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雅思閱讀練習及答案：

1 There\'s a dimmer switch inside the sun thatcauses its brightness to rise and fall on timescales ofaround 100,000 years—exactly the same period asbetween ice ages on Earth. So says a physicist whohas created a computer model of our star\'s core.

2 Robert Ehrlich of George Mason University inFairfax, Virginia, modelled the effect of temperaturefluctuations in the sun\'s interior. According to thestandard view, the temperature of the sun\'s core isheld constant by the opposing pressures ofgravity and nuclear fusion. However, Ehrlich believed that slight variations should be possible.

3 He took as his starting point the work of Attila Grandpierre of the Konkoly Observatory ofthe Hungarian Academy of Sciences. In 2005, Grandpierre and a collaborator, Gábor ágoston,calculated that magnetic fields in the sun\'s core could produce small instabilities in the solarplasma. These instabilities would induce localised oscillations in temperature.

4 Ehrlich\'s model shows that whilst most of these oscillations cancel each other out, somereinforce one another and become long-lived temperature variations. The favouredfrequencies allow the sun\'s core temperature to oscillate around its average temperature of13.6 million kelvin in cycles lasting either 100,000 or 41,000 years. Ehrlich says that randominteractions within the sun\'s magnetic field could flip the fluctuations from one cycle length tothe other.

5 These two timescales are instantly recognisable to anyone familiar with Earth\'s ice ages:for the past million years, ice ages have occurred roughly every 100,000 years. Before that,they occurred roughly every 41,000 years.

6 Most scientists believe that the ice ages are the result of subtle changes in Earth\'s orbit,known as the Milankovitch cycles. One such cycle describes the way Earth\'s orbit graduallychanges shape from a circle to a slight ellipse and back again roughly every 100,000 years.The theory says this alters the amount of solar radiation that Earth receives, triggering the iceages. However, a persistent problem with this theory has been its inability to explain why theice ages changed frequency a million years ago.

7 "In Milankovitch, there is certainly no good idea why the frequency should change fromone to another," says Neil Edwards, a climatologist at the Open University in Milton Keynes, UK.Nor is the transition problem the only one the Milankovitch theory faces. Ehrlich and othercritics claim that the temperature variations caused by Milankovitch cycles are simply not bigenough to drive ice ages.

8 However, Edwards believes the small changes in solar heating produced by Milankovitchcycles are then amplified by feedback mechanisms on Earth. For example, if sea ice begins toform because of a slight cooling, carbon dioxide that would otherwise have found its way intothe atmosphere as part of the carbon cycle is locked into the ice. That weakens thegreenhouse effect and Earth grows even colder.

9 According to Edwards, there is no lack of such mechanisms. "If you add their effectstogether, there is more than enough feedback to make Milankovitch work," he says. "Theproblem now is identifying which mechanisms are at work." This is why scientists like Edwardsare not yet ready to give up on the current theory. "Milankovitch cycles give us ice agesroughly when we observe them to happen. We can calculate where we are in the cycle andcompare it with observation," he says. "I can\'t see any way of testing [Ehrlich\'s] idea to seewhere we are in the temperature oscillation."

10 Ehrlich concedes this. "If there is a way to test this theory on the sun, I can\'t think ofone that is practical," he says. That\'s because variation over 41,000 to 100,000 years is toogradual to be observed. However, there may be a way to test it in other stars: red dwarfs. Theircores are much smaller than that of the sun, and so Ehrlich believes that the oscillation periodscould be short enough to be observed. He has yet to calculate the precise period or theextent of variation in brightness to be expected.

11 Nigel Weiss, a solar physicist at the University of Cambridge, is far from convinced. Hedescribes Ehrlich\'s claims as "utterly implausible". Ehrlich counters that Weiss\'s opinion isbased on the standard solar model, which fails to take into account the magnetic instabilitiesthat cause the temperature fluctuations.

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