July

FIGURE 12.19 Zonally averaged temperature departures (°C) for the land surface for July, for the last 18 ka B.R according to GCM estimates (linearly interpolated between model runs carried out at 3 ka intervals). Dots indicate the value is statistically significant (2-sided t-test) at or above the 90% confidence level based on the model's natural variability (Kutzbach and Guetter, 1986).

FIGURE 12.19 Zonally averaged temperature departures (°C) for the land surface for July, for the last 18 ka B.R according to GCM estimates (linearly interpolated between model runs carried out at 3 ka intervals). Dots indicate the value is statistically significant (2-sided t-test) at or above the 90% confidence level based on the model's natural variability (Kutzbach and Guetter, 1986).

16.000 15,000 12,000 9,000 6.000 3,000

16.000 15,000 12,000 9,000 6.000 3,000

LAND

July

FIGURE 12.20 Zonally averaged precipitation departures (mm day'1) for the land surface for July, for the last 18 ka B.R according to GCM estimates (linearly interpolated and smoothed, between model runs carried out at 3 ka intervals). Dots indicate the value is statistically significant (2-sided t-test) at or above the 90% confidence level based on the model's natural variability. Positive departures are shaded (Kutzbach and Guetter, 1986).

changes were in the opposite direction in the southern hemisphere where January (summer) rainfall amounts fell and P-E was reduced from 12-3 ka B.P. compared to present (Fig. 12.21). The changes are strongly correlated to solar radiation anomalies over the last 18 ka; thus over land areas 0-30° N in July, a 1% increase in solar radiation more or less corresponds to a 3.5% increase in precipitation. This can be thought of as due to the non-linear increase in saturation vapor pressure with temperature, which amplifies (through water vapor feedback) the radiation effect at the surface. A 7% increase in solar radiation outside the atmosphere at 9 ka B.P. is associated with 11 % higher net radiation at the surface due to a decrease in outgoing longwave radiation (because of increased evaporation and higher water vapor levels in the atmosphere, which absorb long-wave radiation). However, this amplification of solar radiation effects is not as important at higher latitudes where precipitation amounts are much less dependent on solar radiation anomalies and show more sensitivity to surface boundary conditions (Kutzbach and Guetter, 1986).

A recurring theme in Chapters 5 and 6 was the evidence for rapid fluctuations in climate during the last glacial period, particularly around the North Atlantic. Many investigators have suggested that these shifts are related to the North Atlantic thermohaline circulation, which may have oscillated between states involving high salinity surface water and deepwater formation ("conveyor on") and low salinity surface water and little or no deepwater formation ("conveyor off"). Both ocean GCMs

0 0

Post a comment