Keeling’s Curve

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How do we know for sure that human activities are resulting in an increase in global carbon dioxide concentrations? The answer involves accurately measuring the amount of CO2 in the air, a seemingly simple problem that’s actually deceptively complex.


If you tried to record the concentration of carbon dioxide near your home, for example, the measurements will fluctuate wildly depending on the time of year, the amount of humidity in the air, the air pressure and even the wind direction. More importantly, if you live in a big city where cars and factories are pumping out CO2, you’d get a much higher reading than if you lived out of town, next to a stand of trees that are actively pulling CO2 out of the air.


Where and how CO2 is measured can dramatically affect the results. So how do we get measurements that are representative of the whole atmosphere? You could record carbon dioxide simultaneously at thousands of different places across the planet and then take the average of all the readings, but that would be incredibly expensive and complex.


A solution to this measurement problem came in 1958 from the American scientist Dr. Charles David Keeling. To avoid interference from human activities and photosynthesizing vegetation, Dr. Keeling helped install a high-altitude gas measurement station on the slopes of Mauna Loa, a Hawaiian volcano (that is actually much taller than Everest, but most of it is undersea). Far from major cities and surrounded by the Pacific Ocean, the air is particularly well mixed here  and therefore a good surrogate for global background concentrations. On top of that, the hardened lava slopes of Mauna Loa prevent carbon-absorbing vegetation from growing and interfering with the readings.


But finding a suitable spot to take a measurement was just the first step. Dr. Keeling wanted to measure the concentration of carbon dioxide as accurately as possible: ideally within one part per million. He had a feeling human emissions of carbon dioxide were noticeably changing the chemistry of the atmosphere, and wanted there to be no room for doubt in his data. However, even very small fluctuations in air pressure and humidity can cause measurement problems at that level of precision, so he needed a testing process that was extraordinarily accurate and reliable.

Check it out: latest CO2 readings from the Mauna Loa observatory.

Measuring how much carbon dioxide is in the air sample is done using infrared light. A beam is sent through the sample, towards a detector on the other side. The less infrared light received by the detector, the more was absorbed by the carbon dioxide and therefore the higher the concentration. When the air pressure is high, the air sample will be slightly more compressed, contain slightly more air molecules and therefore appear to have a higher CO2 reading. The opposite happens when the air pressure decreases. A similar problem occurs when the air is moist. Water vapour displaces other molecules in the air, and so when an air sample has high humidity levels, carbon dioxide readings tend to be lower than they would otherwise be.


Keeling came up with inventive procedures that took these complications into account. Just before the measurement is taken, three canisters of reference air, carefully created in a lab at very specific concentrations of CO2, are used to calibrate the analyzer. This step accounts for variation in air pressure.


It was more challenging to solve the humidity problem. It is vital that the air sample is completely dry before it is analyzed. This is easier said than done. Heating the air to dry it will do nothing—the water is already a vapour and you can’t vent the water vapour without venting other molecules in the sample, ruining it. Keeling’s solution was to cool the sample so that the water vapour would freeze and fall out of the air. However, even at temperatures around -60 °C water can remain in a liquid state (it’s called super-cooled water). In order to make sure all of the water molecules turn to ice, the air is cooled beyond -100 °C.


Just two years after he first started recording carbon dioxide measurements at the Mauna Loa observatory, Dr. Keeling published his findings. He noted that CO2 concentrations were increasing every year, and that this was likely due to fossil fuel combustion . The year was 1960—three decades before the IPCC published its first report on climate change. Dr. Keeling’s iconic graph of changing carbon concentrations over time became known as the ‘Keeling Curve,’ and still today serves as irrefutable proof of the impact humans are having on the atmosphere, and thus the global climate system.


One more thing: Did you notice that the line on the graph wiggles up and down a little bit every year? During the spring and summer months, trees in the Northern Hemisphere grow leaves and actively pull CO2 out of the air, causing the global concentration of the gas to drop. During the fall, when the leaves drop and decompose on the forest floor, most of this trapped carbon is released back into the air, and so the global concentration goes back up. This seasonal effect isn’t balanced out by the opposite seasons in the Southern Hemisphere because the Northern Hemisphere has more land area and more trees, and so a stronger impact on carbon dioxide levels worldwide.

By Ryan Smith

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