Did AGW Start Thousands of Years Ago?

Is this the reason why we're not currently in the early stage of advancing ice sheets?

Ruddiman (2003) assessed the likely effects of temperature on the mean phase of orbital-scale δ18O signals in the 5 cores used to create the SPECMAP time scale and inferred that ice should have begun forming by 5000 yrs BP at the precession cycle and by 4000–3000 yrs BP at the obliquity cycle. All of these results are consistent with Milankovitch (1941) who estimated a 5000-year lag of ice volume behind summer radiation forcing. The lag inferred by Milankovitch predicts new ice sheets by ∼6000 years ago for precession and ∼5,000 years ago for obliquity.

Bill Ruddiman (Climatic Change, 61, 261–293, 2003) recently suggested that early civil-
isations could have saved us from an ice age because land management over substantial areas caused
an increase in atmospheric CO2 concentration. Ruddiman suggests a decreasing “natural course”
of the Holocene greenhouse gases concentrations and sea-level by referring to analogous situations
in the past, namely the last three interglacials. An examination of marine isotopic stage 11 would
perhaps make Ruddiman’s argument even more thought-challenging. Yet, the hypothesis of a natural
lowering of CO2 during the Holocene contradicts recent numerical simulations of the Earth carbon
cycle during this period. We think that the only way to resolve this conflict is to properly assimilate the
palæoclimate information in numerical climate models. As a general rule, models are insufficiently
tested with respect to the wide range of climate situations that succeeded during the Pleistocene. In this
comment, we present three definitions of palæoclimate information assimilation with relevant exam-
ples. We also present original results with the Louvain-la-Neuve climate-ice sheet model suggesting
that if, indeed, the Holocene atmospheric CO2 increase is anthropogenic, a late Holocene glacial
inception is plausible, but not certain, depending on the exact time evolution of the atmospheric CO2
concentration during this period.

My impression is Yes!

People tend to think of global warming in a very negative sense. However, as Ruddiman shows there is a good side too. Normally, greenhouse gases fall following interglacials as he shows in Figure 2. So, if nature had taken its course, CO2 levels might be only 240ppm. That is they should have gradually fallen from 265ppm instead of rising slowing for 6000 years.

If greenhouse gases fell as they did following the during the previous interglacial, the world would be much cooler than it is now. With perihelion occuring in January, summers are much cooler and the glaciers would be growing all over the Northern Hemisphere. A broad region of the Canadian interior would have been subject to glaciation for the last several thousand years. It might only be a few hundred feet high, but that's enough to affect weather patterns as well as stop forest growth.

Remember, the Laurentide ice sheet was huge. It started in Canada, but eventually reached as far south as New York City. If we currently had a large part of Quebec and Ontario (around the Hudson Bay) in the beginning stages of a glacier, it would also have devastating impacts for thousands of mile.

Here is a commentary that might be of interest:
Commentary on "The anthropogenic greenhouse era began thousands of years ago". Crusifix et al. Climate Change (2005).

It has been hypothesised, based on Vostok ice core CO2 data (Petit et al., 1999), that atmospheric CO2 would have dropped naturally by 20 ppm during the past 8 kyr (in contrast with the observed 20 ppm increase) if prehistoric agriculture had not caused a release of terrestrial carbon and CH4 during the Holocene (Ruddiman, 2003; Ruddiman et al., 2005). This hypothesis also suggests that incipient late-Holocene high-latitude glaciation was prevented by these pre-industrial greenhouse gas emissions. However, this hypothesis conflicts with several, independent lines of evidence, including the lack of orbital similarity of the three previous interglacials with the Holocene and the recent finding that CO2 concentrations were high during the entire Stage 11 Siegenthaler et al., 2005a; Figure 6.3), a long (~28 kyr) interglacial (see Section
6.4.1.5). This hypothesis also requires much larger changes in the Holocene atmospheric stable carbon isotope ratio (13C/12C) than found in ice cores (Eyer, 2004), as well as a carbon release by anthropogenic land use that is larger than estimated by comparing carbon storage for natural vegetation and present day land cover (Joos et al., 2004).

It has been suggested that Stage 11 was an extraordinarily long interglacial period because of its low orbital eccentricity, which reduces the effect of climatic precession on insolation (Box 6.1) (Berger and Loutre, 2003).

ice cores cannot provide estimates of changes in atmospheric 13CO2 because of as of yet unresolved technical problems. One of the least understood and important influences on the changes to the isotopic composition of atmospheric CO2 are that of vascular plants. While marine benthic δ13C records have been used to infer past changes in terrestrial vegetation, accurate estimation of changes in carbon storage on land during ice ages has proved elusive. Other estimates have been made from terrestrial biomes of pollen records but a large discrepancy between marine and land based estimates remains.

Good find Andre;

In other words, they should have compared the current interglacial to the interglacial between 399 - 427 Ka. However, notice the IPCCs following comment:

In other words, that interglacial had low orbital eccentricity and I am guessing that our current interglacial has moderately low eccentricity. So, maybe our current interglacial has a stronger orbital forcing than the previous 3.

Anyhow, I agree that the early Anthropocene hypothesis needs more work. However, I don't think it is totally dead.

Let the grant request begin!!!