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When the young Carl Linneaus set off on his journey of botanical discovery to Lappland in 1732, he left on foot from his home in Uppsala. He didn't wait until he reached his destination to start making observations, but found interesting things to think about all along the way, even in the plant life at his doorstep. So it is with climate as well.

To the discerning and sufficiently curious observer, a glance out the window, a walk through the woods or town, a short sail on the ocean, all raise profound questions about the physics of climate. Even without a thermometer, we have a perception of "heat" or "temperature" by examining the physical and chemical transitions of the matter around us. In the summertime, ice cream will melt when left out in the sun, but steel cooking pots don't. Trees and grass do not spontaneously burst into flame every afternoon, and a glass of water left outdoors in the summer does not boil. Away from the tropical regions, it often gets cold enough for water to freeze in the wintertime, but hardly ever cold enough for alcohol to freeze. What is it that heats the Earth? Is it really the Sun, as seems intuitive from the perception of warmth on a sunny day? In that case, what keeps the Earth from just accumulating more and more energy from the Sun each day, heating up until it melts? For that matter, why don't temperatures plummet to frigid wintery values every night when the Sun goes down? Similarly, what limits how cold it gets during the winter?

With the aid of a thermometer, such questions can be expressed quantitatively. The first, and still most familiar, kinds of thermometers were based on on particular reproducible and measurable effect of temperature on matter - the expansion of matter as it heats up. Because living things are composed largely of liquid water, the states of water provide a natural reference on which to build a temperature scale. The Celsius temperature scale divides the range of temperature between the freezing point of pure water and the boiling point at sea level into 100 equal steps, with zero being at the freezing point and 100C at the boiling point 1.

Through observations of fire and forge, even the ancients were aware that conditions could be much hotter than the range of temperatures experienced in the normal course of climate. However, they could have had no real awareness of how much colder things could get. That had to await the theoretical insights provided by the development of thermodynamics in the nineteenth century, followed by the invention of the refrigeration by Carl von Linde not long afterwards. By the close of the century, temperatures low enough to liquify air had been achieved. This was still not as low as temperatures could go. The theoretical and experimental developments of the nineteenth century consolidated earlier speculations that there is an absolute zero of temperature, at which random molecular motions cease and the volume of an ideal gas would collapse to zero; no temperature could go below this absolute zero. On the Celsius scale, absolute zero occurs at —273.15C. Most of thermodynamics and radiation physics can be expressed more cleanly if temperatures are given relative to absolute zero, which led to the formulation of the Kelvin temperature scale, which shifts the zero of the scale while keeping the size of the degrees the same as on the Celsius scale. On the Kelvin scale, absolute zero is at zero degrees, the freezing point of water is at 273.15K, and the sea-level boiling point of water is at 373.15K. Viewed on the Kelvin scale, the temperature range of Earth's climate seems quite impressively narrow. It amounts to approximately a ±10% variation about a typical temperature of 285K. A 20% variation in the Earth's temperature (as viewed on the Kelvin scale) would be quite catastrophic for life as we know it. This remark can be encapsulated in a saying: "Physics may work in degrees Kelvin, but Earth life works in degrees Celsius,"

There is more to climate than temperature. Climate is also characterized by the amount and distribution of precipitation (rainfall and snowfall), as well as patterns of atmospheric winds and oceanic currents. However, temperature will do for starters. In this book we will discuss temperature at considerable length, and venture to a somewhat lesser degree into the factors governing the amount of precipitation. We will not say much about wind patterns, though some of their effects on the temperature distribution will be discussed in Chapter 9.

If you live outside the tropical zone, you will come to wonder why it is hotter in summer than in winter, and why the summer/winter temperature range has the value that it does (e.g. 30C in Chicago) and why the variation is generally lower over the oceans (e.g. 7C in the middle of the Pacific Ocean, at the same latitude as Chicago). If you communicate with friends living in the Arctic or Antarctic regions, and other friends living near the Equator, you will begin to wonder why, on average, it is warmer near the Equator than in the polar regions, and why the temperature difference has the value it does (e.g. 40C difference between the annual average around the Equator vs. the annual average at the North Pole). The physics underlying the seasonal cycle and the pole

1 The scale is named for the Swedish astronomer Anders Celsius, who originally formulated a similar temperature scale in 1742. Celsius' scale was reversed relative to the modern one, putting 100 at the freezing point and zero at the boiling point. The Celsius scale is sometimes called centigrade, but Celsius is considered to be the preferred term. The official definition of the temperature scale is now based on standards that are more precise and unambiguous than the freezing and boiling point of water.

to equator temperature gradient is discussed in Chapters 7 and 9. If you climb a mountain (or even observe the snow-capped peaks of a mountain from the valley floor on a hot summer day), or if you go up in a hot-air balloon, or fly in an airliner which informs you of the outdoor temperature - you will notice that the air gets colder as one goes higher in altitude? Why should this be? This turns out to be a general feature of planetary atmospheres, and the basic physics underlying the phenomenon is discussed in Chapter 2.

The air that surrounds us is itself a matter of interest. We know that it is there because it has a temperature, exerts pressure, and because it is necessary that we breathe it in order to remain alive. But what is the air made of, and why does it have the composition it does? We can see water condense out of the air, but why don't other components condense in the course of natural weather and climate variations? How much air is there? And has it always been there with its present composition, or has it changed over time? If so, how much and how quickly?

We know that our planet journeys through the hard vacuum of outer space, clothed in a thin blanket of air - our atmosphere. It is natural to wonder how our atmosphere affects the Earth's climate. The airless moon shares the same orbit of the Earth, at the same distance from the Sun, so one can look to the Moon to get an idea of what the Earth's climate would be like if it had no atmosphere. We know the moon is airless because a reasonably thick atmosphere would bend the light rays from the Sun and stars, just as objects appear displaced when viewed through the surface of a swimming pool. But how to measure its temperature?

Of course, one could go there with a thermometer (and this did eventually happen) but people became curious about Lunar conditions long before it seemed likely that anybody would ever get there. Dante Alighieri himself, in the Paridiso written between 1308 and 1321, devoted fully one hundred cantos to a learned discussion between himself and Beatrice concerning the source of Lunar light and the solidity of the Lunar surface. By the mid nineteenth century, science had progressed to the point that the questions could be formulated more sharply, and the means for an answer had begun to emerge. With the discovery of infrared light by Sir William Herschel in 1800, astronomy opened a new window into the properties of planets and stars. Over the coming decades, it gradually became clear that all bodies emit radiation according to their temperature. This is known as blackbody radiation and will discussed in detail in Chapter 3. Infrared light from the Moon was detected by Charles Piazzi Smyth in 1856, and the first attempt to use it to estimate temperature was by the Fourth Earle of Rosse in 1870. The instruments available at the time were not up to the task. In 1878, Langley invented the bolometer, which made good observations of Lunar infrared possible. However, while Langley made the first accurate observations of Lunar infrared, theory was not quite up to the task of interpreting the observations. These issues were largely sorted out by 1913, though Langley gave up on his earlier estimates rather reluctantly. By 1913 it was pretty clear that the daytime temperature of the Moon at the point where the Sun is directly overhead is well in excess of 373K (the sea-level temperature of boiling water on Earth). Night-time temperatures were harder to determine accurately, since the infrared emission from cold objects is weak; however it was clear that temperatures at night dropped by well over 140K relative to the daytime peak. Pettit and Nicholson observed the temperature of the Moon during the Lunar eclipse of 1927, using the Mt. Wilson telescope. They found something even more remarkable: over the span of the few hours of the eclipse, the Lunar temperature fell from 342K at the point of observation to 175K. Modern measurements show the daily average temperature at the Lunar equator to be around 220K, while the mean temperature at 85N latitude is 130K

It appears that without an atmosphere or ocean, the Earth would be subject to extreme swings of temperature between day and night. The Moon's "day" is 28 Earth days, since it always shows the same face to the Earth; on that basis, one could imagine that the day/night extremes were due to a longer night offering more time to cool down, but the rapid cooling during an eclipse gives the lie to this idea. Given the rapid cooling of an airless body at night, it is likely that the Earth's summer/winter temperature difference would be far more extreme in the absence of an atmosphere. Further, a comparison of the pole to equator gradient in daily mean temperature with that on Earth suggests that the atmosphere significantly moderates this gradient, too. What is it about the atmosphere or ocean that damps down day/night or summer/winter swings in temperature? This subject will be taken up in Chapter 7, where we'll also learn why summer is warmer than winter and why the poles are on the average colder than the Equator. How does an atmosphere or ocean moderate the temperature difference between pole and equator? We'll learn something of that in Chapter 9.

At its hottest the Moon gets much hotter than Earth, and at its coldest it gets much colder. But how does the Moon's mean temperature stack up against that of Earth? The 220K mean equatorial temperature of the Moon is very much colder than the observed mean tropical temperature on Earth, which is on the order of 300K. If the Earth's mean temperature were as low as that of the Moon, the oceans would be solidly frozen over. The cold mean temperature of the Moon does not come about because the Moon reflects more sunlight than the Earth; the Moon looks silvery but measurements show that it actually reflects less than Earth. Why is the Earth, on average, so much warmer than the Moon? Does this have something to do with our atmosphere, or is it the case that Earth is warmed by some internal heat source that the Moon lacks?

The search for the first stirrings of an answer to this problem takes us back to 1827, when Fourier published his seminal treatise on the temperature of the Earth. Fourier could not have known anything about the temperature of the Moon, but he did know a great deal about heat transfer - having in fact largely invented the subject. Using his new theory of heat conduction in solids, Fourier analyzed data on the rate at which average temperature increases as one descends deeper below the Earth's surface ; he also analyzed the attenuation of day/night or summer/winter temperature fluctuations with depth. (Fourier's solution for the latter problem will be derived in Chapter 7.) Based on these analyses, Fourier concluded that the flow of heat outward from the interior of the Earth was utterly insignificant in comparison to the heat received from the Sun. We'll see shortly that this situation applies to other rocky planets as well: dry rock is a good insulator, and doesn't let internal heat out very easily.

If the Earth is continually absorbing solar energy, it must also have some way of getting rid of it. Otherwise the energy would have accumulated over the past eons, leading to a molten, incandescent uninhabitable planet (see Problem ??) - which is manifestly not the case. Fourier seems to have known that there was little or no matter in the space through which planets plied their orbits, and so he posited that planets lose heat almost exclusively through emission of infrared radiation (called "dark heat" at the time). 2 He also knew that the rate of emission of "dark heat" increased with temperature, which provided a means for an equilibrium temperature to be achieved: a planet would simply heat up until it radiated infrared energy at the same rate as it received energy from the Sun. Finally, Fourier refers to experiments showing that something in the atmosphere emits infrared radiation downward toward the ground, and seems to have been aware also of the fact that something in the atmosphere absorbs infrared. Based on these somewhat sketchy observations, Fourier inferred that the Earth's atmosphere retards the emission of infrared to space, allowing it to be warmer than it would be if it were airless.

Fourier's treatise made it clear the the thermal emission of infrared light was not just useful for astronomical observations - it was in fact part and parcel of the operation of planetary climate.

2Fourier also refers to the importance of heating from what he calls the "temperature of space." It is unclear whether he thought there was some substance in space that could conduct heat to the atmosphere, or whether he was referring to some invisible radiation which pervades space. His inferences regarding the importance of this factor were erroneous — the only real error in an otherwise remarkable paper.

At Fourier's time the state of understanding of infrared radiation emission was not sufficiently developed as to allow him to complete the calculation he set up. Nonetheless, he correctly formulated the problem of terrestrial temperature as one of achieving a balance between the rate at which solar radiation is absorbed and the rate at which infrared is emitted. With this great insight, the modern era of study of planetary temperature had begun. Fleshing out the "details," however, required major advances in several areas of fundamental physics. The basic principles of planetary energy balance, and of the manner in which an atmosphere increases planetary temperature, are introduced in Chapter 3 and elaborated on in the earlier parts of Chapter 4.

One of the many details that needed to be settled was the question of which components of the atmosphere affected the transmission of infrared radiation. In 1859 Tyndall found that the dominant components of the Earth's atmosphere - nitrogen and oxygen - are very nearly transparent to infrared radiation. He found instead that it was two relatively minor constituents - water vapor and CO2 - which accounted for most of the infrared absorption and emission by Earth's air. Gases of this sort, which let solar energy through virtually unimpeded but strongly retard the outward loss of infrared radiation, are known as "greenhouse gases." Their warming effect on the lower portions of a planet's atmosphere, and on its surface (if it has one) is called the "greenhouse effect." The term was not coined by Fourier, and in some ways is misleading, since real greenhouses do not work by blocking infrared emission. However, the glass or plastic enclosure of a real greenhouse does warm the interior by reducing heat loss to the environment while allowing solar heating, and in that sense - viewed as a broader metaphor for the implications of energy balance - the analogy is apt. Besides CO2 and water vapor, we now know of a number of additional greenhouse gases, including CH4 (methane), which may have played a very important role on the Early Earth, and plays some role even today. In fact, it turns out that in some very dense atmospheres such as that of Titan, even nitrogen can become a greenhouse gas. What determines whether a molecule is or is not a good greenhouse gas, and how do we characterize the effects of individual gases, and thus the influence of atmospheric composition on climate? These questions will be taken up in the latter half of Chapter 4.

In thinking about the effect of greenhouse gases on climate, it is important to distinguish between long-lived greenhouse gases which are removed slowly from the atmosphere on a time scale of thousands of years or more, and short-lived greenhouse gases which are removed on a time scale of weeks to years by condensation or rapid chemical reactions. The short-lived greenhouse gases act primarily as a feedback mechanism. Their concentration adjusts rapidly to other changes in the climate, serving to amplify or offset climate changes caused by other factors - including changes due to long-lived greenhouse gases. Long-lived greenhouse gases can also participate in feedbacks, but only on time scales longer than their typical atmospheric adjustment time. Whether a greenhouse gas is long-lived or short-lived depends on environmental conditions. On the Earth, CO2 is a long-lived greenhouse gas but water vapor is a short-lived greenhouse gas; however, on Mars, which gets cold enough for CO2 to condense, that gas can be considered short-lived.

Greenhouse gases are largely invisible, but the atmosphere also holds a readily visible component that exerts a profound influence over our planet's energy balance - the clouds. Clouds on Earth are composed of suspended droplets of condensed water, in the form of liquid or ice. Clouds, like water vapor, act as a short-lived greenhouse gas affecting the rate at which infrared can escape to space. The infrared opacity of clouds is used routinely in weather satellites, since this property makes cloud patterns visible from space even on the night side of the Earth. However, clouds affect the other side of the energy balance as well, because cloud particles quite effectively reflect sunlight back to space. The two competing effects of clouds are individually large, but partly offset each other, so that small errors in one or the other term lead to large errors in the net effect of clouds on climate. Moreover, the effect of clouds on the energy budget depends on all the intricacies of the physics that determine things like particle size and how much condensed water remains in suspension. For this reason, clouds pose a very severe challenge to the understanding of climate. This is the case not just for Earth, but for virtually any planet with an atmosphere. The physics underlying the effects of clouds on both sides of the radiation balance will be discussed in Chapters 4 and 5

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