Celestial climate driver: a perspective from four billion years of the carbon cycle

Geoscience Canada, March, 2005 by Jan Veizer

[FIGURE 20 OMITTED]

In order to test the hypothesis of C[O.sub.2] "piggybacking" on the water cycle, several large watersheds were examined, because there the water balance can be deconvolved into precipitation, discharge, evaporation, interception and transpiration fluxes. Knowing the transpiration flux and the requisite WUE, it is then possible to calculate the photosynthetic sequestration capacity for C[O.sub.2] for a given watershed. Taldng the Mississippi basin (Fig. 20) as an example (Lee and Veizer, 2003), plant transpiration recycles about 60% of precipitation back into the atmosphere and the calculated, water balance-based, annual photosynthetic sequestration of C[O.sub.2] by plants is then 1.16 Pg of carbon. This is essentially identical to the heterotrophic soil respiration flux of 1.12 PgC derived by biological approaches for the same watershed. Hence, the suggestion that the carbon cycle is "piggybacking" on the water cycle is a viable proposition. This scenario is supported also by the satellite data of global productivity for the 1982-1999 period, with "climatic variability overland exerting a strong control over the variations in atmospheric C[O.sub.2]" (Nemani et al., 2003). In these two decades the global biomass grew by 6% (3.4 PgC). Almost one hall of the increase happened, surprisingly, in the Amazon basin, and was caused by a decrease in the cloud cover (decline in CRF?) and to a concomitant 20th century increase in solar radiation (Figs. 13, 14, 15). Again, while C[O.sub.2] may act as an amplifying greenhouse gas, the actual atmospheric C[O.sub.2] concentrations are controlled in the first instance by the climate, that is by the sun-driven water cycle, and not the other way around.

ENVIRONMENTAL IMPLICATIONS

At this stage, two scenarios of potential human impact on climate appear feasible: (1) the standard IPCC model that advocates the leading role of greenhouse gases, particularly of C[O.sub.2], and (2) the alternative model that argues for celestial phenomena as the principal climate driver. The two scenarios are likely not even mutually exclusive, but a prioritization may result in different relative impact. Models and empirical observations are both indispensable tools of science, yet when discrepancies arise, observations should carry greater weight than theory. If so, the multitude of empirical observations favours celestial phenomena as the most important driver of terrestrial climate on most time scales, but time will be the final judge. Should the celestial alternative prevail, the chain of reasoning for potential human impact may deviate from that of the standard IPCC model, because the strongest impact may be indirect, via the formation of cloud condensation nuclei (CCN). The CRF-generated positive and negative ions combine, within minutes, into electrically neutral aerosols, but only if the two ions are large enough. The required size of these "cluster ions" is reached by addition of atmospheric molecules, particularly sulphuric acid. Since [H.sub.2]S[O.sub.4] is highly hygroscopic, it attracts also water molecules. In this way, the ~30-100 nm large CCN required as precursors for droplets can potentially be generated (Carslaw et al., 2002; Lee et al., 2003). Thus, sulphur compounds (and perhaps dust, soot and secondary particles, which are formed by condensation of low-vapour-pressure gases) could playa major role in this seeding process. In the northern hemisphere, the precursor of sulphuric acid, sulphur dioxide gas, originates mostly from anthropogenic activities, but natural sources, such as volcanic eruptions of DMS from marine plankton, are also substantial.