Universe a grand tour of modern science Phần 3

climate change At that time, in Cambridge, Nicholas Shackleton was measuring, as Emiliani had done, the proportion of heavy oxygen in forams from seabed cores. But he picked out just the small animals that originally lived at the bottom of the ocean. When there’s a lot of ice in the world, locked up ashore, the heavy oxygen in ocean water increases. With his bottom-dwelling fossils, Shackleton thought he was measuring the changing volumes of ice, during the ice ages and warmer interludes. In the seabed core used by Shackleton, Neil Opdyke of Columbia detected a reversal in the Earth’s magnetic field about 700,000 years ago. That result, in 1973, gave the first reliable dating for the ice-age cycles and the various climatic stages seen in the cores. It was by then becoming obvious to the experts concerned that the results of their researches were likely to mesh beautifully with the Milankovitch Effect. I When the snow lies all summer Milutin Milankovitch was a Serbian civil engineer whose hobby was the climate. In the 1920s he had refined a theory of the ice ages, from prior ideas. Antarctica is always covered with ice sheets, so the critical thing is the coming and going of ice on the more spacious landmasses of the northern hemisphere. And that depends on the warmth of summer sunshine in the north. Is it strong enough to melt the snows of winter? The Earth slowly wobbles in its orbit over thousands of years. Its axis swivels, affecting the timing of the seasons. The planet rolls like a ship, affecting the height of the Sun in the sky. And over a slower cycle, the shape of the orbit changes, putting the Earth nearer or farther from the Sun at different seasons. Astronomers can calculate these changes, and the combinations of the different rhythms, for the past few million years. Sometimes the Sun is relatively high and close in the northern summer, and it can blast the snow and ice away. But if the Sun is lower in the sky and farther away, the winter snow fails to melt. It lies all summer and piles up from year to year, building the ice sheets. In 1974 a television scriptwriter was in a bind. He was preparing a multinational show about weather and climate, and he didn’t want to have to say there were lots of competing theories about ice ages, when the Milankovitch Effect was on the point of being formally validated. So he did the job himself. From the latest astronomical data on the Earth’s wobbles, he totted up the changing volume of ice in the world on simple assumptions, and matched it to the Shackleton curve as dated by Opdyke. His paper was published in the journal Nature, just five days before the TV show was transmitted. ‘The arithmetical curve captures all the major variations,’ the scriptwriter noted, ‘and the core stages can be identified with little ambiguity.’ The matches were 142 climate change very much better than they deserved to be unless Milankovitch was right. Some small discrepancies in dates were blamed on changes in the rate of sedimentation on the seabed, and this became the accepted explanation. Experts nowadays infer the ages of sediments from the climatic wiggles computed from astronomy. The issue was too important to leave to a writer with a pocket calculator. Two years later Jim Hayes of Columbia and John Imbrie of Brown, together with Shackleton of Cambridge came up with a much more elaborate confirmation of Milankovitch, using further ocean-core data and a proper computer. They called their paper, ‘Variations in the Earth’s orbit: pacemaker of the ice ages’. During the past 5000 years the sunshine that melts the snow on the northern lands has become progressively weaker. When the Milankovitch Effect became generally accepted as a major factor in climate change over many millennia, it seemed clear that, on that time-scale, the next ice age is imminent. ‘The warm periods are much shorter than we believed originally,’ Kukla said in 1974. ‘They are something around 10,000 years long, and I’m sorry to say that the one we are living in now has just passed its 10,000 years’ birthday. That of course means the ice age is due any time.’ Puzzles remained, especially about the sudden melting of ice at the end of each ice age, at intervals of about 100,000 years. The timing is linked to a relatively weak effect of alterations in the shape of the Earth’s orbit, and there were suggestions that some other factor, such as the behaviour of ice sheets or the change in the amount of carbon dioxide in the air, is needed as an amplifier. Fresh details on recent episodes came from ice retrieved by deep drilling into the ice sheets of Greenland and Scandinavia. By 2000, Shackleton had modified his opinion that the bottom-dwelling forams were simply gauging the total amount of ice. ‘A substantial portion of the marine 100,000-year cycle that has been the object of so much attention over the past quarter of a century is, in reality, a deep-water temperature signal and not an ice volume signal.’ The explanation of ice ages was therefore under scrutiny again as the 21st century began. ‘I have quit looking for one cause of the glacial–interglacial cycle,’ said Andre Berger of the Universite Catholique de Louvain. ‘When you look into the climate system response, you see a lot of back-and-forth interactions; you can get lost.’ Even the belief that the next ice age is bearing down on us has been called into question. The sunshine variations of the Milankovitch Effect are less marked than during the past three ice age cycles, because the Earth’s orbit is more nearly circular at present. According to Berger the present warm period is like a long one that lasted from 405,000 to 340,000 years ago. If so, it may have 50,000 143 climate change years to run. Which only goes to show that climate forecasts can change far more rapidly than the climate they purport to predict. I From global cooling to global warming In 1939 Richard Scherhag in Berlin famously concluded, from certain periodicities in the atmosphere, that cold winters in Europe would remain rare. Only gradually would they increase in frequency after the remarkable warmth of the 1930s. In the outcome, the next three European winters were the coldest for more than 50 years. The German army was amazingly ill-prepared for its first winter in Russia in 1941–42. Scherhag is not considered to be directly to blame, and in any case there were mild episodes on the battlefront. But during bitter spells, frostbite killed or disabled 100,000 soldiers, and grease froze in the guns and tanks. The Red Army was better adapted to the cold and it stopped the Germans at the gates of Moscow. In 1961 the UN Food and Agriculture Organization convened a conference in Rome about global cooling, and its likely effects on food supplies. Hubert Lamb of the UK Met Office dominated the meeting. As a polymath geographer, and later founder of the Climate Research Unit at East Anglia, he had a strong claim to be called the father of modern climate science. And he warned that the relatively warm conditions of the 1930s and 1940s might have lulled the human species into climatic complacency, just at a time when its population was growing rapidly, and cold and drought could hurt their food supplies. That the climate is always changing was the chief and most reliable message from the historical research of Lamb and others. During the past 1000 years, the global climate veered between conditions probably milder than now, in a Medieval Warm Period, and the much colder circumstances of a Little Ice Age. Lamb wanted people to make allowance for possible effects of future variations in either direction, warmer or colder. In 1964, the London magazine New Scientist ran a hundred articles by leading experts, about The World in 1984, making 20-year forecasts in many fields of science and human affairs. The meteorologists who contributed correctly foresaw the huge impact of computers and satellites on weather forecasting. But the remarks about climate change would make curious reading later, because nobody even mentioned the possibility of global warming by a man-made greenhouse effect. Lamb’s boss at the Met Office, Graham Sutton, said the issue about climate was this: did external agents such as the Sun cause the variations, or did the atmosphere spontaneously adopt various modes of motion? The head of the US weather satellite service, Fred Singer, remarked on the gratifying agreement 144 climate change prevalent in 1964, that extraterrestrial influences trigger effects near the ground. Singer explained that he wished to understand the climate so that we could control it, to achieve a better life. In the same mood, Roger Revelle of UC San Diego predicted that hurricanes would be suppressed by cooling the oceans. He wanted to scatter aluminium oxide dust on the water to reflect sunlight. Remember that, in the 1960s, science and technology were gung-ho. We were on our way to the Moon, so what else could we not do? At that time, Americans proposed putting huge mirrors in orbit to warm the world with reflected sunshine. Australians considered painting their western coastline black, to promote convection and achieve rainfall in the interior desert. Russians hoped to divert Siberian rivers southward, so that a lack of fresh water outflow into the Arctic Ocean would reduce the sea-ice and warm the world. If human beings thought they had sufficient power over Nature to change the climate on purpose, an obvious question was whether they were doing it already, without meaning to. The climate went on cooling through the 1960s and into the early 1970s. In those days, all great windstorms and floods and droughts were blamed on global cooling. Whilst Lamb thought the cooling was probably related to natural solar variations, Reid Bryson at Wisconsin attributed the cooling to man-made dust—not the sulphates of later concern but windblown dust from farms in semi-arid areas. Lurking in the shadows was the enhanced greenhouse hypothesis. The ordinary greenhouse effect became apparent after the astronomer William Herschel in the UK discovered infrared rays in 1800. Scientists realized that molecules of water vapour, carbon dioxide and other gases in the atmosphere keep the Earth warm by absorbing infrared rays that would otherwise escape into space, in the manner of a greenhouse window. Was it not to be expected that carbon dioxide added to the air by burning fossil fuels should enhance the warming? By the early 20th century, Svante Arrhenius at Stockholm was reasoning that the slight raising of the temperature by additional carbon dioxide could be amplified by increased evaporation of water. Two developments helped to revive the greenhouse story in the 1970s. One was confirmation of a persistent year-by-year rise in the amount of carbon dioxide in the air, by measurements made on the summit of Mauna Loa, Hawaii. The other was the introduction into climate science of elaborate computer programs, called models, similar to those being used with increasing success in daily weather forecasting. The models had to be tweaked, even to simulate the present climate, but you could run them for simulated years or centuries and see what happened if you changed various factors. Syukuro Manabe of the Geophysical Fluid Dynamics 145 climate change Laboratory at Princeton was the leading pioneer. Making some simplifying assumptions about how the climate system worked Manabe calculated the consequences if carbon dioxide doubled. Like Arrhenius before him, he could get a remarkable warming, although he warned that a very small change in cloud cover could almost cancel the effect. Bert Bolin at Stockholm became an outspoken prophet of man-made global warming. ‘There is a lot of oil and there are vast amounts of coal left, and we seem to be burning it with an ever increasing rate,’ he declared in 1974. ‘And if we go on doing this, in about 50 years’ time the climate may be a few degrees warmer than today.’ He faced great scepticism, especially as the world still seemed to be cooling despite the rapid growth in fossil-fuel consumption. ‘On balance,’ Lamb wrote dismissively in 1977, ‘the effect of increased carbon dioxide on climate is almost certainly in the direction of warming but is probably much smaller than the estimates which have commonly been accepted.’ Then the ever-quirky climate intervened. In the late 1970s the global temperature trend reversed and a rewarming began. A decade after that, Bolin was chairman of an Intergovernmental Panel on Climate Change. In 1990 its report Climate Change blamed the moderate warming of the 20th century on man-made gases, and predicted a much greater warming of 38C in the 21st century, accompanied by rising sea-levels. This scenario prompted the world’s leaders to sign, just two years later, a climate convention promising to curb emissions of greenhouse gases. Thenceforward, someone or other blamed man-made global warming for every great windstorm, flood or drought, just as global cooling had been blamed for the same kinds of events, 20 years earlier. I Ever-more complex models The alarm about global warming also released funds for buying more supercomputers and intensifying the climate modelling. The USA, UK, Canada, Germany, France, Japan, China and Australia were leading countries in the development of models. Bigger and better machines were always needed, to subdivide the air and ocean in finer meshes and to calculate answers spanning 100 years in a reasonable period of computing time. As the years passed, the models became more elaborate. In the 1980s, they dealt only with possible changes in the atmosphere due to increased greenhouse gases, taking account of the effect of the land surface. By the early 1990s the very important role of the ocean was represented in ‘atmosphere–ocean general circulation models’ pioneered at Princeton. Changes in sea-ice also came into the picture. 146 ... - tailieumienphi.vn