快猫短视频

NEXT time you complain that the weather is dreadful, take comfort from this
fact. You鈥檙e right, but it can鈥檛 get any worse. Not on a global scale anyhow.
According to a controversial new theory, Earth鈥檚 climate is finely tuned to be
as violent as possible. It whips up every last raindrop, wave and gust of wind
that it can.

No one knows why the climate should be like this. It鈥檚 just an observation,
and for that reason some meteorologists have dismissed it as a coincidence. But
recent evidence suggests that it also holds true for other planets. If so, it
could turn out to be a universal principle that helps us to understand the
climate on Earth and other planets now, in the past, and into the future.

Earth鈥檚 climate is a hugely complex system, with multiple interacting
elements. Because they defy simple expression, meteorologists rely on a 鈥渂ottom
up鈥� approach to modelling the climate. They divide the world into boxes, model
the forces on the atmosphere and oceans in each box, then guess the overall
effect. The approach works鈥攑roviding you toss in hundreds of 鈥渇udge
factors鈥� to make the models fit with what we actually see happening. Try to
apply the model to a different time in Earth鈥檚 history, or a different planet,
and it falls to pieces.

But beneath this complexity is a single force: the Sun. Solar energy warms
the Earth, and it does so disproportionately, heating the tropics much more than
the poles. This creates a temperature gradient which drives the climate. The net
effect is to transport heat from low to high latitudes.

Heat flow from the tropics to the poles is an expression of one of the
fundamental rules of nature. This is the second law of thermodynamics, which
states that heat always flows spontaneously from warm to cold
regions鈥攏ever the opposite. Put a hot brick next to a cold one in an
insulated box and wait, and you get two cool bricks.

One of the consequences of this process is the ability to do mechanical work.
You could rig up a contraption to convert the heat flow from brick to brick into
electricity. The same is true of Earth鈥檚 climate. Our atmosphere and oceans are
hard at work every moment. They鈥檙e busy lifting water to drop as rain or snow,
whipping up waves and blowing sand into dunes. Wind turbines, sailing boats and
hydroelectric power stations all depend on this hard work.

So far so good. But the second law of thermodynamics doesn鈥檛 tell us how
quickly the heat is transported. In other words, it doesn鈥檛 say how hard the
climate works. It would be nice to know, if only to give us an alternative, 鈥渢op
down鈥� approach to meteorological modelling.

We can start by asking how quickly heat flows from the tropics to the poles.
One thing we can say without too much difficulty is that it is neither as fast
nor as slow as possible. If the heat flow was very vigorous, the temperatures of
the zones would be almost the same, since the flows would balance out the
different amounts of sunlight. That鈥檚 obviously not what happens. On the other
hand, if heat flow was very slow, the temperature difference would be enormous.
Northern Europe, New Zealand and the southern part of South America would be
locked in a perpetual ice age.

The reality is somewhere in between. But where? The new idea is that heat
flow adjusts itself so that the climate does the maximum amount of mechanical
work possible.

Work is determined by two factors. First of all there鈥檚 heat flow. With very
weak flows, there is little energy to convert into work. As the heat flow
increases, the amount of work also increases鈥攂ut then it starts to tail
off. That鈥檚 because of the second factor, efficiency. The efficiency with which
heat flow is converted into work is proportional to the temperature gradient.
Very rapid heat flow flattens out the temperature gradient, so little work gets
done.

Disorderly conduct

Like most theories, the idea has been put forward before in other forms. Most
scientists credit Garth Paltridge, an Australian climatologist now at the
University of Tasmania, as the originator of the idea. In the mid-1970s he
experimented with a model of the Earth divided into latitude zones with
different amounts of sunlight and cloud cover. One of the free parameters of the
model was the heat flow between the zones. Paltridge found that if he set this
so it maximised the production of the thermodynamic quantity called entropy, the
results modelled Earth鈥檚 climate well. Entropy production is a measure of the
generation of disorder, and it is closely related to a system鈥檚 capacity for
mechanical work. Both quantities peak at about the same heat flow.

Several other researchers have since confirmed Paltridge鈥檚 result. Yet most
orthodox meteorologists dismiss it as an uninteresting fluke. There鈥檚 no reason
why the climate should maximise entropy, or the work done.

That argument might hold if Earth was the only place the principle works. But
new research suggests it鈥檚 not. With Jonathan Lunine and Paul Withers of the
University of Arizona and Chris McKay at NASA Ames Research Center in Moffett
Field, California, I have shown that Saturn鈥檚 giant moon Titan seems to follow
the principle too.

In a way I found that out by accident. In December 1999 I was supposed to be
working on new data from the Mars Polar Lander, but the craft was lost without
trace, so I was looking around for another problem. At the time I was also
starting to think about what the Cassini mission to Saturn might tell us about
Titan. I decided to take a look at Titan鈥檚 climate.

In 1980, the Voyager 1 spacecraft made some curious measurements of Titan鈥檚
surface temperature. These showed that the moon was intensely cold鈥攁
frigid 鈥�179 掳C at the equator and 4 掳C colder at the poles. No
surprise, given how far Titan is from the Sun. But the temperature gradient was
strange. Conventional meteorological models suggest the difference should be
much less鈥攁bout one-hundredth of a degree.

According to the old models, heat transport depends on three factors: the
size of the planet or moon, its rate of rotation, and the thickness of its
atmosphere. Judged by all these criteria, Titan should transport heat very
quickly. It鈥檚 small. It has a nitrogen atmosphere thicker than the Earth鈥檚. And
it rotates very slowly, so the atmosphere isn鈥檛 whipped into swirling
latitudinal weather systems that interfere with heat transfer.

To find out more, I wrote a computer model to simulate Titan鈥檚 climate. When
I programmed in rapid heat transfer from the equator to the poles the model
refused to settle down. But when I slowed the heat transfer right down the model
stabilised. What鈥檚 more, I could recreate the 4 掳C temperature difference
seen by Voyager. Then came a crucial discovery. I looked at the entropy
production and this peaked just where the temperature difference was 4 掳C.
In other words, Titan, like the Earth, squeezes as much work out of its climate
as it can.

It isn鈥檛 clear why heat transport on Titan should be so low. Perhaps the
atmosphere has strong east-west winds that suppress heat transport towards the
poles. Measurements from the Cassini spacecraft, due to arrive in 2004, should
tell us more.

But the result raised a wider question. If the principle worked for Earth and
Titan, would it work anywhere else? Would it turn out to be a universal feature
of planetary climates? Mars was an obvious place to try next. I looked at
conventional models of the Martian climate and they all assumed that heat
transfer would be very weak, since the atmosphere is so thin. They also did a
good job of modelling the planet鈥檚 climate, which was a problem. For my theory
to hold, the atmosphere would have to transport a hundred times more heat. The
disagreement looked insoluble.

But then I noticed something. None of the studies accounted for the 鈥渓atent
heat鈥� associated with Mars鈥檚 polar ice caps, huge expanses of frozen carbon
dioxide that wax and wane with the seasons. They assumed that the ice was always
at 鈥�103 掳C, the freezing point of CO₂. Even if the models
predicted that the polar temperature would drop below the freezing point, the
researchers just fudged it and held it steady.

Freezing and thawing, though, involves the exchange of heat. Take a glass of
water and cool it at a constant rate. When it hits 0 掳C it鈥檒l start to
freeze, but stay at a steady temperature until all the water has turned to ice.
That鈥檚 because the process of freezing unlocks energy stored in liquid water.
The same happens when Mars鈥檚 ice caps solidify. If you calculate how much heat
is exchanged, it is more or less what is predicted by the new theory. In other
words, the principle of maximum work鈥攐r maximum entropy production, to
give it its formal name鈥攈olds true for Mars too.

Too good to be true

There are places where the theory won鈥檛 work. On Mercury the atmosphere is so
thin that it can scarcely transport any heat at all. To reach the point of
maximum entropy production the wind speed would have to exceed its physical
limit. But constrained climates like this are easy to model anyway.

If the principle is universal on planets with thicker atmospheres, it could
prove very useful. It means we would know something about almost any planetary
climate, such as that of early Mars when life may have existed there, or of
potentially inhabited planets around other stars.

In a way it seems to be too good to be true, and the idea certainly has its
opponents. 快猫短视频s whose careers are built on large, sophisticated computer
models are reluctant to believe that such a simple theory can be true. Many
dismiss the Titan example on the grounds that Voyager鈥檚 temperature measurements
were inaccurate.

They may have a point. The Voyager measurements were made with an infrared
detector which could have underestimated the surface temperature at the poles.
The problem is that part of the infrared signal came from high up in the moon鈥檚
stratosphere, where polar temperatures are known to be cooler. True, all the
workers who analysed the data included corrections for the stratospheric signal,
and believe that the temperature gradient is about right. But we can鈥檛 be
absolutely sure until Cassini measures the gradient with microwaves, which are
unaffected by stratospheric interference.

There鈥檚 another serious objection to overcome. No one knows why the theory
works. At the moment I鈥檓 not sure either. But I鈥檓 working on it.

In the meantime we might as well play around and see what the theory tells
us. One obvious application is the study of the distant past, in particular the
idea of a 鈥淪nowball Earth鈥� 700 million years ago when the entire globe was
covered in ice. Models have shown that once the Earth is sufficiently covered by
ice, it will reflect so much sunlight it triggers 鈥渞unaway glaciation鈥�, freezing
the whole planet. How could life have survived? Existing climate models cannot
agree. We do not know the layout of the continents back then, nor other details
that are needed for accurate simulations.

Enter the theory of maximum entropy production. If it is universal, it must
have been doing its stuff 700 million years ago. And one of the consequences is
that the colder the Earth is as a whole, the weaker the heat transport from the
tropics to the poles. Weaker heat transport means there might have been an
unfrozen refuge, which may be how life survived.

And planetary climatology may not be the only application. There are hints
that heat flows inside planets also stick to the principle. The theory may also
be useful for studying other dynamic systems such as stars or the protoplanetary
discs in which planets form. And if we ever want to design a ship for exploring
another world, we鈥檇 at least know to build it well. Because it sounds like it鈥檚
going to be a rough ride out there.