The Problem with '12 Years'

It's like the more carbon we come across, the more problems we see.

It’s been a long year.

It’s almost exactly a year since the Intergovernmental Panel on Climate Change came out with a major report describing what 1.5°C of warming will look like (I know, it feels like an eternity ago). Today we’re at about 1 degree of warming compared to pre-industrial times, so we’re already two-thirds of the way to this threshold.

This report catapulted the idea of carbon budgets into the zeitgeist. A carbon budget tells us how much carbon dioxide we can continue to emit and still have a reasonable chance of remaining below some particular threshold of warming.

Think of a carbon budget as the amount of money that remains in our carbon bank account, and our annual carbon emissions as our annual spending. As the IPCC famously warned us a year ago, at our current rate of emissions, we’ll burn through our budget for 1.5°C of warming in about 12 years.

Actually, the report didn’t say “12 years”. They leave it to you to do the math. The IPCC came up with carbon budgets for a reasonable shot at staying below 1.5°C of warming. If you divide these budgets by our annual emissions, you’ll find that at our current rate we’ll blow through them in a little over a decade. The report concludes that unless “global CO₂ emissions start to decline well before 2030”, we will surpass the 1.5°C threshold.

So that’s where ‘12 years’ comes from. Some people believe that this a simple and effective slogan that captures the urgency of our climate crisis, and has helped galvanize people into action. Others feel that it’s overly simplistic at best, and misleading at worst, because it singles out a specific number as an all-or-nothing target, whereas in reality the effects of climate change are a continuum. As climate scientist Kate Marvel put it, “Climate change isn’t a cliff we fall off, but a slope we slide down”.

But that’s not what I want to discuss here, there are already many good pieces on this debate. Instead, I want to know where carbon budgets come from in the first place. What’s the justification for these numbers, and how accurate are they?

So in this post, I’m going to take you through how you can look at the data for yourself and come up with a rough estimate of the carbon budget.

Let’s start with the basics

You’re probably very used to seeing graphs depicting global warming that look like this. (I just doodled this, so don’t take the exact shape too seriously. )

These kinds of graphs are great for showing us how much the Earth has warmed. (And as Ronnie Chieng hilariously pointed out, they even work upside down.)

Although these graphs teach us about the past, they aren’t so helpful when it comes to predicting the future. It’s hard to tell by looking at this graph which way we’re headed, because that depends on how much carbon dioxide humans will emit in the future.

Let’s go ahead and create this graph using public data. The Global Warming Index is a measure of how much of Earth’s temperature rise is attributable to humans (so it’s removing all of the natural fluctuations in temperature).

Here’s what this looks like, wen plotted versus time (interactive version here).

Notice how it shoots up around 1970? You can compare that to a historical graph of carbon dioxide emissions and work out why that happens.

The reason this graph isn’t so useful for predicting the future is that its slope (how steeply it rises) depends on how much carbon dioxide is in the air. A future in which we take major climate action will follow a very different trajectory compared to a future that’s business as usual.

It turns out that there’s another simple way to look at our historical climate trajectory, one that’s more helpful for predicting the future.

Another way to think about warming

Here’s how it works. Imagine that you change the x-axis of the graph so that instead of measuring the years go by as we did above, we instead measure how much carbon dioxide we’ve added to the air. In that case, the graph might end up looking like this.

Every year, we add some more CO₂ to the air, so we take a step further to the right. Consequently, the Earth warms up. For every step to the right, we also take a step up. So you’d expect our trajectory to move towards the top-right.

If we make this switch, here’s what this new graph looks like (interactive).

In the graph above, the horizontal axis represents the total amount of carbon dioxide that we’ve emitted, due to both fossil fuels and land use. I pulled these numbers from the Global Carbon Project. The vertical axis measures how much the world has warmed as a consequence. Let’s call this graph a climate carbon curve.

The animation shows us the history of human-caused global warming. Every dot represents a single year of human history. Each year we emit more CO₂, so we take a step to the right. And for every step to the right, we warm up, so we take a step up. Notice that as global annual emissions accelerate, the dots get further and further apart — we’re taking bigger steps along the climate carbon curve.

Do you notice anything interesting about this graph? Let’s draw a line connecting the first and last points on this graph.

When viewed in this way, the history of climate change is surprisingly simple to understand. Aside from a bump in the 1970s, the global historical climate trajectory from 1850 to 2017 essentially follows a straight line on the climate carbon curve.

That’s a simple message. In spite of all of the complexity in climate science, there’s a direct proportionality between how much total carbon dioxide we’ve pumped out, and how much the planet has warmed as a result. For every trillion tonnes of carbon dioxide that we emit, the graph teaches us that we raise Earth’s temperature by about 0.44 degrees Celsius. To paraphrase Notorious B.I.G., the more carbon we come across, the more problems we see.

Although the exact slope might differ, this straight-line relationship is born out both through historical observations as well as through every serious climate simulation. There are dozens of scientific papers demonstrating that this relationship holds.

A caveat: We’re talking about how much the temperature rises soon after we dump greenhouse gases into the air. Climate scientists call this the effective transient response of the climate to cumulative carbon emissions (quite the mouthful). Once this carbon is in the air, the temperature will gradually continue to rise further, over centuries. The long-term temperature rise is known as the equilibrium climate sensitivity, which is a fair bit higher than the transient climate response.

Why a straight line?

It’s somewhat puzzling that this graph is so simple. What causes this straight-line relationship? Why should every unit of carbon dioxide, whether emitted in the past or the future, have the same warming effect?

If you only consider carbon dioxide in the air, then every subsequent unit of carbon dioxide does indeed have slightly less of a warming ‘bite’. That’s because the atmosphere gets increasingly saturated with CO₂ over time. It’s a bit like how throwing mud into murky water has less of an effect than throwing it into clear water.

So by itself, every additional unit of carbon dioxide that we emit would have a slightly diminishing warming effect. However, as we emit more CO₂, the oceans also become more saturated with the gas. This means that they’ll have less room to absorb carbon dioxide in the future, so a larger fraction of our emissions will end up in the air.

So we’ve got two opposing effects. As the air gets saturated with carbon dioxide, every additional unit of the gas has slightly less of a warming effect. At the same time, as the oceans get saturated with carbon dioxide, more of the gas will end up in the air.

As it turns out, these two effects cancel each other out. The result is that every new unit of carbon dioxide has approximately the same warming effect as the previous one. This is why we end up with this straight-line relationship between CO₂ and warming. (You can read more about this here, or check out the papers linked above.)

What comes next?

The big question is, where are we going to end up in the future?

We can take a reasonable guess by extrapolating out trajectory forwards.

Every possible climate future, in one graph

The yellow line represents our current trajectory, extrapolated into the future. I’ve also added a ‘cone of uncertainty’ around it. That’s because, in reality, there are bounded gaps in our knowledge (i.e., uncertainties). This cone approximately represents the uncertainty in our prediction, which comes from the underlying uncertainty in the historical measurements and analyses.

This graph isn’t telling us about just one climate future. Instead, it places a bound on every possible climate future. Trace your finger upwards along the cone of uncertainty and you’re tracing out a possible future. A high carbon future is one where your finger moves further along the cone, a low carbon future is one where your finger doesn’t move as far.

Notice that even if we shrink our annual climate emissions down to a tiny fraction of its current value, this won’t completely halt global warming. We’ll still be inching up along the cone, just taking smaller steps. The only way to stop the warming is to stop taking any steps at all. To stabilize Earth’s temperature, we need to get to zero emissions.

D.I.Y. Carbon Budgets

As promised, let’s use this graph to work out a carbon budget. By the end of 2017, we were at ~1° of warming, having emitted ~2300 billion tons of CO₂.

Trace your finger along the thin grey 1.5° line on the graph until it intersects the cone (for greater accuracy, you can zoom in with your cursor in this interactive graph).

You’ll see that it crosses the red shaded region and yellow line at about 3000, 3500, and 4200 billion tons. (These numbers are all rounded to the closest 100 billion tons.) To calculate our remaining carbon budget, subtract 2300 (where we are today) from these numbers.

So this graph tells us that as of 2018, our 1.5°C carbon budget ranged from 700 to 1900 billion tons of CO₂, with a best guess of 1200 billion tons.

Going through the same exercise for the 2°C budget, we come up with a carbon budget ranging from 1600 to 3200 billion tons, with a best guess of 2300 billion tons.

Compared to the IPCC predictions, these numbers are all on the high side. However, they’re in the ballpark of existing estimates of the carbon budget made from historical observations.

The IPCC predictions are created using climate models combined with observations. In general, this approach predicts a steeper slope for the climate carbon curve than purely historical observations do. The steeper the slope, the faster we’ll warm up. This is why the IPCC arrives at a smaller carbon budget than our simple method.

The Takeaway

We’ve seen how extending our historical trajectory forwards gives us a simple way to predict our climate future, and estimate carbon budgets. However, it’s important to keep in mind that all such estimates tend to have very large uncertainties.

We can confidently say that as we continue emitting carbon dioxide, we’re going to move further up the cone of uncertainty and warm up. We can use this idea to estimate when we’ll cross any particular threshold. But that’s all that these numbers are — approximate, ballpark estimates.

When people say that we have 12 years to use up our carbon budget, we should read that as ‘12 years give or take 10 or 15 years’. 12 years is not a sharp dividing line, it’s just a rough indicator of a wide range of uncertainty. What’s more, small changes in our underlying measurements and assumptions significantly shift these predictions (as the IPCC did last year). So to take this number as literal and unambiguous truth is to deeply misunderstand what it means.

That’s why some critics argue that rather than focus on impossibly precise carbon budgets, we should instead talk about when we’ll reach zero emissions. Because one thing that we can say with certainty is that so long as we’re still emitting carbon dioxide, we’re going to keep warming up.