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Over the past 200 years, humans have started to have a measurable effect on the carbon cycle. Part of this is due to deforestation. If we look at biomass per acre, forests contain the highest density of biomass on the planet. So deforestation removes carbon from biomass, and where do you think it goes? Some of it is turned into wood and paper products, but a lot of it has gone into the atmosphere.
However, an even bigger change to the carbon cycle has occurred as a result of the widespread use of fossil fuels. Burning fossil fuels releases carbon dioxide, also known as CO2, into the atmosphere. This adds carbon that has been out of circulation for millions of years directly to the atmosphere.
The immediate result of these human activities is that in the past 200 years, the concentration of carbon dioxide in the atmosphere has dramatically increased. In 1800, the carbon dioxide concentration in the atmosphere was about 280 parts per million, but the most recent readings from June of 2012 measured the concentration at 395 parts per million. Scientists measure carbon dioxide in the atmosphere in parts per million, which is another way of saying, 'Out of every million molecules of air, how many are carbon dioxide?' If 280 molecules out of every million are carbon dioxide, then the air has a CO2 concentration of 280 parts per million, or ppm.
In 1958, scientists started taking direct measurements of atmospheric CO2 on a continuous basis. From these direct measurements, we know that the concentration of carbon dioxide has been steadily increasing year after year for the past 54 years in a remarkably regular seasonal pattern. The pattern is so steady that CO2 levels have reached a new high each May for the past 46 years straight.
It's also very predictable. In fact, I'll predict that in May of 2013, CO2 levels will rise to a new peak level of 398.6 ppm, plus or minus 1 part per million, and that CO2 levels will break above 400 ppm for the first time in more than a million years in either April or May of 2014. To check the accuracy of these predictions, or to just check out the data yourself, you can go to this National Oceanic & Atmospheric Administration website and you'll see the latest atmospheric CO2 data.
So what does this mean? Why should we care that the concentration of carbon dioxide in the atmosphere is steadily increasing each year? The reason is: carbon dioxide is a greenhouse gas, which is a gas in the atmosphere that absorbs infrared radiation from the Earth and slows the rate of heat loss. The three major greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, and methane.
These greenhouse gases act like a giant blanket for the Earth, helping it to retain heat. And when the concentration of greenhouse gases goes up, it's like adding another layer of blankets. When this happens, then global warming, or an increase in the average worldwide temperature caused by higher concentrations of greenhouse gases in the atmosphere, is the logical result. Now, this may sound theoretical, like I'm guessing that if CO2 levels rise, then we expect the Earth to get warmer. However, the fact is we have actual scientific evidence that when CO2 levels rise, so does the average temperature of the Earth.
These two graphs show the correlation. The top graph shows the carbon dioxide concentration in the atmosphere, and the bottom graph shows the average global temperature. Both graphs cover the 800,000 years preceding 1950. As you can see, every time CO2 concentrations spiked higher, the temperature went up too. And when CO2 levels bottomed out, so did the temperature.
Now, like almost all true scientific data, there are some small anomalies and the data doesn't always match up perfectly, but even with these small imperfections, this is one of the closest correlations that you will ever find in the natural world. The history is clear. In the past 800,000 years, whenever CO2 levels rise significantly, the temperature rises too.
There's one other interesting thing to point out about past CO2 levels. In the past 800,000 years, the concentration of CO2 never rose much above 300 parts per million - that is, until about a hundred years ago, when we blasted right through the 300 mark on the way to our present level in 2012 of a little over 390 parts per million.
So what will happen now? There are lots of different estimates and claims that many people make, but in the past century the global temperature has risen 1.4 degrees Fahrenheit. That may not seem like a lot, but the temperature has been rising at a faster rate in the last few decades. In addition, we have 800,000 years of history that show that temperature is highly correlated to atmospheric CO2 levels. We are continuing to burn fossil fuels, and the atmospheric CO2 levels continue to rise at a steady, predictable rate.
All of the scientific data says that the Earth is getting warmer and will continue to warm up for some time. The polar ice caps are melting, and glaciers around the world are receding. The estimates for how high and how fast the sea level can rise if or when these sheets of ice melt are all over the board and depend on many factors that may come into play. If the oceans warm up, then there will be an increase in water vapor in the atmosphere. In addition, some scientists warn that there are massive stores of methane in the oceans that may be liberated into the atmosphere if the oceans warm up. Since methane and water vapor are greenhouse gases, adding more of them to the atmosphere could further accelerate global warming.
Now, before we get all depressed about how global warming will end the world, let's take a look back in history. If we go far enough back in history, there was a time when there was a lot more of the greenhouse gases in the atmosphere and the Earth was much warmer. Actually, for the entire Mesozoic period that lasted between 65 and 250 million years ago, the Earth stayed between 10 and 25 degrees Fahrenheit warmer than it is now, and CO2 levels ranged between about 700 to 2,500 parts per million.
Now, granted, life forms were much different then, and the dinosaurs ruled the world, but it does serve as a reminder that the Earth was once much warmer for a much longer period of time, and life survived and thrived through many periods of climate change. I'm not saying that dinosaurs will evolve again and dominate if the Earth warms up another 10 to 20 degrees, but some species will go extinct and others will survive. New species will also evolve to fill whatever niches they can, but it's impossible to know what they will look like. Maybe this time around, global warming isn't natural, and maybe we'll find a way to stop it from happening - or maybe we won't. Either way, life will adjust as it always has, and since humans are perhaps one of the most adaptable species on the planet, it's likely that we'll adjust too.
So let's review. The vast majority of the world's carbon has been locked away in sediments for millions of years. However, over the past 200 years, humans have started to have a measurable effect on the carbon cycle by burning fossil fuels and releasing more CO2 into the atmosphere. This carbon that has been out of circulation for millions of years is being added to the active short-term carbon cycle. As a result, the concentration of carbon dioxide in the atmosphere has been steadily increasing year after year for at least the past 54 years in a remarkably regular seasonal pattern.
This is significant because carbon dioxide is a greenhouse gas, which is a gas in the atmosphere that absorbs infrared radiation from the Earth and slows the rate of heat loss. The three major greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, and methane.
In addition, over the past 800,000 years there has been a remarkable correlation between atmospheric concentrations of carbon dioxide and global temperatures. So now that CO2 levels in the atmosphere have reached their highest levels in over a million years, it's not surprising that global temperatures are increasing and glacial and polar ice is receding. There is a lot of scientific evidence to support the claim that global warming isn't just occurring but is in fact accelerating. It also doesn't look like we will reverse the addition of carbon dioxide to the atmosphere anytime soon, so it's logical to expect global warming to continue. And while this may cause the polar ice caps to melt, sea levels to rise, and some species to go extinct, life will adapt to a new global climate, just like it has in the past.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 219 lessons