Millennial Climate Oscillations

Introduction

Millennial climate oscillations (MCOs) are sudden changes in Earth's climate that happen regularly every 1,470 years. An oscillation is a regular back-and-forth change such as the swinging of a pendulum. These climate oscillations are called millennial because the period or time between them is on the order of 1,000 years, that is, a millennium.

At the onset of a typical MCO, climate in the Northern Hemisphere warms by up to 27°F(15°C) in the course of as little as 20 years—a geological eyeblink—followed by cooling over several centuries. High-resolution data from layered ice cores and ocean-floor sediments are available for the last half-million years or so, revealing many MCOs.

Several causes have been proposed and strongly defended, including internal oscillations of Earth's climate, similar to the regular El Niño cycle but larger and slower, and variations in the output of the sun. MCOs are significant because past sudden shifts of Earth's climate may shed light on the power of human beings to trigger more extreme and rapid climate change than has thus far been seen.

Historical Background and Scientific Foundations

The existence of MCOs was first noticed in ice cores. These are long, solid cylinders of ice drilled vertically out of thick ice deposits such as the ice caps of Greenland and Antarctica, which are miles thick in places. Ice cores preserve a record of past climate because an ice cap is formed by the accumulation of annual snowfalls as a pile of thin ice layers. These layers record air temperature at the time the snow fell in their ratio of oxygen–18 to oxygen–16. Oxygen–16 atoms have eight protons and eight neutrons, and are the most common (over 99% of oxygen atoms in nature); Oxygen–18 atoms have eight protons and 10 neutrons. Oxygen–18 atoms are heavier, so water molecules containing oxygen–18 atoms are heavier.

Colder clouds do not mix as well as warmer clouds, and so tend to drop more of their heavier water molecules when they release snow or rain—namely, those containing oxygen–18. Thus, air temperature at the time of a snowfall is recorded in the amount of extra oxygen– 18 present in the resulting ice layer. Oxygen–18 gives a secondhand or proxy record of air temperature that can be read hundreds of thousands of years later.

In 1985, Danish paleoclimatologist W. Dansgaard and colleagues noticed climate cycles in oxygen–18 readings from the Greenland Ice Sheet Project (GISP) ice cores, which had been drilled from 1974 to 1981. These warming events, now called Dansgaard-Oeschger events, showed that the climate stability of the Holocene period— the time period from about 10,000 years ago to the present, including all of recorded human history—has been exceptional, not normal. MCOs continued during the Holocene, but at smaller amplitude, with temperature changes limited to about 3.6–5.4°F(2–3°C) over Greenland and to less than 3.6°F(2°C) over the North Atlantic.

Since the early 1990s, data of many types from around the world have confirmed the existence of MCOs. These data sources include, among others, ocean sediments from the North Atlantic, the coast of Africa, the subtropical Atlantic, and the Arabian sea, as well as Antarctic ice cores and stalactites from Europe and China.

Dansgaard-Oeschger events occur every 1,470 years or at some even multiple of this period (for example, 2 x 1,470 = 2,940 years or 3 x 1,470 = 4,410 years). This time scale points to involvement with changes in large-scale ocean circulation, because Earth's atmosphere changes much more quickly while long-term shifts in climate due to regular changes in Earth's orbit around the sun, Milankovitch cycles, are too slow. Sudden shifts in ice sheets could happen on a millennial time scale, but this is ruled out as the cause of MCOs by the fact that even during the Holocene, when the relatively small Greenland ice sheet has been the only significant ice in the Northern Hemisphere, MCOs have continued to occur, although less intensely.

Therefore, changes in the global overturning thermohaline (warmth-and-salt) circulation of the oceans, also known as the THC or the Great Conveyor Belt, are probably part of the mechanism underlying the MCOs. That part of the THC which overturns the waters of the Atlantic Ocean, the Atlantic thermohaline circulation (ATC), has been central in these theories. The importance of the ATC to climate in the Northern Hemisphere is shown by the fact that a third of the heat in the North Atlantic area is supplied by the warm water transported northward by the ATC, with the other two thirds coming from the sun. Any disturbance in the ATC is likely to be reflected in northern climate.

Attempts to explain the MCOs in terms of changes in ocean circulation fall into three groups:

• ATC bistability: In these theories, there are two stable states of the ATC, like the two settings of a light switch. In one stable state, the sinking flow of cold, salty water in the North Atlantic, also called North Atlantic Deep Water formation, is active. After sinking, this deep water flows south along the ocean floor, allowing a balancing flow of warm surface water to come north to fill its place. In this state, northern climate is warmer. In the other stable state, the North Atlantic Deep Water formation is shut off by some unknown trigger, and the northward flow of warm surface water is shut off with it.
• Convection shifts: In these theories, the ATC does not shut off, but parts of its circulation move northward (producing warming) or southward (producing cooling). As with theories of ATC bistability, the trigger between these two states is unknown.
• Internal THC oscillations: In these theories, MCOs are explained as internal oscillations of the thermohaline circulation of the oceans. On this view, the abrupt climate changes of the MCOs occur somewhat like the ticks of a clock: the clock produces regular ticks without being fed any outside forces or triggers. The internal oscillations of the THC would be driven entirely by the buildup and release of stresses within the system, such as imbalances of heat flow or of saltiness. (Salty water is denser, so uneven saltiness affects the tendency of the oceans to circulate.) The 1,470-year period of the MCOs would, on this view, follow solely from the structure of the THC system, just as the rate at which a clock ticks depends only on the internal mechanism of the clock.

WORDS TO KNOW

BISTABILITY: The property of a system that it can exist in two stable states: a dinner plate, for example, is stable lying on either side, but not balanced on edge. A tipping point often exists in bistable systems, beyond which transition to the other stable state becomes automatic. A plate on edge is at a tipping point, where a slight push will quickly cause the system (i.e., the plate) to move all the way to one of its stable states. Some feedbacks in the climate system may be bistable.

CONVECTION: The rising of warm air from an object, such as the surface of Earth.

CORE SAMPLE: Cylindrical, solid sample of a layered deposit, cut out of the deposit at right angles to its bedding planes. Core samples of lake-bottom and sea-bottom sediments, or of ice layers of the Greenland or Antarctic ice caps, can supply information about past climate variations and changes in atmospheric composition. For example, Antarctic ice cores supply climate data going back 800,000 years.

DANSGAARD-OESCHGER EVENTS: Global climate warming events that occur every 1,470 years or at some even multiple of this period (for example, 21470 = 2,940 years or 31470 = 4,410 years). The cause is uncertain; some scientists hypothesize shifts in ocean circulation triggered by regular changes in Earth's orbit.

HOLOCENE PERIOD: Geological period from 10,000 years ago to the present. Technically an epoch, not a period; the Pleistocene was the first epoch of the Quaternary Period (1.64 million years ago to present), and the Holocene was the second.

MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.

OSCILLATION: A repeated back–and–forth movement.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.

Another line of scientific inquiry into the causes of the MCOs has focused on astronomy. In 2003, German climatologist Stefan Rahmstorf pointed out that the MCO cycle happens with strong regularity (recalling the regular ticks of a clock): it is 95% likely that for at least the last 23 cycles of millennial climate oscillation, the time delay between events has varied no more than 12%. According to Rahmstorf, oscillatory changes in Earth's oceans and other systems would have to be far less regular. The regularity of the MCOs alone, therefore, points to an extraterrestrial cause or trigger, probably in the sun. Yet, although the heat output of the sun is well known to vary on cycles of 11, 22, 87, 210, and 2,300 years, there is no 1,470-year solar cycle (the orbit of Earth does not vary on any 1,470-year cycle either).

In 2005, Holger Braun and colleagues proposed a solution to the problem that extraterrestrial forcing of MCOs seemed both required and unavailable. They proposed that the combined effects of the 87-year and 210-year solar cycles could explain a 1,470-year cycle. Both 87 and 210 are close to being prime factors of 1,470 (1,470/210 = 7; 1,470/87 ≅ 17). This means that the peaks of the two cycles will only line up once every 1,470 years. The coinciding of the peaks could produce rapid glacial melting in the Northern Hemisphere, slowing the ATC by adding freshwater to the ocean in the vicinity of Greenland. Since there is less ice to melt during interglacial periods such as the Holocene, MCOs would naturally be less intense during such periods, as has been observed.

The solar-influence theory would agree with either of the first two types of ocean-circulation theory, which both require some kind of external trigger. It is in direct contradiction to the third type, which would attribute the 1,470-year timing of the MCOs entirely to internal features of the ocean circulation system.

Impacts and Issues

MCOs are important for the understanding of future climate as well as past climate. Their existence, argues leading climatologist Richard B. Alley, “suggest[s] that neither the sensitivity nor the variability of the climate are fully captured in some climate-change projections,” such as that of the Intergovernmental Panel on Climate Change (IPCC). This, Alley argues, means that “the future may be more challenging than anticipated in ongoing policy making.”

Large changes in climate have occurred repeatedly in response to only small forcings or pushes. Computerized climate models predict moderate consequences from the freshening of North Atlantic waters due to human-caused climate change, but such models tend to underestimate the intensity and speed of past changes, namely the MCOs. People will not fully understand the possible consequences of human interference with climate until they completely understand millennial climate oscillations.

See Also Abrupt Climate Change; Heinrich Events.

BIBLIOGRAPHY

Periodicals

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Braun, Holger. “Possible Solar Origin of the 1,470-Year Glacial Climate Cycle Demonstrated in a Coupled Model.” Nature 438 (2005): 208–211.

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McManus, Jerry F., et al. “A 0.5-Million-Year Record of Millennial-Scale Climate Variability in the North Atlantic.” Science 283 (1999): 971–975.

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Larry Gilman