Credit: Reid Wiseman NASA |
We can certainly model the heck out of hypothetical atmospheric chemistries - with climate simulation, photochemistry, and even abstract models of biospheres. The trick is disentangling all of that three-dimensional, diurnal, seasonal, and chemical variation. Sometimes there is significant 'degeneracy', where similar, astronomically derived data (like a spectrum of starlight filtered through a planet's atmosphere) would present itself in quite different physical circumstances. Oxygen for example can be produced by life, but it can also be produced by non-biological atmospheric chemistry.
Carbon dioxide is another tricky compound. A planet can fill its atmosphere with CO2 from volcanoes, for example. It can also show strong seasonal variation in this gas. Both Earth and Mars do this, but for very different reasons. On Earth it's the biosphere that drives an annual oscillation in CO2 concentration. On Mars it's the thawing and freezing of CO2 at polar regions that causes its atmospheric density to vary over a year.
But Earth has also been increasing its greenhouse gas concentration pretty steadily over the past couple of hundred years. This is almost entirely attributable to the burning of fossil fuels (we can tell because of the isotopic mix of carbon in those fuels).
We've all read the reports, or seen the media discussion. Except it can be a little hard to grasp the sheer magnitude of what our species has accomplished. Dire warnings that we've crossed 400 parts-per-million of atmospheric CO2 (a level last seen tens of millions of years ago) may sound pretty bad, but it takes a little intellectual imagination to really grasp what's going on.
Let's give that a shot.
Current data (from direct measurements of the atmosphere to historical records of industry) tells us that between 1751 and 1987 fossil fuels put about 737 billion tons of CO2 into the atmosphere. Between just 1987 and 2014 it was about the same mass: 743 billion tons. Total CO2 from industrialized humans in the past 263 years: 1,480 billion tons.
Now, let's relate that to something a bit easier to visualize. A coniferous forest fire can release about 4.81 tons of carbon per acre. At the low end, about 80% of that carbon comes out as CO2. In other words, to release an equivalent CO2 mass to the past 263 years of human activity would require about 1.5 billion acres of forest to burn every year during that time.
That's 6 million square kilometers of burning forest every year for more than two centuries. That's a square patch about 2,450 by 2,450 kilometers, or about 1,500 by 1,500 miles. Here's what that'd look like:
Equivalent area of burning forest to produce annually averaged carbon dioxide output. Credit: C. Scharf |
Credit: C. Scharf |
Let's also, for a moment, do another thought experiment. You're a long-lived, budding astrobiologist on the planet Proxima Centauri b. You've just spent several hundred Proxima b years using that world's largest space telescope to take a spectrum of the light from an intriguing planet about 4 light years away. Some unknown process on this planet seems to be persistently adding carbon dioxide to its atmosphere. Seasons come and go, but the composition of the planet is changing during your own lifetime.
Your conclusion? If there's life on that world, something potentially catastrophic is happening to it.
Links
- Global Warming Fact: More than Half of All Industrial CO2 Pollution Has Been Emitted Since 1988
- How much carbon does a forest fire spew into the atmosphere?
- Climate Accountability Institute
- Global Carbon Budget
- Intergovernmental Panel on Climate Change
- New Study Links Carbon Pollution To Extreme Weather
- Why The World Economy Has To Be Carbon Free By 2050
- Warming Earth Threatens To Release Huge Amounts Of Carbon – UN Agency
- How the World Passed A Carbon Threshold And Why It Matters
- Very Soon, We'll Have Blown The World's Entire Carbon Budget
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