Australia: The Land Where Time Began

A biography of the Australian continent 

Climate - multiple controls

The data obtained from GISP2 have shown that there are multiple climate forcings involved in global warming that includes most of the known factors that can be weighted according to their overall effect. The climate is a complex subsystem within the complex system of the Earth, with no single factor being responsible for overall climatic behaviour, or over long time periods.

The author1 suggests that much of the GISP2 data can be explained by the amount of solar energy, varying as it does with the changes in the distance of the Earth from the Sun, and changes in area and cover of ice sheets and sea ice, and circulation changes of the ocean. The author1 credits David Meeker, a mathematician, with important new insights being brought to climate research.

Analysis of the GISP2 data has led the researchers to conclude that rapid climate change results from several climate controls operating together, the effects of these factors sometimes acting as additive effects and sometimes cancelling the effects of others, the author1 stating that this reasoning is important to both palaeoclimate and the climate of the present, human contributions acting as additional controls, and depending on the magnitude and timing of the natural controls these anthropogenic controls could be reinforced or reduced.

The GISP2 record indicates that over most of the 110,000 years covered by the ice core, when looked at primarily over time scales of several decades the behaviour of chemical species is fairly similar, though more differences are noted at higher resolutions. All of the chemical species acting as monitors of terrestrial changes, such as dust species, including calcium, magnesium and potassium, and those reflecting changes over the ocean, sodium and chloride, display similar behaviour. The circulation of the atmospheric system is energised to lesser or greater degree, increasing or decreasing wind speeds. During RCCEs it is assumed that most or all of the circulation system of high latitudes is energised, as the atmospheric circulation systems intensify so greatly over the land and ocean, as indicated by dust and sea salt increases on order of magnitude scale, during RCCEs. An increase in the Polar Circulation Index (PCI) is indicated by higher levels of dust and sea salt (Mayewski et al., 1997; Meeker et al., 1997). More than 90 % of the increases and decreases of sea salt and dust is approximated by the PCI, the PCI being used to search for predictable patterns, of which several were found, each of these being approximated by a signal that had a repeating cycle (period) represented statistically by bandpass components that allow some flexibility ± 10% of the period. In the record studied periods were identified that were unmistakable, having a significance of 99.9 %, a 0.1 % chance of the cycle occurring by chance.

It was found that over the last 110,000 years of climate change much of the climate behaviour was recurring or periodic, and according to the author1 the more order is found in nature the greater the possibility of predicting future climate behaviour, and in a warming world, the part controlled by natural factors. It has been found by analysis of the GISP2 data that cycles that are on scales the order of 500 years or longer the climate behaviour is quite regular, though cycles of shorter periods are not quite as regular. The author1 suggests that in order to predict climate behaviour on practical time scales for humans the data need to be refined.

In the PCI 8 major cycles were identified, the total period covered by these 8 cycles approximating almost 90 % of the original record, indicating that the original record is very predictable. According to the author1 these cycles are extremely relevant.

The amount of solar radiation impacting the Earth has been related to the first 4 cycles identified in the GISP2 record, more than 70,000, 38,500, 22,500 and 11,000 years. All of these have previously been observed in other palaeoclimate records, such as those developed for marine sediments where taxa changes indicate climate change, stable isotopes of marine shells, or ocean chemistry changes (Shackleton & Op-dyke, 1976;  Hays et al., 1976; Imbrie & Imbrie, 1979). These cycles have been related to the astronomical theory of climate change.

 See Milankovitch Cycles

Changes in the PCI are represented by these cycles, such as expansion of the Northern Hemisphere ice sheets is associated with expansion and energising of the polar atmospheric circulation system - increased wind speed. Plotting the sea level record for the last glacial/interglacial cycle compared to the PCI record has demonstrated that the PCI is related to volume change of the ice sheets. According to the author1 when periods in their PCI record relating to the orbital cycles of the Earth were compared by Michael Prentice, a sea level researcher, a close association was found between falling sea level and falling PCI (Mayewski et al., 1997). This results from sea level being largely controlled by the volume of water that is present in the ice sheets - lower sea level resulting from more water being involved in ice sheet formation.

There is a slight difference between calculated orbital cycles and PCI cycles, such that a cycle of more than 70,000 years is related to the 100,000 year cycle of orbital eccentricity. As the GISP2 records go back 110,000 years only 1 such cycle would be covered by the period of the GISP2 ice cores. The ice core data have a 38,500 cycle, which is very close to the obliquity cycle of 41,000 years, and the 22,500 year cycle in the ice cores that is very close to the 23,000-year precession cycle. It is suggested by the author1 that the time required for the ice sheets to adjust to changing solar radiation input accounts for the lag period that is observed between the orbital cycle and the PCI cycle, the PCI cycles usually beginning about 6,000 years after the solar radiation input has changed. In previously studied palaeoclimate records it has been seen, and in those cases has been attributed to "ringing" in the climate system. It is suggested by the author1 that when the Earth is "hit" by incoming solar radiation changes in the effects may continue for some time, possibly resulting in half-cycles of the original hit, the response gradually dampening (Mayewski et al., 1997).

In the PCI record seen in the ice cores the 6,000-year cycle very closely approximates the timing of Heinrich Events that are seen in the North Atlantic marine record. These are episodes of massive discharges of icebergs, being detected in marine sediment records as increased coarse debris in amounts that would only be found great distance from shore if it had been dropped by melting icebergs (Bond et al., 1992). The area of ice sheets is limited by bounding structures such as mountains and the margin of the landmass they have formed on. When ice sheets extend past the shore line the outer edge eventually reaches a point where it begins to float. The most likely places for the icebergs to be discharged are at ice stream edges or where the ice is moving fast. If the size of ice sheets is controlled by orbital changes, as the author1 suggests they are, based on association with the sea level record, it must take time for ice volume changes to respond to incoming radiation changes. Variations in the accumulation of snow produce changes in the volume of ice, more open regions of ocean are warmer and increase the amount of evaporation from the surface resulting in higher snowfalls, and cold ice deforms less readily than warm ice. The author1 suggests 6,000 years appears to be the so-called "response time", based on the record in the ice core, with the result that when much of the Northern Hemisphere high latitudes were covered by ice sheets ice was discharged about every 6,000 years (Mayewski et al., 1997). According to the Author1 observations at the present of Antarctic ice sheet edge provides analogs for this theory of ice berg discharge, which responds to climate changes in the past and the present.

It is believed they probably covered much of the North Atlantic during periods of large discharges as recorded by the Heinrich events. The extent of sea ice in the North Atlantic would have been increased by the cooling effect, which would have cooled the air above them by association, and capped off the ocean surface from the atmosphere. The timing of the RCCEs is explained  by the 1,450 cyclical period found in the PCI record, and such a period had not previously been found in any other record of palaeoclimate, though it has now been found in a number of palaeoclimate records, leading the author's1 team to consider it is real, though at the time of writing they were unsure of its cause or causes. Because there is a lack of records that are well-dated the relative timing of these events between sites cannot be determined with the result that it remains one of the mysteries connected with the study of RCCEs that occurred in the ice age. The author1 suggests that if the 1.450-year RCCEs are found to have occurred at the same time in all parts of the world it would be assumed that the cause would be something similar to solar output change. If a phasing sequence is found to apply to RCCEs from different sites, such as occurring first in the Antarctic, following which it moved to the Arctic, it would be concluded that the Antarctic was the site of the cause.

Many marine sediment records document distribution change of surface and deep water that preserve evidence of RCCEs, though they do not record definitive timing relative to GISP2. The author1 asks whether there could be causes other than solar variability. If these occur in the oceans and the atmosphere, is it possible that the ocean might circulate with a 1,450-year period? Climate oscillations operating on periods of about 1,300 years have resulted from models of a so-called "simple deterministic feedback system" in which deep ocean temperature changes, extent of sea ice and the levels of carbon dioxide in the atmosphere are included (Saltzman et al., 1981), demonstrating that aspects of the climate system that are self-regulating may in effect feed off each other, and if these are not perturbed significantly, such as by a new control, may be operating in a mode in which oscillations of climate occur in a periodic manner.

Internal oscillations of the ocean, as well as a number of other subtle factors, may indicate if such a mechanism has caused the RCCEs to occur in a repeating period of 1,450 years. A worldwide conveyor belt phenomenon has resulted from the "turn over" of the oceans, and this conveyor belt stops working on a regular basis, and it appears these shutdowns may be connected with RCCEs.

In the PCI the cycles of 2,200 years and 510 years are, when compared with previous cycles, of relatively small magnitude, though these periods have been suggested to be important in other environmental records.

The author1 suggests a clue to the influence variation of solar radiation may have on climate is provided by tree rings that record the variation of the 14C isotope, as it is believed changes in the solar wind output affect the amount of 14C produced in the upper atmosphere. Periods of low solar activity are believed to be associated with high levels of 14C production (Stuiver et al., 1991). The 14C data from tree ring records covering the last 10,000 years has been analysed statistically and the results indicate a number of cycles, about 2,200-2,400, 500, 200, 80-90, 22 and 11 years (e.g., Suess, 1980; Hood & Jirkowic, 1990; Sonnett & Finney, 1990). The PCI record in the GISP2 ice cores displays the same cycles, suggesting there is a solar influence on this record as well. When the cycles at 2,200 and 500 years were examined in the PCI and the tree rings there was a similarity, in both cases they explain about 40% of the total signal in the Holocene, the last 10,000 years of the record. As 14C data from tree rings are not available earlier than about 10,000 BP it is not possible to test tree ring data against the PCI record. It has been found that in the Holocene a critical climate control was solar activity, but its effect on older climates is less clear because of the lack of other records to compare it against. According to the author1 beryllium 10 will soon be available to provide evidence of pre-Holocene solar activity.

Observational evidence of the importance of the impact of solar activity on climate is provided by sunspot cycles, the 11 year cycle of solar radiation output. As measurements are available only for the upper atmosphere from satellites for the last 2-3 solar cycles, a period for which changes of temperature have been quite small, as measurements from the lower atmosphere would be affected by clouds and atmospheric chemistry. According to the author1 it is currently believed that simple temperature changes resulting from changes in the levels of radiation over a solar cycle would be insufficient to account for the behaviour of RCCEs, and this behaviour is dramatic, indicating that more is going on that causes shifts that are seen in the records.    

Changes in the output of the Sun also transform the atmospheric chemical composition, including the abundance of ozone, according to one suggestion, and according to this view the role of human activity from a different angle is implicated. As anthropogenic pollution increases the amount of ozone in the troposphere, and other pollutants, such as chlorofluorocarbons, decreases the concentration of ozone in the stratosphere, this pollution may be complicating the behaviour of climate.

As the data set is too short there is no solar cycle longer than 11 years that is instrumentally observed (verified) that is believed might show a greater influence on temperature, the result being that the relationship between solar variability and the record present in the GISP2 ice core must remain unclear for the present.

It would make a good case for solar variability if a particular climate event was known to occur approximately simultaneously in all parts of the world, the solar variability being one of the few forces affecting the whole planet, making it a key factor, though it is not conclusive proof. Obtaining records as well dated as the Greenland ice cores is the biggest challenge in determining the cause or causes of RCCEs. It will become possible to determine if RCCEs occur at the same time in all parts of the world, or if the events are regionally phased in, or even possibly there is no relationship.

The author1 suggests that even if it is uncertain whether all events occur at the same time, most evidence that is known points towards RCCEs occurring on a global scale. Evidence is also accumulating that these climate change events are both rapid and regular, and it is becoming apparent that more than a single forcing factor is involved in their regulation and occurrence. As a result the attention of climate scientists is shifting from one of "warming" or "cooling" to "stable" or "unstable". It is beginning to become apparent, to scientists though not yet to the general public, and especially decision makers, that RCCEs are a more realistic view of climate change.

According to the author1 the periodic behaviour seen in the GISP2 record has allowed a new view of the operation of climate that is more complete, the single record of climate in the ice core has displayed a high degree of regularity in the natural climate and an unexpectedly high number of controls on climate, even though the analysis is not near completion at the time of writing. The processes of operation of climate forcing is not shown clearly by this new approach to climate, not allowing the non-linearity in the system to be fully understood. One uncertainty is that it is still not known if there are a critical number of years or a critical degree of forcing required to lead to a response from climate, and whether the eventual response is simple or complex.

Scientists working in this field are still debating these issues, and now with a basis of the data from the GISP2 ice core, which has allowed the compilation of a running list, that the author1 calls a precursor to an equation describing the full processes of climate controls. Included in the list now known from the GISP2 record, that covers the last 10,000 years, are:

Orbital cycles of the Earth

  • ice sheet behaviour oscillations
  • ocean circulation oscillations
  • Solar energy output changes
  • atmospheric greenhouse gas concentration changes - water vapour in clouds, carbon dioxide, methane, nitrous oxide, ozone, all changed by natural and human activity
  • Atmospheric dusts and acid concentrations, resulting from changes in moisture content on land, wind speed changes over land and oceans, volcanoes, human activities and the biological productivity of land and oceans.
  • variables that are still to be identified.

The author1 suggests it is still not possible to predict climate with any degree of precision based on all these known climate controls, and he admits there is still the possibility that climate may turn out to be controlled by events that are purely random or chaotic. He also suggests that a major hurdle that remains to be overcome is extracting the interpretable part of the record (the signal).

Sources & Further reading

  1. Mayewski, Paul Andrew @ White, frank, 2002, The Ice Chronicles: The Quest to Understand Global Climate Change, University Press of New England.
Author: M. H. Monroe
Last Updated 05/02/2013

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