The role of climate
|| A third factor in the deterioration of cave paintings that has attracted some attention is the role of the speleoclimate. The climatic regimes of deep caves operate largely independent of external conditions, particularly where the entrances are small and passages form air syphons. Air residency rates can be very high in caves with poor ventilation. In such conditions three factors are greatly affected by the frequent visits of large numbers of humans: air temperature, air humidity and level of respired carbon dioxide. The temperature in a small enclosed space rises markedly within minutes of the arrival of visitors, and interestingly it has been noted that juveniles give off relatively more heat than adults. The relative air humidity in a cave is so sensitive to humans that even the presence of a single person renders measurements unreliable and recording hygrometers need to be used. The effects of flooding caves with carbon dioxide can be a significant conservation threat because rocks tend to be moist and this renders the dilute carbonic acid the moisture forms with the CO2 corrosive (Fernandez 1986). Many caves are close to the upper level of the aquifer, which retards drainage of the gas which is heavier than air, and so prolongs exposure of the walls to it.Closeness of the water table also prevents intake of warmer air, cool air is trapped, and buffered by both the water body and the rock mass, allowing its humidity to remain equally stable. When this equilibrium is threatened by convective heat in the entrance, the evaporation rate there increases, evaporation effects a lowering in temperature and thus protects the cool air below (Bednarik 1991b). In this system equilibrium is maintained, condensation does not occur, and rock paintings can survive for tens of thousands of years in almost original condition. This provides an illustration of the effectiveness of micro-climate in rock art preservation.
While it is not possible for a rockshelter to exercise the same influence on its local climate, it does nevertheless create its own climatic regime. This factor has been subjected to a good deal of research in several world regions, usually by the placement of recorders of ambient air temperature and relative air humidity. Much less attention has been given to other, similarly relevant factors, such as rock temperature, which in some of the European projects of monitoring deep cave climates has been taken care of. On the whole these programs seem to have provided little tangible benefits in understanding the rather capricious micro-climates of rockshelters. However, it has been demonstrated that levels of relative air humidity and diurnal temperature oscillations in rockshelters are affected by tree cover.
References – Bibliography of rock art conservation