To understand why climate change is occurring, it is first necessary to understand how carbon dioxide is naturally cycled by the biosphere and ocean. The figure below depicts this process, known as the carbon cycle.

The carbon cycle is a relatively simple concept. There are natural processes that release carbon into the atmosphere and natural processes that remove it from the atmosphere. In fact, these natural processes have managed so far to remove about 40% of the carbon dioxide humanity has emitted from the atmosphere. However, this has not been without a cost; adding carbon dioxide to the ocean has lowered its pH, making it more acidic, to the detriment of much of oceanic life.

There are two major natural cycles. The first is on land, involving the biosphere. The carbon dioxide emitted to the atmosphere comes from the respiration of animals and the decay of dead organic matter. That same carbon dioxide is removed from the atmosphere through photosynthesis, the process by which plants produce energy for themselves and release oxygen back into the atmosphere. You’ll notice from the diagram that these two cycles are balanced with each other; effectively the same amount of carbon dioxide is released into the atmosphere as is removed through photosynthesis.

The other major natural cycle is the exchange between the ocean and atmosphere. This is heavily related to the cyclical nature of ocean water, which is constantly cycled from the surface to the deep ocean in some regions, and then returned to the surface in others. When deep water production occurs, carbon dioxide is transferred from the atmosphere into the deep ocean. And where upwelling occurs (deep water coming to the surface), carbon dioxide is transferred to the atmosphere. You’ll notice that these two processes are effectively in balance as well.

The key here is that everything is in balance. For atmospheric concentrations of carbon dioxide to rise, that balance would have to be altered.

If you look on the bottom left of the diagram, you’ll see large amounts of carbon dioxide present in coal, oil, and gas deposits, as well as in calcium carbonate (labeled as marine sediments & sedimentary rocks). According to this diagram, there are 2,300 gigatonnes (GT) of carbon dioxide is the form of fossil fuel deposits, though estimates run up to about 5,000 gigatonnes. For perspective, one gigatonne is one billion metric tonnes, or one trillion kilograms. There are even larger amounts of carbon locked up in the calcium carbonate deposits, and yet more still in the Earth’s mantle, though these are irrelevant to the current changes in climate.

Since 1800, the balance has been altered quite a bit. In that time, the atmospheric concentration of carbon dioxide has risen from 280ppm (parts per million, a measure of the proportion of the atmosphere that contains the gas in question) to 387ppm in 2009. In the past 800,000 years the concentration of carbon dioxide has not risen above 300ppm. Older geological records suggest that carbon dioxide levels have not been this high for over 20 million years.

This begs the question: why is the concentration of carbon dioxide so high now? It is easy to assert that there is a correlation between the beginning of the industrial revolution and the rise in carbon dioxide, but is there a better answer? In my next post I’ll explain the three separate lines of evidence that prove that the increase in carbon dioxide in the atmosphere over the past 210 years is due to the burning of fossil fuels.