As the Milankovic Cycles warm the southern ocean, CO2 is emitted, amplyfying that warming. The Cycles themselves do not have enough change in energy to cause the deglaciations. Only the increase in heat retained through the feedback loop of the increase in CO2 does that.
Now it only takes an increase from 180 ppm to 280 ppm to cause a deglaciation. We have added another 100+ ppm in the last 150 years through the burning of fossil fuels. And are seeing the warming that is the result of that.
However, because our actions have caused the increase of CH4 by 150%, and we have put an abundance of industrial GHGs in the atmosphere, many more than 10,000 times as effective of a GHG as CO2, the real equivelant GHG level is over 450 ppm of CO2.
We have not seen a level like that on Earth in millions of years. And for that level to be reached in only 150 years means that the climate changes caused by that level will be rather adrupt.
Even if the Ocean Clathrates do not kick in. Which they have already started to do in the Arctic Ocean.
CO2 lags temperature - what does it mean?
The combined effect of these orbital cycles cause long term changes in the amount of sunlight hitting the earth at different seasons, particularly at high latitudes. For example, around 18,000 years ago, there was an increase in the amount of sunlight hitting the Southern Hemisphere during the southern spring. This lead to retreating Antarctic sea ice and melting glaciers in the Southern Hemisphere.(Shemesh 2002). The ice loss had a positive feedback effect with less ice reflecting sunlight back into space (decreased albedo). This enhanced the warming.
As the Southern Ocean warms, the solubility of CO2 in water falls (Martin 2005). This causes the oceans to give up more CO2, emitting it into the atmosphere. The exact mechanism of how the deep ocean gives up its CO2 is not fully understood but believed to be related to vertical ocean mixing (Toggweiler 1999). The process takes around 800 to 1000 years, so CO2 levels are observed to rise around 1000 years after the initial warming (Monnin 2001, Mudelsee 2001).
The outgassing of CO2 from the ocean has several effects. The increased CO2 in the atmosphere amplifies the original warming. The relatively weak forcing from Milankovitch cycles is insufficient to cause the dramatic temperature change taking our climate out of an ice age (this period is called a deglaciation). However, the amplifying effect of CO2 is consistent with the observed warming.
CO2 from the Southern Ocean also mixes through the atmosphere, spreading the warming north (Cuffey 2001). Tropical marine sediments record warming in the tropics around 1000 years after Antarctic warming, around the same time as the CO2 rise (Stott 2007). Ice cores in Greenland find that warming in the Northern Hemisphere lags the Antarctic CO2 rise (Caillon 2003).
To claim that the CO2 lag disproves the warming effect of CO2 displays a lack of understanding of the processes that drive Milankovitch cycles. A review of the peer reviewed research into past periods of deglaciation tells us several things:
Deglaciation is not initiated by CO2 but by orbital cycles
CO2 amplifies the warming which cannot be explained by orbital cycles alone
CO2 spreads warming throughout the planet