Prioritize...
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.
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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 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 can be traced back to Eunice Foote's research in 1856 and John Tyndall's work 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.
Summary
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.
KATHERINE HAYHOE: All this worry about warming when it's just a natural cycle. Climates always change. And today is no different, right? If we want to understand why the earth is warming, it just makes sense to start by taking a closer look at all the natural factors that have caused climate to change in the past. Are they to blame for the current warming? Or do these natural suspects have an alibi this time?
Let's start with the sun. We get nearly all our energy from our nearest star. And we know that the sun's energy tends to fluctuate a bit over time, because the sun is actually a variable star. When its energy goes up, the earth gets a bit warmer. When the sun's energy goes down, the earth gets a bit cooler.
What do we see then when we track how the sun's energy has changed over time? Observations show a repeating 11-year cycle that's correlated with sunspots and solar activity. But observations also show a small long-term increase in the sun's energy through the last century, that is until we hit the 1970s. Since then, the sun's energy has been going down, not up.
So over the first part of the 1900s, increasing energy from the sun did contribute to some of the warming of the planet, though not much. But over the last 40 years or more, the opposite has been happening. If the Earth's temperature were controlled primarily by changes in the sun's energy, we would have been cooling, not warming. So the sun can't be causing us to warm now. It has a perfect alibi.
What about volcanoes? Volcanoes can affect climate in two ways. First of all, when they erupt, they can spew enormous amounts of soot and dust and ash into the atmosphere. If the volcano is powerful enough, that dust and ash can reach all the way up into the stratosphere where the particles can circle the globe for months and even years.
But volcanic ash and dust doesn't warm the Earth. Instead, it acts like a giant umbrella, reflecting the sun's energy back to space, making the earth cooler. So, clearly, volcanic eruptions can't be responsible for the warming.
What's the second way that volcanoes can affect climate? Well, in geologically active areas like Sicily or Yellowstone National Park, powerful heat-trapping gases like carbon dioxide and methane can seep out from deep inside the Earth's crust. This can occur during volcanic eruptions. But the majority of it actually happens through what they call mud volcanoes, or mud domes.
We know that these gases are an important part of the Earth's natural blanket that keeps us nearly 60 degrees Fahrenheit or over 30 degrees Celsius warmer than we would be otherwise. So are the heat trapping gases that are seeping out of the ground in these areas responsible for our current warming? Well, no. It turns out regular volcanoes, little mud domes, and other geologic activity produce around 1% of the carbon dioxide and less than 15% of the methane that we humans produce every year. So while it's true that long-term geologic activity can warm the planet, its contribution over the past 100 years is pretty much minimal compared to how much of those same gases are being produced by humans.
So if it can't be the sun, and it's not volcanoes, what about natural cycles? Could they be causing our current warming? Natural cycles like El Niño and other lesser known cycles like the Atlantic Multidecadal Oscillation are all part of the variability that's built into the climate system. Because these cycles are internal to the climate system, they can't spontaneously generate heat. That would violate the fundamental physical law of conservation of energy. Rather, these cycles effect the Earth's temperature by redistributing heat around the world, usually by altering the balance of heat stored in the atmosphere versus the ocean.
During a La Niña episode, for example, more heat is transferred from the atmosphere into the ocean, so global average temperature tends to dip a little below average. In contrast, during an El Niño year, more heat is relocated from the ocean into the atmosphere, so global temperature tends to tick up. And El Niño years, like 2016, tend to be a little warmer than average. If natural cycles have been causing the atmosphere to warm, they could only be doing this by moving heat from the ocean into the atmosphere. Over the last century then, if the heat content of the atmosphere has increased due to natural cycles, then the heat content of the oceans should have decreased to balance out the total heat in the system. So is this happening? No, it isn't. The heat content of both the ocean and the atmosphere is going up. Not only that, but the increase in the ocean heat content is 20 times greater than the increase in the atmosphere and the land and the ice all put together.
If the entire climate system is warming, natural cycles inside the system can't be causing it to warm because all they do is move heat around. So the warming must be coming from somewhere else. And that leaves us to our last natural suspect, orbital cycles. Over time, the Earth's orbit around the Sun becomes more circular and then more elliptical. The Earth's axis of rotation also precesses, just like a top when you spin it. These slow, periodic, and, most importantly, predictable changes affect how sunlight falls on the earth, which in turn affects the Earth's temperature. We've known for almost 100 years that these orbital cycles are responsible for the ice ages and the warm periods in between, like the one we're in right now.
So it just makes sense to ask, are we still coming out of the last ice age? Well, no. If we look at the Earth's history, we see that warming after the last ice age peaked about 6,000 to 8,000 years ago. Since then, the Earth's temperature has been very gradually, slowly decreasing on a long slide into the next ice age. We can even calculate where we are in these orbital cycles. And when we do, it explains what we see in the observations. The next thing on our geologic calendar sometime in the next few thousand years was another ice age, was, that is, until that long, slow cooling trend stopped and then abruptly reversed just about 200 years ago.
The sun, volcanoes, and natural cycles, they all have an alibi. Now it's time to look for a new factor that might be causing the planet to warm. And it turns out that scientists have known about such a factor for over 150 years. The heat-trapping gases we produce whenever we burn coal or gas or oil as well as from deforestation, land use change, and agriculture, these gases are wrapping an extra blanket around our planet. This blanket is trapping heat inside the climate system that would otherwise escape to space. That's why we're warming. For the first time in the history of our planet, it's us.
Thank you for watching Global Weirding. Be sure to check out globalweirdingseries.com every other Wednesday for a new episode. Don't forget to subscribe to our YouTube page, like us on Facebook, and follow me on Twitter. Have a question about climate change? We want to hear it. We'll have a live Q&A on our Facebook page every other Thursday at 7:00 PM central. See you next time.
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!