The Influence of Vegetation-Atmosphere-Ocean Interaction on Climate During the Mid-Holocene

Abstract

surement precisions (0.5%) (20) in the travel times themselves. 20. Single-crystal samples of MgO (purity Ͼ99.9%) were oriented with x-ray diffraction method described in (13) to [100] and [110] orientations (better than 1°); the samples were cored, cut, and polished to cylinders of about 1.8 mm in diameter and 1.2 to 1.5 mm in length. The original sample lengths were measured to micrometers, that is, Ϯ 1 m. Measurements of the sample travel time were achieved by overlapping the glass buffer rod echo and the sample echo and recording the amplitudes of the interference signal as a function of the frequency (from 20 to 70 MHz). Precision in the travel time measurements is about 2 ns or 0.5% for the sample used in this study. We used Pt (20 m thick) and Au (2 m thick) foils to couple the acoustic wave from the glass buffer rod to the sample. These metal foils alleviate the problems of the compressibility and thermal expansivity mismatches between the buffer and the sample, but they also introduce uncertainties in the acoustic travel time [ (25) after correcting for the effect of the bonds (Pt or Au foils), the sample travel times agree within the measurement uncertainties (2 ns). 21. In extracting the cross pressure-temperature derivatives of the elastic moduli ‫ץ(‬ 2 C ij /‫ץ‬P‫ץ‬T ), our modulus data at elevated P and T are fit by polynomials [with the constraints of fixing the pressure derivatives (‫ץ‬C ij /‫ץ‬P) at ambient temperature (from our high pressure-ambient temperature data) and the temperature derivatives (‫ץ‬C ij /‫ץ‬T ) at ambient pressure [from averaging the rectangular parallelopiped resonance (RPR) data (5)]. Such fits are mathematically robust because the boundary conditions (along the pressure and temperature axes) are well defined: the RPR method provides data if very high precision along the temperature axis, and the acoustic data along the pressure axis also have high precision [in light of their good agreement with the results obtained from the gas pressure vessel (Fig. 3) Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean leads to a warming of the Southern Hemisphere.Numerous paleodata suggest that the climate of the mid-Holocene around 6 thousand years ago (ka) was quite different from that of today. Generally, the summer in many mid-to high-latitude regions of the Northern Hemisphere was warmer, and paleobotanic da...

The Holocene temperature conundrum

“…However, given different cloud parameterization schemes across models, it is not obvious why all of the models tend to exhibit a global warming trend bias. Vegetation feedback has been suggested to be able to induce warming in the Early to Mid-Holocene in the mid-to high-latitude boreal forest region through its impact on surface albedo (25). However, the CCSM3 and LOVECLIM both include dynamic vegetation models, which apparently are insufficient to generate a cooling trend in these models.…”

Timing and spatial distribution of mid‐Holocene drying over northern China: Response to a southeastward retreat of the East Asian Monsoon

“…According to Berger and Loutre [1991], Northern Hemisphere summer solar insolation has been decreasing since the early Holocene. Numerical modeling experiments have shown that a decrease in insolation would have resulted in a weakened monsoon circulation [Ganopolski et al, 1998;Kutzbach et al, 2001]. The forcing mechanism lies in that the cooling of the continental interior gives rise to a decrease in thermal contrast between the east Asian continent and the adjacent ocean, thus leading to a weakened monsoon circulation, i.e., a southeastward retreat of the east Asian summer monsoon.…”

Simulation of an abrupt change in Saharan vegetation in the Mid‐Holocene

“…Likewise, a more vegetated land surface appears to be darker than bare soil, which not only increases absorption of solar radiation but also evaporation and, finally, the greenhouse effect. model, a multibasin, zonally averaged ocean model including sea ice, and, in contrast to [Ganopolski et al, 1998a], a dynamic model of terrestrial vegetation [Brovkin et al, 1997[Brovkin et al, , 1999. Vegetation cover is represented as a composition of trees, grass, and desert (bare soil).…”