Increased insolation 20,000 years ago caused deglaciation in the Norther Hemisphere, according to a new report in the August 7, 2009, edition of Science. Further more, it was the onset of deglaciation of the West Antarctic Ice Sheet, which occurred between 14 - 15 thousand years ago, that was the source of sea-level rise at the beginning of the Holocene warming. Such events are often associated with rising CO2 levels by climate catastrophists but the evidence says otherwise.
The Last Glacial Maximum (LGM) is typically defined as the most recent interval in Earth history when global ice sheets reached their maximum volume. This is conventionally calculated from sea-level records but, according to Peter U. Clark et al. this is an overly simplistic approach. Sea-level records do not distinguish between globally synchronous ice-sheet maxima and temporary regional ice-sheet maxima that can combine to produce apparent sea-level low points that can last for a thousand years or more. In their report
the author's describe their improved approach as follows:
We drew on 4271 14C ages and 475 terrestrial cosmogenic nuclide (TCN) ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of maxima in global ice-sheet extent. For all but the Barents-Kara and Greenland Ice Sheets, the spatial distribution of ages is sufficient to evaluate regional variability in the timing of maxima for different sectors of individual ice sheets. Because ice-sheet extent scales with ice volume, our constraints on regional variability in ice-sheet maxima allow us to evaluate the temporal evolution of individual ice-sheet contributions to global sea-level change. Because mountain glaciers are highly sensitive to climate change, we used an additional 172 14C ages and 786 TCN ages to constrain mountain-glacier fluctuations from five widely distributed regions of the world, allowing a more comprehensive assessment of the response of the cryosphere to climate change.
The sea-level change at a number of the sites studied reflected relative changes in sea-level and not the change on a global basis. This is because of the variations in Earth's gravitational field, deformation of the planet's crust, and rotational effects on local sea levels driven by the shift of water mass from glacial ice to the ocean. “In order to evaluate these effects, we used a state-of-the-art theory that includes a realistic glaciation phase to predict the RSL change at these far-field sites,” states the report. From these up-to-date theoretical constraints a new model was built to predict the actual LGM.
Glacial extent during the last glacial maximum.
Using this improved approach the researchers found that the prediction for Barbados indicated a net sea-level fall of ~10 ft (~3 m). This was attributed to “peripheral bulge dynamics and, to a lesser extent, the continental (lithospheric) levering effect dominate the anti-syphoning effect during the LGM.” In contrast, the remaining four “far-field sites” were well outside the peripheral bulges, allowing anti-syphoning effects to cause a net sea-level rise of ~13 to 16 ft (~4 to 5 m) rise in sea level at these sites. Showing how tricky calculating global sea-level changes from local changes can be, during the LGM test period the differential sea-level change between Barbados and the other sites approached 32 ft (10 m). The model showed no change in overall ice volume and hence no change in overall sea-level.
Using insights gained from the model to construct a more accurate time-line for events during the LGM. From this time-line the researchers then examined thee major forcings—high northern latitude insolation, atmospheric CO2, and tropical Pacific sea surface temperatures—to see what part they may have played in the climatic changes during that time. The results are summarized in the figure below, taken from the report online.
Temporal relation between the LGM and various climate-forcing factors. Clark et al.
The vertical purple bar represents the time of the LGM as defined from the RSL data, whereas the vertical gray bar represents the earliest interval when sea level began to fall to the LGM lowstand, corresponding to the time when the first ice-sheet LLGM were reached. (A) Summer energy for 45°N (red line, τ = 400) and 65°N (purple line, τ = 400). (B) 21 June–20 July insolation for 45°N (red line) and 65°N (purple line). (C) Atmospheric CO2 from the Dome C ice core (light purple circles) and Byrd ice core (dark purple circles) (60). (D) The 500-year average NINO3 index from the Zebiak-Cane model forced with orbital-scale solar variations (gray line) compared to SST records from the tropical Pacific [deep yellow, RC13-110; ruby red, ODP 846B; light orange, TR163-19; magenta, MD98-2176; red, MD98-2181]. (E) The 20-year-resolution δ18O record from the Greenland NGRIP ice core (blue line) and the SD of that record calculated with a centered, 3-ky sliding window (purple line).
[see the Science article online for sources, sub. required]
As a result of their analysis the researchers concluded that the responses of the Northern and Southern Hemispheres differed significantly. Furthermore, the differing responses reveal how the evolution of specific ice sheets affected sea level and provide insight into how insolation controlled the deglaciation. Of the three major forcings investigated it was change in insolation, the amount of radiant energy received from the Sun, that seems to be the driving factor in shifting from ice-house to green-house conditions on Earth. Here is how the researchers put it:
[O]ur geochronology for the LGM clearly demonstrates that only northern insolation led the termination and was thus the primary mechanism for triggering the onset of Northern Hemisphere deglaciation. Moreover, the fact that ice sheets of all sizes, as well as Northern Hemisphere mountain glaciers, began to retreat at approximately the same time (19 to 20 ka) suggests that the primary insolation control on initial deglaciation was through increased summer ablation, which can substantially reduce the long response times of large ice sheets by enabling dynamical processes that lead to rapid mass loss.
This investigation is an excellent example of how computer modeling of climatic conditions should be used. As stated in the paper: “Our constraints in support of an extended LGM sea-level lowstand provide important insights into the origin of the carbonate δ18O signal measured in benthic foraminifera (δ18Oc), which is often used directly as a proxy for sea-level change.” The model outputs were not being presented here as a result or conclusion but rather as a way to gain insight into what might be happening. Armed with those insights, the authors were able to further analyze actual proxy data and draw new conclusions.
While the authors cautiously allow a role for increasing CO2 levels and tropical Pacific sea surface temperatures as amplifiers of climate change, the fundamental conclusion is that changes in insolation was the primary forcing, whether caused by orbital variations (the Milankovitch Cycles) or other factors. “Whether these changes in CO2 and SSTs were induced by deglaciation of Northern Hemisphere ice sheets or high southern latitude insolation, however, remains an open question,” the paper concludes. In other words, though they don't want to diminish the role of carbon dioxide too much, they have no conclusive idea of why CO2 levels increased following the change in insolation. Not being constrained by climate change political correctness I am free to say that once again the Sun and orbital variation trumps CO2 levels.
Be safe, enjoy the interglacial and stay skeptical.
Hoping the interglacial comes soon.