Climate science is complicated business, and understanding the extent to which climate change is man-made also requires an understanding of Earth's powerful natural cycles. One of those natural cycles involves Earth's orbit and its complicated dance with the sun.
The first thing you need to know about Earth's orbit and its effect on climate change is that orbital phases occur over tens of thousands of years, so the only climate trends that orbital patterns might help explain are long-term ones.
Even so, looking at Earth's orbital cycles can still offer some invaluable perspective on what is happening in the short term. Most notably, you might be surprised to learn that Earth's current warming trend is happening in spite of a relatively cool orbital phase. It's therefore possible to better appreciate the high degree that anthropogenic warming must be taking place in contrast.
Not as simple as you might think
Many people might be surprised to learn that Earth's orbit around the sun is much more complicated than the simple diagrams studied in childhood science classrooms. For instance, there are at least three major ways that Earth's orbit varies over the course of millennia: its eccentricity, its obliquity and its precession. Where the Earth is within each of these cycles has a significant effect on the amount of solar radiation — and thus, warmth — that the planet gets exposed to.
Check out this must-see educational video for a visual presentation on Earth's complicated orbit:
Earth's orbital eccentricity
Unlike what is portrayed in many diagrams of the solar system, Earth's orbit around the sun is elliptical, not perfectly circular. The degree of a planet's orbital ellipse is referred to as its eccentricity. What this means is that there are times of the year when the planet is closer to the sun than at other times. Obviously, when the planet is closer to the sun, it receives more solar radiation.
The point at which the Earth passes closest to the sun is called perihelion, and the point furthest from the sun is called aphelion.
It turns out that the shape of the Earth's orbital eccentricity varies over time from being nearly circular (low eccentricity of 0.0034) and mildly elliptical (high eccentricity of 0.058). It takes roughly 100,000 years for Earth to undergo a full cycle. In periods of high eccentricity, radiation exposure on Earth can accordingly fluctuate more wildly between periods of perihelion and aphelion. Those fluctuations are likewise far milder in times of low eccentricity. Currently, the Earth's orbital eccentricity is at about 0.0167, which means its orbit is closer to being at its most circular.
Earth's axial obliquity
Most people know that the planet's seasons are caused by the tilt of the Earth's axis. For instance, when it is summer in the Northern Hemisphere and winter in the Southern Hemisphere, the Earth's North Pole is tilted toward the sun. The seasons are likewise reversed when when the South Pole is tilted more toward the sun.
What many people don't realize, however, is that the angle at which the Earth tilts varies according to a 40,000 year cycle. These axial variations are referred to as a planet's obliquity.
For Earth, the tilt of the axis varies between 22.1 and 24.5 degrees. When the tilt is at a higher degree, the seasons can likewise be more severe. Currently the Earth's axial obliquity is at about 23.5 degrees — roughly in the middle of the cycle — and is in a decreasing phase.
Perhaps the most complicated of Earth's orbital variations is that of precession. Basically, because Earth wobbles on its axis, the particular season that occurs when Earth is at perihelion or aphelion varies over time. This can create a profound difference in the severity of the seasons, depending on whether you live in the Northern or Southern Hemisphere. For instance, if it is summer in the Northern Hemisphere when Earth is in perihelion, then that summer is likely to be more extreme. By comparison, when the Northern Hemisphere instead experiences summer in aphelion, the seasonal contrast will be less severe. The following image may help to visualize how this works:
This cycle fluctuates on roughly a 21- to 26,000-year basis. Currently, summer solstice in the Northern Hemisphere happens near aphelion, so the Southern Hemisphere should experience more extreme seasonal contrasts than the Northern Hemisphere, all other factors being equal.
What's climate change got to do with it?
Quite simply, the more solar radiation bombarding Earth at any given time, the warmer the planet should get. So Earth's place in each of these cycles should have a measurable effect on long term climate trends — and it does. But that's not all. Another factor has to do with which hemisphere happens to be receiving the heaviest bombardment. This is because land warms faster than oceans do, and the Northern Hemisphere is covered by more land and less ocean than the Southern Hemisphere is.
It has also been shown that shifts between glacial and interglacial periods on Earth are most related to the severity of summers in the Northern Hemisphere. When summers are mild, enough snow and ice remains throughout the season, maintaining a glacial layer. When summers are too hot, however, more ice melts in the summer than can be replenished in the winter.
Given all of this, we might imagine a "perfect orbital storm" for global warming: when Earth's orbit is at its highest eccentricity, Earth's axial obliquity is at its highest degree, and the Northern Hemisphere is in perihelion at summer solstice.
But that's not what we see today. Instead, Earth's Northern Hemisphere currently experiences its summer in aphelion, the planet's obliquity is currently in the decreasing phase of its cycle, and Earth's orbit is fairly near its lowest phase of eccentricity. In other words, the current position of the Earth's orbit should result in cooler temperatures, but instead the average temperature of the planet is on the rise.
The immediate lesson in all of this is that there must be more to Earth's average temperature than can be explained through orbital phases. But a secondary lesson also lurks: Anthropogenic global warming, which climate scientists overwhelmingly believe is the prime culprit in our current warming trend, is at least powerful enough in the short term to counteract a relatively cool orbital phase. It's a fact that should at least give us pause to consider the profound effect that humans can have on the climate even against a backdrop of Earth's natural cycles.
Image credit (Eccentricity): NASA
Image credit (Obliquity): NASA
Image credit (Precession): Greg Benson/Wiki Commons/GNU