Jy blaai in die argief vir paleoclimate.

Climate Change: The debate in Geobulletin (continued)

28/01/2013 in Uncategorized

Climate Change – The Short Story ( William McClenney). (continued)

A reconstruction from fossil algae in sediments from Drought Lake in North Dakota of the past 2,000 years found that dry conditions were far and away the rule in the High Plains, with the Dust Bowl conditions of the 1930’s one of the lesser dry spikes found in the record. Half of the warming that brought us out of the last ice age (the Wisconsin) occurred in less than a decade. There were 24 Dansgaard-Oescher oscillations between this interglacial, the Holocene, the interglacial in which all of human civilization has occurred, and the last one, the Eemian, in which the first fossils of Homo sapiens are to be found. D-O oscillations average 1,500 years, and have the same characteristic sawtooth temperature shape that the major ice-age/interglacials do, a sudden, dramatic, reliable, and seemingly unavoidable rise of between 8-10 degrees C on average, taking from only a few years to mere decades, then a shaky period of warmth (less than interglacial warmth), followed by a steep descent back into ice age conditions. Each D-O oscillation is slightly colder than the previous one through seven oscillations, then there is an especially long, cold interval, followed by an especially large, abrupt warming up to 16 degrees C (a Bond cycle). During the latter part of the especially cold intervals, armadas of icebergs are rafted across the North Atlantic(Heinrich events), their passage recorded reliably by the deep ocean sediment cores which capture the telltale signature of these events in dropstones and detritus melted out of them.

We know with absolute certainty that these events happen, with evidence of D-O oscillations extending back some 680 million years. We do not know, yet, precisely what causes them. What we do know is that the past six interglacials (dating back to Mid Pleistocene Transition) have lasted roughly half of a precessional cycle, or currently 11,500 years, which just happens to be the present age of the Holocene. What we know is that N65 latitude insolation values are very close now to what they were at the close of the Eemian. What we also know is that GHGs (Green House Gases), like us, seem to have played only a spectator role to all of these alarming natural transitions, with temperature changes leading GHG concentrations by a considerable margin of time (800-1,300 years). What we do not know is if the reverse could occur, anthropogenic sourced GHGs triggering a climate change event, for perhaps the first time ever. What we do know is that earth’s climate is bimodal, cold 90% and warm 10% of the time for the past million years or so, with the transition times (such as at the end of an interglacial) well known from proxy records to be quite sensitive to forcings we do not yet understand, and the forcings we have identified seemingly incapable of producing the responses we see in the paleoclimate record. Including the recent paleoclimate record.

(to be continued)