Rapid Climate Change Events (RCCEs) "Rickies" in the Holocene

Australia: The Land Where Time Began

Rapid Climate Change Events (RCCEs) "Rickies" in the Holocene

The ice core obtained by the Greenland Ice Sheet Project Two (GISP2) has been studied to find if RCCEs have occurred in the Holocene. Throughout much of the record in the ice core it was found that change in continental dust concentration and sea salt concentration in the ice core rise and fall together. When there is increased levels of continental dust deposited on the snow that eventually became the GISP2 ice core it indicates the atmospheric transport is intensified. Plant cover on land surfaces was sparser at times such as the coldest parts of the glacial phases than it was during the warmer phases, leading to the increased dust levels recorded in the ice. Continents such as North America and Asia were the sources for this dust, so the dust arriving at Greenland was transported by zonal winds, west-to-east winds. The author¹ suggests that the increased levels of sea salt in the Greenland ice were probably picked up over the Atlantic Ocean and transported to Greenland by meridional winds, south-to-north. The disposition of both the dusts and sea salt on Greenland resulted from the intensification of both meridional and zonal winds that occurs during RCCEs. Examination of the Holocene section of the GISP2 ice core revealed there were 4 major periods during which both zonal and meridional winds intensified, and a number of minor periods, that occurred since the close of the Younger Dryas, which was the last RCCE prior to the start of the Holocene. It was found that during the Holocene the levels of sea salt and dust were 1 order of magnitude lower than in the previous glacial period.

According to the author¹ there are 2 ways of determining if the shifts that occurred in the zonal and meridional winds in the Holocene are true RCCEs, which he says are important because, if they are rapid climate change events, they occurred in the period during the development and rise of civilisation, about 10,000-8,000 BP. The author¹ suggests the importance of such a discovery lies in the fact that it would indicate that natural climate variability still has a key role to play, though the Holocene was a period in which the climate was significantly milder than the glacial period.

One test for the existence of RCCEs in the Holocene is to search for periods which display the same change characteristics as are found in RCCEs such as the Younger Dryas. Intensified atmospheric circulation patterns, coupled with temperature decrease and the rate of accumulation of snow in places such as Greenland, and decreased atmospheric methane levels, a greenhouse gas, as a result of freezing of the standing water bodies in which plants grow that emit methane. These associations can be searched for in Holocene atmospheric circulation data, the 4 largest zonal and meridional atmospheric circulation changes were found to occur about every 2,600 years (O'Brien et al., 1995). In the glacial phase RCCEs occurred about every 1,500 years, the author¹ suggesting it is still possible a 1,500-year pattern for the Holocene may be present in the GISP2 ice core that is less prominent than in the glacial age section of the ice core. Marine sediments deposited in the North Atlantic during the Holocene record iceberg discharge events that occur with a pattern of almost 1,500 years (Bond et al., 1997) that have been found to correlate with the record of continental dust for potassium from the GISP2 ice core, which the author¹ suggests probably relate to atmospheric circulation changes over Siberia and Asia. As with the glacial section of the ice core record, the marine sediment record displays similarities with the Holocene section of the ice core record. There are decreases that are coincident in accumulation rate, temperature, (based on the stable isotope proxy), and methane that is expected from the characterization of RCCEs in the glacial age, for the oldest rapid climate change event, from 8,800-7,800 BP recorded in the GISP2 Holocene ice core. Similar associations are shown in large rapid climate change events that are successively younger at 6,100-5,000, 3,100-2,400 and since 600 BP.

Knowing that in the Holocene changes in sea level, volume of ice, solar variability, and human involvement, complicate the the task of determining if they are still RCCEs is made more difficult, possibly meaning the characteristics of RCCEs from the glacial age will be difficult to apply to the Holocene. The author¹ suggests that it is possible human activities may have begun to affect methane levels by 4,000-2,000 BP, instead of just the natural processes, as by this time there had been a dramatic increase in agriculture, especially rice cultivation, which produces a relatively large amount of methane. He also suggests that the expected decrease in methane expected with a RCCE could have been negated by human involvement.

Holocene climate complexity

The northern Hemisphere has undergone significant environmental changes since the Younger Dryas, with rapidly decreased extent of ice sheets (ice volume) from the Younger Dryas to about 8,000 BP, by which time the extent of Northern Hemisphere ice was similar to that of the present. The sea level, that were more than 131 ft (40 m) below the present level, rose to the level of the present, about 11,500 BP to within 10 m (33 ft) of the current sea level by about 8,000 BP, and there was a dramatic change in insolation resulting from the lower albedo as the area of ice shrunk. There was a greater difference between the seasons, winters were colder and summers were warmer than at present, about11,500 BP.

There were several episodes during which solar variability was particularly weak, and there were dramatic floral and faunal changes, and the distribution of deserts, all of which combined to make the Holocene a climatically complicated period.

Another way of proving that RCCEs occurred in the Holocene has been proposed by George Denton, University of Maine and Wibjorn Karlen, University of Stockholm. A paper written by Denton & Karlen summarised their field work in the Yukon Territory and Scandinavia in which they mapped changes in the margins of alpine glaciers in the Holocene, as well as the work of other scientists on the same subject, the conclusion of the paper was that the spacing of globally distributed fluctuations of glaciers was close to 2,600 years (Denton & Karlen, 1973), a spacing coinciding quite well with the timing of RCCEs as observed in the record seen in the GISP2 ice core.

These glaciers are much more sensitive to climatic changes than ice sheets with much larger ice volumes, though they are too small to dramatically affect the sea level the way the ice sheets can. The size of a glacier determines how rapidly it can respond to climate changes, the smaller the glacier the more rapidly it can respond to climate changes. The Northern Hemisphere glacial age ice sheets and of the Antarctic ice sheets have response times of thousands of years; that of alpine glaciers is from years to decades.

The author¹ concludes that a result of this correlation is that cooling can be seen to have occurred on a global scale, leading to the advances and retreats of alpine glaciers coinciding with the timing of the largest RCCEs that have been shown in the GISP2 ice core to have occurred in the Holocene.

Atmospheric Cooling as Greenhouse gases are Rising

Over the last century atmospheric temperatures have risen, though during this time there have been periods, years and decades, over which periods of cooling have interrupted this warming trend of the troposphere. In the period from 1940-1970 the atmosphere cooled, primarily over the North Atlantic. Insight into this cooling is provided by a simple comparison between patterns of change in some of the primary parameters controlling climate.

When the temperature trend for the Northern Hemisphere is compared with a single potential climate controller, carbon dioxide, the overall temperature increase on the scale of a century follows the same trend as carbon dioxide, but in the period from 1940 to 1970 the cooling is not associated with decreasing carbon dioxide, so another cooling cause needs to be investigated.

Increased atmospheric sulphuric acid levels is a potential cause of the cooling as it has a shielding effect in the atmosphere. It has been known for some time that following volcanic eruptions large amounts of sulphur dioxide are often added to the atmosphere, and when this combines with water to form sulphuric acid it cools the atmosphere, leading to a temperature decrease of 1-2^oC (2-4^oF) following major eruptions such as Tambora in 1815, in North America and western Europe leading to the "year without summer".

Among the first to note the cooling effect of volcanic dust was Benjamin Franklin. Atmospheric sulphuric acid travels further than volcanic dust and often has a cooling effect that is more far-reaching and of longer duration. The data from the GISP2 ice core shows that the amount of sulphuric acid (non-sea salt sulphate) in the atmosphere has increased dramatically since the beginning of the 20th century as industrial activity has increased and the burning of fossil fuels that are rich in sulphur. In the 1940s to 1970s a response to the industrial demand of World War II and the post-war industrial boom lead to increasing levels of atmospheric sulphuric acid. The potential cooling of the North Atlantic region resulted from the most intense source of sulphur emissions being located in that region. This regional cooling is a clear instance of human activity affecting the climate, this time in the form of cooling.

There was also a decrease in sunspot activity for part of the cooling period. The association between sunspot activity and climate, such as the quasi-11-year cycle, is not understood very well, though it has been noted that there are many associations that are highly significant between sunspot activity and temperature, increased or decreased sunspot numbers and increased or decreased surface temperatures.

The author¹ speculates that the Little Ice Age (LIA) may have been terminated naturally, the increases in carbon dioxide levels accelerating a warming trend, though some researchers believe the Little Ice Age has not yet ended, the warming trend of the present running counter to the normal direction of natural climate. If this is the case the warming trend is not simply accelerating the direction of natural climate change, but is a countervailing force that is putting it into reverse.

Assuming this is the case, the warming trend, according to the author¹ is even more significant than expected and requires more explanation. The onset of the LIA was indicated by other factors besides temperature, such as the globally distributed cooling that foreshadowed it, as indicated by the response of mountain glaciers that expanded in Asia, North America, South America, Europe, New Zealand and at the poles. Increased storm activity, at least in the higher latitudes, resulting from an atmospheric circulation change.

Storm activity - the Medieval Warm Period and the Little Ice Age

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