**4. The effect of redox carbon cycle on the climate in the past**

The role of CO2 in climate formation is a well-known fact. It is the main component of "greenhouse" gases [20]. The periodic filling of the "atmosphere–hydrosphere" system with CO2 in orogenic time and the following depletion of CO2 due to photosynthetic assimilation in geosynclynal period provide alternating warming–cooling change. It is even possible to use the relation between CO2 concentrations and Earth temperature for the determination of paleotemperatures [21], although the validity of this correlation is limited due to the contri‐ bution of other greenhouse gases.

Following the logic of this model, the existence of climatic cycles is a result of the orogenic cycles. The beginning of the orogenic cycle may be considered as the warmest time of the cycle and the end as the coldest one. The latter is often accompanied with glaciations.

The mentioned *ε* parameter may be used as the indicator of orogenic and climatic cycles. At the beginning of the orogenic cycle, when CO2/O2 ratio is maximal and the contribution of photorespiration is low, *ε* parameter is also at its maximum and corresponds to the warming period. Conversely, at the end of the cycle, when CO2/O2 ratio is minimal and photorespiration increases, *ε* parameter reaches its minimum and corresponds to the cooling period.

Popp et al. [15] found a coherence of *ε* values and climatic cycles in the Cenozoic. Hayes and others [16], having examined carbon isotope composition for more than 5000 samples of coeval carbonates and sedimentary organic matter spanning the Precambrian and Phanerozoic, found statistically significant differences in *ε* values in interglacial periods and those in periods of glaciations. The results were supported by other researchers [22, 23].
