Howdy everyone and thank you for continuing on with module 9. In this video, we're going to start taking a look at the human impact on global climate. Human influence on regional and global climate did not begin with the onset of the modern industrial revolution like some like to believe. There is evidence that people have been modifying the environment over extensive areas for thousands of years. The use of fire and the overg grazing of marginal lands by domesticated animals have both reduced the abundance and distribution of vegetation. And by altering the ground cover, humans have modified important climate factors such as surface albido and evaporation rates. The most significant impacts are the recent post-industrial revolution additions though of carbon dioxide and other greenhouse gases into the atmosphere in addition to some human generated aerosols. Let's first take a look at probably the biggest contributor to human impact on global climate change. Now, in the last module, we learned that carbon dioxide makes up such a tiny fraction of our total atmospheric composition. Nonetheless, carbon dioxide is influential because it is transparent to that incoming shortwave radiation, but it's not transparent to some of the longwave radiation being emitted by the Earth. This is similar to how a greenhouse works. A greenhouse lets the sunlight in, but it does not let the heat escape, if that makes more sense. The world's industrialization over the past two centuries has been fueled mainly by the burning of fossil fuels like coal, natural gas, and petroleum. Combustion of these fuels adds great amounts of carbon dioxide into the atmosphere, which we'll take a look at next. Here on the right, I have shown the US total energy consumption in 2017. Now, this graph shows energy consumption measured in quadrillion BTU. A quadrillion is 10 raised to the 15th power or a billion million for context. And a BTU is short for British thermal unit which is a unit of measurement for the heat content of a fuel or energy source. So the burning of fossil fuels which includes the first three listed here contributes to about 80% of the total energy that was consumed that year. Now, the Keeling curve is a graph of the accumulation of carbon dioxide in Earth's atmosphere based on continuous measurements that have been taken at Monaloa Observatory on the island of Hawaii from 1958 to today. Now, this red zigzag pattern that you see here reflects the seasons. During the spring and summer in the northern hemisphere, we have photosynthesis by plants that are absorbing carbon dioxide and that actually pulls the line down. However, during the fall and winter, photosynthesis slows down and then we have the decay of plant material which releases that carbon dioxide back into the atmosphere sending the line back up. But the overall trend of the line has been steadily increasing since measurements began. Clearing of tropical rainforest is a serious environmental issue. In addition to the loss of biodiversity, which we could spend a lot of time talking about, deforestation is a significant source of carbon dioxide as well. Fires are frequently used to clear the land. This is a scene from Brazil's Amazon basin where humans are intentionally burning down the vegetation. And according to the UN, more than 25 million acres of tropical forests are permanently destroyed each year during the decades of the 1990s and the early 2000s. Carbon dioxide is clearly the most important contributor to human enhanced greenhouse warming, but other trace gases also have a role to play, such as methane and nitrous oxide. Even though methane is less abundant than carbon dioxide, it is 20 times more effective at absorbing outgoing radiation. Methane is produced by anorobic bacteria. And anorobic bacteria typically exists in areas with little oxygen. So swamps, bogs, wetlands, and the guts and termites of grazing animals like cattle and sheep. And this is why the meat industry gets a bad reputation when it comes to global warming. Agriculture is also responsible for an increase in anorobic bacteria and flooded patty fields used for growing rice too. And as our human population grows, so do the rice patty fields. And so does the number of cattle contributing to more methane to our atmosphere. Now, agriculture isn't the only culprit for adding methane. Mining coal and drilling for oil and natural gases are other sources of methane as well. Even though methane is a much more potent gas, it does not contribute as much because it rapidly oxidizes and brings the concentrations back down. Now, nitrous oxide, otherwise known as laughing gas, is also a greenhouse gas, and it can last in the atmosphere for 150 years. Agriculture is also to blame for the increase in this gas, although not as rapidly as methane. When farmers use fertilizers with nitrogen to boost their crop yield, some of that nitrogen enters the air as nitrous oxide. This gas is also produced by high temperature burning of fossil fuels. So, let's take a look at the atmosphere's response to these increases in greenhouse gases. Here's a graph showing global temperature change from 1880 to 2017. On the y-axis, we have the temperature difference measured in Celsius compared to the average temperature between 1951 and 1980. Over the 137ear span from 1880 to 2017, 17 of the 18 warmest years on record occurred after 2001. And as you probably know, since 2017, we've broken those records a few times already. Now, here's a map and chart showing the global temperature trend. The map on the left shows the average temperature in 2017 deviated from the average temperature between 1951 and 1980 base period. Now, all of the warm colors like yellow, orange, and red show an increase in average temperature while the cooler colors like blue shows a decrease in temperature. Do y'all see what colors dominate this map? On the right, we have a chart showing the change in temperature by decade. The decadeby decade temperature trend is increasing since 1950. Each record has been warmer than the previous one. And continued increases in atmosphere's greenhouse gas levels are driving a long-term increase in global temperature. Global climate is also affected by human activities that contribute to the atmosphere's aerosol content. Remember, aerosols are the tiny particles and drops of liquid suspended in the atmosphere. Most aerosols reflect sunlight back to space and indirectly make clouds brighter. Natural sources are numerous and include wildfires, dust storms, breaking waves, and volcanoes. However, most human generated aerosols come from the burning of vegetation to clear agriculture land, which we saw happening in the tropics and from the sulfur dioxide emitted during the burning of fossil fuels. One category of aerosols is called black carbon, which is soot generated by the combustion process in fires. Unlike most aerosols, black carbon warms the atmosphere because it is an effective absorber of incoming radiation. Also, when black carbon is deposited on snow and ice, it reduces the surface albido, thus increasing the amount of radiation absorbed. Nonetheless, despite the warming effects of black carbon, the overall effect of aerosols is actually to cool the Earth. Okay. Wow. So, Earth's climate is complex and it's a very difficult system to predict. In our next video, we're going to explore some feedback mechanisms in our climate system that makes prediction even more difficult. I'll see you there.