Response of the Himalayan glacial cycles to multiple equilibrium of climate system: A review

The astronomical theory of Milankovitch supports the waxing and waning of ice sheets and polar glaciers by orbital forcing, excluding the fact that small changes in solar radiation flux are difficult to reconcile with the amplitude of glacial cycles. The numerical model explaining the 100 kyr problem has now revealed that the phase locking mechanism of orbital forcing has started to modulate the climatic transitions since ∼1.2 Myr back and remained locked till present climatic state (Rial et al., 2013). However, the true evidence to confirm it is not present. The present study is an attempt to comprehend the concept of synchronization of summer insolation modulation and internal climate forcing, responsible for glacial/interglacial termination present in and around the Himalaya-Tibet and worldwide. We present geographical evidences of glaciations from the Himalayan-Tibet region pointing out that responses (i.e. 18–24 kyr and 60–70 kyr BP glaciation events) are similar to the progression in climatic data and glaciations cycles for the last 100 kyr. The compelling evidence presents a possible explanation towards synchronized modulation of 100 kyr orbital cycles with 41 kyr obliquity cycles, coincidently resulting in unprecedentedly large glaciation around the last glacial cycle (60–70 kyr BP) and the Last Glacial Maxima (18–24 kys BP). By considering the influence of internal and external factors on glacier mechanism, we underline the most probable cause of 41 kyr cycle (obliquity) as the foremost driver of glaciations in the Himalaya and Tibet on longer timescales. We provide several lines of evidence to establish the possible connection of glaciation events and orbital cycles with the help of geographical and geological records consistent with the fact that resonance might be faded through time but the effect of frequency modulation still persists in the system.