In viticulture, consistent wine quality and style depend on reasonably stable environmental conditions. Of the many environmental factors that can affect both




НазваниеIn viticulture, consistent wine quality and style depend on reasonably stable environmental conditions. Of the many environmental factors that can affect both
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A perspective on climate change: I Causes and predicted effects http://www.wynboer.co.za/recentarticles/200709climate.php3



John Wooldridge
John Wooldridge
ARC
Infruitec-Nietvoorbij, Stellenbosch Sept. 2007

Summary
Geographic
and orbital factors indicate that the world is approaching the end of an interglacial period in the Cenozoic Ice Age. However, rather than cooling, a warming trend is in progress and temperatures are likely to increase by 1.8 to 4°C this century. This warming, which is driven by rising concentrations of greenhouse gases in the atmosphere, will be accompanied by changes in rainfall and has the potential to affect viticulture worldwide

Introduction
In viticulture, consistent wine quality and style depend on reasonably stable environmental conditions. Of the many environmental factors that can affect both the grapevine and its product, climate (defined as the long-term average weather pattern experienced at a given locality, whereas weather is the state of the atmosphere at a particular time and place) is probably the most important. Because of its potential to affect populations and food security worldwide, climate change has been a controversial and sensitive topic over several decades. Climatologists fell into both anti and pro warming lobbies. Some pointed out that the variability in weather within seasons, from year to year, and from decade to decade, and the shortness of the period for which instrumented weather recordings are available are such that it is difficult to be certain about what constitutes a normal climate, let alone a climate change. This uncertainty and other apparent anomalies were used, on occasion, to obfuscate and trivialize the issue. Other observers cited early flowering and late leaf fall, species migrations, receding glaciers, insomniac bears, thawing permafrost, spells of unusually high temperature and rising sea levels as evidence of warming. From a viticultural viewpoint the conflicting viewpoints were confusing. Recently, however, evidence has accumulated to the point where there appears to be widespread consensus among climate researchers that warming is a reality. With that consensus, the controversy has largely shifted from the scientific to the political arena where it mainly concerns such issues as control and mitigation. On a more practical level the merits of such northern localities as Scotland, Alaska and Norway as sites for end-of-21st century premier dry white wine production, and of southern England for sparkling wines, are apparently being seriously debated.

Inherent in most discussions of climate change is the unspoken desire for conditions to remain the same. This is the most unlikely scenario of all. Change is integral to the functioning of a geologically and biologically active planet where climates function in an interactive system that includes the continents, oceans and atmosphere, and which now includes a new factor: man. Climates are driven by absorbed solar energy. This varies from place to place, with time of day and with season. Winds and storm systems are, in turn, atmospheric responses to shifting contrasts in temperature, pressure and moisture content, superimposed on global wind patterns created by convection and given direction by the earth’s spin. Climates evolve, grading from the past, of which there is usually a record, through the present, which is recorded, into a future, which science attempts to predict, usually from ongoing tends. Prediction of long term climate change usually entails the parallel running of large ensembles of models, each testing a slightly different set of assumptions (scenarios) concerning such variables as future fossil fuel use, emissions control and population dynamics. The output is a range of values which reflect the variation in the input scenarios. These become progressively more uncertain as the prediction period lengthens. There is considerable scope for error if the wrong scenario is followed. Ultimately, the best way to determine the accuracy of a model, and of the assumptions on which it is based, is to wait for the climate to evolve. In practice, assessments of likely changes in climate are usually based on historical trends (Midgley et al., 2005. The models may themselves be tested and refined by running past climatic data for which the outcome is known. Knowledge of past climates is therefore extremely important, and ingenious methods for obtaining this information have been devised. Although organised instrumental recording only began in the mid to late 1800’s, palaeoclimatic information dating far back into geological time can be derived from proxies such as tree rings, pollen, ice core data, cave carbonate deposits, isotopic ratios in sediments, and palaeogeographic reconstructions. Apart from providing data for model testing and predictive purposes, these studies provide evidence that certain aspects of global climate were, and remain, linked to geographic and other associated factors.

The aim of this article is to summarise and review the latest technical information on climate change. To enable this information to be interpreted in the widest possible context, the article begins with a discussion of how palaeogeographic factors have influenced the evolution of the present climate, and of how modern temperature and atmospheric compositions compare with those of past times. Possible effects of climate change on global and South African viticulture will be discussed in a subsequent article (Wooldridge, xxxx).

Temperatures, past and present
The earth is currently passing through a warm interval in an ice age that began in the northern hemisphere around three million years ago (Mya) (Redfern, 2000). (An ice age may be described as a succession of periods of low temperature, associated with the expansion of glacial ice (glacials) and falling sea levels, alternating with shorter periods of relative warmth, receding ice cover and rising sea levels (interglacials)). Ice ages do not occur frequently. During the Phanerozoic, which dates back 542 My (Gradstein, et al., 2004), and includes most of the time that complex, multicellular life has existed on earth, there have been only three ice ages and one cool spell. These occurred at irregular, roughly 140 My intervals. Once established, each ice age lasted for 50 My or more (Holmes, 1965; Global Warming Art(a)). The present northern ice age is therefore still in its early to mid stages. Reconstructions of Phanerozoic palaeoclimates by Scotese (2002) suggest that global average temperatures alternated between hot (22°C) and cold (12°C), with a mean of 17°C. By comparison the global average for the year 2000 was about 14.4°C (IPCC, 2007a). If reliable, these reconstructions show that the present interglacial temperatures are several degrees cooler than those which prevailed over most of the last half billion years.

Climatic zones, continental drift and the onset of the Cenozoic Ice age
The world’s climatic zones form concentric belts, north and south of the equator. These belts are reflected in the Köppen-Geiger system of climate classification (Kottek et al., 2006). The polar and cool temperate climatic belts expand during cooling phases, whereas the arid and warm temperate belts expand during warm phases. In the early Eocene (55 to 50 Mya), for example, tropical conditions extended 10 to 15 degrees of latitude poleward of their present limits. The belts do not change their relative positions. In contrast, the continents drift slowly across the latitudinal climatic belts and poles, driven by convection in the upper mantle. Climate is thus dependent on, inter alia, the shifting distribution of landmasses, relative to one another and to the climatic belts and, by extension, to the intervening ocean basins and current systems.

As is the case at present, past ice ages occurred at times when the distribution of the continental landmasses prevented effective heat transfer between the thermal equator and higher, cooler latitudes. The cooling that led to the present ice age began about 55 Mya when a warming trend was terminated by tectonic (mountain building) activity which raised the Himalayas and several other major mountain chains. The principal agents of heat distribution are ocean currents. That oceans should influence climates is unsurprising in view of the fact that around 70% of the earth’s surface is covered by water. These currents change in pattern and strength in accordance with shifts in temperature and salinity, and with geographic changes such as occurred in the late Eocene, after 39 Mya, when the Drake Passage between South America and Antarctica began to open, permitting the development of a cold, circum-Antarctic current. Thus thermally isolated, the Antarctic continent, over which the south geographic pole is located, lost its vegetation and began to develop an ice sheet. This grew extensively around 33.5 Mya. By the end of the Oligocene, 23 Mya, glaciers were discharging into the sea, and water temperatures had fallen to the point where cold, saline bottom water was spiraling outwards and flowing northward into the Atlantic, Indian and Pacific oceans, all of which began to cool. Cooling continued as this newly established interpolar thermohaline (meridional overturning) circulation strengthened, relative to the pre-existing equatorial circulation (McCarthy & Rubidge, 2005). Since about 13 Mya, one of these currents (the Benguela) has upwelled along the west coast of southern Africa, brought to the surface by the prevailing offshore easterly winds which drive the warm surface waters westward. South Africa once received moisture-bearing winds from both the west and the east. However, because evaporation from cold water surfaces is slow, the effect of this upwelling current, combined with the blocking effect of the South Atlantic high pressure system on moisture-bearing winds from the west, was primarily that of inducing progressive aridification. This lead to the formation of the Kalahari desert and the spread of savannah across the western interior. Further contributing factors were two episodes of uplift which, beginning 20 Mya, elevated the eastern side of the southern African subcontinent by 900 to 1150 m, as opposed to 250 m in the west (McCarthy & Rubidge, 2005). Thereafter, moisture from the Indian Ocean mainly precipitated over the uptilted eastern areas whilst the lower areas to the west fell into the associated rain shadow zone (McCarthy & Rubidge, 2005). Similarly, orographic rainfall over the north-south and west-east ranges of the Cape Fold Mountain Belt reduces the moisture content of air masses flowing into the interior of the southern Cape from the west and south (Midgley et al., 2005).

Cooling of the northern hemisphere did not become significant until about 3.5 Mya, perhaps 15 My after Antarctica had frozen over. Again, the main triggering factor was geographic. This time it involved the closure, rather than the opening, of a seaway. This seaway, which separated the North and South American landmasses, became increasingly restricted as the Isthmus of Panama developed. Final closure terminated the latitudinal flow of warm equatorial Atlantic water into the Pacific, causing the temperature of the eastern north Pacific to decline. An additional effect of this closure was to direct warm equatorial Atlantic water, with its high overlying atmospheric humidity, into the north Atlantic, causing an increase in cloud cover and a higher ratio of reflected to absorbed solar energy (albedo). Precipitation fell as snow at high northern latitudes, further increasing heat reflection, and by 3.1 Mya glaciers had formed on the higher northern mountains. Regional cooling was exacerbated by rapid heat loss from the northern landmasses (land gains and loses heat faster than water) (Redfern, 2000). Between 3.0 and 1.6 Mya there were long periods of glacial advance and retreat and, at some point during this interval, the Arctic Ocean, which is largely landlocked, developed a perennial ice cover. Beneath the sea ice, dense, cold, highly saline water began to sink to the bottom and flow southward, drawing in warm Atlantic water behind it, thereby further strengthening the inter-polar thermohaline circulation. The northern continental ice sheets reached their greatest extent in the Pleistocene, 1.8 Mya to 11.5 thousand years ago (Kya) (Gradstein et al., 2004). Between the extremes of each cycle, of which there were at least four, climatic zones shifted by 20 to 30 degrees of latitude (2400 to 3200 km) (Redfern, 2000). These shifts presented the plant and animal populations with enormous evolutionary challenges, resulting in a number of species, including most of the cold-adapted giant mammals, becoming extinct. On at least one occasion, sufficient water was locked into the continental ice sheets that sea levels around southern Africa fell by up to 130 m, exposing the continental shelves, causing deep incision of watercourses and promoting increased aridity and dune formation. Dust carried by winds from unprotected surfaces to high altitudes blocked sunlight, leading to further cooling. Nevertheless, the mountains of the Western and Southern Cape remained glacier-free throughout the Pleistocene. In consequence, the landscapes and soils of the region did not benefit from the rejuvenating effects of glacial action, as on the higher-latitude northern continents, where fertile soils formed in mineral-rich parent materials derived from freshly ground rock. Neither did the region gain from the formation of lakes by glacial action.

Waning of the last glacial period
The last major glacial period peaked about 21 Kya, then receded, to be followed between 12.9 and 11.5 Kya by a short glacial period called the Younger Dryas. The Younger Dryas was remarkable for its rapid onset. Mean annual temperatures in Greenland and the UK decreased by 15°C and 5°C, respectively, below today’s temperatures, apparently within a few decades. Recovery was even more rapid. Thus, although climate change is generally a slow process it may, on occasion, be extraordinarily fast. A wildcard in times of rapid global warming is disruption of the inter-polar thermohaline circulation by the release into the North Atlantic of large volumes of fresh water from rapidly melting continental ice sheets, such as could result from the melting of the Greenland ice cap, which may be approaching a stability threshold. Such disruptions have the potential to promote abrupt, but probably transient cooling at high northern latitudes, even in the face of a warming trend (Redfern, 2000). Paradoxically, exposure of the Arctic Ocean by melting of the sea ice could lead to sufficient evaporation, followed by precipitation as snow and the spreading of high-albedo snow and ice surfaces, to trigger the onset of another glacial period (Holmes, 1965). Other wildcards that could, perhaps, promote rapid change include the release of methane from warming permafrost, the sudden formation of methane gas from hydrates on the sea bed, and widespread changes in land use.
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