My second rebuttal for Skeptical Science. Thanks to all the folks who helped to review it! Further suggestions are welcome, as always. -Kate
“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 variability, especially 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.
ClimateSight 
Nice job, Kate.
>However, internal forces do not cause climate change.
How do you know this? It seems you are contradicting yourself when later you say that it is possible but highly unlikely it is happening naturally.
ENSO and other such internal cycles don’t change the energy balance of the earth. I suppose if they swung a certain direction it could set off feedbacks, but the way they’re operating right now is with no trend. Natural cycles that could actually cause climate change are external – solar, etc. -Kate
Your article doesn’t cover the possibility of both effects, greenhouse and natural variation, happening simultaneously, with one dominating the other.
Um, yes it does. See the paragraph about natural variability in the Atlantic likely contributing to the warming on some timescales, and masking it on other timescales. The linked papers are really interesting too. -Kate
**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.**
How are you comparing global temperatures 1500 years ago to the present?
Are you saying the Medieval Warm Period was not global or not as warm as the present?
Probably. But that’s not the point – we are looking at whether known cycles explain current climate change, and DO events don’t. Also, the MWP was more like 1000 years ago, not 1500. -Kate
This is mostly good, Kate, but I think it’s a bit odd that people construct these arguments without resort to deep-time paleo, which is IMHO (plus, erm, Jim Hansen’s opinion) the sledgehammer argument. Taking that into account, one has to make a case why CO2 should be expected to not affect the climate system as it always has. (Lindzen has the only effective answer to this, which is to ignore it.)
Re “the Medieval Warm Period ended due to a slight increase in solar output,” I think recent work has rendered that long-standing presumption shaky. It can’t yet be excluded, but that’s a different matter. This new paper (haven’t read it yet) may be informative. Tom Crowley stated several years back that he thinks what has been thought to be solar influences will ultimately turn out to be volcanic effects. This seems consistent with the trend in assessments by the solar physics community. What we may be left with is an MWP explainable by the relative absence of volcanoes, and an LIA that is simply the result of a slight Milankovitch trend that was trumped by AGW.
Sorry, I inadvertently sliced the reference to the LIA from the front of that quote.
Exc. post Kate! I just finished my second for John’s site as well. This argument has to be one of the top three myths circulating in the denialosphere.
I agree with Steve Bloom that Hansen’s argument on deep paleo-climate data is a sledgehammer. To explain away climate change one has to come up with a plausible reason based on physics as to why the CO2 /temp. ratio of the last several million years is no longer valid. Especially since it fits the known physics well.
Good luck doing that!
I’m rereading this as I’ve only ready it quickishly so far, but have you more/mostly addressed the Cyclic part of the statement “What if global warming is just a natural cycle?” & not fully addressed the it’s natural part of the Statement ?
Admirch:
It can’t be a natural cycle if it’s not a cycle.
The question about whether it’s natural or anthropogenic (or more appropriately, since climate responds to both, to what extent is it natural and anthropogenic) is a completely different question. And more appropriately, it’s several independent questions – for instance, “natural” causes could include solar activity, orbital shifts, cosmic rays or volcanic activity, and each of these would need to be addressed in kind to answer the natural part. (Oh, and note that some of these *are* cyclical, such as orbital forcing, so if we know it isn’t a cycle we can rule them out.)
Fortunately, this post was written for Skeptical Science, which has dozens of other articles at assorted levels of complexity explaining what we know and linking directly to the peer-reviewed literature so you can check for yourself. Look under “It’s Not Us” to find oodles trying to answer your question. (Note that this post is listed there too, as part of that overall whole.)
Or, if you’d like, let Dr. Richard Alley explain. It’ll take about an hour but it’s honestly the best pure science summary I’ve ever seen.
I do understand your point, but it still doesn’t address my question (I think ?)
If the response is purely only meant to address the “Natural Cycle” only question, then so be it.
Steve Bloom,
I think what you could potentially see is a stochastic resonance in the system. But what seems to be becoming evident is that the LIA and MWP seem to be demonstrating the characteristics of oceanic conveyor changes. In particular we see a positive AMO during the MWP and a negative AMO during the LIA. The AMO is viewed by many (many but not all) as being a proxy for the thermohaline circulation strength. In particular warm AMO phases coincide with greater North Atlantic heat transport and cold phases with lower heat transport. There is evidence that the LIA and MWP could be demonstrating these sort of patterns. But the question is what caused it? Could it be that solar modulates some ocean circulation patterns which changes the circulation? The evidence that the AMO was not present following the maunder minimum gives some credence to my theory. (TAMINO Disagrees though I think haha)