boswell is supposed to have said to dr. johnson, "i'm greatly affected by music. "some music will make me weep. "some music will make me feel so brav "i could march into the thick of a battle and not be scared of anything." johnson says dryly, "if anything could make me act so foolish, i would not have anything to do with it." [low hum and sticks clacking] [cultural music montage] (man) music has got an incredible power, presence. you know, sound is something that you can't escape when you're in its presence. [chanting and drumming] with music, there's no looking away. [spirited clarinet and guitar playing] that is part of the power of music. when you're near it, there's no escape. it can focus your attention on a particular ceremony for example. it can act as a kind of social glue that helps a ceremony to go on over the course of several days. it keeps people's attention focused. music can be used in a work situation. you can find examples of that in africa where people are working in a field. let's say that they are planting millet or they're threshing. any kind of situation where you have got large numbers of people, you need to coordinate their efforts, you can use music to do that. [folk music] (man) music is something that people can dance to. it can move them out of one frame of mind into another. it can move them from being separate people to being one group of people. it can remind them of who they are. it also can get them out of the state that they're in now into some other state. it transforms them. why it's powerful we can't say. we just know that music moves people. in the middle of a church service, somebody will fall out, and a lot of that has to do with the build-up that's been achieved through the music. there are many ways in which it's powerful. it's not necessarily powerful in a good way. [drum roll] [trumpet fanfare] hitler used music brilliantly to organize people ,, so the power of music is not exactly something we can put our finger on scientifically. but we can observe it, and we can talk to people and have them tell you what it feels like to be in a music situation that does something important. (narrator) music plays a strong role in ceremonies and rituals of all kinds. in southern africa, the medicine dance of the kung is a ceremony that is usually held once a week to heal the sick and ward off evil. (brown) among the kung, music itself is a medicine which heals people physically. it heals their ailments, and that is testament to the power of music. [chanting and wailing] the kung are also known as bushmen. they are a people who live in namibia and botswana, in southern africa. they live in a semi-arid or desert region. it's a very tough environment in whi to survive. and they are hunter-gatherers. they are people who live off the land. people survive really on the margins in that environment. it's very easy to go across the line and to get into real trouble through sickness. so the alleviation of sickness and suffering is a regular need that has to be addressed. the healing ceremonies that the kung perform are one way of addressing those needs of restoring balance and harmony and health. the sound of the music itself is a healing sound. the music comes to people on a subconscious level. it gets right to the core. and it has a way of transforming you. one of the most interesting things about the healing ceremonies of the kung is that there aren't any words. there's no text at all. yet that music is very powerful, very moving, very, very emotional music. and how does that work? you know, you hear people singing, and they're yodeling. you hear this wonderful melody, little fragments sung by one person put together with little fragments sung by another person, and together, you get a kind of composite melody that's not sung by any one person. it needs a community. there is no audience who is sitting and watching. everybody is performing some role. you can be involved by singing. you can be involved by clapping your hands. you can be involved by dancing. but the important thing is that everybody is connected in the musical experience. [abrupt shrieks] there's a close connection between music and trance. among the kung, the men in particular, at some point in there lives, usually become healers and become capable of entering into trance. the state of trance is something that is brought about in part by the music and in part by the whole social occasion that is going on. and in that state of consciousness, a human being leaves his own body, or his consciousness leaves his body and another consciousness comes into that body and is able to heal in that state. so the music helps the human being to bridge the gap between the natural world and the supernatural world. that's why the music itself has power. the music itself is medicine. (narrator) life cycle rights and rituals mark important moments of transition in people's lives. music often plays a vital role in elevating these events from the ordinary to the extraordinary. (cohen) i did a film in greece and in astoria queens about this terrific community of epirot musicians. every time they have one of their celebrations-- it could be a wedding or it could be a feast day-- they have to have music. [tambourine, drums, and clarinet playing] you can't have that ritual unless you have that good music. and the best musicians are up there in the mountains of greece, and they'll fly into athens for that one festival. the issue of money isn't important-- that he's there. [music continues] you know, most places, when you want to get excited, you go faster, but somehow with the epirots, they go slower when they want to show how intense they are and expressions of agony and pain and ecstasy-- slow-moving, exotic things... it's crazy. and they dance, and they kind of outdo each other. and they lead each other around doing these slow, slow, slow things. it's very elegant. well, by the time you get involved in that, everybody's sweating and pouring on the energy and slowing down. they're no longer where they were when they came into the room. it's a different place, and i think it's magic. ♪ on my way. (narrator) the power of music itself can be the force that draws people together. ♪ i would like to reach out my hand. ♪ at rock and roll concerts world-wide, musical performance often facilitates the creation of community. large concerts such as those by the pittsburgh-based band rusted root are highly interactive and transformative events that bring participants together in ways that often transcend the performance itself. ♪ send me on my way. ♪ on my way. ♪ send me on my way. (man) i think the question of what is powerful about music or musical experience is very interesting, because i think it goes beyond the scales that are being performed or the particular rhythms or even necessarily the execution of it. i think that the environment that's created between the band and the audience provides some sort of emotional venue in which many magical moments happen. ♪ i may tell you to run. ♪ no ba de say no ba de ohm. ♪ from the stage to the audience, you can see how the sound pulls people together into a collective because they're all moving to the same pulse. the music is providing something for them to move to, and they're in agreement with us on stage, and somehow, some way, we're all in this together. [music continues] (man) they'll come and watch the show, and they'll dance together, and afterwards, they'll go outside and get their drums out of their trunks and play drums in the parking lot. it's a way to bring people together in an age where it's really hard to do that because in this society, it's increasingly not set up that way. [lilting flute plays] (dispirito) i think the performance of music is very cathartic even for the people performing it. i know that i get into a very meditative space myself when i'm performing. you get lost in the music. ♪ amen. ♪ oh, yeah. ♪ amen. ♪ we're called... (narrator) music plays a fundamental role in religious expression around the world. american gospel music is a uniquely african-american musical genre. during the service, music acts as a catalyst that draws the whole congregation together in active participation. it comes from the word evangel, and the evangel is the spirit of good news. and so when we speak of evangelism in the new testament, we're speaking about the good news of god's love in jesus christ. all who are able, stand to your feet, and let us call ourselves together for worship and celebration. technically, what happens with the music in our service is, it begins with a prelude, and from that point on, music is almost analogous to the old steam engine. it starts rather slow, but as it builds up speed, it increases its energy. we call our service of worship, a service of celebration and jubilation, primarily because of the good news. when you hear good news, you are celebrative. but i also recognize that on any sunday gathered in the pews of this sanctuary are persons who have not had a great deal to celebrate during the week. and so music serves not only as a rallying point, but it also serves as a therapeutic means. (man) this music has one message. you've got to feel something. you leave here, you should feel better than when you came in the door. tom dorsey was the godfather, if you will, of gospel music. who was he? the blues musician, jazz musician, playing in the honky tonks and all that. and then one day, according to tom dorsey, god spoke to him and said, "no, no, no. "you're going to stop playing this music. we're going to bring this into the church." now, at that time, i mean, gospel music was resisted. who are these people bringing this boogey-woogey music into the church? there was a lot of opposition to it. but slowly, over time, you know, cream rises to the top. and you couldn't hold back thisvalanche, this feeling, this desire, because it does speak to the people in a way that, you know, no other music form in our church really speaks to us. [choir singing] the lyrics of gospel music being scripturally based do give a message of hope. but gospel music in and of itself has its power from also the musical accompaniment. there is a beat. there is a cadence--a rhythm. so the music must connect with people's interior. ♪ if you get there before i do, ♪ ♪ the angels up in heaven done signed my name. ♪ (woman) when you walk into a service, you may feel down, you may feel troubled, you may be distraught. ♪ the angels up in heaven done signed my name. ♪ but then the music begins, and it's soothing, and it's uplifting, and then you begin to feel you have strength. ♪ i know i've been saved. you know you've come home. ♪ i know i've been saved. ♪ the angels up in heaven done signed my name. ♪ i'm a kid of the '60s, so for me, i came to this gospel music not through the church, but really through the movement. i mean, whenever we were in demonstrations, or i'd see demonstrations, there was always music being played. ♪ we shall overcome. (narrator) music has the power to unite people in common cause. it is often able to convey a political message in stronger and more emotional ways than speech. ♪ we shall overcome. in the 1960s during the civil rights and anti-war movements, music became a driving force in the struggle for social change. (seeger) there wasn't a single meeting that didn't have singing. "we shall overcome" was the most famous song, but there were hundreds of others. they'd change over a gospel song, put new words to it. very common technique. it's been done for centuries. "we shall overcome" was originally a fast song. [clapping] ♪ i'll overcome. ♪ i'll overcome someday. ♪ we shall overcome. when you sing "we shall overcome," your shoulders are touching because you're crossing your arms in front of you, and swaying across from right to left. [softly] ♪ we shall overcome. well, a month after the founding of sncc, this song was sung throughout the whole south. it was the song; it wasn't a song; it was the song. in it's own quiet way, it was taking confidence. you can kill me, you can beat me, but i know we shall overcome. (scott-mclaughlin) in the prisons, they would sing songs. when they we're being beaten by the dogs, they would sing songs. and you'd have to ask yourself what was this thing about, why were they singing these songs as they're being beaten? and the reason why they sang the songs was just like when the priest chanted gregorian chants or when a buddhist has a mantra. or when you say, "hail mary mother of god" in the catholic religion. it was a means of going inside of yourself to find the strength within to deal with the outside world. one of my mentors was bill kunseler. there was one scene he told me about when he was in birmingham where he was representing dr. martin luther king. and they had come from a demonstration or rally, and king had been told that there were men looking to kill him that night. and they drove up to a house, and bill and king were staying in the house together with a group of other people. bill told the story that people were petrified that night. i mean, they thought that the house was going to be bombed, and they darkened the house so there were no lights at all. and he was afraid for his life. and king must have sensed this fear in the room. and he walked over to a piano and started to play "this little light of mine," and the whole group just started to sing this song, and they sang songs all through the night. and bill said at the end of this time, it was like there was nothing to be afraid of. i mean, he himself, a non-religious person, was moved by this music and himself strengthened by it. [light piano music] [explosion] [folk guitar strumming] ♪ it was back in 1942, ♪ i was a member of a good platoon. ♪ (seeger) plato was supposed to have said, "it's very dangerous for the wrong kind of music to be allowed in the republic." there's an old arab proverb-- "when the king puts the poet on his payroll, he cuts off the tongue of the poet," and when people ask me, "can songs really change people's minds?" and i say i can't prove a darn thing. all i know is that the people in power think so 'cause they keep songs off the radio and off the television that they think are dangerous for the people to hear. especially during the vietnam war, my song, "waist deep in the big muddy" [plucking banjo strings] ♪ waist deep in the big muddy, ♪ the big fool says to push on. ♪ it didn't mention president johnson by name, it didn't mention vietnam, but everybody knew what i was singing about. ♪ the captain told us to ford the river. ♪ ♪ that's how we all begun. ♪ we were knee deep in the big muddy. ♪ ♪ the big fool said to push on. ♪ it was a song which was inspired by seeing a photograph showing american troops wading through, waist deep in the water of the mekong delta probably. ♪ we were waist deep in the big muddy. ♪ ♪ the big fool said to push on. ♪ i was asked to be on the smothers brothers program, and the first time i sang it, it was scissored out of the tape, in october of 1967. but the smothers brothers took to the print media and said, "hey, cbs is censoring our best jokes. it censored seeger's best song. what's going on here?" and finally in january of 1968, cbs said, "okay, okay, he can sing the song." ♪ all at once, the moon clouded over. ♪ ♪ we heard a gurgling cry. ♪ a few seconds later, ♪ the captain's helmet was all that floated by. ♪ a friend of mine was working in the distribution office for columbia records in denver, colorado. he says, "pete, you know, my boss took one listen to this record and exploded." he says, "those people in new york must be nuts to think i could promote a record like this." he said, "pete, your record did not leave the shelf." so as i say, i can't prove that songs do anything, but people in control of the country think they do. ♪ another stream had joined the big muddy ♪ ♪ about a half mile from where he'd gone. ♪ ♪ we were lucky to escape from the big muddy ♪ ♪ when the big fool said to push on. ♪ ♪ i know i've been saved. ♪ i know i've been saved. [clarinet plays] [chanting] [rock music] ♪ waist deep in the big muddy. ♪ the big fool said to push on. ♪ funding for this program was provided by the annenberg/cpb project. funding for this program is provided by annenberg media. narrator: given the earth's distance from the sun, its global-average temperature should be a chilling -18 degrees celsius. fortunately, gases in our atmosphere trap some of the sun's heat, creating a hospitable, average temperature of 15 degrees celsius. but the burning of fossil fuels is changing the composition of the atmosphere by increasing the concentrations of these heat-trapping gases. scientist pieter tans and a team of researchers are continuously keeping watch on this change -- one that could ultimately alter the environment for all living things. dr. tans: we use the atmosphere as a diagnostic to monitor that the earth is really doing what we hope it is doing. narrator: but the atmosphere does not just trap heat, it moves it around the world, and together with the oceans shapes earth's climate and weather patterns. one powerful example of this is the tropical cyclone, more commonly known in the united states as the hurricane. dr. kerry emanuel has spent much of his career trying to understand these lethal storms and the larger role they might play in regulating climate. there's something rather unexpected that's turned up from research -- that hurricanes may have a profound effect on the climate. narrator: both of these studies offer us deeper insight into the complicated, ever-changing, active system that is the outermost layer of our planet -- the atmosphere. all around the world, scientists from the national oceanic & atmospheric administration, commonly known as noaa, are monitoring trace gases in the atmosphere. one of their findings is that carbon dioxide is increasing due to the burning of fossil fuels on earth -- an increase that could have significant effects on the climate. for lead investigator pieter tans, the idea that our behavior could have any influence on the planet came as a surprise. dr. tans: i ran into a little book, and it was called "inadvertent climate modification." this was in 1972. my first reaction was, "that's nonsense. "i mean, how can we humans "influence the climate of this planet? "you know, we're just too small for that." but i started -- i leafed through it and i saw about infrared absorption by certain gases, and i thought, "okay, there may be something to that." so i bought the book. and i was convinced right then that this was going to be an important problem. narrator: the problem, human-induced global warming, is caused by the heat-trapping characteristics of carbon dioxide and other so-called greenhouse gases -- the same characteristics that make the earth habitable. dr. tans: it's called the greenhouse effect. you can look at it like a blanket. so the earth receives energy from the sun, and, in fact, when we sit in the sun we feel that we're being heated. but it's not visible to the naked eye. it's infrared radiation. but the earth also emits that energy back into space. and when we put more of these absorbing greenhouse gases into the atmosphere, the infrared radiation will be prohibited from reaching space. it will be trapped in the atmosphere and emitted back to the surface. narrator: greenhouse gases in the atmosphere control the earth's temperature. the greater the amount of greenhouse gases, the higher the temperature. less greenhouse gas means a lower temperature. carbon dioxide is cycled through the atmosphere through many natural processes such as photosynthesis, respiration and decay of vegetation, and sea surface gas exchange. this natural transfer, known as the carbon cycle, is in near balance. the amount being emitted to the atmosphere is close to the amount being absorbed. but the burning of fossil fuels is adding co2 faster than natural systems can respond. pieter tans is taking careful measurements of the atmospheric concentration of carbon dioxide to better understand the carbon cycle. dr. tans: when you study the carbon cycle, what matters is what happens to co2 exchange between the atmosphere and the oceans. what are terrestrial ecosystems doing? are they losing carbon? gaining carbon? why? so we use the atmosphere as a diagnostic to get a handle on these processes to quantify where these processes take place and how large they are. we want to diagnose how the carbon cycle is developing, and we want to understand why. now, the first accurate measurements of carbon dioxide in the atmosphere were done in 1956 by david keeling. he was also the first to start continuous monitoring of carbon dioxide from a mountaintop in hawaii. the mauna loa volcano. narrator: noaa's global monitoring division has continued taking measurements at mauna loa for the past 30 years. but to enhance the accuracy of these measurements, they have expanded their data collection effort to many other remote locations across the globe. dr. tans: for many decades, we've had a global-flask-sampling network, whereby people send us air from specific locations that are downwind from a large stretch of ocean water. or we get these samples from deserts or mountaintops. typically away from terrestrial vegetation. so we get air that is very clean and well mixed so that the weekly sample actually means something. it really indicates this is a concentration over a very large area. narrator: these samples are shipped to noaa's lab in boulder, colorado, where they are analyzed. in each sample, we measure carbon dioxide, methane, carbon monoxide, hydrogen, nitrous oxide, and sulfur hexafluoride. we'll take air from the bottle and send it to three different instruments. this is the one used to measure carbon dioxide. it's interesting -- the way we measure carbon dioxide is by absorption of infrared energy, which is the same principle as the greenhouse effect. so we're using the same property of the gas -- how it affects climate -- to actually measure it. then at night, we connect these suitcases that contain flask samples collected by aircraft. come back in the morning and they're all measured. combined, ground samples and aircraft -- up to 100 a day. narrator: from these hundreds of thousands of air samples, noaa has compiled a comprehensive database of climate-changing gas concentrations over a long period of time. their findings are consistent with those first discovered by david keeling. in the 1950s, keeling's first measurements showed co2 at the level of 315 parts per million. in 2005, noaa's measurements pegged co2 at 380 parts per million. beyond the steady rise in average co2 concentrations every year, noaa's data also showed that these levels fluctuate, mainly due to the capture of carbon dioxide through photosynthesis. for example, during the northern hemisphere winter, carbon dioxide levels peak because so much vegetation is dormant at that time. on the other hand, during the northern hemisphere's summer growing season, co2 levels are at their lowest. this fluctuation can even be seen in a 24-hour period, reflecting the daytime uptake of carbon dioxide by plants when they are photosynthesizing and their release of carbon dioxide at night through respiration. the next step in noaa's diagnosis of the atmosphere is to take measurements close to where greenhouse gases are emitted or absorbed. carbon dioxide is emitted by natural processes, such as plant respiration and decay, as well as m-me processes, such as the burning of fossil fuel ese gas emitters are knowasources. carbon dioxide is absorbed by terrestrial and oceanic plants through photosynthesis and also through sea surface gas exchange. these gas absorbers are known as sinks. if you talk about the budget of the greenhouse gases -- that is, to the atmosphere. that is the sum of the sources and the sinks. so if there's more sources than sinks, the concentration will go up. now, especially for north america, we do think that there's a significant sink -- significant absorption of carbon dioxide on this continent. right now we really can't tell where or why. so we need to do more specific measurements closer to where these sources and sinks are to try to untangle that. narrator: in north america, these measurements are being taken by noaa's tall-tower program led by noaa scientist arlyn andrews. andrews is attempting to tease out north america's specific contributions to the global carbon cycle. we're trying to get measurements that are as close as possible to where all the action happens. so we want to be very close to the forests, close to the cities so that we can see the biggest signals. so what we do is we use a system like this, which measures carbon dioxide and carbon monoxide continuously. so 24 hours a day, we get a measurement every 5 minutes. narrator: john lee of the university of maine monitors one of the tall-tower sites. dr. lee: all we do is basically suck air all the way down the tower and send it to an analyzer that's down below. you want to be in the atmosphere, otherwise you'd have too much of the surface influence affecting your measurements. if we were right next to the surface, we'd only be measuring what's immediately below us and upwind of us. by going up higher, we get all this well-mixed air from a much larger area. this is a pretty good example of northern forests that happen somewhere around this latitude all around the globe. there's kind of this green ring around the earth comprised of these northern forests. without forests like these, the levels of carbon dioxide would likely be higher, increasing the effect of global warming, because carbon dioxide is our major greenhouse gas. dr. andrews: north america does take up a lot of co2. one reason that we've taken up a lot of co2, historically, is that when the eastern part of north america was settled, a lot of forests were cleared for agriculture. and those have been regrowing over the past 100, 150, or 200 years. and so those forests have taken a lot of carbon dioxide out of the atmosphere, but they're nearing maturity, and so they probably won't be such a strong net sink for atmospheric carbon dioxide in the future. narrator: by increasing the number of tall towers, noaa scientists will enhance their ability to understand the rate that natural ecosystems are absorbing or releasing carbon dioxide. dr. andrews: we're really trying to develop a sampling network that will allow us to kind of keep our finger on the pulse of the biosphere continuously. so something that's sustainable and that we can use to monitor net carbon dioxide emissions for many years. narrator: taken together, the programs at noaa's global monitoring division have revealed not only how quickly greenhouse gases are rising in the atmosphere, but are also providing a better understanding of sinks and sources in the cbon cycle. this information, in turn, will allow us to more accurately predict earth's future climate. dr. tans: when you're talking about increasing greenhouse gases, you're talking clite change. see, the's one thing that is pretty certain. the biggest contributor to the rise in carbon dioxide is the emissions caused by burning coal and gas and oil. that's the biggest factor. it's bigger than natural processes that counteract these emissions a little bit. so if you can get an understanding of what the natural system -- what the earth itself is doing with this excess of gases that we've caused, we can actually make better prognoses of what future climate change might be like. narrator: scientists are pondering if future climate change will affect hurricane activity on earth. some say hurricane katrina, which in 2005 devastated the gulf coast of the united states, is evidence that it already has. dr. kerry emanuel of the massachusetts institute of technology has been studying hurricanes for over 20 years. dr. emanuel: i mean, there was nothing meteorologically very exciting or interesting about katrina. what was unusual about it is it hit a very vulnerable place. but people have worried for some years about whether climate change affects hurricane activity. you know, everything from, "well, were there more or fewer hurricanes during the last ice age?" for example. "will there be more or fewer hurricanes if we have global warming?" there's a lot of argument about how this works. narrator: while this argument continues today, historic records do point to a relationship between the recent warming of the oceans and the intensity of tropical cyclones. this graph shows a pronounced upward trend in the total energy generated by hurricanes over the past 50 years. this trend is well correlated with the upward trend in average tropical sea surface temperatures. but there is another question about the relationship between hurricanes and the climate that has received much less attention. could hurricanes serve a vital role in moderating earth's climate? dr. emanuel: very few people have asked the question, "do hurricanes play some central role in the climate?" we tend to think of them as freak storms that don't really affect the climate in an important way. but there's something rather unexpected that's turned up from research -- that hurricanes may have a profound effect on the climate. narrator: for emanuel, it's questions like these that piqued an interest in hurricanes early in his career. dr. emanuel: often you think you understand something, and it's not until you have to teach it that you understand that you don't understand it. and this happened to me in the case of hurricanes. i was asked to teach a course in tropical meteorology. i thought i understood how hurricanes worked. i knew what the conventional explanation was. so i started to teach it, and i had one of those horrible experiences where you start talking about something and you realize it doesn't really make any sense. and that leads you down a path of trying to understand what does make sense. and that's what we call research. narrator: one of the most important tools emanuel uses in his work is computer modeling. dr. emanuel: most people believe that models are used principally to make forecasts, and of course they are. but a lot of modeling is done not for the purpose of forecasting a particular phenomenon, but for understanding it. narrator: using models, emanuel can run simulated hurricanes hundreds of times a day and compare the results with data taken from real hurricanes. when differences are seen between the simulated hurricanes and the real ones, he can pinpoint the cause of those differences in the models and achieve not only better understanding of real hurricanes, but also create more accurate models for future research and forecasts. recently, one of those differences arose which may show hurricanes serving a function never before considered. in order for a tropical cyclone to reach the greatest intensity -- category five, like katrina did in 2005 -- it must have an unusually abundant supply of its raw fuel -- warm seawater. but not all real hurricanes reach this maximum intensity. in fact, they rarely do. dr. emanuel: it's a speed limit. no storm ever exceeds this limit, and a very small percentage of them actually come right up to the limit. most of them fall far short. now, the computer models behaved in some ways very differently from real hurricanes in that they always spun up to their speed limit. narrator: essential to answering why the hurricanes in the models were behaving differently from real hurricanes is an understanding of how real hurricanes work. dr. emanuel: if you look at a photograph of a hurricane from space, what you see, of course, is this beautiful, white spiral mass of clouds. it's a little bit hard to tell, but those clouds extend from very close to the ocean surface up about 10 miles high. and the ocean surface is at a pretty high temperature -- 25 to 30 degrees centigrade. but the tops of the clouds in a hurricane represent the temperature that you find in the upper part of the atmosphere. it's very, very cold. it's around -80 degrees centigrade. so the hurricane's operating across a rather spectacular temperature gradient. narrator: this temperature gradient is just one of the many reasons that hurricanes grow into such powerful storms. the warm ocean water evaporates into the cool air above it, taking heat energy out of the ocean and storing it in the form of water vapor. when that water condenses into clouds, this energy is transferred to the air, warming it and increasing wind speeds even further. dr. emanuel: once this process gets under way, the feedback that allows the hurricane to grow is one between wind and evaporation. the stronger the hurricane winds, the more evaporation. the more evaporation, the more heat goes into the hurricane. the more heat goes into the hurricane, the stronger it gets. and that would just go on forever, but there's one other thing that's stopping it from doing that -- it's friction. narrator: friction is caused by the same thing that's aiding evaporation -- sea spray. dr. emanuel: sea spray is fundamental to a hurricane. when a spray droplet goes up and partially evaporates, it turns out that process transfers an enormous amount of heat to the air. and so sea spray is a very efficient heat-transfer mechanism. now, those spray drops are also slowing the atmosphere down. they're exerting a drag on the winds. the physics of that is very complicated. very complicated. and we don't understand it. and we're trying to understand those physics. we're trying to carefully measure the friction and the heat transfer of the spray. so we flew airplanes into hurricanes over the atlantic to make measurements to help us understand how sea spray affects this heat-transfer and momentum-transfer process. and when we start to better understand this process, we can, with any luck, make better forecasts of how hurricanes will evolve. narrator: but overflights are limited in the data they can provide, so emanuel also studies hurricane machines to get a closeup view of what may be happening when the atmosphere and the ocean interact. dr. emanuel: so we have built a laboratory apparatus in which we can simulate these very fantastic conditions -- very high wind speeds, air blowing across water, the air is filled with spray. when the winds start to blow more than about 80 or 90 miles per hour, so much spray is lofted into the air that eventually it becomes hard to even talk about the surface of the ocean. and i'm not speaking metaphorically. you just go from water, which is filled with bubbles, to air, which is filled with spray gradually. and there's no longer anything you can call the surface of the ocean. so it becomes a real, real chaos in the core of a hurricane. narrator: the interface between ocean and atmosphere is a vital clue to why all of the simulated hurricanes in emanuel's models were reaching maximum intensity, compared to real hurricanes, which rarely do. it also may show hurricanes playing a significant role in controlling the climate. dr. emanuel: in the idealized models, we hold the ocean temperature fixed. we don't let it change. it's just whatever it is. but a real hurricane profoundly changes the temperature of the seawater. not because it's taking heat out of the ocean -- it is doing that, and that does cool the water, but it's maybe a tenth of a degree or so. it's not really noticeable because it's such a huge heat reservoir. what the real hurricanes do is they churn up the ocean. you don't have to go very far down in the tropical ocean before you find very cold water. it's only hot right within the first 100 feet or so of the surface. the hurricanes come along and they mix this cold water up to the surface. and you can look at a satellite picture to see these really cold wakes that are left behind by hurricanes. and so the hurricane is cooling off the ocean temperature. narrator: this ocean cooling offers some explanation as to why real hurricanes do not always grow to their maximum potential intensity. when this effect is incorporated into the computer models, the simulated hurricanes act much more like real storms. this finding implies that a hurricane's relationship with the atmosphere and the ocean may be much more dynamic than previously thought, leading to further implications for how the atmosphere and the oceans moderate earth's climate. dr. emanuel: the reason that the climate isn't more extreme than it is is because both the atmosphere and the ocean take heat from the tropics and bodily move it to the poles. so we have warm air flowing from the tropics toward the poles. and we have cold air flowing from the poles to the tropics. the ocean does the same kind of thing. we have warm gulf stream waters flowing poleward. and underneath them, we have very cold water moving southwards. and if you change those transports somehow, you've changed the climate. narrator: emanuel's research is showing that hurricanes help the oceans to play this heat-transporting role. dr. emanuel: it was actually proved about 100 years ago that the only way you could make the ocean transport a lot of heat from the tropics to the poles is if you could turbulently mix hot water in the tropics down into the deep tropical ocean. that's what you need to do. and oceanographers argued for years about what's doing this mixing. we've done some calculations that suggest that global tropical cyclone activity -- "tropical cyclone" is a generic name for a hurricane -- is what's doing this mixing. narrator: putting this theory to the test, emanuel has built hurricane models where the hurricanes control the heat flux in the oceans. these models may provide an explanation for the climate on earth 50 million years ago -- the time known as the eocene heat wave, when the temperatures in the tropics were about the same as they are today, but the poles were much warmer. this is when there were crocodiles wandering around greenland and alligators in london and places like that. and this is a great mystery. but we think it was a very stormy climate and there were a lot of hurricanes and that this was driving a fantastic poleward heat flux in the ocean, which was responsible both for keeping the tropics relatively cool and for keeping the high latitudes warm. narrator: the idea that hurricanes are in some way driving the earth's climate complicates our understanding of how the climate system works, presenting new challenges as we try to predict future climate and hurricane activity. dr. emanuel: now, if this is true, it means that we have to completely rethink our understanding of how the climate works in general. because in the big, huge computer models that are used to simulate climate, this mixing is just specified. it's constant. it doesn't change with time or climate. it's specified out of ignorance. we don't know what else to do. if it's hurricanes that are driving this mixing, we have a different problem altogether. we have a different system dynamic, because the hurricanes themselves are functions of the climate. so this is a new branch of research that now ties hurricanes into the whole climate system in a way that might prove to be very interesting.