The cornerstone argument of climate change deniers is that our current warming is just a natural cycle, and this claim is usually accompanied by the statement, “the planet has warmed naturally before.” This line of reasoning is, however, seriously flawed both logically and factually. Therefore, I want to examine both the logic and the evidence to explain why this argument is faulty and why we are actually quite certain that we are the primary cause of our planet’s current warming.



The fact that natural climate change occurred in the past does not mean that the current warming is natural

I cannot overstate the importance of this point. Many people say, “but the planet has warmed naturally before” as if that automatically means that our current warming is natural, but nothing could be further from the truth. In technical terms, this argument commits a logical fallacy known as non sequitur (this is the fallacy that occurs whenever the conclusion of a deductive argument does not follow necessarily from the premises). The fact that natural warming has occurred before only tells us that it is possible for natural warming to occur. It does not indicate that the current warming is natural, especially given the evidence that it is anthropogenic (man-made).

To put this another way, when you claim that virtually all of the world’s climatologists are wrong and the earth is actually warming naturally, you have just placed the burden of proof on you to provide evidence for that claim. In other words, simply citing previous warming events does not prove that the current warming is natural. You have to actually provide evidence for a natural cause of the current warming, but (as I’ll explain shortly) no such mechanism exists.



Natural causes of climate change

Now, let’s actually take a look at the natural causes of climate change to see if any of them can account for our current warming trend (spoiler alert, they can’t).

Sun

The sun is an obvious suspect for the cause of climate change. The sun is clearly an important player in our planet’s climate, and it has been responsible for some warming episodes in the past. So if, for some reason, it was burning hotter now than in the past, that would certainly cause our climate to warm. There is, however, one big problem: it’s not substantially hotter now than it was in the recent past. Multiple studies have looked at whether or not the output from the sun has increased and whether or not the sun is responsible for our current warming, and the answer is a resounding “no” (Meehl, et al. 2004; Wild et al. 2007; Lockwood and Frohlich 2007, 2008; Lean and Rind 2008; Imbers et al. 2014). It likely caused some warming in the first half the 20th century, but since then, the output from the sun does not match the rise in temperatures (in fact it has decreased slightly; Lockwood and Frohlich 2007, 2008). Indeed, Foster and Rahmstorf (2011) found that after correcting for solar output, volcanoes, and El Niños, the warming trend was even more clear, which is the exact opposite of what we would expect if the sun was driving climate change (i.e., if the sun was the cause, then removing the effect of the sun should have produced a flat line, not a strong increase).

Finally, the most compelling evidence against the sun hypothesis and for anthropogenic warming is (in my opinion) the satellite data. Since the 70s, we have been using satellites to measure the energy leaving the earth (specifically, the wavelengths of energy that are trapped by CO 2 ). Thus, if global warming is actually caused by greenhouse gasses trapping additional heat, we should see a fairly constant amount of energy entering the earth, but less energy leaving it. In contrast, if the sun is driving climate change, we should see that both the energy entering and leaving the earth have increased. Do you want to guess which prediction came true? That’s right, there has been very little change in the energy from the sun, but there has been a significant decrease in the amount of energy leaving the earth (Harries et al. 2001; Griggs and Harries. 2007). That is about as close to “proof” as you can get in science, and if you are going to continue to insist that climate change is natural, then I have one simple question for you: where is the energy going? We know that the earth is trapping more heat now than it did in the past. So if it isn’t greenhouse gasses that are trapping the heat, then what is it?



Milankovitch cycles

Other important drivers of the earth’s climate are long-term cycles called Milankovitch cycles, which involve shifts in the earth’s orbit, tilt, and axis (or eccentricity, precession, and obliquity, if you prefer). In fact, these appear to be one of the biggest initial causes of prominent natural climate changes (like the ice ages). So it is understandable that people would suspect that they are driving the current climate change, but there are several reasons why we know that isn’t the case.

First, Milankovitch cycles are very slow, long-term cycles. Depending which of the three cycles we are talking about, they take tens of thousands of years or even 100 thousand years to complete. So changes from them occur very slowly. In contrast, our current change is very rapid (happening over a few decades as opposed to a few millennia). So the rate of our current change is a clear indication that it is not being caused by Milankovitch cycles.

Second, you need to understand how Milankovitch cycles affect the temperature. The eccentricity cycle could, in concept, directly cause global warming by changing the earth’s position relative to the sun; however, that would cause the climate to warm or cool by affecting how much energy from the sun hits the earth. In other words, we are back to the argument that climate change is caused by increased energy from the sun, which we know isn’t happening (see the section above).

The other cycles (precession and obliquity), affect the part of the earth that is warmed and the season during which the warming takes place, rather than affecting the total amount of energy entering the earth. Thus, they initially just cause regional warming. However, that regional warming leads to global warming by altering the oceans’ currents and warming the oceans, which results in the oceans releasing stored CO 2 (Martin et al. 2005; Toggweiler et al. 2006; Schmittner and Galbraith 2008; Skinner et al. 2010). That CO 2 is actually the major driver of past climate changes (Shakun et al. 2012). In other words, when we study past climate changes, what we find is that CO 2 levels are a critically important factor, and, as I’ll explain later, we know that the current increase in CO 2 is from us. Thus, when you understand the natural cycles, they actually support anthropogenic global warming rather than refuting it.



Volcanoes

At this point, people generally resort to claiming that volcanoes are actually the thing that is emitting the greenhouse gasses. That argument sounds appealing, but in reality, volcanoes usually emit less than 1% of the CO 2 that we emit each year (Gerlach 2011). Also, several studies have directly examined volcanic emissions to see if they can explain our current warming, and they can’t (Meehl, et al. 2004; Imbers et al. 2014).



Carbon dioxide (CO2)

A final major driver of climate change is, in fact, CO 2 . Let’s get a couple of things straight right at the start. First, we know that CO 2 traps heat and we know that increasing the amount of CO 2 in an environment will result in the temperature increasing (you can find a nice list of papers on the heat trapping abilities of CO 2 here). Additionally, everyone (even climate “skeptics”) agree that CO 2 plays a vital role in maintaining the earth’s temperature. From those facts, it is intuitively obvious that increasing the CO 2 in the atmosphere will result in the temperature increasing. Further, CO 2 appears to be responsible for a very large portion of the warming during past climate changes (Lorius et al. 1990; Shakun et al. 2012). Note: For past climate changes, the CO 2 does lag behind the temperature initially, but as I explained above, the initial warming triggers an increase in CO 2 , and the CO 2 drives the majority of the climate change.

At this point, you may be thinking, “fine, it’s CO 2 , but the CO 2 isn’t from us, nature produces way more than we do.” It is true that nature emits more CO 2 than us, but prior to the industrial revolution, nature was in balance, with the same amount of CO 2 being removed as was emitted. Thus, there was no net gain. We altered that equation by emitting additional CO 2 . Further, the increase that we have caused is no little thing. The concentration has increased by over 40% since the start of the industrial revolution, and the current concentration of CO 2 in the atmosphere is higher than it has been at any point in the past 800,000 years. So, yes, we only emit a small fraction of the total CO 2 each year, but we are emitting more CO 2 than nature can remove, and a little bit each year adds up to a lot over several decades.

Additionally, we know that the current massive increase in CO 2 is from us because of the C13 levels. Carbon has two stable isotopes (C12 and C13), but C13 is heavier than C12. Thus, when plants take carbon from the air and use it to make carbohydrates, they take a disproportionate amount of C12. As a result, the C13/C12 ratios in plants, animals (which get carbon from eating plants), and fossil fuels (which are formed form plants and animals) have more C12 than the C13/C12 ratios in that atmosphere. Therefore, if burning fossil fuels is responsible for the current increase in CO 2 , we should see that ratio of C13/C12 in the atmosphere shift to be closer to that of fossil fuels (i.e., contain more C12), and, guess what, that is exactly what we see (Bohm et al. 2002; Ghosh and Brand 2003;Wei et al. 2009). This is unequivocal evidence that we are the cause of the current increase in CO 2 .

Finally, we can construct all of this information into a deductive logical argument (as illustrated on the left). If CO 2 traps heat, and we have increased the CO 2 in the atmosphere, then more heat will be trapped. To illustrate how truly inescapable that conclusion is, here is an analogous argument:

1). Insulation traps heat

2). You doubled the insulation of your house

3). Therefore, your house will trap more heat

You cannot accept one of those arguments and reject the other (doing so is logically inconsistent).

Note: Yes, I know that the situation is much more complex than simply CO 2 trapping heat, and there are various feedback mechanisms at play, but that does not negate the core argument.

Putting the pieces together

So far, I have been talking about all of the drivers of climate change independently, which is clearly an oversimplification, because, in all likelihood, several mechanisms are all acting together. Therefore, the best way to test whether or not the current warming is natural is actually to construct statistical models that include both natural and man-made factors. We can then use those models to see which factors are causing climate change. We have constructed multiple of these models, and they consistently show that natural factors alone cannot explain the current warming (Stott et al. 2001; Meehl et al. 2004; Allen et al. 2006; Lean and Rind 2008; Imbers et al. 2014). In other words, including human greenhouse gas emissions in the models is the only way to get the models to match the observed warming. This is extremely clear evidence that the current warming is not entirely natural. To be clear, natural factors do play a role and are contributing, but human factors are extremely important, and the models show that they account for the majority of the warming.



Correlation vs. causation

It is usually about now that opponents of climate change start to argue that scientists are actually committing a correlation fallacy, and simply showing a correlation between temperature and the CO 2 that we produce does not mean that the CO 2 is causing the temperature increase. There are, however, several problems with that argument.

First, correlation can indicate causation under certain circumstances. Namely, situations where you have controlled all confounding factors. In other words, if you can show that Y is the only thing that is changing significantly with X, then you can reach a causal conclusion (even placebo controlled drug trials are really just showing correlations between taking the drug and recovery, but because they used the control, they can use that correlation to reach a causal conclusion). In the case of climate change, of course, we have examined the confounding factors. As I explained in the previous section, we have constructed statistical models with the various drivers of climate change, and anthropogenic greenhouse gasses are necessary to account for the current warming. In other words, we have controlled for the other causes of climate change, therefore we can reach a causal conclusion.

Second, and perhaps more importantly, there is nothing wrong with using correlation to show a particular instance of causation if a causal relationship between X and Y has already been established. Let me give an example. The figure to the right shows the smoking rates and lung/bronchial cancer rates in the US. There is an obvious negative correlation between the two (P < 0.0001), and I don’t think that anyone is going to disagree with the notion that the decrease in smoking is largely responsible for the decrease in lung cancers. Indeed, there is nothing wrong with reaching that conclusion, and it does not commit a correlation fallacy. This is the case because a causal relationship between smoking and cancer has already been established. In other words, we know that smoking causes cancer because of other studies. Therefore, when you see that the two are correlated over time, there is nothing wrong with inferring that smoking is driving the cancer rates. Even so, we know from laboratory tests and past climate data that CO 2 traps heat and increasing it results in more heat being trapped. In other words, a causal relationship between CO 2 and temperature has already been established. Therefore, there is nothing fallacious about looking at a correlation between CO 2 and temperature over time and concluding that the CO 2 is causing the temperature change.



Ad hoc fallacies and the burden of proof

At this point, I often find that people are prone to proposing that some unknown mechanism exists that scientists haven’t found yet. This is, however, a logical fallacy known as ad hoc. You can’t just make up an unknown mechanism whenever it suits you. If that was valid, then you could always reject any scientific result that you wanted, because it is always possible to propose some unknown mechanism. Similarly, you can’t use the fact that scientists have been wrong before as evidence, nor can you argue that, “there are still things that we don’t understand about the climate, so I don’t have to accept anthropogenic climate change” (that’s an argument from ignorance fallacy). Yes, there are things that we don’t understand, but we understand enough to be very confident that we are causing climate change, and, once again, you can’t just assume that all of our current research is wrong.

The key problem here is the burden of proof. By claiming that there is some other natural mechanism out there, you have just placed the burden of proof squarely on your shoulders. In other words, you must provide actual evidence of such a mechanism. If you cannot do that, then your argument is logically invalid and must be rejected.



Summary/Conclusion

Let’s review, shall we?

We know that it’s not the sun

We know that it’s not Milankovitch cycles

We know that it’s not volcanoes

We know that even when combined, natural causes cannot explain the current warming

We know that CO 2 traps heat

traps heat We know that increasing CO 2 causes more heat to be trapped

causes more heat to be trapped We know that CO 2 was largely responsible for past climate changes

was largely responsible for past climate changes We know that we have greatly increased the CO 2 in the atmosphere

in the atmosphere We know that the earth is trapping more heat now than it used to

We know that including anthropogenic greenhouse gasses in the models is the only way to explain the current warming trend

When you look at that list of things that we have tested, the conclusion that we are causing the planet to warm is utterly inescapable. For some baffling reason, people often act as if scientists have never bothered to look for natural causes of climate change, but the exact opposite is true. We have carefully studied past climate changes and looked at the natural causes of climate changes, but none of them can explain the current warming. The only way to account for our current warming is to include our greenhouse gasses in the models. This is extremely clear evidence that we are causing the climate to warm, and if you want to continue to insist that the current warming is natural, then you must provide actual evidence for the existence of a mechanism that scientists have missed, and you must provide evidence that it is a better explanation for the current warming than CO 2 . Additionally, you are still going to have to refute the deductive argument that I presented earlier (i.e., show that a premise is false or that I committed a logical fallacy), because finding a previously unknown mechanism of climate change would not discredit the importance of CO 2 or the fact we have greatly increased it. Finally, you also need to explain why the earth is trapping more heat than it used to. If you can do all of that, then we’ll talk, but if you can’t, then you must accept the conclusion that we are causing the planet to warm.

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