By VÁCLAV CÍLEK
We are nostalgiac when we lose the phenomena we have fought with – open unknown seas, rainy forests, high plateaus or deep caves. We as the mankind we have grown through these fights and we have gradually becomed stronger than most of the sea storms, underground rivers and deep forests. Civilisations are evolving when they fight with nature and shrinking when they start to fight with themselves. The last wilderness that remains is climate. This relationship is far from being friendly or melancholic one. It is a threat of the magnitude of Pacific Ocean in 15th century. We know now that climate is stronger and does not care about consequences. What we should talk about are not only the real impacts of climate change but the archaic processes how we deal with wilderness – fears, awe, conflicts, reconciliations and finally protection. But we are at this moment at the very beginning of a deal with a new God of Climate Change.
CLIMATE IS NOT AN ISOLATED SYSTEM
The climate is not an isolated system. It is closely connected with the Sun and other parts of the Earth system such as the ocean, cryosphere, biosphere and so on. The connection with other parts of the Earth is a function of the spatial and temporal scale. For limited time scales – years and decades – the climate depends on the composition of the atmosphere and on solar activity, but for large time scales the other parts of the Earth, such as the lithosphere and biosphere, must be taken into consideration. The climate cannot be considered only as the behavior of the atmosphere but as the total response of the Earth and Solar systems to an ever-changing flow of influences. The biosphere activity buffers some of these changes and makes the Earth more habitable.
Changes in climate involve factors both external to and within the climate system. External factors include solar variability, volcanic activity (especially dust and aerosol eruptions), astronomical characteristics of the Earth’s orbit and meteorite impacts. Internal factors include variability within the atmosphere and ocean, and anthropogenic activities. The climatic system can be viewed on three hierarchical levels:
Level 1: Solar energy. The solar input at a given point on the Earth’s surface is the result of the distance between Sun and Earth, the Earth’s axis inclination and the internal solar activity. The amount of solar energy captured by the Earth depends on the interactions of the Earth’s orbital cycles and the internal cycles of solar energy production. Some of Earth’s orbital characteristics happen at very different time scales from the solar luminosity cycles (11 years to 400 thousand years) so a variety of interactions must be expected.
Level 2: Greenhouse effect. Part of the incoming solar energy is reflected back to outer space but entrapped by the greenhouse gases. Without greenhouse gases, the present Earth would be about 32º C cooler. The uneven carbon burial could have caused temperature shifts of about 50o C during the Proterozoic.
Level 3: Function of sea currents. The changing solar output is entrapped by the Earth’s surface. In the Quaternary, more than 80 % of the incoming energy is captured by the ocean for two reasons: 1) the equatorial part of the Earth receives the maximum of solar energy; 2) the ocean is predominant in the equatorial part of the Earth. The captured heat is distributed by a system of sea currents in a manner similar to that of a house’s central heating. The distribution of land and sea is crucial for heat and humidity transfer.
The gradual development of climate can be interrupted by rapid or even abrupt climatic changes caused by reorganization of the sea currents, meteorite impacts and volcanic events. Temperature rises or falls on the scale of 5-10º C were observed to happen about 20 times during the last glacial within a few decades or even years.
ONSET OF THE QUATERNARY GLACIATION
We observe throughout geological history several factors implying the coming of ice ages:
The continent or islands must be at the right place or high enough to collect and build up the ice masses.
The equator-pole gradient should be high in order to transfer the water vapor evaporated in the tropical zone to the pole regions. The glaciations are not only the result of low temperatures, but also of the humidity transfer to the low-temperature area.
The global pattern of sea currents should be organized in such a way as to bring the moisture poleward but not to melt the ice sheets.
The concentration of greenhouse gases, mainly CO2, should be low.
The total solar irradiance should be diminished.
Between 3.2 and 2.4 Ma, the climate system evolved through another cooling stage ending with the abrupt cooling and ice expansion at 2.5-2.7 Ma. Rapid tectonic movements possibly triggered these changes. The Central American Isthmus became closed and the poleward heat and humidity transport increased. The Greenland-Scotland ridge sank down more than 1000 m and the warm near-surface Atlantic currents were able to reach the Greenland-Norwegian regions, while the cold bottom currents could spread over the Atlantic. The Himalayan uplift may have caused a change in the upper troposphere winds. The Northern Hemisphere glaciation probably represented the switching mechanism of the global ice age onset, because the Antarctic ice sheet was isolated by mid-ocean ridges.
PLEISTOCENE ICE AGES (1.8 MA – 11700 YEARS BC)
We live in the Quaternary period, which has been assigned a beginning at 1.8 Ma, while other scientists propose a ”long” Pleistocene starting at approximately 2.7 Ma. The Earth climates have, during this time, undergone periodic glaciations and cyclic environmental changes. These were accompanied by changes in the geochemical fluxes of many atmospheric elements, in biosphere productivity and oceanic and in atmospheric circulation patterns. Deep-sea sediments show that during the Pleistocene the climatic changes are dominated by orbital forcing, i.e. by the changing distance and inclination between the Sun and the Earth. Temperature and ice volume was mainly controlled by the so-called Milankovitch orbital cycles – changes in precession (19 and 23 ky), obliquity (41 ky) and eccentricity (100 ky). Several other cycles (1 Ma, 400 ky and 60 ky) sometimes appear in the fossil record.
Even if the astronomical theory of Quaternary climate changes is accepted, one should be aware that the global climatic state is not a simple function of the incoming solar energy. The Earth modulates the amount of incoming solar energy by changes in the surface reflectivity, thermal retention in ocean and heat redistribution by sea currents. The process of climate modulation is made even more complex by biological feedback and biogeochemical cycling of atmospheric elements.
The Pleistocene epoch can be divided into a series of ice ages or glacials and warm periods or interglacials. The term ”ice ages” should be restricted to the polar and temperate areas covered by the continental glaciers, because although the tropical and subtropical zones cooled by some 4-7º C, mild tropical weather prevailed even during the ”ice age”. More exact terms such as ”cryomere” or ”Pleistocene cool mode” have been proposed but are not widely used.
The Antarctic ice sheet was not significantly larger than it is now because dry conditions and partial isolation did not allow the build-up of a more extensive ice sheet. The most drastic changes of the glaciated area (shifts from 10 % to 29 % of global ice-covered land) occurred in the Northern Hemisphere. The Laurentide ice sheet covered the northern part of the U.S., reaching as far south as New York. Iceland and Greenland were almost entirely ice-covered. The Scandinavian ice sheet extended south to Kiev, Warsaw and Berlin. Much of Siberia was overspread by the Siberian ice sheet. High mountains on all continents (even in Japan and Hawaii) carried glaciers of varying dimensions.
The effects of the glacial climate and continental ice sheets included a fall in sea levels of up to 120 m, because the water cumulated in ice sheets. The cyclicity of glacial and interglacial modes remains puzzling for the early Pleistocene. The 40 ky cycle seems to have been dominant until some 0.9 Ma ago the 100 ky cycle took control. The early Pleistocene ice ages were shorter and less severe, while the interglacial climates were warmer and possibly (as evidenced by red soils in loess sequences) more arid. The 40 ky orbital cycle is, with regard to insolation, more influential than the weaker 100 ky cycle. In that case, why is the climate of the last million years driven by the latter cycle? The possible explanation may depend on the extent of the ice sheets. They grew so large about 1 Ma ago that due to thermal inertia the longer cycle prevailed. The climatic change during glacials is more severe in the polar and temperate zone where the annual temperatures dropped by about 10º C and the total precipitation decreased by about 50 %. The equatorial zone witnessed less pronounced changes. The glacial seasonality of the temperate zone can be compared to present-day Siberia – short, hot summers alternating with long, dry, windy winters.
The last million years are marked in the temperate zone by 9 large ice ages lasting about 100 ky and 9 interglacials lasting about 20 ky. We have no reasons to conclude – in spite of the present warming – that the interglacial period we live in will not change into another glacial. The orbital parameters suggest significant cooling 5 ky from the present. The last ice age was interrupted by abrupt climatic changes. They are well documented during the last ice age but will very probably also be found in the other glacial intervals. They are known as follows:
Dansgaard-Oeschger oscillation is a rapid, relatively short warm and cold oscillation that punctuated the last glaciation and lasts up to three thousand years. The mean annual temperature may rise by up to 10º C within a few decades and then fall abruptly back into glacial conditions.
Heinrich event is a period of massive iceberg discharge into the North Atlantic, which occurred several times during last ice ages and probably many times throughout the Pleistocene. The meltwater blocked the upwelling sea currents and it reduced the thermohaline circulation and thus the poleward heat transfer. One of the most surprising and alarming climatic findings of the last years holds that the thermohaline conveyor belt, represented by a complex of sea currents, can be switched off and on within several years. The result is a drastic global climatic change. However, thermohaline circulation during interglacials seems to be less variable and less sudden.
The Holocene Epoch, also referred to as the Recent, is the latest interglacial interval of the Quaternary Period. It is unique because it is coincident with the development of post-Paleolithic human civilization. The influence of Man on nature and the environment and, recently, on the climate becomes more and more visible during this epoch. The Holocene has been extensively studied in Europe during the last century and thus a ”eurocentric” view of this epoch still prevails. New studies, however, show that the major climatic changes are simultaneous over the majority of the globe, although smaller or short-lived climatic changes are often restricted to small areas. For example, the Holocene climate fluctuations of the last centuries in the Rhine catchment are slightly different and asynchronous from the developments in the Elbe area. The minor climatic oscillations in Switzerland do not correspond to the oscillations recorded in Greece or Poland. The smaller and shorter the climatic fluctuations are, the more they tend to be restricted to different landscapes and regions.
The Holocene was originally considered a remarkably stable epoch, because major coolings occurred within the range of 1.2 – 1.8º C and major warmings were some 2-3º C higher than at present. The precipitation varied considerably within at least –50 % to +100 % on decadal, centurial and millennial time scales. There is growing evidence of at least three crises when the Holocene climate deteriorated abruptly, only to return to its previous stage within a few centuries. These are recorded in the abrupt falls of many African lakes and are dated to 12 000, 8200 and 5200 years before the present. Other droughts took place during the end of the Late Chalcolithic period some 4500-3900 years before the present, during the Early Iron Age at 2800-3300 years before the present and possibly on many other occasions. These aridity waves are often associated with human ethnic and cultural migrations and societal changes. The droughts, not the temperature, have always represented the principle obstacle to biome or civilization development during the Holocene.
Some of the world’s regions were more vulnerable to these fluctuations than other parts of the Earth; for example, the Saharan desert did not exist during most of the early Holocene. The Saharan Neolithic has appropriately been labeled the ”aqualithic culture” because turtles, mollusks and fish were important food components. The abrupt events are recorded by the ice core record as sharp troughs in atmospheric methane, implying a reduction in tropical wetlands. The main causes of the Holocene climate change are not properly understood, but the most frequent explanations are as follows:
The first half of the Holocene was generally warmer in the Northern Hemisphere, because this part of the Earth received during the summer months some 8 % more of the insolation than it does at present. The elevated temperatures and humidity led to a so-called thermal optimum (9000-5500 years before the present). As the vegetation cover diversified under warm and more humid conditions a so-called forest optimum was reached in the temperate zone some 6000 years ago. Subhumid forest dominated Europe and temperate Asia during this interval.
The climates in the tropical and subtropical zones were largely controlled by the position of the Intertropical Zone of Convergence (ITCZ) and its associated rains. ITCZ moved several times to the north and back again, bringing and withdrawing rains in the very climate-sensitive semi-arid belt of Asia and Africa. It very probably affected the monsoon regime of the Indian Ocean, which was stronger in the early Holocene.
The periodical switching of the North Atlantic Oscillation (periodicity: 7-30 years) and El Niño Southern Oscillation (periodicity: 4-6 years) and the complex interplay among these large oceanic and atmospheric circulation systems led to abrupt short-lived temperature and humidity fluctuations.
The human role in nature was far from being passive. The Mesolithic forest clearing, Neolithic agriculture, pasture and changes in land-use and thus surface reflectivity, massive recent emissions of greenhouse gases, nitrous oxides, aerosols, industrial dust and many other factors are either contributing to the natural climatic ”cycles” or in some cases even counterbalancing them.
THE LAST MILLENNIUM
The conventional view of the climate of the last millennium is concerned with the simple sequence of the Medieval Warm Epoch, Little Ice Age and Industrial Warming. Recent investigations have found no evidence for synchronous climate changes on the global level, but many regional anomalies happening on decadal time scales. The most significant environmental risks are not associated with mean temperatures but with changes of circulation patterns resulting in seasonality shifts and prolonged drought periods. The other important criterion of climate change is the frequency of extreme states; for example, the Medieval Warm Epoch (9th to early 15th century) represents an interval of abundant hydrological anomalies. Some of these may have led to anomalous warmth in some (but not all) regions.
The Little Ice Age (conventionally 1550-1850) is one of the coldest intervals in the entire Holocene. It displays wide temperature variation unevenly distributed in different regions. Some areas were warm at times when others were cold and vice versa. The solar variation and volcanic eruptions can be correlated with the climate oscillations of the last millennium. However, it is more difficult to find such a correlation for the last century. Humans seem to be reaching the point of becoming one of the prime factors of short-term climate change.
DIALOGUE BETWEEN THE PLANET AND LIFE
The atmosphere composition and the resulting climate represent the general response of the Earth system to an interconnected web of feedback loops among the ocean, lithosphere, biosphere and outer space. The general cause-effect scenario is often invalid, because complex systems behave like neural nets where direct causality is diminished and cycles or cascades of effects appear. Many synchronous changes of different parts of the Earth system shape the climate. The concept of planetary co-evolution fits our experience with the Earth system better than the idea of a chain of linear causes. Our climatic models are often too simplistic because we are required to choose one cause from the cycle of causes as the beginning, and then to deduce the consequent chain of climatic effects. The climate can be thus expressed as a never-ending and ever-changing dialogue between the planet and life.
Old house in Bahariya Oasis in Western Desert, Egypt. We sometimes believe that a dialogue between humans and nature takes place, but in fact we often hopelessly sit in front of the closed doors.
Baviti in Bahariya Oasis in Western Desert. The travellers and pilgrims frequently found abandoned cities in the desert and they asked if the climate is really as stable as we suppose.
The Valley or the World has always represented for Old as well Modern Egyptians the place of order encircled by chaotic barbarian forces of desert. We conquered all wilderness on the Earth but climate remained one of the last strange gods of our changing world ( all photos by V. Cilek).