The last few million years have been punctuated by many abrupt climate transitions many of them occurring on time scales of centuries or even decades. These sudden stepwise climate transitions provide a disturbing scenario when we consider the possible effects that humans might have on the present climate system through global warming. In order to better understand our current climate system we need to understand how the past climate transitions occurred. In this session we examine and review the paleoclimate proxies and some modeling results for each of the key climate transitions in the Quaternary period. I will introduce those transition which have been identified as the most important: 1) the Onset of Northern Hemisphere Glaciation (ONHG) which heralded the start of the Quaternary period, 2) the Mid-Pleistocene Revolution (MPR) and the switch to a non-linear global climate system, 3) Glacial-interglacial cycles, 4) Heinrich events and glacial Dansgaard-Oeschger (D-O) cycles, 5) the last glacial-interglacial transition (LGIT) and 6) interglacial climate transition such as the Intra-Eemian cold event and Holocene D-O cycles.

The continuing debates on causuation centre on external verses internal mechanisms. External mechanisms include tectonic changes, Milankovitch ?orbital? forcing and solar variations. While internal feedback mechanism include: atmospheric, surface ocean and deep-ocean circulation, atmospheric concentrations of greenhouse gases such as carbon dioxide, methane and water vapour, sea level, snow and ice cover, dust and haze particles. In general most of these transitions appear to be threshold changes where by external forcing leads to a change in the state of global climate. I will also introduce the theory that these changes are non-reversible because of bifurcation in our climate system. Hence we suggest that the evidence indicates that long-term climate change occurs in sudden jumps rather than incremental changes; which does not bode well for the future.