Universe a grand tour of modern science Phần 2

biosphere from space By the time that report was published, in 2000, Sellers was in training as a NASA astronaut, so as to observe the biosphere from the International Space Station. The systematic monitoring of the land’s vegetation by unmanned spacecraft already spanned two decades. Tucker collaborated with a team at Boston University that quarried the vast amounts of data accumulated daily over that period, to investigate long-term changes. Between 1981 and 1999 the plainest trend in vegetation seen from space was towards longer growing seasons and more vigorous growth. The most dramatic effects were in Eurasia at latitudes above 40 degrees north, meaning roughly the line from Naples to Beijing. The vegetation increased not in area, but in density. The greening was most evident in the forests and woodland that cover a broad swath of land at mid-latitudes from central Europe and across the entire width of Russia to the Far East. On average, the first leaves of spring were appearing a week earlier at the end of the period, and autumn was delayed by ten days. At the same mid-latitudes in North America, the satellite data showed extra growth in New England’s forests, and grasslands of the upper Midwest. Otherwise the changes were scrappier than in Eurasia, and the extension of the growing season was somewhat shorter. ‘We saw that year to year changes in growth and duration of the growing season of northern vegetation are tightly linked to year to year changes in temperature,’ said Liming Zhou of Boston. I The colour of the sea Life on land is about twice as productive as life in the sea, hectare for hectare, but the oceans are about twice as big. Being useful only on terra firma, the satellite vegetation index therefore covered barely half of the biosphere. For the rest, you have to gauge from space the productivity of the ‘grass’ of the sea, the microscopic green algae of the phytoplankton, drifting in the surface waters lit by the Sun. Research ships can sample the algae only locally and occasionally, so satellite measurements were needed even more badly than on land. Estimates of ocean productivity differed not by percentage points but by a factor of six times from the lowest to the highest. The infrared glow of plants on land is not seen in the marine plants that float beneath the sea surface. Instead the space scientists had to look at the visible colour of the sea. ‘In flying from Plymouth to the western mackerel grounds we passed over a sharp line separating the green water of the Channel from the deep blue of the Atlantic,’ Alister Hardy of Oxford recorded in 1956. With the benefit of an aircraft’s altitude, this noted marine biologist saw phenomena known to fishermen down the ages—namely that the most fertile water is green and 65 biosphere from space murky, and that the transition can be sudden. The boundary near the mouth of the English Channel marks the onset of fertilization by nutrients brought to the surface by the churning action of tidal currents. In 1978 the US satellite Nimbus-7 went into orbit carrying a variety of experimental instruments for remote sensing of the Earth. Among them was a Coastal Zone Color Scanner, which looked for the green chlorophyll of marine plants. Despite its name, its measurements in the open ocean were more reliable than inshore, where the waters are literally muddied. In eight years of intermittent operation, the Color Scanner gave wonderful impressions of springtime blooms in the North Atlantic and North Pacific, like those seen on land by the vegetation index. New images for the textbooks showed high fertility in regions where nutrient-rich water wells up to the surface from below. The Equator turned out to be no imaginary line but a plainly visible green belt of chlorophyll separating the bluer, much less fertile regions in the tropical oceans to the north and south. But, for would-be bookkeepers of the biosphere, the Nimbus-7 observations were frustratingly unsystematic and incomplete. A fuller accounting began with the launch by NASA in 1997 of OrbView-2, the first satellite capable of gauging the entire biosphere, by both sea and land. An oddly named instrument, SeaWiFS, combined the red and infrared sensors needed for the vegetation index on land with an improved sea-colour scanner. SeaWiFS surveyed the whole world every two days. After three years the scientists were ready to announce the net primary productivity of all the world’s plants, marine and terrestrial, deduced from the satellite data. The answer was 111 to 117 billion tonnes of carbon downloaded from the air and fixed by photosynthesis, in the course of a year, after subtracting the carbon that the plants’ respiration returned promptly to the air. The satellite’s launch coincided with a period of strong warming in the Eastern Pacific, in the El Nino event of 1997–98. During an El Nino, the tropical ocean is depleted in mineral nutrients needed for life, hence the lower global figure in the SeaWiFS results. The higher figure was from the subsequent period of Pacific cooling: a La Nina. Between 1997 and 2000, ocean productivity increased by almost ten per cent, from 54 to 59 billion tonnes per year. In the same period the total productivity on land increased only slightly, from 57 to 58 billion tonnes of fixed carbon, although the El Nino to La Nina transition brought more drastic changes from region to region. North–south differences were already known from space observations of vegetation ashore. The sheer extent of the northern lands explains the strong seasonal drawdown of carbon dioxide from the air by plants growing there in the northern summer. But the SeaWiFS results showed that summer productivity 66 biosphere from space is higher also in the northern Atlantic and Pacific than in the more spacious Southern Ocean. The blooms are more intense. ‘The summer blooms in the southern hemisphere are limited by light and by a chronic shortage of essential nutrients, especially iron,’ noted Michael Behrenfeld of NASA’s Laboratory of Hydrospheric Sciences, lead author of the first report on the SeaWiFS data. ‘If the northern and southern hemispheres exhibited equivalent seasonal blooms, ocean productivity would be higher by some 9 billion tonnes of carbon.’ In that case, ocean productivity would exceed the land’s. Although uncertainties remained about the calculations for both parts of the biosphere, there was no denying the remarkable similarity in plant growth by land and by sea. Previous estimates of ocean productivity had been too low. I New slants to come The study of the biosphere as a whole is in its infancy. Before the Space Age it could not seriously begin, because you would have needed huge armies and navies of scientists, on the ground and at sea, to make the observations. By the early 21st century the political focus had shifted from Soviet grain production to the role of living systems in mopping up man-made emissions of carbon dioxide. The possible uses of augmented forests or fertilization of the oceans, for controlling carbon dioxide levels, were already of interest to treaty negotiators. In parallel with the developments in space observations of the biosphere, ecologists have developed computer models of plant productivity. Discrepancies between their results show how far there is to go. For example, in a study reported in 2000, different calculations of how much carbon dioxide was taken in by plants and soil in the south-east USA, between 1980 and 1993, disagreed not by some percentage points but by a factor of more than three. Such uncertainties undermine the attempts to make global ecology a more exact science. Improvements will come from better data, especially from observations from space of the year-to-year variability in plant growth by land and sea. These will help to pin down the effects of different factors and events. The luckycoincidence of the SeaWIFS launch and a dramatic El Nino event was a case in point. A growing number of satellites in orbit measure the vegetation index and the sea colour. Future space missions will distinguish many more wavelengths of visible and infrared light, and use slanting angles of view to amplify the data. The space scientists won’t leave unfinished the job they have started well. E See also Carbon cycle. For views on the Earth’s vegetation at ground level, see Biodiversity. For components of the biosphere hidden from cameras in space, see Extremophiles. 67 O n a visit to bell labs in New Jersey, if you met a man coming down the corridor on a unicycle it would probably be Claude Shannon, especially if he were juggling at the same time. According to his wife: ‘He had been a gymnast in college, so he was better at it than you might have thought.’ His after-hours capers were tolerated because he had come up single-handedly with two of the most consequential ideas in the history of technology, each of them roughly comparable to inventing the wheel on which he was performing. In 1937, when a 21-year-old graduate student of electrical engineering at the Massachusetts Institute of Technology, Shannon saw in simple relays—electric switches under electric control—the potential to make logical decisions. Suppose two relays represent propositions X and Y. If the switch is open, the proposition is false, and if connected it is true. Put the relays in a line, in series, then a current can flow only if X AND Y are true. But branch the circuit so that the switches operate in parallel, then if either X OR Y is true a current flows. And as Shannon pointed out in his eventual dissertation, the false/true dichotomy could equally well represent the digits 0 or 1. He wrote: ‘It is possible to perform complex mathematical operations by means of relay circuits.’ In the history of computers, Alan Turing in England and John von Neumann in the USA are rightly famous for their notions about programmable machinery, in the 1930s and 1940s when code-breaking and other military needs gave an urgency to innovation. Electric relays soon made way for thermionic valves in early computers, and then for transistors fashioned from semiconductors. The fact remains that the boy Shannon’s AND and OR gates are still the principle of the design and operation of the microchips of every digital computer, whilst the binary arithmetic of 0s and 1s now runs the working world. Shannon’s second gigantic contribution to modern life came at Bell Labs. By 1943 he realized that his 0s and 1s could represent information of kinds going far wider than logic or arithmetic. Many questions like ‘Do you love me?’ invite a simple yes or no answer, which might be communicated very economically by a single 1 or 0, a binary digit. Shannon called it a bit for short. More complicated communications—strings of text for example—require more bits. Just how many 68 bits and qubits is easily calculable, and this is a measure of the information content of a message. So you have a message of so many bits. How quickly can you send it? That depends on how many bits per second the channel of communication can handle. Thus you can rate the capacity of the channel using the same binary units, and the reckoning of messages and communication power can apply to any kind of system: printed words in a telegraph, voices on the radio, pictures on television, or even a carrier pigeon, limited by the weight it can carry and the sharpness of vision of the reader of the message. In an electromagnetic channel, the theoretical capacity in bits per second depends on the frequency range. Radio with music requires tens of kilocycles per second, whilst television pictures need megacycles. Real communications channels fall short of their theoretical capacity because of interference from outside sources and internally generated noise, but you can improve the fidelity of transmission by widening the bandwidth or sending the message more slowly. Shannon went on polishing his ideas quietly, not discussing them even with close colleagues. He was having fun, but he found writing up the work for publication quite painful. Not until 1948 did his classic paper called ‘A mathematical theory of communication’ appear. It won instant acceptance. Shannon had invented his own branch of science and was treading on nobody else’s toes. His propositions, though wholly new and surprising, were quickly digestible and then almost self-evident. The most sensational result from Shannon’s mathematics was that near-perfect communication is possible in principle if you convert the information to be sent into digital form. For example, the light wanted in a picture element of an image can be specified, not as a relative intensity, but as a number, expressed in binary digits. Instead of being roughly right, as expected in an analogue system, the intensity will be precisely right. Scientific and military systems were the first to make intensive use of Shannon’s principles. The general public became increasingly aware of the digital world through personal computers and digitized music on compact discs. By the end of the 20th century, digital radio, television and video recording were becoming widespread. Further spectacular innovations began with the marriage of computing and digital communication, to bring all the world’s information resources into your office or living room. From a requirement for survivable communications, in the aftermath of a nuclear war, came the Internet, developed as Arpanet by the US Advanced Research Project Agency. It provided a means of finding routes through a shattered telephone system where many links were unavailable. That was the origin of emails. By the mid-1980s, many computer scientists and 69 ... - tailieumienphi.vn