Volcanic activity: Aerosols (SO2, CO2, H2O, N2) and dust are injected into the atmosphere during explosive volcanic eruptions which affects the Earth’s radiative balance through enhanced absorption and scattering of solar radiation (Beer and Van Geel 2008). This leads to warming in the upper atmosphere and cooling in the lower atmosphere and can significantly affect the climate on an annual to decadal time-scale (Crowley 2000; Robock 2000). Deglaciation can trigger volcanic activity as ice-sheet unloading and/or sea-level rises may alter the loading on the geological structures confining the magma (Capra 2006; Self 2006).
Ocean Circulation: Due to their volume, heat-capacity and inertia the oceans
store and transport an immense amount of heat energy, and consequently play a crucial role in the regulation of the global climate system. Wind-driven currents, such as the Gulf Stream, carry heat polewards in the North Atlantic. As the warm water meets cold polar air, the water cools by evaporative cooling and sinks. This process is assisted by the formation of sea ice which increases the salinity and density of the unfrozen water. The cold, dense water sinks and flows southwards forming the North Atlantic Deep Water current (NADW). The NADW flows, at depth, as part of the ‘ocean conveyer belt’ into the Pacific Ocean. The North Atlantic is warmer than the North Pacific. Evaporation rates, and therefore salinity, are higher relative to the North Pacific and this salinity
gradient drives global thermohaline circulation (THC). The circulation is perpetuated by the ‘pull’ created by the sinking of the NADW which draws surface water from the Gulf of Mexico.
Periodic changes in grain size within deep sea sediment cores, from the North Atlantic, suggest the strength of the NADW is linked to climate oscillations (Bianchi and McCave 1999). During glacial periods, formation of the NADW is reduced or, possibly, shut down by the southwards displacement of the polar front and reduced evaporation due to cooler sea surface temperatures (Broecker and Denton 1989). Broecker (1987) proposed that salinity changes between the North Atlantic and North Pacific could be sufficient to reverse the pattern of global thermohaline circulation. The change to a salinity-thermal driven halothermal circulation (HTC) mode may explain the rapid (<1,000 years) termination of the YD (Dansgaard et al. 1989). However empirical evidence in support of mode changes is inconclusive and reduction or shutdown of ocean heat transport would not be sufficient to initiate global temperature changes and ice sheet development without the involvement of other internal feedback mechanisms such as changes in atmospheric composition (Broecker and Denton 1989).
Atmospheric Composition: Changes in atmospheric composition, particularly of
‘greenhouse gases’ CO2, CH4 and H2O vapour, act to amplify the effects of radiative forcing by absorbing outgoing long-wave radiation and re-radiating the energy back to the Earth’s surface. In addition compositional changes impact on the formation of clouds and aviation-induced contrails and surface albedo. Changes in composition occur as a result of both natural and anthropogenic factors. Natural factors include solar changes and volcanic emissions which have been previously discussed together with processes such as biogeochemical cycles in soils and the ocean (Forster et al. 2007). Over geological timescales changes in atmospheric greenhouse gas composition are associated with transitions between glacial and interglacial periods. Ice-core records from Antarctica show that atmospheric greenhouse gases co-vary with temperatures over glacial-interglacial cycles, with a lag of several centuries to millennium (Mudelsee 2001). Therefore changes in atmospheric composition
act as a feedback mechanism to enhance and sustain climate change rather than as a primary forcing mechanism.
Anthropogenic change: Human activity, through forest clearance, land use,
burning of fossil fuels and other industrial processes, has increased the concentration of CO2, CH4, N2O and other greenhouse gases since the eighteenth century (Andres et al. 2000; Forster et al. 2007). Analysis of gases trapped in ice cores suggests recent concentrations of CO2, CH4 and N2O far exceed their pre-industrial values and the average rate of increase in their radiative forcing is greater than at any time during the past 13000 years of the Late Quaternary (Jansen et al. 2007). Shifts in atmospheric CH4 and CO2 gas concentrations over the Late Quaternary, prior to the industrial period, were generally small-scale, with the exceptions of declines in CH4 concentrations at 8200 yrs BP and CO2 concentrations at 1200 yrs BP. Therefore Mayewski et al. (2004) suggested changes in greenhouse gas concentrations played a negligible forcing role and acted to amplify climate variations until approximately 250 yrs BP. Ruddiman (2003), however, argues that atmospheric CO2 and CH4 concentrations have diverged from trends of previous interglacial cycles and steadily increased from 8000 years BP due to the development of agriculture. However the hypothesis conflicts with evidence of high CO2 concentrations during the interglacial MIS 11 (Siegenthaler et al. 2005). Atmospheric concentrations of carbon dioxide (CO2) have increased from a pre-industrial value of 280 ppm to 385 ppm in 2008 (Keeling et al. 2009) and exceeds the natural range (of 180 – 300 ppm) over the past 650,000 years (EPICA community members 2004). Climate models only reproduce the observed 20th century global mean surface warming when both anthropogenic and natural forcings are included (Zwiers 2007). The evidence for warming in the Arctic over the past 150 years is reviewed in section 1.3.2.
Tectonic processes: Orogeny (the uplift of mountain ranges) influences global
climate by affecting atmospheric circulation patterns and the drawdown of atmospheric CO2 through enhanced rates of physical and chemical weathering of the rocks (Raymo and Ruddiman 1992). Epeirogeny (changes in the global arrangement of land masses) influences surface albedo, oceanic circulation
patterns and increases atmospheric CO2 during periods of intense sea-floor surface spreading. However these mechanisms operate over timescales of tens to hundreds of millions of years so their influence on variability in the Late Quaternary - Holocene climate is likely to be minimal.