“It’s Just a Natural Cycle”

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 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.

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The Energy Budget

I’ve decided to take this blog more in the direction of communicating science – there are only so many sociological musings to come up with. This is the first of many planned posts explaining basic climate science so people have better context for what they read in the newspaper.

Every post is a work in progress, and will be continuously edited when necessary, so please leave comments with suggestions on how to improve the accuracy or clarity. Enjoy!

What determines the temperature of the Earth?

The temperature in your backyard, the warmth of the equator, the frigid polar regions, the average global temperature for the whole planet…..they might seem like very different things to measure, but they’re all caused by the same process. It all comes back to energy.

This energy comes from the Sun, but it’s not as simple as a single transfer. Remember, at any time of the day or night, the Sun is shining on some part of the Earth. That energy can’t just stay on our planet, otherwise it would keep building up and up and we would fry after a couple of weeks.

Therefore, incoming energy from the Sun has to be balanced by outgoing energy from the Earth for the planet’s temperature to stay relatively constant. So when the Sun’s rays hit the ground, as a mixture of light, infrared, and UV radiation, the Earth absorbs the energy. Then it converts it to all to infrared radiation, which we perceive as heat when it hits us, and releases it upward.

All objects perform this absorption and emission when they are hit with radiation. If they receive enough energy, they can release some of it in the form of light – think of how a stove element glows when it’s turned on. However, the energy hitting the Earth is nowhere near this level, so it all comes out as infrared.

It is this emission of infrared radiation that determines the temperature of the Earth. The second step, not the first, is the important one, the one that we actually feel and experience. So on a hot summer’s day, it isn’t actually energy coming down from the Sun that’s making the air warm. It’s energy coming up from the Earth.

The air doesn’t warm up instantly, either – there’s a bit of a lag. This allows warm air to be transported away from the Equator and towards the poles, in the global circulation system of wind currents. Without this lag time, many regions of our world would have far more extreme temperatures.

Additionally, not all the radiation the Sun sends down gets absorbed by the Earth. Some of it is bounced back by clouds, which is why sunny days tend to be warmer than cloudy days. Some of it reaches the surface of the planet, but is bounced back too, before it’s even absorbed. This reflection of energy is particularly common when the surface is light in colour. That’s why it seems so bright outside after a snowstorm – because the snow is bouncing the energy back up as light, instead of absorbing it and releasing it upward as heat. It also explains why dark concrete, which absorbs almost all the radiation that hits it, is so much warmer than a light-coloured deck.

The amount of energy that the Sun sends down to us is greater than the amount that the surface of the Earth actually absorbs. However, the amount absorbed has to be equal to the amount released, and the amount released is what we witness as the temperature outside.