The Keeling Curve

In a previous post, I discussed why most discussions about the climate crisis revolve around CO₂ concentrations in the atmosphere. A short recap: the climate is changing because the concentrations of Green House Gases (GHGs) in the atmosphere is increasing at a dangerous pace. The top GHG of concern is CO₂, but there are many other GHGs in focus such as CH₄ (methane) and N₂O (nitrous oxide).

The Keeling Curve shows CO₂ concentrations in the atmosphere. It is maintained by the Scripps Institution of Oceanography at UC San Diego (UCSD). It can be freely accessed here.

For example, here is the Keeling Curve for the week ending March 19, 2020.

Source

Pretty unimpressive, huh? There doesn't seem to much information to be gleaned out of it -- no obvious pattern or events. One thing we can notice is that the y-axis is in units of ppm or parts per million. A concentration of 415 ppm means that there are 415 parts (say, molecules) of CO₂ per every million or 10⁶ parts (say, molecules) in the atmosphere.

How bad is 413.75 ppm?

There is no absolute, universal "safety limit" to the CO₂ concentration. From the toxicity point-of-view, a concentration of about 2000 ppm starts leading to medical problems and concentrations of 5000 ppm can be fatal. Hence, that's not what we need to worry about. Our primary cause of concern is that beyond a certain threshold, atmospheric CO₂ traps dangerous levels of heat, leading to global warming and consequent climate change.

Okay, so how bad is 413.75 ppm, climate-wise?

Most climate scientists converge to the answer -- "it's very bad" (which is also my personal answer). There is no single answer to what is the optimum CO₂ level we should be aiming for. As I like to joke, the day when all couples could agree on the ideal temperature to set the AC/thermostat at, we might perhaps start hoping that ~200 countries of the world would converge on a single value for the global average temperature. It doesn't seem likely that we will ever find a single value that works just as well and profitably for all regions. Fortunately, coming to an agreement regarding that exact value is not our concern - it is a question that we will hopefully leave future generations capable of discussing. As of now, moving mankind out of the "very bad" zone, towards lower CO₂ concentrations is both challenging and rewarding enough a task for current generations.

 

The UCSD website has several options above the Keeling Curve. Hover over to the options (do not click) to see different plots covering different lengths of time.

Here is the Keeling Curve for the year ending March 19, 2020.

Source

We do see a pattern now -- a wave that hits its highest point in May and lowest point in October. This is related to the global uptake of CO₂ by vegetation -- when plants start their growing season in the Northern Hemisphere. (Why is the Northern Hemisphere more important than the Southern Hemisphere in this regard? Because there's more land and hence more vegetation.) We will see that this is a consistent annual pattern.

Here is the full record ending March 19, 2020.

Source

We notice two things:

1. The vegetation effect is quite consistent and persists over the years.
2. Keeping the vegetation effect aside, CO₂ concentrations have been increasing every year.

One thing not obvious from the plot is that not only is the CO₂ concentration increasing, but the rate of increase is also increasing. In the 1980s, this rate of increase was about 1.6 ppm per year. It is now close to 2.2 ppm per year. This is deeply worrisome and suggests that we are not yet making sufficient progress in tackling the problem.

The current value of 413.75 ppm is not the highest value so far (since we're in March). The highest value was reached in May 2019 at 414.8 ppm. For a baseline, the Keeling Curve started at 313 ppm in the year 1958.

Let's move beyond (behind) the instrumental record. Between 1958 and now, the record is based on direct measurements at Mauna Loa; but if we want to look at the years before 1958 we resort to indirect or proxy measurements such as ice-core data (How does this work? This is an excellent page to start.)

This is the record from the year 1700 to March 19, 2020.

Source

First, the record before 1958 seems a lot more stable. That's partly because of the temporal resolution -- proxy records can rarely match instrumental records in terms of resolution. This means that while instrumental records have daily or even sub-daily measurements, proxy records tend to have one measurement per season, or year, or even more. Climate scientists keep all these factors in mind when using proxy records for assessing climate models.

The unmatchable benefit of proxy records is that they go far back in time. For example, without proxy records we would simply not know what atmospheric concentrations were like before 1958.  There were some discrete measurements at some places, but a continuous record was not available. Proxy records give us a baseline and also tell us more about the variety of climates the Earth has seen before humans started keeping their records.

Going back to the 1700 - present record: the stable, flat-ish line we see on the left is what scientists refer to as Pre-Industrial (PI) conditions. During the pre-industrial period, atmospheric concentrations were around 280 ppm.

 

In fact, extending this record further back for the last 10,000 years shows us even lower "natural" values. Notice the sharp rise at the end? That's why the modern climate crisis is worrisome -- because it essentially throws us out the "comfortable" zone where any of the previous civilizations have thrived. The Earth can manage with higher CO₂ levels, we can't.

Source

 

Is 280 ppm a potential goal?

It is tempting to consider 280 ppm as a "natural" baseline, since it's the level of CO₂ concentrations before humans started messing things up. However, considering it as a goal is problematic for at least two reasons:

1. The 280 ppm conditions were the conditions before the Industrial Revolution. I don't think any of us is really looking to go back to life without modern day amenities. Even though I'm deeply worried about the climate crisis, I don't find it practical to aspire to a time before machines, railways, electricity, internet, etc.
2. We're not returning to a 280 ppm level anytime soon. Especially since we're at 413 ppm right now, especially since the concentrations are still increasing, especially since the rate of increase is still increasing. On a personal level, I wouldn't like to aim at such an impractical goal.
   

To conclude

What is the optimum level of CO₂ in the atmosphere? There is no single answer. The optimum CO₂ level depends on the ability of societies to cope with climate change. This means that some societies would likely survive and thrive even if CO₂ levels reach 500 ppm, many won't. 400+ is fatal for a smaller proportion of people than 500+ is. In the interests of a common goal, I'd advocate setting eyes on the popular value of 350 ppm, but I hope the reader understands the uncertainty around this answer.

What we can say with certainty though is that

1. The modern climate crisis began with us and must be faced and solved by us.
2. All civilizations have historically developed in the range between 280 - 320 ppm.
3. Modern society is seeing a LOT of problems at 414 ppm already. And we will likely cross 500 ppm, the way things are going.
4. We are quite far from reversing our effects on the climate. Reversal would begin with a dip in the rate of increase of CO₂ concentrations, and then a dip in the CO₂ concentrations. Recovery would begin long after that (in the order of decades and centuries). Full recovery will not be achieved for several millennia, and mankind will likely adjust to a >280 ppm level anyway (which is okay).
5. Monitoring CO₂ levels is important. It's a good indicator of how far we are on the road to climate solutions.

I hope this post will make you more comfortable keeping up with the climate conversation since it's often centred around the atmospheric CO₂ levels.


Suggested reading:
How the World Passed a Carbon Threshold and Why It Matters

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← The Greenhouse Gases (Part II)     Climate Change Fundamental (Part II) →

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