Wednesday, April 24, 2013

The Case for Global Warming, Part 2: How We Know It's Us

In Part 1 of “The Case for Global Warming,” I presented an overview of the evidence that the Earth is indeed warming, and at a rate unprecedented in over 10,000 years. Today, I cover the evidence that we are its primary cause, and how, as a species, we manage to accomplish such a task.

There is no doubt that the globe is getting hotter, and at a brisk pace. But what is the underlying cause? The evidence points unambiguously towards human interference. Nonetheless, how are we, weak, half-naked apes that we are, able to affect climate on a global scale? Isn’t it a little arrogant to think we have so much power?

To answer these questions, we must understand how something so stupendously large as the Earth can heat up or cool down in the first place. The Earth is a big place: a spheroid rock with a girth of over 40,000 kilometers and massing in at 5.97 million billion billion (5.97 x 1024) kilograms. (Wikipedia, 2013) It stands to reason, then, that the largest contributors to terrestrial climate must be themselves of substantial scale.

Indeed, the three most relevant players to our question are massive in and of themselves. The first is the Earth itself—or rather, its mantle. The ground beneath our feet is only the Earth’s crust: a shell of solid rock 35 kilometers deep. Given the Earth’s radius averages 6,371 kilometers, this is not so deep as it may sound at first. (Wikipedia, 2013) To use a culinary analogy: the planet’s turf is more like the thin shell of a deviled egg than the crust of a deep dish pizza. Moreover, the rock beneath the crust consists of roiling, molten rock and metal. The decay of radioactive elements prevents Earth from cooling, keeping the core a scorching 4,000 °C, and the upper parts of the mantle around 400-500 °C. (Wikipedia, 2013)

The Earth’s core and mantle can contribute to surface temperature, but generally not by radiating heat (as I explain later). The largest direct contributor is our second player, the sun. The sun, of course, is a gigantic, fusion-powered fireball over a hundred million kilometers away. That is a good thing, because its surface temperature exceeds 5,000 kelvins (about 4,723 °C). (Wikipedia, 2013) Our closest planetary neighbor, Venus, is the sweltering acid-swathed world it is largely because of its proximity to the sun. At Earth’s distance, the sun’s rays warm our planet with greater moderation, permitting liquid water to form on the Earth’s surface.

But while the Sun is the largest direct contributor to the Earth’s temperature, it is not sufficient to explain the climate by itself. After all, while nights are generally cooler than days, we also have warm nights and hot days. The last player is our own atmosphere. Our planet is massive enough to hold on to a robust comforter of nitrogen, oxygen, carbon dioxide and water vapor. The atmosphere absorbs and retains heat that would have been reflected into space in a vacuum.

The atmosphere is actually the immediate cause of global warming. The key is the greenhouse effect. As with actual greenhouse roofs, our atmosphere is transparent to visible light. These visible rays pass through without problem, and are absorbed by the ground, increasing the surface temperature. Of course, this absorbed light is later re-emitted as heat in the form of infrared light.

This infrared light, however, does not exit the atmosphere unmolested. Like the glass of the greenhouse roof, the atmosphere is comparatively opaque to infrared radiation. Some of the infrared rays bounce back, and the heat remains in the atmosphere, making the surface temperature warmer than it would have been otherwise.

Carbon dioxide is the largest contributor to the greenhouse effect in our atmosphere, though other trace elements also contribute. The website Hyperphysics, hosted at Georgia State University, explains the greenhouseeffect in more detail. They also link to graphs demonstrating the change in atmospheric concentrations of greenhouse gasses over the last thousand years.
IPCC graph showing greenhouse gas levels over time.
Graph re-posted from Hyperphysics.
In the last two hundred years, concentrations of carbon dioxide, methane, and nitrous oxide—all known greenhouse gasses—spiked. This correlates almost exactly with the temperature spike observed in the past century and a half.

But what is behind the increase? Volcanic eruptions do release many greenhouse gasses into the air in large quantities, and we have seen reports of eruptions increase over the past two centuries. However, this is due to a bias in reporting, as this article explains, and not actual increases: the world’s population has grown by leaps and bounds, so we’re more likely to notice an eruption than we were before. Over geological time periods, there has been little change in Earthbound volcanism. The culprit for warming must be something else.

Perhaps the sun is growing hotter? The sun does shine brighter and brighter over time, and will eventually grow hot enough to scorch our planet. That will happen… in 500million years. Solar output is linked to stellar evolution: a process that takes billions of years to play out in full. There is no way the sun could have brightened so quickly.

What else could it be? When the world started growing hotter and smoggier, in the 19th century, human civilization reached a turning point. We call that time the Industrial Revolution. The rise of factories, steam power and economy of scale spurred a massive growth in our consumption of natural resources. We started by burning coal in factories and steam engines. Then we burned coal for electricity. Then we burned gasoline and diesel for cars, trucks, and energy. We cleared forests for factories, for homes, for agriculture, and just about any other reason you can think of. This process continue to this day, as few countries remain unindustrialized and less developed powers struggle to increase their output.

Burning coal releases carbon dioxide. Burning oil releases carbon dioxide. The forests we clear are full of trees, which breathe in carbon dioxide and exhale oxygen. Ever since the 1800s, we have been pumping more carbon dioxide into the air while killing the plants responsible for filtering CO2 out of the air. The timing of these events correspond exactly with the sudden rise in greenhouse gas contributions and the global rise in temperature.

This is the smoking gun. We are responsible for global warming. In our pursuit of industry, we wrap the Earth ever more tightly in a sweltering blanket of carbon emissions.

But how bad can it get? I explore that question next week, in the final part of this series.

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