6.5 The Current Interglacial

A variety of proxy records provide detailed temporal and spatial information concerning climate change during the current interglacial, the Holocene, an approximately 11.6 kyr period of increasingly intense anthropogenic modifications of the local (e.g., land use) to global (e.g., atmospheric composition) environment. The well-dated reconstructions of the past 2 kyr are covered in Section 6.6. In the context of both climate forcing and response, the Holocene is far better documented in terms of spatial coverage, dating and temporal resolution than previous interglacials. The evidence is clear that significant changes in climate forcing during the Holocene induced significant and complex climate responses, including long-term and abrupt changes in temperature, precipitation, monsoon dynamics and the El Niño-Southern Oscillation (ENSO). For selected periods such as the mid-Holocene, about 6 ka, intensive efforts have been dedicated to the synthesis of palaeoclimatic observations and modelling intercomparisons. Such extensive data coverage provides a sound basis to evaluate the capacity of climate models to capture the response of the climate system to the orbital forcing.

6.5.1 Climate Forcing and Response During the Current Interglacial

6.5.1.1 What Were the Main Climate Forcings During the Holocene?

During the current interglacial, changes in the Earth’s orbit modulated the latitudinal and seasonal distribution of insolation (Box 6.1). Ongoing efforts to quantify Holocene changes in stratospheric aerosol content recorded in the chemical composition of ice cores from both poles (Zielinski, 2000; Castellano et al., 2005) confirm that volcanic forcing amplitude and occurrence varied significantly during the Holocene (see also Section 6.6.3). Fluctuations of cosmogenic isotopes (ice core ¹⁰Be and tree ring ¹⁴C) have been used as proxies for Holocene changes in solar activity (e.g., Bond et al., 2001), although the quantitative link to solar irradiance remains uncertain and substantial work is needed to disentangle solar from non-solar influences on these proxies over the full Holocene (Muscheler et al., 2006). Residual continental ice sheets formed during the last ice age were retreating during the first half of the current interglacial period (Figure 6.8). The associated ice sheet albedo is thought to have locally modulated the regional climate response to the orbital forcing (e.g., Davis et al., 2003).

The evolution of atmospheric trace gases during the Holocene is well known from ice core analyses (Figure 6.4). A first decrease in atmospheric CO₂ of about 7 ppm from 11 to 8 ka was followed by a 20 ppm CO₂ increase until the onset of the industrial revolution (Monnin et al., 2004). Atmospheric CH₄ decreased from a NH value of about 730 ppb around 10 ka to about 580 ppb around 6 ka, and increased again slowly to 730 ppb in pre-industrial times (Chappellaz et al., 1997; Flückiger et al., 2002). Atmospheric N₂O largely followed the evolution of atmospheric CO₂ and shows an early Holocene decrease of about 10 ppb and an increase of the same magnitude between 8 and 2 ka (Flückiger et al., 2002). Implied radiative forcing changes from Holocene greenhouse gas variations are 0.4 W m^–2 (CO₂) and 0.1 W m^–2 (N₂O and CH₄), relative to pre-industrial forcing.