Sensitivity of glacial inception to orbital and greenhouse gas climate forcing G. Vettoretti and W.R. Peltier 2010/01/05 大氣所碩一 闕珮羽

n n To conduct specific sensitivity studies of orbital parameter influences on glacial inception and to compare these influences with those associated with variations in the atmospheric concentration of CO 2 and with SST feedback. n Orbital forcing and CO2(fig4-9) n Ocean forcing (fig10,11)

fig1 n n It is apparent from Fig. 1d that the initiation of glaciation is coincident with strong eccentricity-precession forcing arising. n n When obliquity values are low, this of course also amplifies the NH summer cooling. n n We note that modern obliquity is not in phase with the modern eccentricity-precession forcing as it was in the onset of glaciation in MISs 11, 7 and5. → This raises a reoccurring question in the ongoing debate concerning the impact of greenhouse gas warming.

Table1 ─ 8 experiments n n In Figs. 1b–d, a minimum and maximum set of values is specified by the red and blue lines which indicate conditions that are more (blue) or less (red) favorable for glacial inception. The atmospheric CO2 concentration for a glacial inception sensitive epoch is chosen to be either 260 or 290 ppmv. n n The obliquity is chosen to be either 22 or 23.5 and the eccentricity-precession forcing is chosen to be either 0.15 or 0.04.

fig2 30W/m 2 15W/m 2 25W/m 2

fig3 n Model and observed

fig4 n n decrease in the atmospheric CO2 concentration by 30 ppmv. n n This result reveals a reduction in summer radiative forcing reaching 6 to 9W/m2 at high latitudes north of 60N latitude, with the minimum occurring between 70N and80 N latitude in all of the experiments. n n Except for a slight increase in south polar latitudes, the CO2 induced radiative forcing has little effect at equatorial and mid-latitudes in this model. n The changes in CO2 radiative forcing are stronger with the larger decreases in high latitude insolation as would be expected.

Fig5/fig6 n n There is significant perennial snow accumulation in the Canadian Arctic (Fig. 5d). n n If the CO2 concentrations in the atmosphere are now reduced to 260 ppmv in the four experiments shown in Fig. 5, the perennial snow accumulation is intensified( Fig. 6). not much some

fig7 n n The occurrence of the Arctic insolation minimum in late spring as a result of the stronger obliquity forcing apparently delays spring and summer snow melt.

fig8 n The minimum reaches greater than 12C and extends from July to October when the obliquity cooling, eccentricity-precession cooling and CO2 cooling are strong. n Upon inspection of Figs. 8b and c and Figs. 8f and g, it appears that the CO2 forcing is slightly greater in magnitude than the eccentricity- precession forcing.

fig9 n n It was found that the orbital cooling in the model caused the evaporation to drop more quickly than precipitation as the climate cooled.

fig10 n (10a) sea surface cooling due to reduced insolation leading to reduced CO2 levels which in turn lead to lower SSTs. 8C a reduction in NH sea ice during winter and an increase in sea ice in summer n The increases in sea ice may inhibit the melting of perennial snow cover during the late summer season, thus favoring the onset of glacial inception.

fig10 n n (10a) sea surface cooling due to reduced insolation leading to reduced CO2 levels which in turn lead to lower SSTs. n n (10b) There is a cooling of greater than 8C at high northern latitudes in summer and fall. n n The changes in sea-ice concentration and sea-ice mass (Figs. 10c and d ) reveal a reduction in NH sea ice during winter and an increase in sea ice in summer as compared with modern. n n The increases in sea ice may inhibit the melting of perennial snow cover during the late summer season, thus favoring the onset of glacial inception.

fig11 n n Approximately 3m after 40 years. n n This strikingly implies that the ocean forcing is on the same order of magnitude as a change in the eccentricity-precession forcing or a change in the CO 2 forcing at high latitudes under the low obliquity regime. n n The ocean and/or sea-ice forcing appears to impact glacial inception most strongly at higher latitudes. (compared to lower latitudes)

conclusions n n Obliquity may act so as to delay the spring melt season at higher latitudes and thus extend winter snow accumulation. n n The eccentricity-precession forcing is approximately of the same magnitude as the CO2 forcing at high latitudes and each is approximately half the magnitude of the obliquity forcing in this region. n n The eccentricity-precession forcing and the CO2 forcing can combine in such a way as to produce the same forcing as may derive from a change in obliquity. n n CO2 radiative forcing was found to have a greater influence in concert with eccentricity-precession forcing rather than with obliquity forcing.