PROFESSOR BRYAN KARNEY: Welcome to week one of Our Energetic Earth.This is a MOOC that we're going to use to explore this amazing planet andsome of its remarkable energy transformations and systems.And we welcome you.We're hoping that you're going to come along for the ride and explore thesefascinating systems.In fact, I hope when you think back that you can think or some of theexperiences that you've had with energetic systems--a time that you've gone out, and you've perhaps enjoyed whitewaterrapids on a river.Or perhaps you've enjoyed the sound or the experience of waves at a beach.Or perhaps you've been in a thunderstorm or even amore violent event.The Earth is full of the textures and the varieties of energytransformation systems.And that's what this course is all about.And what we're going to do in each week is we're going to start each weekwith a brief introductory overview that I'm going to give you of whatwe're going to be doing for the week.And then we're going to reflect at the end of each week on some of thesignificance and some of the observations.And that will be a kind of a pattern for us for the next few weeks.This first week, what we're going to think about is the biggest conceptsthat relate to energy for the planet as a whole.Those really big concepts are really remarkable.It's taken humans a great deal of time and effort to come togrips with these things.But they're very much worth the effort and worth the trouble.One of the first ideas that we have is that really fascinatingconcept of a budget.I don't know if you've ever planned a major event in your life, if you'veplanned a wedding or a major family event where you've wanted to do abunch of special things but you've realized that you couldn't spend asmuch money as you might want to.You've had to have a budget to make things work, a sort of upper bound onthe amount of money that you could spend.The Earth has a budget as well.The Earth has an upper bound in the amount of energy that it uses.And that upper bound is set primarily by the energy input that weget from the sun.And the first thing we're going to trace in the first week is what isthat energy input and how it varies.One of the things that's fascinating is, much like money, energy on theEarth is not distributed particularly uniformly.Energy is, in fact, distributed quite non-uniformly.And we have areas of the Earth which are very rich in the amount of energythat they receive and other areas of the Earth which are much poorer in theamount of energy that they get from the sun.It turns out that there's a fair bit a transfer that takes place between theenergies that receive a lot and the energies or the parts of the Earththat receive less energy.And we want to look at those transformations and see how thisenergy conveyor works between different parts of the system.So we're going to look at that budget and look at the way it's allocated anddistributed and look at the way it relates to the Earth.And it turns out that one of the very fascinating and central ideas is ageometric argument.The Earth has a spherical shape, which means that there are certain parts ofthe Earth's surface which are oriented to be perpendicular to the sun andcertain parts of the Earth which are oriented that in a sense slant away orbend away from the directions of the sun's rays.This, of course, gives us the idea of the polar regions andthe equatorial regions.And those are marked by this decisive difference in their energy inputs andin the energy budgets that they have to live under.So we get that first idea of the geometric influence.And it turns out that that geometric influence is very strongly biasedtowards different times of year.And that's because the Earth is oriented not perpendicular to theplane of its orbit around the sun but in fact slanted away from that, whichgives a very decided bias to either the northern hemisphere or thesouthern hemisphere.So one of the first things we've learned is that there is a singleexplanation, really, for why the Earth is cold when we move towards the polesand why, in fact, we experience seasons.And the origin of both of those things arises from this concept of a budgetand the concept of a fixed input of energy perpendicular, but then bendingaway from this geometric component.One of the background questions which we're not going to answer at the verybeginning-- but of course you should be thinking about--is what is energy, anyway?What is this mysterious thing that we use to explain so many differentprocesses and so many different variations?One of the key things you probably already know about energy is energy istransformed in a simple device like a pendulum, which swings up and turnsits motion into potential energy, and then swings down and turns it intokinetic energy.We have a conversion process of two different kinds of energy.If you watch a pendulum for some time, though, you'll see that it willgradually decay.It will no longer have a complete conversion of potential energy intoenergy of motion, but in fact it will gradually dissipate.And the bottom level of this energy transformation is thermal energy.And that turns out to be quite general.In fact, almost all forms of energy are interchangeable, to a very largeextent, except for thermal energy.It's the sort of the bottom foundation.We can get energy into thermal form.But it takes a lot more cleverness even to get any of it out.And it turns out there's very good arguments that we can'tget all of it out.Some of it is irrecoverable in that form.And these ideas go together for one of our other key concepts that we'regoing to get right at the beginning of this course, which is the idea ofenergy cascade, of energy coming in at a top level in a fairly availableform, and then through a sequence of transformations coming out at thebottom level having experienced this process of decay.And in order to illustrate this, I thought I'd put on my screen behind mehere a picture a cascade of water.This is a waterfall where the water comes in the top level with a maximumof potential energy.And it cascades through a sequence of steps.It comes out at the bottom level with much the same basic properties that ithad at the top except it's lost that gravitational potential energy.And that's going to give us that secondary process, which is the ideathat even within these transformations where energy isconserved, energy also decays.