In the first few videos of this module will review the fact that the planet is warming and then discuss the science behind such warming. In the process I hope you'll see why scientists are convinced that the observed increase in global average temperatures over the past few 100 years is a direct result of all the greenhouse gasses we humans have been putting into the atmosphere. And once you understand the science behind global warming, which by the way, isn't all that complicated, you'll have the confidence to explain it to others. That may also be looking for answers. To get started we'll introduce the basic idea behind the greenhouse effect and the relationship between greenhouse gasses, a warming planet and subsequent climate change. As we'll see, greenhouse gasses are actually a good thing, but like so many things in life, too much of a good thing creates problems, and that is our topic for the next few lessons. Initially, we'll focus on carbon dioxide, one of the most important greenhouse gasses, and the dramatic increase in atmospheric carbon dioxide levels over the past few 100 years. Finally, we'll show the very strong evidence that this rise in carbon dioxide over the last century has largely been a result of burning fossil fuels such as coal, oil and natural gas. That means the increase in temperature we've discussed so far is because of us and as a result, it's up to us to fix it. Let's get started. In the last module we showed that the planet was warming here is another look at that same data plotted in a slightly different way. The graph is the average global temperature from 1880 to today. It is the same temperature data that we showed last time only now plotted relative to the average temperature across the entire 20th century. In other words, relative to the average between about the years 1,900 and 2,000. This plot generated by the United States Noah Organization is another very commonly used graph you might see when studying climate change. Yet, while it might look a bit different from the first one we discussed in great detail last time, it shows the same thing, that the planet has warmed an average of one 2°C or 2.2° Fahrenheit since 1,900. And is continuing to increase today. The question that scientists have been thinking about for decades now is what is actually causing this global warming? Certainly there are natural causes to global temperature changes. As we've seen how these play out over history, for instance, ice ages followed by periods of warming. Some natural causes are related to the earth's position relative to the sun. Other causes include the sun's intensity, which in fact varies over time. We also know that volcanoes and forest fires emit quite a lot of carbon dioxide, which is a greenhouse gas. And as we'll see increasing levels of greenhouse gasses can also have an impact on the climate for a long time. Now, scientists have been able to create very sophisticated mathematical models based on all these effects, which are all very well understood from a physics standpoint. The problem is none of these explain the increasing temperature we've observed over the last 125 years or so. The only way to explain global warming is to take man made causes into effect, also known as anthropogenic effects. For instance, if you look at greenhouse gasses that are due to us, then the picture gets a lot clearer. When climate scientists from around the world take their mathematical models about global warming and include the human contribution, they all tend to agree, global warming is mostly due to anthropogenic greenhouse gasses. What does this mean in a nutshell that global warming is due to human effects, specifically to the greenhouse gasses that we've added to the atmosphere since the Industrial Revolution began a few 100 years ago. But before we dive into where all those greenhouse gasses are coming from, let's review the whole idea of the greenhouse effect to make sure we understand the link between greenhouse gasses and the earth's temperature. I'm sure many of you are familiar with the phenomenon known as the greenhouse effect, but let's go over the basic ideas. So, we're all on the same footing. As we see in this animation, energy from the sun impacts the planet all day long. This is shown as the yellow energy rays coming in from the left hand corner. Some of this energy is reflected off the Earth's atmosphere back into space shown by the small yellow ray. Even more energy is reflected off the surface of the earth and back out into space. But a significant amount of the sun's energy is absorbed by the planet, warming it up just like it warms us up when we're outside in the sunshine and this is where things become important. As the Earth's surface warms up it gives off thermal energy, technically known as infrared radiation, that is shown by the red energy ray emitted by the Earth. Much of this energy ends up going back into space, but some of it gets absorbed by specific gasses in the atmosphere. And reflected back towards the planet, as shown by the smaller red energy ray to the far right. Importantly, the gasses that absorb and re radiate the earth's warming thermal energy are known as greenhouse gasses. By definition greenhouse gasses or GHGs as well often call them are those gasses that absorb and then emit infrared radiation. We'll get into the details of that in a later lesson. For now, you should start to see a major implication of all of this, heat in the form of thermal radiation is trapped in the atmosphere by greenhouse gasses. In more practical terms, greenhouse gasses act like an insulating blanket around the planet, keeping the planet warmer than it would be otherwise. And this is known as the greenhouse effect. A common misconception is that all greenhouse gasses or GHGs are bad, but imagine what would happen if they didn't exist at all? We had no greenhouse gasses in our atmosphere. In this case as shown in this animation, all the heat radiating from the planet would just go right out into space. Nothing would be reflected back to us. In other words, we would lose our thermal insulating layer around the planet and it would be a whole lot colder. In fact, scientists calculate that without greenhouse gasses keeping the planet warm, the average temperature would plunge to minus 16° C are only 3°F. And what does that mean? The world would be one big ice box in a perpetual state of an ice age. So, why do greenhouse gasses get such a bad reputation? Because as we'll see, we humans have greatly disrupted the normal levels of greenhouse gasses in our atmosphere, adding more and more ever since the Industrial Revolution. So, much more that we've increased the influence of the greenhouse effect and now it's warmer than the planet should really be. Let's go back to our high school chemistry class once again and understand a bit more about our atmosphere. Our atmosphere is made up of about 79% nitrogen gas or N2, and about 21% oxygen or O2. There are small amounts of other gasses such as water vapor, H2O, carbon dioxide CO2, methane chemically known as CH4 and lots of other types of gasses. Some good and some not so good. While we need most of these gasses in our atmosphere, not all of them are greenhouse gasses. As we'll see in the next few lessons, nitrogen and oxygen are not greenhouse gasses at all. And though they make up most of our atmosphere, they don't do anything related to the greenhouse effect. Heat from the earth would just go right through the atmosphere and into space, bringing us back to the ice ages once again, and most of us would just freeze to death like we saw in the last video. So, the presence of greenhouse gasses or GHGs is essential to keeping our planet warm enough for all of us and our fellow plants, and animals to thrive. And for a very long time that nice warm temperature was an average of 15°C and 59°F. An analogy I often think about, is relating the presence of greenhouse gasses to having a blanket when you sleep at night. If you didn't have any blanket, your body heat would just radiate into your bedroom and you would be pretty darn cold. This is the same as if there were no greenhouse gasses, the Earth's heat would radiate out into space and it would be a lot colder than we would like at the other extreme. What if we had a really thick blanket? In this case the blanket is capturing most of your body heat and you might be uncomfortably hot. Maybe even sweating. This isn't such a great situation either. If greenhouse gasses get too high, then they too have the same effect as a very thick blanket. They trap the heat generated by the earth and the earth would become uncomfortably warm. To get the best rest at night you need just the right blanket, one that keeps you warm, but not too warm. Similarly, there is an optimum level of greenhouse gasses too, just the right amount to keep the planet warm, but not too warm. And this is where things get interesting. If we didn't have greenhouse gasses, it would be a whole lot colder. If we have levels of greenhouse gasses that are too high, then it would be a whole lot hotter for us. We've managed to be quite comfortable with the level that has been around for the last 100,000 years or so. And that's a topic for our next lesson. Let's summarize our discussion of the greenhouse effect with a few key takeaways. For one while there are clearly natural causes that contribute to global warming, none of these account for the increase of 1.2 degrees Celsius or 2.2 degrees Fahrenheit that we've observed so far. The only explanation for global warming over the last century or so is due to a rise in greenhouse gasses, mostly coming from us as well, see enhancing the greenhouse effect. But remember for the most part, greenhouse gasses are a good thing. They have kept the planet at a very comfortable and stable temperature for thousands of years. But like so many things in life, too much of a good thing is no longer such a good thing. As we'll soon see, we've added quite a lot of greenhouse gasses to the atmosphere. And those higher levels of greenhouse gasses trap more heat, thus leading to the observed increase in global temperatures. And so in the end, we're the ones responsible for global warming and therefore it's our responsibility to figure out a solution to it before it gets too late. For our next lesson, we're going to show you just how much greenhouse gasses have been increasing. We'll explore something known as the keeling curve, which is one of the first indicators of rising greenhouse gasses in our atmosphere. Well then link this back to the greenhouse effect, there's a lot more to come. I'm Michael Reedy and I'll see you next time.