Today I’d like to talk a bit about the observations of climate change. Observations are used both to set up climate models and to test them. That is a bit circular – and where independent data sets exist, different data sets are used for these two roles – but usually the observations are used to tune the model using a method called “data assimilation” which is a mathematical process that tries to minimise the average difference between prediction and observation.

There are three types of observation we need to consider: observations of the quantities that affect the climate, observations of the changing climate and observations of the effects of changing climate. In practice, these three categories are blurred (many observations are both cause and effect).

Today we’ll consider the first of these, and in particular the graph that was published widely in the last week because it measured the highest carbon dioxide levels yet: the Mauna Loa observation of carbon dioxide levels in the atmosphere. As we considered in lesson 7, carbon dioxide is a powerful greenhouse gas that affects the Earth’s radiative energy balance (though not in a simple manner). The Mauna Loa Observatory is on a volcano in Hawaii – right in the middle of the Pacific, and, most significantly, a very, very long way from any meaningful industry. The instruments are at the top of the mountain – 3397 m above sea level – again conditions that keep the observations pure. The observatory has measured carbon dioxide daily since March 1958 by taking samples of air and analysing which gases are inside them.

There is an excellent video at https://youtu.be/gH6fQh9eAQE, which I will embed here:

In the video you can see the observations of carbon dioxide from observatories since 1989. The red dot is Mauna Loa (the black dots are other stations around the world – over time the number of black dots changes as stations come in and out of operation). The upward trend is clear – and this has to be factored into the climate models. The zig-zag pattern is due to the seasons – and in particular due to the summer leaf growth in the northern hemisphere which temporarily removes carbon dioxide from the atmosphere. But the unceasing upward trend behind this is because we’re burning fossil fuels (and, to a more minor extent, because we’re cutting down forests and there are more forest fires).

One problem with these observations is that they are made at only a few sites and these sites are intentionally chosen to be well away from the places where fossil fuels are burnt. There are some satellites that are now measuring global CO₂ levels – and these can show where the CO₂ is. These work by observing the absorption of the spectrum (seeing how black the black lines are) of sunlight reflected by the Earth in wavelengths we know carbon dioxide absorbs (see back to earlier lessons). In particular they make measurements in a “weak-CO<sub>2<\sub>” band, a “strong-CO<sub>2<\sub>” band and an oxygen O<sub>2<\sub> band. The strong band is a band where carbon dioxide strongly absorbs: this band gives information about the overall absorption of carbon dioxide. The weak band is one where carbon dioxide only partly absorbs. This means it goes through most of the atmosphere undisturbed and gives information about carbon dioxide absorption near the surface: in other words it gives information about whether the surface is a source (e.g. factory) or sink (e.g. forest) of carbon dioxide and to what extent. The oxygen band is a reference band to compare the carbon dioxide against.

The main current CO₂ sensor is the NASA OCO-2 satellite which has run since 2014 (OCO failed on launch in 2009).

You can get a video of OCO-2’s observations on YouTube too (https://youtu.be/x1SgmFa0r04)

There’s a joint French-British satellite mission called Microcarb that is currently being built to be launched in 2021 that will also perform satellite-based carbon dioxide measurements.