2011-01-10 10:52:13INTERMEDIATE Rebuttal 136: It's a natural cycle

Author notes: I chose to do this as more of a logical argument than direct discussion of data, although I touched on that at the end. I see this piece as sort of a unifying article connecting rebuttals such as "1500-year cycle" and "PDO causes global warming." What do you think?

I would also like suggestions for images...I wasn't really sure what/where to put them in. Maybe something from the image page? -Kate

A natural cycle requires a forcing, and no known forcing exists that fits the fingerprints of observed warming - except anthropogenic greenhouse gases. 

"What if global warming is just a natural cycle?" This argument is, perhaps, one of the most common raised by the average person, rather than someone who makes a career out of denying climate change. Cyclical variations in climate are well-known to the public; we all studied the ice ages in school. However, climate isn't inherently cyclical.

A common misunderstanding of the climate system characterizes it like a pendulum. The planet will warm up to "cancel out" a previous period of cooling, spurred by some internal equilibrium. This view of the climate is incorrect. Internal variability will move energy between the ocean and the atmosphere, causing short-term warming and cooling of the surface in events such as El Nino and La Nina, and longer-term changes when similar cycles operate on decadal scales. However, internal forces do not cause climate change. Appreciable changes in climate are the result of changes in the energy balance of the Earth, which requires "external" forcings, such as changes in solar output, albedo, and atmospheric greenhouse gases. These forcings can be cyclical, as they are in the ice ages, but they can come in different shapes entirely.

For this reason, "it's just a natural cycle" is a bit of a cop-out argument. The Earth doesn't warm up because it feels like it. It warms up because something forces it to. Scientists keep track of natural forcings, but the observed warming of the planet over the second half of the 20th century can only be explained by adding in anthropogenic radiative forcings, namely increases in greenhouse gases such as carbon dioxide. 

Of course, it's always possible that some natural cycle exists, unknown to scientists and their instruments, that is currently causing the planet to warm. There's always a chance that we could be totally wrong. This omnipresent fact of science is called irreducible uncertainty, because it can never be entirely eliminated. However, it's very unlikely that such a cycle exists.

Firstly, the hypothetical natural cycle would have to explain the observed "fingerprints" of greenhouse gas-induced warming. Even if, for the sake of argument, we were to discount the direct measurements showing an increased greenhouse effect, other lines of evidence point to anthropogenic causes. For example, the troposphere (the lowest part of the atmosphere) is warming, but the levels above, from the stratosphere up, are cooling, as less radiation is escaping out to space. This rules out cycles related to the Sun, as solar influences would warm the entire atmosphere in a uniform fashion. The only explanation that makes sense is greenhouse gases.

What about an internal cycle, perhaps from volcanoes or the ocean, that releases massive amounts of greenhouse gases? This wouldn't make sense either, not only because scientists keep track of volcanic and oceanic emissions of CO2 and know that they are small compared to anthropogenic emissions, but also because CO2 from fossil fuels has its own fingerprints. Its isotopic signature is depleted in the carbon-13 isotope, which explains why the atmospheric ratio of carbon-12 to carbon-13 has been going down as anthropogenic carbon dioxide goes up. Additionally, atmospheric oxygen (O2) is decreasing at the same rate that CO2 is increasing, because oxygen is consumed when fossil fuels combust.

A natural cycle that fits all these fingerprints is nearly unfathomable. However, that's not all the cycle would have to explain. It would also have to tell us why anthropogenic greenhouse gases are not having an effect. Either a century of basic physics and chemistry studying the radiative properties of greenhouse gases would have to be proven wrong, or the natural cycle would have to be unbelievably complex to prevent such dramatic anthropogenic emissions from warming the planet.

It is indeed possible that multidecadal climate variabilityespecially cycles originating in the Atlantic, could be contributing to recent warming, particularly in the Arctic. However, the amplitude of the cycles simply can't explain the observed temperature change. Internal variability has always been superimposed on top of global surface temperature trends, but the magnitude - as well as the fingerprints - of current warming clearly indicates that anthropogenic greenhouse gases are the dominant factor.

Despite all these lines of evidence, many known climatic cycles are often trumpeted to be the real cause, on the Internet and in the media. Many of these cycles have been debunked on Skeptical Science, and all of them either aren't in the warming phases, don't fit the fingerprints, or both.

For example, we are warming far too fast to be coming out of the last ice age, and the Milankovitch cycles that drive glaciation show that we should be, in fact, very slowly going into a new ice age (but anthropogenic warming is virtually certain to offset that influence).

The "1500-year cycle" that S. Fred Singer attributes warming to is, in fact, a change in distribution of thermal energy between the poles, not a net increase in global temperature, which is what we observe now.

The Little Ice Age following the Medieval Warm Period ended due to a slight increase in solar output (changes in both thermohaline circulation and volcanic activity also contributed), but that increase has since reversed, and global temperature and solar activity are now going in opposite directions. This also explains why the 11-year solar cycle could not be causing global warming.

ENSO (El Nino Southern Oscillation) and PDO (Pacific Decadal Oscillation) help to explain short-term variations, but have no long-term trend, warming or otherwise. Additionally, these cycles simply move thermal energy between the ocean and the atmosphere, and do not change the energy balance of the Earth.

As we can see, "it's just a natural cycle" isn't just a cop-out argument - it's something that scientists have considered, studied, and ruled out long before you and I even knew what global warming was.

2011-01-10 12:42:46Great rebuttal
John Cook


I like that you bring up fingerprints but also the important point of the 'natural cycle' not explaining why GHGs are not having an effect. This is an important point. One possible suggestion - not a deal breaker - when you mention the effect of greenhouse gases, what about slipping in that the increased greenhouse effect has been observed while linking to http://www.skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse-effect.htm

When you mention the observed fingerprints, what about linking to http://www.skepticalscience.com/its-not-us.htm - the more interlinking between rebuttals, the better.


2011-01-10 14:40:32Notes
Robert Way

Not to be the perpetual complainer of the group on this subject but I think we should make note that there is indeed natural oscillating behavior but its magnitude cannot explain all the current warming although it is likely contributing.





I see more and more papers illustrating the link between atlantic/arctic multidecadal variability with surface air temperatures and I think it is important that we are the first to acknowledge any link once it becomes established (not fully at this point). Regarding your commentary on the LIA and the MWP, I think it might be useful to note that changes in ocean circulation and pressure differences likely help to explain both (Trouet et al 2007? Mann et al. 2009) as well as decreased/increased volcanism.

Outside of the commentary I gave above, I think it is very well done. You are quite a writer for someone of your age and for any age really.
2011-01-10 17:46:19Young writers
John Cook

It's good having these young, bright whippersnappers like Kate and James to continue to fight the good fight after us oldies give up the ghost :-)
2011-01-10 21:32:12Young Writers part 2
Robert Way

It's a pity there aren't more out there who are interested in collaborating because there's sort of an appeal for people to listen to young people who actually know what they're talking about and are willing to let their voice be heard.

Frankly I'm not a huge fan of the activist young people who take on causes such as AGW but do not even bother to learn what it is really about. It is refreshing to see people take an interest in what is behind the activism.
2011-01-10 22:28:49
Rob Painting
Very well written. Whenever I see some "skeptic" go on about it's a natural cycle, I can't help but laugh. Dude, how stupid are you?.
2011-01-11 05:10:52nice job
Dana Nuccitelli

I agree with John on adding those links to the other SkS articles, and Robert makes a good point about acknowledging that natural cycles do impact the temperature, but can't explain the current warming.

I'd also suggest when you talk about the stratosphere, mentioning that the higher layers of the atmosphere are cooling as well.  I prefer not to just focus on the stratosphere because of the ozone complication, even though you touched on that.  I discussed the higher layers in the advanced "it's not us" rebuttal if you want a reference/link.

2011-01-11 05:14:34Thanks!
Thanks to John, Robert, and Dana - all very good points. I will make those changes tonight, when I am at home with my laptop, rather than my iPod with the campus wifi occasionally cutting out!
2011-01-11 12:37:00

I've made the changes as suggested - do you think it is better now?

Robert, your first link is not loading (at least in Chrome), perhaps the FTP is not open-access? The second link is fascinating, but I can just picture the WUWT postings taking it out of context when it gets published...

I also agree with what you say about young activists, they generally drive me crazy, and trivialize the issue in my opinion. 

Nice to know I am a "bright young whippersnapper", that made me laugh! BTW, who is James? I'm not sure I have heard of him. 

2011-01-11 14:08:02Got my thumb
Daniel Bailey
Daniel Bailey

Like it; well-written, like all of Kate's posts.


BTW, emailed you that paper Robert linked to that wouldn't load.

2011-01-11 17:39:37I'm James
James Wight


Good rebuttal, particularly the point that “skeptics” have to explain not only what’s causing the warming, but why GHGs aren’t causing it. I wonder if SkS should put more emphasis on this – I don’t think it’s mentioned in the “It’s not us” rebuttal, for instance.

In David Archer's book The Long Thaw, he uses the analogy of a murder mystery where all the evidence points to the butler. You can't convict someone else of the murder - say, the chauffer - without also unconvicting the butler.

2011-01-11 18:00:00Published
John Cook


I've gone ahead and published this. Look forward to the discussion it provokes.

Kate, remember you can always tweak the rebuttal text if you wish to update this - just look for the Edit Rebuttal link beneath the rebuttal:


2011-01-12 10:16:26
Awesome! Thanks, John! I'm enjoying reading the discussion. I will cross post on my blog too.