CO2 Concentrations and Temperature
The strength of the greenhouse effect—how much extra energy it directs toward the earth’s surface—depends on how many greenhouse gas molecules there are in the atmosphere. When greenhouse gas concentrations are high, they absorb a greater percentage of the earth’s infrared energy emissions. This means that more energy gets reemitted back toward the earth’s surface, raising its average surface temperature. The reverse is also true: taking CO2 out of the atmosphere would reduce the amount of infrared energy that it absorbs and cause the earth to cool. Vascular plants first appeared on earth around 420 million years ago. Before then, land plants had been restricted to only the wettest environments, but the evolution of vascular systems allowed them to colonize a much wider variety of terrestrial environments. Over the following 90 million years, the resulting increase in photosynthetic activity removed so much CO2 from the atmosphere that it helped set off a 60 million year ice age.
Think of the atmosphere as a heat-trapping “net” surrounding the earth. Carbon dioxide and other greenhouse gasses are the “ropes,” while nitrogen and oxygen are the open spaces between the ropes. When infrared energy hits an open space, it escapes into outer space and dissipates; but when it hits a rope, the rope heats up and reradiates some portion of the energy back toward the planet, raising its overall temperature. The more CO2 molecules there are in the atmosphere, the more ropes there are in the net, shrinking the open spaces and making it harder for infrared energy to escape into space without hitting a rope. The tighter the net, the more energy it absorbs, and the hotter the earth gets.
The Carbon Cycle
The carbon found in atmospheric CO2 is just a tiny fraction of earth’s total carbon reserves. Carbon is also found in rocks, oceans, fossil fuel deposits, and all living things. The movement of carbon atoms between these reservoirs is known as the carbon cycle. Carbon leaves the atmosphere when photosynthesizing organisms such as plants, algae, and some kinds of bacteria pull it out of the air and combine it with water to form carbohydrates. It gets returned to the atmosphere as CO2 when humans and other animals breathe it out, or when plants die and decompose. Sometimes, instead of rotting and releasing their carbon back into the atmosphere, photosynthesizers get buried deep underground, locking their carbon away in the earth for millions of years. With the right underground conditions, the remains of those photosynthesizers will become fossil fuel deposits such as coal, oil, or natural gas. This underground carbon is effectively removed from the short term carbon cycle. It will return to the atmosphere slowly, over tens or hundreds of millions of years, if ever.
Upsetting the Balance
For the past 10,000 years, this balance of intake and emission has kept the amount of carbon dioxide in the atmosphere constant. But by burning an ever-increasing amount of fossil fuels, we are putting our finger on the scale, tipping the balance toward CO2 emission. When we mine fossil fuels and burn them for energy, we are taking carbon that would have taken millions and millions of years to reenter the atmosphere and putting it back there all at once. There is a finite amount of carbon dioxide that plants and other photosynthesizers can remove from the atmosphere in a year, and almost all of that photosynthetic capacity goes toward balancing out natural CO2 emissions. Once the CO2 emitted by animal respiration and organic decomposition has been accounted for, there’s not enough photosynthetic capacity left to scrub all the extra CO2 emitted by fossil fuel combustion. As a result, the amount of carbon dioxide in the atmosphere has increased steadily since the beginning of the Industrial Revolution, with CO2 concentrations rising especially sharply in the latter half of the 20th century. There is another way that organic carbon (carbon found in living things) gets returned to the atmosphere: fire. When organic matter burns, it releases CO2 and water vapor. This is as true for fossil fuels, which are just the geologically modified remains of ancient organisms, as it is for a wood fire in your fireplace. Normally there is a balance between the CO2 that enters the atmosphere from sources such as decomposing organic matter, animal respiration, and wildfires, and the CO2 taken out by photosynthesis. Sometimes one process slightly outpaces the other, but in the long run they even out.
Human CO2 Emissions
A ton of carbon dioxide has a volume of 136,324 gallons. In 2012, humans emitted 34.8 billion tons of CO2 into the atmosphere, a volume of 4.3 quadrillion gallons, or 3,912 cubic miles. That’s enough CO2 to fill Lake Mead, the 120-mile long, 489-foot deep reservoir formed by the Hoover Dam, once every seventeen hours, for a year. Since the Industrial Revolution, about 250 years ago, the amount of CO2 in the atmosphere has increased 37%. Before 1750, one out of every 3,571 molecules in the atmosphere was a carbon dioxide molecule. As of 2013, that number has risen to one out of every 2,500.5 If one out of 2,500 molecules doesn’t sound like a lot, consider that Venus’s atmosphere is 96.5% carbon dioxide (or more than 96 out of every one hundred molecules), and its surface temperature is 863°F. An increase of a couple CO2 molecules per thousand can have a significant effect on a planet’s temperature. More CO2 in the atmosphere means more opportunities for earth’s infrared energy to be absorbed and reradiated back toward its surface. And we’re seeing the effects: our planet is 1.3°F warmer than it was a century ago. By 2100, it could be another 3.2° to 7° warmer.
Other Sources of Warming
Another effect of warmer global temperatures will be the thawing of permanently frozen ground in places like Siberia and Alaska. This permafrost contains vast amounts of methane, another potent greenhouse gas. As the ice holding it in the ground begins to melt, the methane will escape into the atmosphere. As with CO2, the more methane there is in the atmosphere, the stronger the greenhouse effect will become and the warmer earth will get. Human CO2 emissions are the driving force behind climate change, but as the earth gets warmer, it will trigger a number of self-reinforcing processes (vicious circles) that will intensify its effects. For example, the earth’s surface only absorbs about 70% of the sunlight that falls on it. The amount of sunlight that gets absorbed depends on the color of the surface the light falls on. Dark colors absorb more light (and get hotter) than lighter colors. Snow and ice, which are white, reflect almost all the light that hits them, and the reflected light goes back into space without warming earth’s surface. When snow or ice melts, it exposes the darker, more absorptive materials (rocks, dirt, water) beneath. With less snow on the ground due to climate change, earth will absorb more incoming energy from the sun, raising its temperature even further.
Are Humans Causing Climate Change?
But can we be certain that human activities are responsible for that warming? Yes. There are no natural carbon dioxide sources that could account for this rapid increase in atmospheric CO2. Volcanoes, which are one of the largest natural CO2 emitters, only emit about 220 million tons of CO2 per year. In 2012, humans emitted 34.8 billion tons of carbon dioxide, more than 150 times as much CO2 as volcanoes. And volcanic activity doesn’t change much from millennium to millennium, so there’s no reason to think that they are giving off more CO2 now than in previous centuries. Earth’s rising surface temperature isn’t due to increased solar intensity, either. Satellite observations of the sun show that it hasn’t been getting any hotter in recent decades. We know that atmospheric carbon dioxide traps infrared energy from earth, and that the more CO2 there is in the atmosphere, the warmer earth gets. We also know that atmospheric CO2 levels have risen sharply in recent decades, reaching 400 parts per million (one out of every 2,500 molecules) in 2013. Measurements of atmospheric composition taken at the Mauna Loa Observatory in Hawaii show that since 1960, the amount of CO2 in the atmosphere has jumped from 320ppm to 400ppm. CO2 emissions from fossil fuel use have increased from 2.16 billion tons per year in 1900, to over 34.8 billion tons per year in 2012. As a result, earth is now 1.3°F warmer than it was at the beginning of the 20th century.
Climate scientists are nearly unanimous in their agreement that human CO2 emissions are the driving force behind climate change. Over 97% of climate scientists say earth is warming, and that human activities are causing it. As far as scientists are concerned, there is no debate about whether climate change is real, or whether we are causing it.
Why We Need to Do Something About it
Once extra CO2 gets into the atmosphere, it stays there for a long time. Most of the carbon dioxide that we emit today will still be there centuries from now. Even if we stopped burning fossil fuels right now, it would take the better part of a millennium for photosynthesis and other carbon-consuming processes to remove all our excess CO2 from the atmosphere. For all practical purposes, the effects of climate change are permanent, and the more CO2 we add to the atmosphere, the more severe those effects will be. It is true that the earth’s climate could return to normal after a few thousand years, but most of us don’t have a few thousand years to wait for things to get back to normal.