.