Here's another conclusion of the most recent IPCC report. Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven largely by economic and population growth, and are now higher than ever. This has led to atmospheric concentrations of carbon dioxide, methane, and nitrous oxide that are unprecedented in at least the last 800,000 years. Their effects, together with those of other anthropogenic drivers, have been detected throughout the climate system and are extremely likely to have been the dominant cause of the observed warming since the mid-20th century. So a lot of you are probably already familiar with the greenhouse effect, but let's go through it anyway as reminder and for those of you who aren't familiar with it. So of course, solar radiation comes in, hits the Earth's surface, and most of that radiation is absorbed by the Earth's surface, and that's what provides the Earth with all of its energy and we of course depend completely on the sun for energy. Now some radiation is emitted by the Earth surface as infrared radiation, and some of that infrared radiation just goes off into space. But other of that infrared radiation is absorbed by greenhouse gases, and we'll talk about what gases those are in a minute, and those greenhouse gases then re-emit that energy, that infrared radiation, and some that goes back to the Earth's surface and helps to warm the Earth. So this is actually a natural process. There are naturally greenhouse gases in the atmosphere. In fact, having those greenhouse gases in the atmosphere is really essential for life. So greenhouse gases are not bad. So the Earth's mean temperature without this greenhouse effect, if there were no greenhouse gases in the atmosphere would be minus 18 degrees centigrade or minus 0.4 degrees Fahrenheit, but with this natural greenhouse effect, the mean temperature is 15 degrees centigrade or 59 degrees Fahrenheit. The problem is that humans have perturb the carbon cycle resulting in this kind of unnatural emission of greenhouse gases that has increased the levels of greenhouse gases in the atmosphere substantially in a very short period of time. So how have humans perturb the carbon cycle? So first and foremost is the extraction and combustion of fossil fuels. The timescale of fossil fuel formation was millions of years, very slow process, the timescale of extraction and burning of fossil fuels has just been hundreds of years, basically since the start of the Industrial Revolution around 1750 or so. The second way that humans have perturb the carbon cycle is through land-system changes. Those being deforestation and soil degradation through unsustainable agricultural practices. So both trees and soil holds a tremendous amount of carbon and that carbon, as carbon dioxide, is released into the atmosphere with deforestation and soil degradation. So this slide shows time trends before 1850 to the near present of global anthropogenic CO_2 emissions. You can see that there's an important increasing trend starting around 1940 or so and going really up to the near present and that most of that or really all of that increase has been due to increase in the burning of fossil fuels. Here we focus on anthropogenic carbon dioxide emissions from 1990 through 2018. These data from the Global Carbon Project released in the fall of 2018, we see that emissions are still increasing with the projection of a 2.7 percent increase between 2017 and 2018. So what happens when you emit all this CO_2 in the atmosphere? Well, that increases the concentration of CO_2 in the atmosphere. So let's focus in this graph on the top graph and which is for CO_2. Here we're going all the way back to the year zero to the near present. So we can see that atmospheric CO_2 was constant at roughly 280 parts per million for many years and that it starts increasing around 1800 or so, again once the Industrial Revolution gets going and we see an acceleration of that increase starting maybe let's say here, around 1940 or so. There are other greenhouse gases apart from CO_2. There's methane, this graph shows the trends for methane, and there's nitrous oxide. This graph shows the trends for nitrous oxide. We're not going to go into those in too much depth, but it's important to note that they have been increasing as well. One more slide on the atmospheric CO_2 increase. So the Mauna Loa Observatory in Hawaii has been measuring atmospheric CO_2 continuously since before 1960. So this shows the trends that they've seen since 1960 going from around 310 parts per million and the latest figure, I just looked it up, is 411 parts per million. So as we've seen, greenhouse gases absorb and readmit infrared radiation, not every gas is a greenhouse gas. In fact, nitrogen and oxygen which are the two most common gases in the atmosphere are not greenhouse gases at all. So only certain gases will absorb and readmit infrared radiation. Also this may be surprising, water vapor itself, water is a greenhouse gas. It turns out it's the most important greenhouse gas. But water is not a direct concern with regard to the climate change and global warming because human activity is not putting more water into the atmosphere as it is putting more carbon dioxide and methane and other greenhouse gases into the atmosphere. So I've already mentioned methane and nitrous oxide the other ones to note briefly or ozone which is a greenhouse gas as well as halocarbons. Halocarbons are human-made chemicals that are used as refrigerants, also they had been used in aerosol cans until that activity was banned, and they're the same class of compounds that cause ozone depletion. So this slide shows global greenhouse gas emissions by gas. We can see that about two-thirds of emissions are carbon dioxide from centrally fossil fuel burning. Then another 11 percent of emissions are also carbon dioxide from forestry and other land use as described earlier. So the three-quarters in other words, of human greenhouse gas emissions are carbon dioxide, but we should also note that methane is important, it represents 16 percent of global greenhouse gas emissions. So the lesson here is that it's important to think about methane when thinking about how to control and mitigate climate change. So sticking with methane, what are the sources of anthropogenic methane? So to understand the sources we need to know that methane is a product of metabolism of certain microbial microbes called methanogens, and so they live for example, in the guts of cows and pigs, so enteric fermentation represents more than a quarter of anthropogenic methane emissions and there are also some other minor agricultural components to that. The other major source is the extraction of fossil fuels and where we have oil and gas being 24 percent, coal mining being nine percent. So natural gas consists mainly of methane. So when we mine natural guess it leaks at various points of the mining and then also at the transport, etc. So that's an important source of methane. Natural gas is also and methane is also associated with oil as well as with coal. So that's why we see that emissions from the fossil fuel industry.
