After completing this section, you should be able to define anthropogenic and discuss anthropogenic contributions to climate change. Specifically, you should be able to identify anthropogenic greenhouse gases that are increasing in concentration, discuss their sources, and discuss the impacts of these increased concentrations. You should also be able to discuss anthropogenic sources of sulfur aerosols and the impacts of increased concentrations.
The natural drivers of climate change that we covered in the last section don't tell the whole climate-change story, because they can't account for the observed temperature changes that have occurred since the late 1800s (especially the warming in the last 50 years or so). So, there must be other factors affecting Earth's climate. While you've already seen some ways that humans can affect weather and climate locally, now we're going to look at ways that human activities can affect climate on a global scale.
These human-caused changes to the climate are referred to as "anthropogenic climate change" (anthropogenic means human caused). A major component of anthropogenic climate change is global warming, which refers to a gradual warming of the earth caused by an unnatural (human-induced) increase of the greenhouse effect, as concentrations of greenhouse gases increase primarily from the burning of fossil fuels (coal, oil, and natural gas). To explore this contribution to climate change, let's first quickly review the greenhouse effect.
Remember that so-called "greenhouse gases," such as water vapor, carbon dioxide, methane, and nitrous oxide absorb and emit infrared radiation, and the contributions of downwelling infrared radiation from greenhouse gases to warming the planet are called the greenhouse effect. If you recall from our study of energy budgets, the emissions from clouds and invisible greenhouse gases contributed to the "downwelling infrared" traces on our energy graphs, like the one below from Penn State University. This particular graph came from a perfectly sunny day on March 11, 2012, so the downwelling infrared contributions primarily came from greenhouse gases.
Without greenhouse gases, Earth would be much, much colder; its average temperature would be nearly 60 degrees Fahrenheit lower! The greenhouse effect is natural, and the warming it causes is essential to sustaining life as we know it on Earth. But, since the Industrial Revolution in the late 1700s, humans have been burning carbon-rich, "fossil" fuels like coal, oil, and natural gas on a large scale, releasing additional carbon dioxide into the atmosphere. Emissions of carbon dioxide grew very slowly and gradually in the 1800s, but with population growth and still a heavy reliance on fossil fuels today, global carbon dioxide emissions have grown (Credit: U.S. Department of Energy) more than ten times from 1900 through recent years. The percentage of the world's energy coming from fossil fuels has dropped a bit in recent decades with the growth of nuclear power, wind, solar, and other renewable energy sources, but still remains at nearly 80 percent.
Before the Industrial Revolution, the atmospheric concentration of carbon dioxide was around 280 parts per million, but through the burning of fossil fuels like coal, oil, and natural gas, humans have added carbon dioxide to the atmosphere. The concentration of carbon dioxide in the atmosphere now exceeds 400 parts per million, and you can see the upward trend in atmospheric carbon-dioxide concentration since the late 1950s in the data from the Mauna Loa Observatory in Hawaii below.
Remember that carbon dioxide is the second most important greenhouse gas (behind water vapor) so increasing its concentration gradually results in a stronger greenhouse effect, which means more downwelling infrared being emitted toward Earth, causing the planet to warm additionally (causing a "global warming"). The anthropogenic increase in the greenhouse effect in particular helps explain Earth's warming since roughly 1970, during a time when the major natural drivers of climate change have favored a slight cooling.
While global warming from a strengthening of the greenhouse effect effect didn't start making mainstream news until the late 1980s and 1990s, it's hardly a new idea scientifically. As early as 1903, a Swedish scientist named Svante Arrhenius (a Nobel Prize winner in chemistry) noted that the burning of carbon-rich coal would likely lead to a warming of the planet because of increased carbon dioxide concentrations. His ideas were largely ignored at the time, not because other scientists doubted the greenhouse effect (indeed, knowledge of the greenhouse effect goes back to John Tyndall's research starting in 1859), but because of incomplete knowledge of Earth's carbon cycle, which we'll study more in depth shortly.
While the concentration of atmospheric carbon dioxide varies naturally (like many aspects of the earth system), studies of historical atmospheric composition based on air bubbles trapped in ice cores reveal that current-day concentrations of carbon dioxide are unprecedented in hundreds of thousands of years. As you can see from the graph below, carbon dioxide concentrations largely remained between roughly 180 parts per million and 300 parts per million for hundreds of thousands of years...until about 1950. Since then, carbon dioxide concentrations have continued to climb, and are now above 400 parts per million thanks in large part to the burning of fossil fuels.
While increased carbon dioxide concentrations from human activities get a lot of attention when it comes to anthropogenic climate change (for good reason), human impacts on the climate don't stop there. For starters, carbon dioxide isn't the only greenhouse gas that has increased in concentration. Atmospheric concentrations of methane have more than doubled since pre-industrial times, primarily from decomposition of organic matter (such as carbon-based garbage in landfills), agricultural and biological processes (livestock digestion and rice cultivation are two examples), and the production and distribution of fossil fuels. Nitrous oxide has also increased in concentration since pre-industrial times, mostly through agriculture (adding nitrogen to soils, which eventually gets released into the atmosphere), and through other various industrial activities that involve burning solid waste and fossil fuels.
I should add that while methane and nitrous oxide concentrations have increased from human activity, their concentrations remain much smaller than the concentration of carbon dioxide. However, each molecule of methane and nitrous oxide is actually more efficient at absorbing and emitting infrared radiation than is carbon dioxide, so while their concentrations are extremely small, we can't ignore these other greenhouse gases. Sometimes in our atmosphere, a little bit of something goes a long way!
The net result of the increases in greenhouse gases from human activities is that the atmosphere now retains about one percent more energy compared with pre-industrial times. That may not seem like much, but the increase in emission of downwelling infrared radiation helps explain the nearly 1 degree Celsius (1.8 degrees Fahrenheit) of warming that has occurred globally since 1970.
Interestingly enough, not all aspects of human activity, and the burning of fossil fuels in particular, lead to warming on a global scale. Burning coal, oil, and natural gas is also a source of air pollution, which includes sulfur gases and tiny solid particles (soot, ash, etc.). Additionally, sulfur gases (especially sulfur dioxide) can react with other substances in the atmosphere to form tiny liquid drops or solid particles (aerosols), which can serve as cloud condensation nuclei. The net effect of these aerosols (and any subsequent cloud formation) is to increase the amount of solar radiation that gets scattered back to space, which reduces the amount absorbed by Earth's surface. So, the aerosol byproducts of industrial activities actually favor a cooling of the planet.
Scientists estimate that during the first half of the 20th century, much of the human-induced warming from increasing greenhouse gases was actually offset by decreased incoming solar radiation because of aerosols. But, with many governments more heavily regulating air pollution by the 1970s (the Clean Air Act of 1970 in the United States is a good example), aerosols are having less of a cooling effect in recent decades. The reduction in sulfur dioxide in the atmosphere is directly beneficial to human health (air pollution is bad for your respiratory system), but it has increased the rate of earth's warming due to human activities, as greenhouse gas emissions have become the dominant factor.
The video below (8:07), which features Dr. Katharine Hayhoe, Director of the Climate Science Center at Texas Tech University, provides a nice summary of how scientists have come to realize that recent global warming is driven by human activity. It combines many of the topics we've discussed in the last two sections. Before watching, however, I want to offer one caveat: In an effort to simplify the topic, Dr. Hayhoe uses common analogies for greenhouse gases, likening them to blankets that "trap heat." Remember that this is an oversimplification, and doesn't really describe how these gases keep the earth warmer. As you've learned, these gases are important because they actively absorb and emit infrared radiation. In other words, they're more like space heaters than a blanket (which warms you up by limiting convection away from your body). As long as you keep that in mind, I think the video will help you put the pieces of natural and anthropogenic climate change together.
Since the dominant human influence on global climate has become the emission of greenhouse gases, particularly carbon dioxide, we need to spend some time exploring how carbon cycles throughout the earth system. The fact that carbon gets exchanged between the earth and atmosphere naturally complicates our climate picture somewhat. Read on!