Climate Forcing During the Holocene

What caused Holocene climate variations? On the very longest, multi-millennial timescales, the main factors affecting Holocene climate change are related to orbital forcing (changes in obliquity, precession and eccentricity). Decadalto century-scale changes may be related to solar forcing (changes in total solar irradiance) and changes on shorter timescales (decadal to inter-annual) are most likely related to volcanic forcing (Bradley, 2003). Not all climatic variability seen in the Holocene can be ascribed to specific external forcings. For example, the ~8200 calendar year BP “event”, seen in many paleoclimatic archives, resulted from catastrophic pro-glacial lake drainage at the margins of the Laurentide Ice Sheet (Alley et al., 1997; Barber et al., 1999). This rapid flooding of the North Atlantic with freshwater had a significant regional climate impact, unrelated to any external forcing. Internal modes of climate system variability (e.g. El Niño-Southern Oscillation, North Atlantic Oscillation, Pacific Decadal Oscillation) that probably vary on both long and short timescales—though we know relatively little about their long-term behavior—may also produce significant changes in regional climate. And stochastic resonance in the climate system—by which a weak quasi-periodic forcing signal may be amplified into a non-linear, bi-stable climate signal—may have brought about relatively abrupt changes in the past by pushing the system across critical thresholds (Ruzmaikin, 1999). Thus, the spectrum of Holocene climate variability reflects the combined effects of all of these forcings and feedbacks within the climate system. Orbital forcing involves a significant redistribution of energy, both seasonally and latitudinally (Fig. 1). In the early Holocene, precessional changes led to perihelion at the time of the northern hemisphere summer solstice (whereas today, it is closer to the winter solstice). This resulted in higher summer insolation in the early Holocene at all latitudes of the northern hemisphere (ranging from ~40W m-2 higher than today at 60oN to 25W m-2 higher at the Equator). Thus, July insolation (radiation at the top of, or outside, the atmosphere) has slowly decreased over the last 12,000 years (Fig. 1). Anomalies during southern hemisphere summers were smaller, centered at lower latitudes, and were opposite in sign (that is, insolation increased over the course of the Holocene). For example, January insolation anomalies at 20oS were ~30W m-2 below current values in the early Holocene. Changes in solar irradiance (energy emitted by the Sun) might be expected to affect all parts of the Earth equally. However, this is not so because the response to solar irradiance forcing is amplified regionally, as a result of feedbacks and interactions within the atmosphere (Rind, 2002). The record of solar variability in the Holocene is discussed further in the accompanying article by Goslar. Here, I would only note that there are many Holocene paleoclimatic studies that claim solar forcing has driven observed changes, based largely on comparisons of proxy records with the 14C anomaly series. It remains to be seen how robust these relationships are, and what plausible mechanism might link solar activity/irradiance variations with climate in widely separated parts of the globe, from the Arabian Sea and Africa, to the North Atlantic and Greenland. One possibility (seen in some model simulations) involves solar variations influencing the Hadley circulation (intensity and/or extent), which then leads to tropical and sub-tropical precipitation anomalies, and to further teleconnections to extra-tropical regions. It is well known, from studies of instrumental records, that explosive volcanic eruptions can have short-term cooling effects on overall hemispheric or global mean temperatures (Robock, 2000). This results from direct radiative effects, with the volcanic aerosol reducing energy receipts at the surface, plus associated circulation changes that may result from such effects. Most temperature effects are not detectFigure 1: Schematic representation of insolation anomalies at the top of the atmosphere relative to 1950 levels. Anomalies are color-coded in W m-2. The x-axis of each monthly panel shows changes from 10,000 calendar years BP (left) to today (right). The y-axis shows latitude from 90oN (top) to 90oS (bottom) (figure prepared by Anne Waple).