Energy 2.0

Questions about CO2 and Climate Change

Posted on: May 23, 2008

What are CO2  emissions?

 Carbon dioxide (CO2) is a gas that makes up a tiny fraction of the Earth’s atmosphere. It occurs naturally, mostly as a result of breathing, of decay, from the burning of wood and the release of CO2 from the oceans. CO2 emissions also result from the burning of fossil fuels and other human activities.  It is this human-generated CO2 that we are showing in our simulation.

What are CO2 removals?

Carbon sinks remove carbon from the atmosphere. The main carbon sinks responsible for removals are photosynthesis and absorption by the oceans.

The oceans are both a carbon sink and a source of CO2. There is an ongoing exchange of CO2 between the atmosphere and the oceans. The balance depends upon factors including water temperature and the concentrations of CO2 in both the oceans and the atmosphere.

For hundreds of thousands of years emissions and removals remained roughly in balance with the concentration of CO2 in the atmosphere varying between 180 and 300 parts per million (ppm). This was true until humans began to burn fossil fuels during the Industrial Revolution. These additional CO2 emissions are the problem. Currently much more CO2 is being released than can be taken up by plants or absorbed by the ocean. The concentration of CO2 in the atmosphere is now 380 ppm and rising.

Why do removals seem to follow emissions?

Carbon dioxide flows between the atmosphere, biosphere, and oceans in order to maintain a balanced distribution. When the concentration of CO2 in the atmosphere increases, two things happen:

  • “CO2 fertilization” occurs. Plants use more CO2 for photosynthesis, growing more leaves and woody material.
  • The surface ocean—mixed by wind-driven waves— quickly absorbs CO2, which then diffuses more gradually into the deep ocean.

Both processes have limits. The oceans can only absorb so much CO2 before releasing as much CO2 back to the atmosphere as was taken up. For plants, the limitations on growth from water and other nutrients become important. This is called “sink saturation.”

In the “Allow Increased Emissions” future, removals increase because the rapidly-growing concentration of CO2 in the atmosphere continues to drive uptake. Part of the excess CO2 is absorbed by plants and the oceans.

In the “Reduce CO2 Emissions” future, removals fall because the excess of CO2 in the atmosphere above that in the biosphere and oceans is not so great.

What’s the connection between CO2  and climate change?  

We know that CO2 absorbs heat from the Sun and releases it into the atmosphere. Going back millions of years, when the concentration of CO2 was higher, the Earth was warmer. Eventually CO2 concentration dropped and the world became cooler. Since the 1740s CO2 concentration has increased significantly, and the average temperature on Earth has also increased.

 Why does the CO2 level in the atmosphere continue to rise even when emissions are leveled off?
This scenario corresponds to clicking the middle button in our simulation: “LEVEL OFF CO2 EMISSIONS.” After about 2045 emissions are no longer increasing. At that point removals are also level from year to year. But since emissions are greater than removals, each year more CO2 goes into the atmosphere than is removed. So the amount of CO2 in the atmosphere continues to rise. 

It’s like a bus traveling through the city with people getting on and off. Let’s say that at one stop 5 people get on the bus and 3 get off. At the next stop the same thing happens: 5 people get on and 3 get off. If this pattern continues the bus will get very crowded. The number of people getting on the bus is level: 5 at each stop. But since only three people get off there is an increase of 2 people each time the bus stops. In order to keep the crowding from getting worse, the same number of people have to get off the bus as get on.  And to reduce the crowding, more people have to get off than get on.

In order to keep the concentration of CO2 in the atmosphere at a given level, say 450 ppm, emissions and removals have to be equal.  In order to reduce the concentration of CO2 in the atmosphere, removals have to be greater than emissions.


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