now we come to the final question question seven on your review quiz most scientists consider that the single most serious environmental problem facing mankind is and you had to write in something of your choice you gave a wide variety of answers but any of you picked the right answer which is global warming also known as climate change some of you are probably beginning to realize that climate change is the defining environmental issue of today it's important that you do understand it i think i can say that your future depends on it so global warming or as it's usually called now climate change what is it here's the globe why is it getting warmer is the sun sending more heat to the earth now than it used to no it's not sun's been sending pretty much the same amount of heat to the earth for millions of years unchanged the important thing is not how much heat comes to the earth it's how much of that heat gets to leave again long term all the heat that arrives from the sun has to leave the earth the amount leaving must exactly balance the amount arriving think about what would happen if that wasn't true if day by day the earth received more heat got rid of less every day the temperature would have risen for the last four billion years by now the planet would be red-hot no survivors conversely if the earth consistently lost more heat than it receives it would get colder and colder we'd be in a permanent ice box the fact that we have pleasant livable conditions means that all the heat that arrives from the sun also leaves question is how soon how quickly if we go to a place that doesn't have an atmosphere like the moon moon is the same distance from the sun that we are without that atmosphere conditions are very extreme if you stand in the sun temperatures rise above the boiling point of water if you step into the shade temperatures instantly drop far below zero no middle ground nothing little here's a little interesting point on that when we sent astronauts to the moon back in the late 1960s early 1970s we were very careful to plan the landing sites every landing took place at a point on the moon that was close to dawn what does that mean when the sun is low in the sky think of pictures that you may have seen of astronauts on the moon long black shadows low sun now the idea of landing near dawn here on earth doesn't make a whole lot of sense you land near dawn and two or three hours later it's not near dawn anymore the earth spins once every 24 hours the moon however spins in about 28 days so you can land on the moon close to dawn spend two or three days doing field work studying the area and then take off again and it's still pretty close to dorm so why did we pick landing spots close to dawn because with the low sun one side of the astronaut's body was receiving all the heat from the sun the other side of the astronaut's body was in shade one side above boiling the other side below freezing so inside the astronaut suits there were systems of water little water pipes running all the way around the body and a pump in that big backpack that they wore circulating the water carrying the heat from the hot side to the cold side so the astronaut inside remained comfortable without the hot side and the cold side it would have been very difficult to effectively maintain body temperatures so those extremes where there's no atmosphere here on earth we have an atmosphere around the planet that atmosphere contains certain gases which hold onto the heat and delay its departure it gets away eventually but we hold on to it for a while first this phenomenon is called the greenhouse effect what is it how does it work well first of all for you guys what is a greenhouse people in southern california don't know what greenhouses are okay a greenhouse is a small building freestanding or lean to up against a wall entirely made of glass and it's got a wooden frame but all the panels of the roof the walls the door are all glass inside the greenhouse we grow plants flowers fruits vegetables okay the energy coming from the sun comes in as short wave electromagnetic radiation which goes through the glass like it wasn't there hits the interior of the greenhouse the ground the plants whatever heats it up the heated surfaces inside the greenhouse give off longer waves why the difference well the sun is at a temperature of thousands of degrees and therefore gives off the short waves the earth is only heated up to temperatures on tens of degrees and therefore gives off the longer waves the longer waves reach the glass of the greenhouse and they are turned back they can't get through something you could try yourself if you're in your car roll down the window the sun shining on you is hot roll up the window and the sun doesn't feel as hot anymore when it's shining directly on if you have a fire in your fireplace hold up a sheet of glass in front of you you can still see the fire looks just the same but as soon as you hold the sheet of glass up in front of your face suddenly most of the heat is gone feels much cooler okay so the short waves get in and heat up the interior the long waves can't get out the interior stays nice and warm take a look at this picture so in somewhere like alaska minnesota wisconsin we can grow tomatoes tropical vegetables tropical orchids summer and winter without a phenomenal heating bill here in southern california we don't need extra heat so we don't have greenhouses where we grow plants inside a building the purpose of the building is to reduce the amount of sunlight control the humidity keep out wind things like that and so we are very careful not to make those buildings out of glass we make them out of plastics and stuff like that but don't exhibit a greenhouse effect okay some of the gases in the atmosphere behave in a similar way to the sheet of glass in the greenhouse they let the short waves come in and they block absorb keep hold on to the longer waves and as a result this warms up the surface of the earth what gases are we talking about the atmosphere consists of 78 nitrogen and 21 oxygen which by the way adds up to 99 neither of those gases is a greenhouse gas nitrogen is not a greenhouse gas oxygen is not a greenhouse gas pay close attention to this animation it shows how you can tell whether a gas is a greenhouse gas or not notice that greenhouse gases have molecules with at least three atoms so that there's a middle atom that can vibrate and thus trap energy do you know what the most important greenhouse gas is it's water vapor water vapor is responsible for most of the greenhouse effect the process that keeps the earth warm and livable it's not carbon dioxide then why is it you hear about carbon dioxide all the time well carbon dioxide is the second most important greenhouse gas the reason that we worry about this one and not about that one has to do with something that scientists call residence time once the gas goes into the atmosphere how long is it going to stay there the residence time for water vapor is around three days think about if we somehow add a whole lot of extra water vapor humidity into the atmosphere what's nature going to do about it right more rain gets rid of it and back to normal in three days in other words nature is completely self-regulating where water vapor is concerned we can't mess it up well more on that later basically we can't mess it up nature takes care of it carbon dioxide on the other hand has a residence time of around 250 years which means that if we put a load of carbon dioxide into the atmosphere it is going to stay there for a long long time and we are putting carbon dioxide into the atmosphere steadily principally from the burning of fossil fuels coal oil and natural gas the amount of carbon dioxide in the atmosphere before we started burning all this fossil fuel before the industrial revolution was around 280 parts per million it has now passed 400 and goes up about another 10 every year so we haven't doubled the amount of carbon dioxide in the atmosphere yet but we are headed in that direction a major major increase