The above graph is a screenshot taken from An Inconvenient Truth. There are many versions of this graph available; I chose this one because the image in the movie was on such a large scale that the last few decades are actually visible. The graph shows temperatures and concentrations of carbon dioxide over the past 650,000 years, a brief period on geological timescales, but a period that is three times longer than the age of our species. The white line shows temperature; the blue line shows carbon dioxide concentration; the red line shows the projection for 2050 if business as usual continues. (It’s worth noting that, with the exception of the current period of warming, temperatures always rise before the concentration of carbon dioxide. Warmer temperatures lead to greater production of carbon dioxide, which in turn leads to higher temperatures, and so on. This is what is known as a feedback loop. Because of our greenhouse emissions, we’ve started this period of warming at step two).
It’s important that we understand what exactly we are doing to the climate. I’ve already explained what carbon dioxide and other greenhouse gases are doing to the energy equilibrium of the Earth. And in my first post on climate change I discussed the normally slow pace of climate change. With this post I’d like to begin looking at what exactly is happening today, specifically by addressing the rate at which carbon dioxide is accumulating in the atmosphere.
One recent significant period of warming occurred during the period from 60 million years ago to 34 million years ago. From 60 million years ago to 50 million, what is now India was moving rapidly through what is now the Indian Ocean. By rapid I mean about 20 centimeters per year, as opposed to the far more typical inch or two. When continents move, they move over the ocean crust, generating a massive amount of heat and pressure. This creates carbon dioxide and methane, both greenhouse gases, from the calcium carbonate and organic sediments on the ocean floor.
By 50 million years ago, the concentration of carbon dioxide in the atmosphere reached an estimated 1,400 parts per million (ppm). (For reference, the current concentration is 387 ppm. Before the Industrial Revolution, it was 260-270 ppm). Now, this estimate has a large uncertainty; the actual amount could have been in a range of 500 ppm in either direction. The important point is that this caused about 8 degrees Celsius worth of warming by 34 million years ago.
The concentration of carbon dioxide in the atmosphere began to fall once India crashed into Asia. (Though, remember that the carbon dioxide that had already been formed stays in the atmosphere for centuries, and thus continues to cause warming, though at diminishing rates). By 34 million years ago, the concentration had fallen to 450 ppm, with an uncertainty of 100 ppm. At this point, ice began forming in Antarctica; it had been too warm before. This is one reason why scientists place the target for carbon dioxide concentration at 350 ppm, but I will address this point in a later post.
The point I’d like to address in this post is the rate at which carbon dioxide concentrations increased. During the time India was passing through the Indian Ocean, the concentration of carbon dioxide was increasing by about one ten thousandth of 1 ppm per year. (In other words, by 0.0001 ppm per year). This means that it would take 1 million years for carbon dioxide levels to increase by 100 ppm. However, as I noted above, in the past 200 years, carbon dioxide levels have risen by over 100 ppm.
The current rate of emissions stands at 2ppm per year; this is ten thousand times faster than the faster natural increase in atmospheric carbon dioxide we know of. While the graph at the top of this post only shows a brief period of time, it does illustrate this rapid pace quite nicely.
Make no mistake; this rapid increase in carbon dioxide concentration is the result of human activity. However, this will not always be the case. If the globe warms enough, a warming feedback loop will be established and we will experience warming on a scale not seen since the Eocene. I will address this in a future post.