2016 was the hottest year on record, making it the third year in a row to set a new temperature record. Indeed, the past few decades have been so warm, that anyone who is 31 years old or younger has never experienced a single month when the earth’s temperature was below average (that’s using the 1951–1980 average that is generally used for benchmarking global temperatures; NASA data). That’s pretty striking evidence that the planet is warming. Nevertheless, there are many who object to this and claim that we simply don’t have enough data to know what is really going on. They point to the fact that our global records only go back to 1880 and insist that 130+ years is nothing when you consider the entire history of planet earth. However, this argument ignores several key points that I will elaborate on. First, we actually do know a lot about the climate prior to 1880, and we have used those data to understand how the climate works. Second, by using our understanding of past climate, we have been able to test the current drivers of climate change and determine with an extremely high degree of confidence that the current warming is not natural.

How long is long enough?

Before I deal with the actual evidence itself, I want to point out a fundamental problem with this argument. If you think that 130+ years isn’t enough data, then how much will be enough? How long does the trend need to go to convince you? Think about it, if the argument is that 130+ years is nothing compared to the billions of years that earth has been around for, then there is probably no realistic amount of data that would convince you. For example, would 200 years convince you? I doubt it, because in the grand scheme of things, those extra 70 years are also minuscule. To put this another way, people have been saying “there isn’t enough data” for decades now, even as more data has continued to accumulate. Back in 2000, for example, tons of people said that we don’t have enough data and the trend probably won’t continue, but 16 additional unusual (and often record setting) years of data have been collected since then, and many of them still aren’t convinced. So how long does the trend have to last for before you’ll accept it?

My point is that this argument sets up a non-falsifiable situation that allows people to appear rational by demanding more evidence, when the reality is that no amount of evidence will convince them. To put that another way, people who use this argument are picking an arbitrary amount of data that is necessary, rather than actually looking at the quality and content of the data that we already have. This argument is really a cop-out. It is an excuse for blind ignorance, rather than a logical argument. So if you are prone to using this argument, I hope that you will set it aside for a minute and actually look at the data that we have gathered.

Past climate data

When we talk about global temperature records, we are generally referring to the period starting with 1880, because that is when we had sufficient, real-time measurements from around the world to confidently state the global temperature. However, we do in fact have climate data going back much, much further thanks to things like ice cores, the ratios of certain chemicals, etc. So although we may not be able to confidently state a precise global mean temperature before 1880 that is directly comparable to our modern measurements, we still have a really good understanding of what the climate was like. For example, Martin et al. (2005) used the Mg/Ca ratios of benthic foraminifera (a marine protist) to trace ocean temperatures back for 90 thousand years (which is substantially longer than 130+).

This is really important, because we can use those past climate data to understand things like how solar cycles and CO₂ affect the climate. Scientists have, of course, done this, and here is what they found in a nutshell. The earth goes through prolonged cycles known as Milankovitch cycles that affect the earth’s eccentricity, precession, and obliquity (i.e., orbit, tilt, and axis) and this affects the amount of energy entering the earth or the location/season at which it enters. This (as well as other factors like solar activity) often causes a small amount of warming (often regional), and that warming causes the oceans to release the CO₂ that is stored in them (Martin et al. 2005; Toggweiler et al. 2006; Schmittner and Galbraith 2008; Skinner et al. 2010). That CO₂ then drives the bulk of the warming (this is why there is a brief “lag” in the climate data; Lorius et al. 1990 ; Shakun et al. 2012). So in short, past events of global warming occurred whenever something caused enough small-scale warming (often regional) for the oceans to release CO₂, and that spike in CO₂ caused most of the actual warming (more details here).

Our current warming

Now that we have established the drivers of past climate change, we can look at our current situation to see whether or not natural factors are causing the current warming (spoiler alert, they aren’t). You see, the thing that the “only 130+ years of data” argument ignores is that we already understand causal relationships when it comes to the drivers of climate change. In other words, scientists aren’t just sitting back and saying “well it’s been warming for 130+ years, therefore it must be us and it will keep on warming until we change.” Rather, they have carefully examined the cause of the warming, and it is the understanding of that cause that allows us to be so confident.

Image from the Scripps Institution of Oceanography

So what is the cause? You probably guessed it; it’s CO₂. The image to your right shows the earth’s atmospheric CO₂ concentration over the past 800,000 years. You’ll notice that it is substantially higher now than it has been during any point in that time period (surely 800,000 years is long-term enough). Also, remember that we know from our studies of past climate change that CO₂ is a major factor in driving planetary warming. At this point, the situation is really quite simple: we know that CO₂ traps heat and is important for regulating our climate, we know that CO₂ has been a major driver of past climate change, and we know that we have greatly increased the CO₂ in our atmosphere, therefore, the only logical conclusion is that we are causing the climate to change. Please note that this conclusion is not based on 130+ years of warming. Rather, it is based on understanding physics and the factors that drive climate change. Indeed, scientists predicted that burning fossil fuels could change the climate way back in 1896. So the recent warming is simply the confirmation what scientists predicted over 100 years ago.

To put this another way, we can demonstrate that we are the cause using a simple syllogism.

1. In the past, large increases in CO₂ have caused the planet to warm
2. We have caused a large increase in CO₂
3. Therefore, we are causing the climate to warm

This figure from Hansen et al. 2005 shows the effect of both the natural and anthropogenic drivers of climate change. Notice how only anthropogenic sources show a large warming trend. Also, see figure 2 of Meehl et al. 2004.

Nevertheless, you may protest and say, “what about volcanoes, the sun, etc.” Well, first off, volcanoes actually only emit a very tiny amount of CO₂ (less than 1% of what we produce Gerlach 2011). Second, scientists have carefully looked at volcanoes, the sun, and other natural drivers of climate change, and they simply cannot explain the current warming without including our greenhouse gas emissions (Stott et al. 2001; Meehl et al. 2004; Hansen et al. 2005; Allen et al. 2006; Lean and Rind 2008; Imbers et al. 2014).  In other words, we know that the current warming isn’t natural, because we have tested the natural causes of climate change, and they all come up short (even when combined together; more details here). Output from the sun, for example, simply doesn’t correlate with our current warming, therefore we know that it isn’t the sun that is driving it (note: the actual tests are far more complicated than simple correlations, because they both control for confounding variables and combine the effects of variables, but it is often convenient to think about it as simple correlation to start with, then build complexity on top of that). To be clear, these tests have also shown that the current warming is not 100% human induced. There is some natural background fluctuation, but the bulk of it is man-made, and it will continue if we don’t reduce our fossil fuel use.

It is probably also worth mentioning that we are certain that the increase in CO₂ is from us because of isotope ratios. I explained this in more detail here, but in short, carbon has two stable isotopes (C13 and C12), and the ratio of those two isotopes in fossil fuels is different from the ratio that is in the atmosphere. Therefore, if the CO₂ is from us burning fossil fuels, we would expect that atmospheric ratio to shift to be more like our fossil fuels, which is exactly what has happened (Bohm et al. 2002; Ghosh and Brand 2003;Wei et al. 2009). These ratios are like fingerprints that demonstrate beyond any reasonable doubt that the CO₂ is from us.

Confirmation via models

At this point, you may be tempted to think that all of this is well and good, but we can’t really know what is happening without more data. I disagree, but there are still additional pieces of evidence that we can bring to bear. For example, we can be highly confident that our current understanding is correct thanks to models (as well as other data that I’ll talk about later). Scientists are a pretty clever bunch, and what they realized is that we can construct models based on our current understanding of how climate works, and use those models to make predictions about the future. If our understanding is correct, then the models should make accurate estimates, and if our understanding is wrong, the models should make incorrect estimates (science thrives on testable predictions like this). Granted, there is a certain amount of uncertainty around this, because the models are reliant on the skills of their programmers, as well as natural factors that are out of our control (e.g., if the output of the sun changes and is different from the model input, that will cause the model to be wrong, even if our basic understanding of the climate is correct). Nevertheless, these models have actually done a remarkably good job of predicting the current warming (Hansen et al. 2006; Frame and Stone 2012; Rahmstorf et al. 2012; Cowtan et al. 2015; Marotzke and Firster 2015). Now, yes, I know, you have read on the internet that the models have all been wrong. Well, I hate to break it to you, but not everything on the internet is true. The claims that the models have been wrong are based on cherry-picking and misrepresenting how the models work. The actual scientific analyses have found that they have done a good job of predicting the current warming (more details here and here). Also, it is worth mentioning that the models have been refined over time because our understanding of the climate has improved over the years. However, those refinements simply improve the accuracy of the models, they don’t change the overarching trend or predictions.

Satellite measurements

Finally, I want to draw your attention to one final piece of evidence which, in my opinion, is the single most conclusive demonstration that we are causing the planet to warm. When energy from the sun hits the earth’s surface, much of it is radiated back of as long-wavelength, low-energy infrared radiation (IR, aka heat energy), and CO₂ in our atmosphere is capable of absorbing those long wavelengths. Thus, it traps some of that heat energy before it leaves the earth. This is a scientific fact that no one disputes, and it is actually a good thing, because the earth would be inhospitably cold without CO₂ and the other greenhouse gases. Nevertheless, it can be a problem, because if that CO₂ increases, then more heat is trapped and the earth warms. Again, we know that this is largely what drove past climate changes.

Now that we have those facts in place, we can make another testable prediction. If our CO₂ is causing the planet to warm, then the amount of energy entering the earth should be roughly constant, but the amount of energy that is leaving the earth should decrease significantly because some of it is being trapped by our CO₂. Scientists have actually measured this using satellites, and just as expected, the energy from the sun is basically unchanged, but less energy is leaving at the specific wavelength that CO₂ traps (Harries et al. 2001; Griggs and Harries. 2007; Huang et al. 2010). This is extremely clear evidence that we are causing the planet to warm, and if you want to say that the warming isn’t from us, then I want to know where that IR is going. We know that the roughly same amount is coming in, but less is going out, so if it’s not our CO₂ that is trapping it, what is it? (note: recently, the sun’s output has decreased slightly, but it is not enough to explain the decrease in IR, and the pattern hasn’t been consistent; i.e., the sun’s output does not correlate well with the decrease in IR, but our CO₂ does).

In conclusion, let’s put all of the pieces together. We know that CO₂ traps heat and warms the planet, we know that increases in CO₂ caused past climate change, we know that we have greatly increased the CO₂ in the atmosphere, and we know that CO₂ is currently causing the planet to trap excess heat. The conclusion from that is inescapable: we are causing the planet to warm. We do not need more data to be confident in that conclusion.

Note: To be clear, I’m not suggesting that we should stop studying the climate, we certainly should and there is still much to learn, but the evidence that we have so far is sufficient to be highly confident that we are causing the climate to change. To use an analogy, there is still a lot to learn about the human body and we should keep studying it, but we have plenty of data for me to confidently state that your liver isn’t the organ that is responsible for pumping blood.

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