The view that all manner of processes connected with climate – volumes of land ice, ocean temperature and flow, aspects of atmospheric composition and its motion, and the expansion and contraction of biological communities – are locked into the cycles of changing solar input steadily evolves into something less mechanical, as new data flows in. The first serious doubts about Milutin Milankovic’s theory of astronomical forcing of the world’s climate, since oxygen isotope fluctuations in sea-floor sediments began to reveal the periodicities predicted by him, stemmed from a very different kind of deposit. Devil’s Hole near Las Vegas, a fissure being slowly filled by calcite flow stone that precipitates from groundwater, presented a detailed record of oxygen-isotope variations over the last 600 thousand years. Though showing the same patterns as ocean cores available at the time, Devil’s Hole revealed changes in continental climate that differed from those in land-ice volume by thousands of years. Ice-core time series of Antarctic air temperatures also show that warming began up to 9000 years before the last four terminations of glaciation. As more proxies for climate are devised, the more complex global climate shifts appear to have been.
The latest measure stems from fluctuations in the structure of compounds produced by marine algae as a result of shifts in sea-surface temperature. Applied to sea-floor sediments deposited off California, an area influenced today by the southward, cold California Current, they reveal regional warming of the sea that began 10 to 15 thousand years earlier than the last five deglaciations of the northern hemisphere (Herbert, T.D. and 8 others 2001. Collapse of the California Current during glacial maxima linked to climate change on land. Science, v. 293, p. 71-76). In cores south of the modern cold current, no such large discrepancies emerged. In fact they accompanied the maximum extents of land ice. It seems that, like the Gulf Stream, the California Current is prone to shutting down, but as a result of changed Pacific wind patterns in response to the North American ice sheets rather than to thermohaline deep circulation. Here is an explanation for the vexing record from Devil’s Hole – regional climate shifts that do not “knock” Milankovic.
There is no doubt that changes in ice volume on the northern continents are the main characteristic of environmental change going back more than 2 Ma. However, the mechanistic view that lots of ice means a cold, dry world and a great deal less points to warmth and more moist conditions is dead in the water as a useful paradigm. Yet all models of climate are little more than Heath Robinson tangles of such reductionism, despite claims for their increasing incorporation of ideas that stem from measured realities. As always, the devil lies in the detail, and Herbert et al.’s paper also shows from pollen records in the marine cores that dense warm-climate forests cloaked the Pacific seaboard during the last 5 glacial maxima. For a vast area of western North America to be warm while ice sheets elsewhere were at their maximum should be a warning of unpredictable future climate shifts.
Growing concern about unpredictable and contrary change was amply expressed by a meeting of 1800 climate specialists in Amsterdam in early July. They endorsed the distinct possibility of sudden shifts in regional climates that may stem from increased global warming, such as return of vegetation to the Sahara, aridity in the Amazon basin, and Europe’s plunging into a frigid climate as the Gulf Stream slows because of reduced thermohaline circulation (Pearce, F. 2001. Violent future. New Scientist, 13 July 2001, p. 4-5).