Iconic Figure # 1: CO2 trends
Iconic Figure # 1: "Keeling" CO2
In my class I have a set of figures that I call the "iconic figures" of climate change. There are only a handful of them, and they are the figures that I think all of my students should be aware of and understand. One of the exercises that I suggest for my students is to write a figure caption for each of the figures. (Perhaps an extended figure caption.)
Here is the first figure, which has been mentioned by a number of people commenting on recent blogs. This is often known as the "Keeling curve," named for the C. David Keeling who started to take CO2 observations at Mauna Loa in 1958. These data are now taken and maintained by NOAA's Earth Systems Research Laboratory, Global Monitoring Division. This web site is excellent, provides references, as well as access to the observations. Here is a recent update of the Keeling Curve.
Figure 1: Carbon Dioxide at Mauna Loa Observatory.
There are two obvious things in this record. The first is that there is a steady upward trend of CO2. The second is that there is an annual oscillation of CO2. What is not so obvious on this scale is that the amplitude of the annual cycle is increasing. Here is a link to the NOAA site which has much more information on this figure, as well as global averages and numbers of how much the CO2 increases each year.
The steady upward trend is mostly attributed to the release of CO2 by fossil fuel burning. There has been significant effort to account for all of the sources (and sinks) of carbon dioxide, and the increase both correlates with the increased burning of fossil fuels, and it is generally consistent with amounts that are estimated based on fuel consumption.
Some have criticized the use of these particular observations because they are at Mauna Loa, which is a volcano. While this is true, the observatory at Mauna Loa was chosen because, to a very good approximation, it sees clean maritime air. This is constantly checked. One way it is checked is to calculate trajectories to see where the air being sampled comes from. Here is a link to recent trajectory calculations. If there is active volcanism, then is accounted for in the data quality control. It is found, more and more, that the air at Mauna Loa sees emissions and pollution from Asia. This is much more likely than seeing local volcanism.
There has been some significant effort to calculate the CO2 emissions from volcanoes. In the recent time, last 100 years, this amount is estimated to be more than 100 times smaller than that from fossil fuels. Here is the link to the USGS web site on volcanoes. With satellites and other observing systems, there are not any volcanoes in some hidden part of the world that are unknowingly spewing large amounts of CO2 or SO2 or aerosols into the atmosphere. Really.
Back to the Keeling Curve: The annual cycle in the CO2 is caused by the "breathing" of the terrestrial biosphere; that is, plants. Plants use CO2, and when the northern hemisphere blooms in spring and summer, the plants take up CO2. In fall and winter, there is release of biospheric CO2.
Not completely obvious in this figure, but more obvious in stations from high northern latitudes, the amplitude of the annual cycle is increasing. This increase is directly correlated with the "greening" of both North America and Siberia. Because of the warming at higher latitudes, there is greater growth of trees (easily measured by satellites). This greater growth takes up more CO2. On one hand, this increased "breathing" is consistent with the predictions of global warming; hence, it is part of the finger print that contributes to the validation of the theory. On the other hand, some have maintained that the increased biological activity would "take up the extra CO2." There seems to be no evidence to support this assertion, and the observations suggest that increased biological activity cannot keep up - at least on the time scales we have observed.
While it is possible to substantiate that the Mauna Loa station is not, in general, contaminated by local pollution, this substantiation is not generally accepted as adequate. It needs to be validated. One way to do this is to take observations at many other sites. Here is a list of other sites in the carbon observing network. If you were to study the observations from these sites, you would see a consistent signal. CO2 is increasing; there is biological breathing, and the amplitude of the breathing is increasing. However, the amount of CO2 does vary, especially as a function of latitude. This variation reflects a number of items. First, it reflects the enhanced emissions of the industrialized nations. Second, since these stations are located primarily in the northern hemisphere, there is more CO2 in the north than the south. This difference between the hemispheres can be used to estimate how long it takes the two hemispheres to mix.
I will leave it there. Wait for comments, and ask - what are the differences between the northern and southern hemispheres in terms of CO2?