Universe a grand tour of modern science Phần 3 docx

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

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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

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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

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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

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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.

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Next to be added was sulphate, a common form of dust in the air, and by 2001

non-sulphate dust was coming in too. The carbon cycle, in which the ocean and

the land’s vegetation and soil interact with the carbon dioxide in the air, was

coupled into the models at that time. Further refinements under development

included changes in vegetation accompanying climate change, and more subtle

aspects of air chemistry.

Such was the state of play with the largest and most comprehensive climate

models. In addition there were many smaller and simplified models to explore

various scenarios for the emission of greenhouse gases, or to try out new

subroutines for dealing with particular elements in the natural climate system.

But the modellers were in a predicament. The more realistic they tried to make

their software, by adding extra features of the natural climate system, the

greater the possible range of errors in the computations.

Despite the huge effort, the most conspicuous difficulty with the models was

that they could give very different answers, about the intensity and rate of global

warming, and about the regional consequences. In 1996, the Intergovernmental

Panel promised to narrow the uncertainties in the predictions, but the reverse

happened. Further studies suggested that the sensitivity of the climate to a

doubling of carbon dioxide in the atmosphere could be anything from less than

18C to more than 98C. The grand old man of climate modelling, Syukuro

Manabe, commented in 1998, ‘It has become very urgent to reduce the large

current uncertainty in the quantitative projection of future climate change.’

I Fresh thinking in prospect

The reckoning also takes into account the natural agents of climate change,

which may have warming or cooling effects. One contributor is the Sun, and

there were differences of opinion about its role. After satellite measurements

showed only very small variations in solar brightness, it seemed to many experts

that any part played by the Sun in global warming was necessarily much less

than the calculated effect of carbon dioxide and other greenhouse gases. On the

other hand, solar–terrestrial physicists suggested possible mechanisms that could

amplify the effects of changes in the Sun’s behaviour.

The solar protagonists included experts at the Harvard-Smithsonian Center for

Astrophysics, the Max-Planck-Institut fu¨r Aeronomie, Imperial College London,

Leicester University and the Dansk Rumforskningsinstitut. They offered a variety

of ways in which variations in the Sun’s behaviour could influence the Earth’s

climate, via visible, infrared or ultraviolet light, via waves in the atmosphere

perturbed by solar emissions, or via effects of cosmic rays. And there was no

disputing that the Sun was more agitated towards the end the 20th century than

it had been at the cooler start.

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A chance for fresh thinking came in 2001. The USA withdrew from the

negotiations about greenhouse gas emissions, while continuing to support the

world’s largest research effort on climate change. Donald Kennedy, editor-in￾chief of Science magazine, protested, ‘Mr. President, on this one the science

is clear.’

Yet just a few months later a committee of the US National Academy of

Sciences concluded: ‘Because of the large and still uncertain level of natural

variability inherent in the climate record and the uncertainties in the time

histories of the various forcing agents (and particularly aerosols), a causal linkage

between the build-up of greenhouse gases in the atmosphere and the observed

climate changes during the 20th century cannot be unequivocally established.’

At least in the USA there was no longer any risk that scientists with

governmental funding might feel encouraged or obliged to try to confirm a

particular political message. And by the end of 2002 even the editors of Science

felt free to admit: ‘As more and more wiggles matching the waxing and waning

of the Sun show up in records of past climate, researchers are grudgingly taking

the Sun seriously as a factor in climate change.’

Until then the Intergovernmental Panel on Climate Change had been headed by

individuals openly committed to the enhanced greenhouse hypothesis—first

Bert Bolin at Stockholm and then Robert Watson at the World Bank. When

Watson was deposed as chairman in 2002 he declared, ‘I’m willing to stay in

there, working as hard as possible, making sure the findings of the very best

scientists in the world are taken seriously by government, industry and by

society as a whole.’ That remark illustrated both the technical complacency and

the political advocacy that cost him his job.

His successor, by a vote of 76 to 49 of the participating governments, was

Rajendra Pachauri of the Tata Energy Research Institute in New Delhi. ‘We

listen to everyone but that doesn’t mean that we accept what everyone tells us,’

Pachauri said. ‘Ultimately this has to be an objective, fair and intellectually

honest exercise. But we certainly don’t prescribe any set of actions.’ The

Australian secretary of the panel, Geoff Love, chimed in: ‘We will be trying to

encourage the critical community as well as the community that believes that

greenhouse is a major problem.’

E The link between carbon dioxide and climate is further examined in Carbon cycle. For

more about ice and climate change, see Cryosphere. Uncertainties about the workings

of the ocean appear in Ocean currents. Aspects of the climatic effects of the variable

Sun appear in Earthshine and Ice-rafting events. Natural drivers of brief climate

change are El Nino˜ and Volcanic explosions.

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