 So let's think about, for a moment, the sustainability of exploitation of a resource. How long can we do it for? And that depends on your perspective. You might wish to make a lot of money very quickly out of exploiting a resource and be thinking, oh, if it lasts 10 to 20 years, I'm not going to be around after that. Let's go for it. And that's the classic fisherman's dilemma, in fact, that you don't want your competitors to take the last cod out of the ocean. You're going to go get it first. So there's a degree of mutual distrust about the way you exploit a resource. But there are many resources that we regard as sustainable. So sustainable or renewable. I might use that sort of interchangeably. And there are some that are quite clearly, like the sun. Sun, solar, sustainable for billions of years. We'd be crazy, you know, not to use this as a source of energy. Of all the possible sources of energy we have, that beats the definition of sustainable. If you like, as far as we're concerned, almost infinitely in terms of time. There are other renewable resources, though, which fundamentally depend on the time scale that you're thinking about. For example, if you were somebody living in an age when wood burning was the primary source of heat for warmth and cooking, you'd think that trees were a renewable and sustainable resource because the population size wasn't sufficient to diminish the size of the stock. As population increases, then clearly your view of what was sustainable or not in terms of forest would change. For example, the British used so much wood in the construction of their navy and merchant fleet that they had driven the stock of timber to very low levels and were forced to start importing timber to supply their needs in that regard from other countries. And there are plenty around that they could go to and acquire timber. As you probably know, one of the first places that was settled in Australia very quickly after the first fleet of the British arrived was to send a group to Norfolk Island. And they were sent there, not originally as the hardcore criminals, but as people to exploit the timber and the flax, as reported by early explorations, by Captain Cook. Now the timber turned out to be useless for what they wanted it for. It was a hope that that would supply spas and equipment for the ships. And it turned out that the Norfolk pines were actually not good enough for that. And eventually the flax proved to be the wrong kind of flax so that they ultimately, having treated it as a hardcore penal colony, abandoned the island for some period of time. So it depends on your view of the resource and how quickly you're going to use it, or how fast, whether it's sustainable or not. I would like to now turn to energy resources. I've already said you're crazy not to think about that. By the way, the way the sun differentially heats up our atmosphere leads to movement of the atmosphere itself, and we call that wind. So as long as the sun shines, you'll get wind with an atmosphere on this planet. We also, as you well know, are located at just the right distance from the sun with the available incoming radiation and the atmosphere we have that most of the H2O water occurs as liquid water. And we can also, I'll just abbreviate that, so long as that keeps going and as long as we've got winds and we've got an atmosphere we will get liquid water so we can exploit water running downhill as an infinitely available resource to power supplies with energy and supplies with other needs like being able to drink it. So there's this group. Timescale, irrelevant as far as humans are concerned, that's always going to be there as far as we're concerned. But I would like to address another source of energy that, of course, is a hot topic in terms of our current view of how we should use it. And these are the carbon-based energy forms, so oil, gas, coal, peat, et cetera. Now, I should tell you that I'm a geologist and you might immediately think that would label me as an arch exploiter, a pillager of the earth and all those epithets that are cast at some geologists. That's an acceptable criticism, but I would say that geologists actually have probably a view of the earth that is longer and more balanced than many other sciences. And there are ways of saying we recognize everything that's wrong with taking these materials and burning them to supplies with energy. Now, all of these, in a sense, are potentially non-renewable. Why would I label them non-renewable? Well, all of these primarily require some length of time and elevated pressures and temperatures bearing them in the earth's crust to convert them to a form where we can use them. Oil, after all, is essentially the product of the death and preservation of marine plankton. So all of these are carbon-hydrogen-based forms. There's some oxygen in there, too, but CHO compounds. Gas is essentially a gaseous form of oil. Just is made at a lower pressure and temperature window than oil. If you pressurize gas to greater depths and heat it up a little more, you convert gas to oil. They occur together. All of this, though, takes millions of years to form. So a plankton falling to the bottom of the ocean now, and it has to be anoxic. The bottom of the ocean has to be anoxic to allow it to stay as CH and not respire back to carbon dioxide and water. The bottom of an anoxic ocean, and you bury that carbon-hydrogen compound with other sediment, mud, you can eventually, after a long period of time, convert it to these forms of energy. It takes a long time. The rate at which we extract these exceeds the rate at which it's being replenished in the Earth. These are going to run out because we're consuming them at such a rate. Coal, similarly. These are land-based plants. We're lucky there are plants and photosynthetic plants and planks on around that have given us this as an energy source for several hundred years. Pete is a soggy, damp version of coal. It's on its way to becoming coal, but it has not been compacted, pressurized, heated long enough yet to turn it into coal. But you can burn Pete as long as you get the water out of it. So here are, I would argue, non-renewable energy sources. We are using them faster than they're being replenished.