Climate science after Kyoto

With the Official Science monkey gone from our backs, what can we do with climate and climate science?

Over the last year, I've outlined on this blog a set of open questions that are frequently ignored and often not even seen correctly as questions, but that need to be answered if we're going to talk sense about "climate" and "climate change." Not able to predict weather beyond about two weeks ahead, we need a simplified and abstracted definition of "climate" whose "state" can be defined, analyzed, and predicted with some confidence. The fallacies of temperature averaging and "climate parameterizations" were a failed attempt to do this. Whatever notions of "climate," "climate state," and "change of climate state" we end up with will have to withstand - as temperature averages and the "hockey stick" cannot - probing criticism and emerge free of the hefty list of fallacies associated with the "global warming" hysteria.

In retrospect, the generational dead-end of climate and allied sciences with "global warming" and general circulation models (GCMs) can be viewed as an attempt to force a premature integration of theory and observation. It would be best for all involved if theory and observation were to remain aware of one another, but go their separate ways until such time as they have sufficient means to meet each other honestly. Until such a synthesis is possible, it's best to maintain a pluralistic and agnostic attitude about the Big Picture, resisting the forces bent on our "salvation" from the "evils" of industrial civilization or plying us with supposed alternatives to "knowing" nature.

What follows is a personal and partial view, informed by this failure to get a grip on these issues and by my own scientific experience along the edges of the problem and in related fields. It should not be taken as definitive or complete.

Theory: It's a really hard problem. The Earth's climate is the most complex scientific problem ever posed and almost certainly unsolvable in its full generality. Any progress we make with this problem will therefore necessarily involve approximations. The essential point is that, if we're going to attempt actual predictions, we need better and controlled approximations at every step. These are currently lacking.

Theory: Science is hypothesis and deduction. That is, it's not just a piling up of facts. Certainty of conclusions requires control of assumptions and reasoning.

There is thus an important role for mathematical deductive modeling, with simplified and controlled approximations applied at every step. The ideal should be to make these mathematically simplified and controlled models closer and closer, with each step, to the real climate. The failure of the "parameterized" GCM approach underscores the need to keep the modeling within controlled approximations at each step and not jump into the deep end of the pool right away.

Theory: Don't BS - simplify and smooth. A revealing way to look at the "climate state" problem is to grasp the motive behind "climate parameterizations": it was to "force closure" on the dynamical-structural equations of climate. In general, there are never enough equations to match the number of unknown variables. "Forcing closure" on the system means guessing or making up extra equations to close the gap.

But the gap could equally well be closed the other way: reduce the number of variables. A simplified "climate state," less complex than "the exact, instantaneous state of the whole atmosphere," is just such a proposal. It's also likely that such a state will not only involve flows and topology in space, but time and space integrals of the basic variables (equivalent to what statisticians call cumulants). Such integrals are usually better-behaved than the original variables.

Theory: Boil, mist, and trouble. Climate is chaotic, in the technical sense: exponential sensitivity to errors in initial conditions. Alternatively, climate is essentially nonperiodic, and not all climate disturbances die away. The atmosphere is a fluid, in the physicist's sense; its chaos is turbulence. Turbulence is the largest unsolved problem in physics. A partial or complete solution would have immediate impact on many areas of science and engineering, pure and applied, theoretical and practical - everything from understanding convection in planetary atmospheres and stars to improving your airplane or boat ride to reducing turbulence losses in your car engine.*

Climate needs new and better techniques for coping with chaos. Many such techniques have been developed in the last 25 years in various areas of science, but they haven't penetrated far into the climate world, partly because of the paralysis induced by Official Science. They include exceptionally relevant techniques like the following.

• Renormalization. This technique is a powerful generalization of the dimensional analysis we learned in school. (It's sometimes goes under the guise of "homology" or "rescaling.") It relates one mathematical problem posed at one set of space and time scales to a different problem at a different set of scales. Sometimes, impossible problems posed at one scales can be recast into other problems at different scales, and those different problems are solvable, either exactly or by controlled approximation.
  
  Renormalization for climate means imagining a scale at which decades, centuries, or even millennia seem modest and slow cycles like El Niño, say, wink by in rapid succession. On those scales, we can see more clearly the invariant and almost-invariant structure that must define, at a deeper level than everyday weather, what "climate" is.
  
  
• Dynamical reconstruction of phase space. This requires some contact with observed climate (see below), but the essential technique amounts to isolating the relevant degrees of freedom in the very complex climate system, the ones that operate on scales of tens to thousands of miles. Only a small subset of the possible changes in the climate system are actually important. Isolating them is a big step toward defining "climate" in a simplified sense. It will undoubtedly involve flows of heat, water, etc. (not local temperatures or humidities) and how they're connected in space (their topology).
  
  
• Non-Gaussian statistics, for extreme weather analysis. This is an application of the great progress that has been made in understanding how energy and other conserved physical quantities move through "open" systems, like the climate. Again, the issue straddles both theory and observation. People just have to stop assuming Gaussian (classical central limit or bell-curve) conditions in analyzing weather "events." There's never been any reason to do so.
  
  
• Pattern formation. This is an intersection of renormalization, non-Gaussian statistics, and "complexity," as an earlier posting discussed. The locus classicus for these techniques is understanding the perpetually landsliding sand pile. (There's even a cute book on the subject by Per Bak.) In complex, open systems with "flow-through" of air, water, and heat (or sand grains for that matter), long-range patterns with "almost" (but never quite!) repetitive behavior form and dissipate over and over - just like the weather: cyclones, storms, fronts, and so on.
  
  Pattern formation is especially germane to understanding clouds - their nature and lifecycle - better. Clouds are the most important feature of climate not easily captured by simplified models; convective turbulence is actually secondary in importance, at least for heat flow, although it's still crucial for the complete picture. And the big, difficult pieces of climate - clouds, turbulence, water transformations - are all linked together. Convection doesn't just transport heat; it lifts water vapor to higher altitudes than it would otherwise go, making clouds form more often and last longer than they would otherwise.
  
  The presence of clouds in turn transforms the climate by changing how radiation flows into and out of the atmosphere and providing a greatly enhanced form of upward heat convection. The main source of IR-active gas in the clear air is not CO2 or CH4, but the feedback effect of enhanced, clear-air water vapor. But even limited condensation of the enhanced water vapor into clouds changes the radiation flow drastically.

Observation: Go forth and squint hard. In the end, a chaotic system is its own best computer: no model we devise or limited set of observations we make can ever capture every aspect of its behavior. But observation nonetheless remains essential for understanding climate. Modern scientific observation of the atmosphere, increasingly detailed since the 18th century, has a lot to tell us about the repertory of possibilities.

In understanding actual climate, we must always keep in mind the proviso that chaotic systems feature an unending stream of unique events. We also have to face repetitive trends that repeat on time scales longer than the modern scientific record captures. Climate is, in this sense, a unique problem, in that we're inside the system being studied, and we're myopic observers with only hints and partial clues about the long term. Although laboratory experiments are essential for isolating general physical laws, the actual conditions of climate do not constitute a laboratory experiment. It's not controlled, and we're not outside the system in a position to aspire to know and control everything about it.

Observation: The Sun will have its say. It always does. It's the ultimate factor in charge of Earth's climate. Like other stars, the Sun is variable, at a small but measurable level. What limited observations have been made of our Sun already strongly hint at important solar modulations of Earth climate. The more basic solar physics in control here, and how the Earth responds to solar changes, are still poorly understood, and the whole problem remains at the frontier of research. But the base of raw data needed is now available in a way not true 20 or 30 years ago. Studying other planets' response to the Sun's variability will help.

Observation: All things green and blue. Plants and oceans need to be understood better as well. Over scales of decades and longer, they play a critical role in absorbing and recycling carbon dioxide. Current climate models capture the ocean part only imperfectly and plants barely at all. Yet there's a 0.2/0.00038 = 530 ratio of diatomic oxygen (O2) to CO2 in the air, which large ratio is made entirely possible by plant metabolism.** The annual plant-driven variations in atmospheric CO2 concentrations are about eight percent of the total. Since 100/(8/year) ~ 12 years, every CO2 molecule in the atmosphere gets captured by a plant in a little more than a decade.

What's not understood is how plants are responding in their annual cycle of growth and decay to increasing CO2 in the atmosphere. With more "food," there will be more and bigger plants. How much is unknown, although the Ice Age results give a very rough idea. What little research here has been done so far has been strongly tainted by people out to "prove" that plants aren't important - even though they clearly are. It's an obvious place for Gaia-philes to speak up. One of the few geoengineering ideas with any merit involves humans enhancing an already old and thoroughly proven means for removing CO2 from the atmosphere: more plants, bigger plants, maybe even über-plants. More on that next.
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* Indeed, the wild claims made by the IPCC for GCMs and "climate parametrizations" can be put into sharp relief when we consider that, were these results actually definitive answers for climate, we would also have a solution to fluid turbulence.

In fact, we don't. Over at the Clay Institute web site, you'll see there's a Clay Millennium prize for solving turbulence (Navier-Stokes equations) - and it remains unclaimed. Given the true state of affairs (turbulence remains an unsolved problem in physics and engineering), we can then rightly reason backward and conclude that the climate problem remains unsolved as well, since the turbulence problem is embedded within it.

** Without constant plant replenishment, the O2 would rapidly disappear from the atmosphere by oxidation weathering and water absorption. Animal metabolism would be impossible without plants, although plants can and, long ago, did do fine without us and our animal relatives.

You read it here first

... sorry, can't help but gloat:

Global warming isn't to blame for the recent jump in hurricanes in the Atlantic, concludes a study by a prominent federal scientist whose position has shifted on the subject.

Not only that, warmer temperatures will actually reduce the number of hurricanes in the Atlantic and those making landfall, research meteorologist Tom Knutson [of NOAA] reported in a study released Sunday.

[... snip ...]

Another group of experts, those who study hurricanes and who are more often skeptical about global warming, say there is no link. They attribute the recent increase to a natural multi-decade cycle.

Both groups are right, albeit in different ways. But you read it here first. Kudos to Instapundit for staying on top of this.

And the NOAA website puts it this way:

NOAA Model Projects Fewer, But More Intense Hurricanes Late This Century

(Emphasis added.) You read that here first too.

Fat tails and outliers: Extreme weather

It's often claimed, on little physical basis, that "global warming" with rising concentrations of infrared-active gases will lead to more extreme weather. We need to define more clearly what "extreme weather" is and apply some of principles learned from other open, "driven" systems, with non-conserved totals of energy, matter, and so on. The weather lives in Extremistan, at least to an extent.

Concentrate on just the atmosphere. The amount of dry air is essentially fixed, but not so the water in the air (both vapor and condensed), which is highly variable. The amount of heat (which is also rather variable) is less important than the heat flow. While the heat flow is split among radiation, evaporation, and turbulent convection, the total flow is relatively fixed. The division among the three mechanisms of flow is less stable.

Start with the total available. Instead of taking each "extreme" weather event one at a time, let's reverse the logic, with the same reasoning used by geophysicists when they consider earthquakes.* Imagine you're a storm god with a more or less fixed annual budget of heat and water. Taking the totals as the staring point, how do you divide them up every year? All at once? Blow your budget on a few big events? Or on many, many little events?** The "storm-generating system" is an open, not a closed, system; one with stuff flowing through, not isolated and in thermodynamic equilibrium. Extremistan statistics (fat tails) should be the default way to analyze it. It's natural then to assume that "storm budget" for a year will be dissipated by the cumulative effect of a range of events, some small, some medium, some extreme, all together making a distribution of event frequency versus event size.

Unfortunately, a wholly bogus case for "increasing extreme weather" has been built on using misguided Mediocristan (Gaussian) statistics to analyze extreme events. The fundamental problem is the same as in the other misapplications of the classical Central Limit Theorem to such events: the distribution moments are supposed to finite, but in fact, are not. So the sampling of the distribution by observations is misinterpreted to infer that the total of such events is increasing in time (non-stationary), when in fact the moments (like the variance) are simply diverging.

A subtle point: The number versus the energy release of extreme events. A "global warming" world is one where many differences in climate fade, leading to a fall in certain kinds of "weather events," like fronts and tropical storms. In such a world, it's reasonable to conjecture that both the number of extreme events and the cumulative energy dissipated by them go down.

But that reasoning alone is not precise enough to tell us how much each will go down. Periods of warming (like 1910-40 and late 70s to mid-90s) had fewer tropical storms. But a few of them were nonetheless memorable for their size; for example, the unnamed 1938 hurricane that hit Long Island and New England, or Hurricane Gilbert, which hit the Caribbean, Central America, and Mexico in 1988.

It could be that both the number and cumulative energy of extreme weather events goes down under "global warming," but that the energy per storm or event can go up. The number of events can go down by more than the amount of energy dissipated goes down. Thus the ratio (energy/event) can go up. A possibility to think about.
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* This is the Gutenberg-Richter law, one of a large set of modified power-law distributions with "fat tails" (slowly falling probability for larger and larger events, instead of a sharp fall-off). The upper cut-off is not literally infinite, but very large. In practice, it's set by the total amount of energy available - in the earthquake case, the total amount stored as a potential energy in the Earth's crust under tension. Conceivably, all that stored energy could be released in one giant earthquake.

** Don't laugh. If the Greeks had known about Extremistan, they would have deified it. Actually, they did: Tyche, or Fortuna.

Hurricanes, typhoons, and cyclones: Variations

What impact would an infrared (IR) opacity-dominated climate ("global warming") have on cyclones, tropical cyclones in particular?

The tropical heat/moisture engine is modified. The basic features of an IR-opacity dominated climate are:

• A rise in temperature overall, across space and time
• A reduction of temperature differences by latitude, by hour, and by season
• Reduced poleward flow of excess tropical heat

Putting back in turbulent convection and the hydrologic cycle gets us a few key compensating effects as consequences:

• Enhanced vertical convection, moving heat upward more quickly and speeding up the air circulation "merry-go-round"
  
• Enhanced evaporation, precipitation, and cloudiness

The air circulation "merry-go-round," on the largest scale, is just our friends, the Hadley cells. These will be enhanced in both speed and effectiveness of heat transfer. The earlier discussion of convection perhaps made it sound as if it is just a vertical, upward affair. It is for the heat. But the air, unlike the heat, does not escape the lower atmosphere. So there has to be some horizontal motion as an intermediate step between up motion and down motion. And there has to be compensating downward motion of cooler air, after it's given up its heat to the upper atmosphere, to preserve the overall mass of tropospheric air.

Consequences for tropical cyclones. If you think of the Second Law at work in the Earth's atmosphere by latitude, you realize that tropical cyclones serve the atmosphere's "need" to even out heat flow, overcoming the large difference between equator and poles and between high summer and autumn. If the temperature difference between poles and equator and between summer and winter narrows, even as the overall temperature goes up, the Earth's atmosphere has less "need" for tropical cyclones.

The specific mechanism that drives topical cyclones with excess heat and moisture is the nighttime temperature difference between the still-warm water and the cooler air.* "Global warming" will raise air temperatures more than it does water temperatures. Hence, the air-water temperature gap narrows, and the cyclone-driving "heat/moisture pump" is weakened. Some consequences of this change are difficult to trace out in full. But the essentials are unequivocal:

• Poleward transfer of heat and moisture: reduced
• Duration of tropical cyclones: reduced
• Intensity of tropical cyclones: reduced
• Frequency of cyclonic storms: hard to say, but an educated guess is: reduced
• Poleward movement of tropical cyclones: enhanced by stronger Hadley cell and other convective motions

Overall, in a "globally warming" world, expect a (probably) reduced frequency of tropical storms, with each storm being less intense and shorter-lived, and moving more quickly poleward away from the equator.

Further thoughts on tropical cyclones. People living in tropical and subtropical areas obviously have a big stake in this question, and it's been manipulated and misrepresented by "global warming" fanatics eager to chase ambulances. In particular, whenever a major hurricane like Katrina or Rita hits a populated and developed area like the Gulf coast, we can expect more "global warming" hysterics. Little attention is paid to the big fact about hurricane-prone areas, that they're far more populated and developed now than they were 30 or 50 years ago. As a result, they will necessarily suffer more human damage when a large storm lands than they would have back then.

The irony of it all is this: more frequent, more intense, and longer-lasting Atlantic hurricanes are almost certainly an indicator of a summertime atmosphere that is cooling year on year. It's one piece of a growing body of evidence that the Earth's atmosphere, or at least the north Atlantic, has been cooling - in fact, since the late 90s.

Put simply, periods of warming (1920s and 30s; late 70s to mid 90s) are periods of reduced hurricane activity. Periods of cooling (late 1940s to mid 70s; since late 90s) are periods of increased hurricane activity. The reasons are just those spelled out above. An upcoming summary of the present climate situation will return to this point.
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* As summer passes on into fall, the tropical air cools day by day faster than does the water. Recall that water has a much larger heat capacity than does air.

Hurricanes, typhoons, and cyclones: Basics

Cyclones, anticyclones, and fronts. Near the Earth's surface, vertical convection often creates localized low-pressure areas where surface air is pulled in towards a center of exceptionally low pressure, rises strongly toward the tropopause, then flows aloft outward from the low-pressure center. Compensating such low(er)-pressure areas are areas of high(er)-pressure where air aloft flows toward an center of exceptionally high pressure, flows down toward the surface, then outward along the surface from the high-pressure center.

In the Earth's rotating frame, any vertical or north-south air motion is deflected by the Coriolis pseudoforce.

• In the northern hemisphere, low-pressure systems are set spinning counterclockwise. In the southern hemisphere, they are set spinning clockwise.
• In the northern hemisphere, high-pressure systems are set spinning clockwise. In the southern hemisphere, they are set spinning counterclockwise.

Low-pressure systems (marked by "L" on a weather map) are also called cyclones and often bring with them rain and strong winds. If they form, as they often do, over water or humid land, the strongly heated, rising air at their centers takes with it a lot of moisture, which condenses into large cloud banks that eventually dump their moisture as precipitation (rain or snow). High-pressure systems (marked by "H" on a weather map) are also called anticyclones. Their air is typically much drier than the air in a low-pressure system, being pulled from aloft. They compensate for the presence of cyclones not only with respect to horizontal air flows, but also with respect to angular motion. They ensure that the atmosphere has no net angular momentum.*

Cyclones can be broadly divided between those associated with weather fronts (see here for more about fronts) and independent storms. A weather front is a sharp boundary between two air masses, one typically colder and drier, the other typically warmer and more humid. A cold front is a front where the colder air mass is pushing the warmer air mass; a warm front, a front where the warmer air mass is doing the pushing. Where a cold and a warm front meet, a "hinge" forms that gets spinning (by the Coriolis force) and forms a "low." A weather front can bring its own precipitation and strong winds, driven by the temperature, humidity, and pressure differences across the front boundary.

Front-associated cyclones and anticyclones are dragged along by their fronts in a direction determined by the basic Hadley wind pattern associated with the climate zone in question. Fronts in the temperates are pushed eastward. In the tropics and polars, they are pushed westward.

Tropical cyclones. Independent cyclones form away from any front boundary and live out a typical lifecycle that strongly depends on latitude and season. Some form near the equator (0^o lat) and, while still in the tropics, drift westward and (steered by the upper air tropical Hadley cell flow) away from the equator. Once they cross into the temperate zone, the prevailing eastward Hadley cell air flow takes over, and they reverse course to eastward, while continuing to move poleward.** Tropical cyclones are universally called "tropical storms." If they feature winds above a certain threshold, they are called hurricanes in the Atlantic (after the Carib storm god, Hurikán), and typhoons in the Pacific.

Tropical cyclones derive their power from upward convection of warm, moist air. They pick up this power at night, when the warmer nighttime ocean water dumps heat by upward convection into the cooler nighttime atmosphere. (Recall that during the day, the heat flow right over the water is downward, supplying the latent heat needed for evaporation - the daytime air temperature is higher than the daytime water temperature.) The longer they move over warm tropical water, they more powerful they get. They lose that power rapidly when they cross over land. The strong upward convection of warm, moist air at the cyclonic center, combined with the Coriolis force, gets the cyclone to spinning faster and faster, with a well-defined and relatively cloud-free vertical column of upward convection (the "eye" of the storm) whose walls have high circulating winds. If these winds exceed a certain limit, the storm is classified as a hurricane/typhoon. While Atlantic hurricanes can be spectacular and destructive, Pacific typhoons are typically stronger and more frequent, because the Pacific features a much larger region of unobstructed warm tropical water.

Northern hemisphere tropical cyclones form during the period June-November (from right after summer solistice to almost winter solistice), but mainly in August and September. It is during these months that the nighttime contrast between warmer water and cooler air temperatures is largest. The upward convection of warm, moist air is most enhanced in these months, leading to the highest frequency of storms and the strongest storms. In the rest of the year, conditions are not right for tropical cyclone formation. For southern hemisphere storms, shift by six months.

Extratropical cyclones. Independent cyclonic storms also form near the ±60^o lat lines. They drift westward in the polar zone, but switch to eastward if they cross into the temperate zone. Depending on geography, they can become trapped and sit for weeks or even months before dissipating. They typically pull warmer moist air from the south, convert it to rain or snow, then dump it at higher latitudes. They play an important role in transferring moisture to subarctic regions and replenishing polar ice caps. At somewhat lower latitudes, they form the nucleus of long-lived springtime storm systems that can afflict the northeast American and Asian coasts.†

The basic driving force of these systems is the contrast between the warmer, moister air of the high temperate zones and the colder, drier air of the polar regions. This contrast reaches its maximum (in the northern hemisphere) in the middle to late spring (April to early June), exactly the period when these storms are most frequent and strongest. For the southern hemisphere, again shift by six months.

Poleward mass, moisture, and heat transfer. The overall effect of these moist and warm low-pressure cyclones is to help in the transfer of excess heat and humidity from the tropical zone towards the poles. There is no net mass transfer, but rather a conveyor belt where mass transfer evens out to zero because of the balancing of low- and high-pressure systems.

While poleward heat transfer in the atmosphere is not as powerful as it is in the oceans, the effect is still significant. The net heat transfer is a result of the fact that the net air flow is, rising at the equator and falling at the poles. The presence of three Hadley cells per hemisphere, rather than one, merely complicates the details. Poleward atmospheric moisture transfer is also crucial in maintaining polar ice and snow covers. Water is evaporated at lower, hotter latitudes and dumped as rain and snow at higher, cooler latitudes.
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* That is, as seen by an observer rotating around the Earth's axis with the surface. As seen by an outside inertial observer, the Earth's atmosphere does have a net angular momentum associated with its overall rotating motion.

** If the temperate eastward air flow ("westerlies") is weakened, a hurricane can keep drifting westward, even as it continues to move northward. From the Atlantic, such storms get into the Caribbean Sea and Gulf of Mexico, where they can wreak exceptional havoc in a small confined area. Such storms also get new strength from the warm, shallow waters of the Gulf.

If the temperate "westerlies" are strong, OTOH, Atlantic hurricanes get strongly deflected back eastward once they get into the temperate zone. They can then miss landfall altogether. As they move over the colder north Atlantic waters, they rapidly lose strength.

† These include the famous New England-Maritime-Greenland "nor'easters."

Atmosphere in motion: Hadley cells

A permanent, year-round feature of the Earth's current climate regime is the movement of warm air away from the equator and toward the poles, with the convective motion broken into a set of three distinct latitudinal bands or "cells", one set per hemisphere, for a total of six cells all told. These cells now carry the name of the British natural philosopher George Hadley who, in the 18th century, first described these cells and grasped the associated wind patterns that so influence weather at different latitudes and played a critical role for seafarers in the Age of Sail.

It's difficult to describe these cells in words and better to see them in pictures or, preferably, as dynamic animations. Here's a static picture. And here's a more detailed diagram.

The Hadley cells originate from the warm air that rises at the equator (0^o lat), sinks at ±30^o lat, rises at ±60^o lat, and sinks at the poles (±90^o lat). The basic north-south movement is:

• Between 0^o and ±30^o lat, southward at the surface and northward aloft (tropical zone)
• Between ±30^o and ±60^o deg lat, northward at the surface and southward aloft (temperate zones)
• Between ±60^o and ±90^o deg lat, southward at the surface and northward aloft (polar zones)

The consequence is a permanent tendency to form low-pressure systems at 0 and ±60^o lat and high-pressure systems at ±30^o and ±90^o lat. Tropical cyclones (such as hurricanes and typhoons) form close to the 0^o lat line, while subarctic (extra-tropical) cyclones form near the ±60^o lat lines.

Now add in the effect of the Earth's rotation and the resulting Coriolis force (or pseudoforce, a feature of rotating, noninertial reference frames). Poleward motion at the surface is deflected eastward, while equator-ward motion is deflected westward. This causes the basic wind patterns we see at the surface: "westerlies" in the temperate zones (coming from the west, but headed east - I know that's confusing) and "easterlies" in the tropical and polar regions (coming from the east, but headed west). Hence, in the temperate zones, weather systems tend to move SW to NE in the northern hemisphere and NW to SE in the southern. In the polar and tropical regions, weather systems tend to move NE to SW in the northern hemisphere and SE to NW in the southern.

The Hadley cells, together with the Coriolis force and seasonal cycle, explain most of what you need to know about geographic weather patterns. The calm region of high-pressure, sinking air at ±30^o causes the "horse latitudes," where ships in the Age of Sail were becalmed from lack of wind and had to throw their horses overboard to conserve on water. The tropical easterly "trade winds" were discovered and exploited by Columbus and other European explorers in the Age of Sail as the basic means of getting east to west, while the temperate westerlies (familiar to us living in the North America and Europe) were the basic means of getting west to east.

Local weather phenomena near the surface tend to move SW to NE in the northern temperates. But weather systems with strong vertical structure move NW to SE in the same latitudes. The controlling flow (steering wind) in such cases is the southward Hadley flow aloft, not the northward surface flow.

Hadley cells on other planets. The terrestrial inner planets Venus and Mars also feature atmospheric convective cells. Venus' atmosphere is so thick and stable that it only has one Hadley cell per hemisphere. Mars' atmosphere, OTOH, is so thin and unevenly heated that it apparently has no stable configuration of Hadley cells.

The outer gas giants of our planetary system also have convective cells in their atmosphere. However, these gas giants lack a significant solid surface and their atmospheres are very opaque. So the pattern of heating and convective "rolls" in their atmospheres are essentially different from those of the terrestrial planets. Our sort of Hadley cells arise from having a mostly solid planet with a thin atmosphere that is mostly heated from below.

Hadley cells in your kitchen. You can form Hadley cells in mildly heated water, if you introduce differential heating; that is, more heating at one end of a pan and less at the other. The exact pattern is a function of liquid density and viscosity. But you should expect rising liquid at the more heated end and falling liquid at the less heated end.

Of course, it's pretty hard to getting the Coriolis rotational effect in your kitchen. But you can get something like it if you use fancy lab equipment.

Rayleigh-Bénard convection. Hadley cells are an example of a well-studied phenomenon in fluid dynamics called Rayleigh-Bénard convection or the Rayleigh-Bénard instability. See here and here for more.

A third look: The doctor is in

In medical school, students learn about diseases and syndromes defined by a list of symptoms and underlying causes. When medical students become doctors, however, they don't work with disease entities; they work with patients. Patients present symptoms, not abstract disease labels or underlying causes, and what doctors see is collections of symptoms. Each symptom typically can match a number of illnesses. But the whole list of symptoms will usually match only one or two diseases, and any remaining ambiguity is usually eliminated by tests. The more symptoms you know - and test results can be viewed as just more symptoms to add to the list - the more a doctor can narrow down the possible diagnosis. Using multiple symptoms to isolate a unique disease is called differential diagnosis.

The same idea can be applied to the Earth's climate. Any changes humans make to the climate take place in a context of pre-existing natural trends. How can we be sure that one climate symptom indicates, say, enhanced infrared (IR) opacity ("global warming" in the popular sense), or solar cycle changes, or internal climate cycles? Actually, you can't be sure with one symptom. But a whole list of symptoms narrows down the scope of possible causes. If many symptoms simultaneously match a hypothesized cause, you can be much more sure that you've identified that cause correctly.

If enhanced IR opacity increases were dominating climate variability, would there be a distinctive or "smoking gun" list of symptoms? The answer is yes, all linked by a single theme: A "globally warming" climate of this type would become somewhat more like the tropics. Overall, the surface air temperature and pressure would rise slightly; evaporation would be enhanced, leading to more clouds and precipitation. Plant growth would be enhanced.

But what's really interesting is that this "tropicalization" is not geographically uniform. As always, it's differences that drive climate and everyday weather, and a slightly more tropical world would see disparate and distinctive climates impacts. A sense of these changes is communicated best by making a simple latitudinal division of the Earth into tropics (±30^o lat of the equator), temperates (+30-60^o lat away from the equator), and polars (within 30^o lat of the poles).*

The tropics would become slightly more tropical; the temperates would shift a little more toward tropical; the polars would shift noticeably more toward tropical. The temperates would experience longer growing seasons, but certain types of plants would be favored. All plants need CO2, water, and sunlight; they would get more of two but less of the third. Some plants like that, some don't, and the former would be favored relatively more.

What are tropics anyway? They're places that get more "vertical weather" (a steeper temperature lapse rate) and thus more locally generated storms, but less "horizontal weather" due to reduced horizontal differences in temperature and saturation water vapor pressure. Remember that "climate is driven by differences": fewer differences, less "weather." The tropics generally disfavor large, laterally moving storm systems and weather fronts. Storms and fronts are less frequent, less intense, and shorter-living. The temperates are a good comparison. These latitudes experience the most "horizontal weather" - storms and fronts - because they span the greatest differences in temperature, all the way from the semi-polar to the semi-tropical.

Simply put: Reduced horizontal temperature, pressure, and water vapor differences between the polar and equatorial regions; enhanced vertical temperature, pressure, and water vapor differences between the surface and the upper troposphere/lower stratosphere.

This overall qualitative picture is robust and consistent. The exact numbers don't matter as much as the correlated multiple trends of definite nature and direction - the whole symptom list: horizontal differences would decline and vertical differences would rise, even as temperatures would rise overall. That's an exact way of saying the Earth would become "more tropical." Since the tropics are already tropical, the change wouldn't be so big there. It would be bigger in the temperates and biggest in the polars.

Whatever the quantitative uncertainty in the numbers, the qualitative picture is rigorous and ultimately a result of the Second Law: the extra heat content in the lower atmosphere (heat capacity times temperature change + change in latent heat of water vapor) wouldn't be distributed evenly. The Second Law requires that it go more to where it's most "needed," namely the polars. Less of the increase would linger in the tropics. (The dominant exchange mechanism would be the poleward ocean currents.) Overall the Earth would shift closer to global thermal equilibrium (reduced spatial variation in temperature), even as the overall temperature distribution rises.

The atmosphere overall would still be a nonequilibrium system, so this idealized equilibrium would only apply to an instantaneous "snapshot" of climate. However, greater geographic uniformity of temperature still indicates the Second Law at work. Solar radiation is converted from its incoming narrow beam to isotropic outgoing terrestial IR radiation; this transformation from unidirectional to omnidirectional, along with the upward shift in radiation wavelength, represents a "randomization" of photon energy - an increase in entropy. The more uniform the Earth's geographic temperature distribution, the more complete the "randomization" of the outgoing radiation. An upcoming posting will introduce a neighboring planet - Venus - where this "hotter, but more uniformly hotter" principle holds to a far greater extreme.**
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* A more rigorous definition uses the tropical latitudes (±23.5^o of the equator) and the polar circles (±23.5^o of the poles), based on the obliquity or tilt of the Earth's axis with respect to its orbital plane (ecliptic). This figure determines which latitudes near the poles experience at least one full day of polar winter night and which latitudes near the equators experience at least one summer day with the Sun straight overhead at noon. A later posting will make it clear why I've used divisions based on 30^o latitude intervals instead.

** At least it's not a spherical cow emitting milk equally in all directions.