Climate researchers at MIT in Cambridge, Massachusetts have analysed Northern Hemisphere climate data from 1972 to 1999, in the search for correlations that might help improve long-term weather forecasting. The most striking match to emerge is that of winter climate with the extent of autumn snow cover in Siberia. Snow reflects back to space a far greater proportion of incoming solar energy than any other kind of surface, with the exception of salt. More snow results in less warming in the area. Although Siberia is at the heart of the Asian continent, and therefore pretty dry, it has cold winters, so that when snow falls it covers large areas and tends to remain. It is the focus for an enormous mid-continent high-pressure area in winter, appropriately named the Siberian High, which is one of three systems that dominate northern climate.
High-pressure areas do two things: air spills from them into surrounding areas; they isolate the area beneath them from warming, moist winds blowing from the oceans. In winter the second creates cooling so intense that temperatures can steadily drop to -50°C or below , further building pressure because of the increase in air density. Siberia sheds cold air westwards into Europe and over the North Pole into North America. The MIT study bears out the obvious prediction based on this tendency. However, it may also add the Siberian High to the range of large-scale terrestrial processes – shifts in air pressure over oceans, such as the El-Niño of the tropical Pacific and the North Atlantic Oscillation, and thermohaline controls over Atlantic surface currents – that make ice-age climate patterns so complex.
Cooling of northern Europe and the Canadian Shield does not have to be very extreme to lower the topographic elevation at which snow remains permanently, the glaciation limit – at present that level is only a couple of hundred metres above the tops of Britain’s highest mountains. Should permanent snow cover return to the highest areas around the North Atlantic, that would amplify the present effect of Siberian autumnal snow and expand the high-pressure area. That is a positive feedback driving climate towards increased frigidity, and larger winter highs would hold back maritime warming influences.
Computer modelling of the air-flow patterns over Asia shows that the primary influence is the Himalaya and Tibetan Plateau. In particular, they dry out air passing over them during the South Asian Monsoon, and hinder its influence further into central Asia. The two huge massifs seem to have risen rapidly and recently, beginning about 8 million years ago, despite the fact that India collided with Asia about 50 million years ago. Together with other roughly E-W high mountain ranges in central Asia, they also channel Siberian cold air to spill westwards and eastwards, and over the pole. Behaviour of the Siberian High almost certainly dates from the uplift of the Himalaya and Tibetan Plateau.
Adding another controlling factor to long-term northern climate has an intrinsic potential in refining academic studies of Pleistocene climate. However, there is an immediacy to the observations. For snow to cause cooling by reflecting away solar heat it does not have to be thick; a few centimetres will suffice. The critical factor is the area covered by it. Siberia is so cold in autumn and winter that it will snow there, provided moist air can enter. Should more get in then more snow will cover a greater area, to feed the positive feedback to cooling. Perversely, the more the climate warms globally, the more moisture evaporates from tropical and mid-latitude oceans to move polewards and towards continental interiors……
Mismatches from north to south proven
Whether or not climate changes, especially those of shorter duration than the full glacial-interglacial cycle, occur at the same time everywhere is something that vexes all climatologists. It encapsulates all the problems of causation: orbital forcing, thermohaline circulation, shifts in the Polar Front and Intertropical Convergence Zone, etcetera. The problem mainly stems from uncertainties in the correlation of time series that show proxies for climate change. This is particularly bad for ocean-floor sediment cores, which depend upon radiometric dates for calibration from depth to time sequences and an assumption of constant rates of sedimentation between dated samples. Imprecision often means that correlations are not believable, except at a very general level. Many analyses end up by correlating the patterns shown by the proxies, which defeats the object of assessing the degree of global synchronicity of climate changes.
Cores taken through ice sheets offer a way out, for annual layers of ice are there to be counted, but only in the upper parts. For deeper parts, converting depth to time relies on models of how ice compacts and how it thins by glacial flow. Another seeming advantage of ice-core records is that a great deal more ice accumulates than does ocean-floor sediment over a particular time. That means that the resolution of ice core records can be finer – potentially at the level of decades compared with hundreds of years for sediment cores. A seeming key to correlation between ice cores lies in the way that ice traps air. Being rapidly mixed, the atmosphere should have the same composition everywhere. This is particularly so for methane, partly because it soon becomes oxidised to carbon dioxide, and partly because its level is highly variable from emissions by rotting vegetation and unstable gas hydrate on the shallow ocean floor. Thomas Blunier and Edward Brook of Princeton University and the University of Berne used the methane records of Greenland and Antarctic ice to correlate the other proxies therein over the last 90 thousand years (Blunier, T. and Brook, E.J. 2001. Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science, v. 291, p. 109-112). They show a consistent mismatch between rapid warmings of the air over the two polar ice sheets, where Antarctic changes precede those over Greenland by 1500 to 3000 years. Interestingly, when frigidity gave way to comparative warmth in a matter of a few decades over Greenland, the Antarctic was shifting from warm to cool conditions.
Commenting on the paper in Science’s Compass, Nicholas Shackleton of Cambridge University shows yet more emerging oddities (Shackleton, N. 2001. Climate change across the hemispheres. Science, v. 291, p. 58-59). In the North Atlantic Ocean, surface water temperatures apparently changed according to Greenland’s pace, while those for deep water match that of the Antarctic. To add to the complexity of climate change through the last glacial period – until a few years ago it was all supposed to link to the astronomical forcing of solar heating at high northern latitudes – the oxygen isotope changes in the same deep water of the North Atlantic match those of ice volume around the north pole.
Whereas Blunier and Brook have proved that air-temperature changes above ice sheets at high northern and southern latitudes are not synchronous, this still leaves problems in correlating between ice and sediment cores, and between the oceanic record at the many sites world-wide, especially those at low latitudes. With a growing number of hypotheses for climate changes of the order of a few thousand years – driven by changes associated with northern ice sheets, Antarctica and the tropics – onlookers await with interest the development of a means of precise correlation among all the time series.