The Sun and Earth
Consider the total solar eclipse – a thing of undeniable beauty. When the Moon passes before the Sun, all that remains visible for a few minutes is the shimmering corona. Perhaps even more remarkable than the sheer beauty of an eclipse is its sheer improbability. The Sun is far larger than the Moon, but it's much farther away, so the two bodies – as we see them in the sky – are within one percent of each other's size. If one or the other were just a bit closer or a bit larger, there could be no eclipse as we know it. The coincidence is truly cosmic.
The more routine pas de deux of the Sun and Earth is an anchor of daily life. We watch the Sun travel across the sky each day, its path edging north or south with the seasons. Yet it's the wobblings of Earth – spinning on its axis and circling the Sun – that not only produce our seasons, but play a huge role in natural climate change (as opposed to the kind in which humans are implicated).
The source of it all
Without the Sun, there wouldn't be any weather, not to mention anyone to notice its absence. The Sun derives its power from nuclear fusion within its superheated core, whose centre simmers at roughly 15 million degrees C/27 million degrees F. As the nuclei of two hydrogen molecules fuse, they produce helium and release energy that radiates upward to the Sun's outer layer. That zone of overturning gases and intense magnetism channels the energy in complex ways, forming sunspots, flares and other solar features. It's this varied structure and the constant changes within it that make the Sun far more than a homogeneous blob.
The atmosphere surrounding the Sun isn't nearly as blistering as the solar core, but at more than a million degrees C, it's still so hot that the atoms within it can't hold onto their electrons. The result is a stew of charged particles (plasma) that expands outward into the coolness of deep space as the solar wind, moving at speeds in the range of 320km/200 miles a second. Meanwhile, the plasma churning inside the Sun emerges at the surface in the form of active regions – a crude equivalent of disturbed weather on Earth. Sunspots are the best-known type of active-region disturbance. Although sunspots appear darker than the rest of the Sun, they're bursting with magnetic energy, so their overall effect is to increase the amount of plasma streaming out from the Sun and help fuel “gusts” in the solar wind. From the Sun's perspective, Earth is like a pinpoint in space, so only a few of these plasma-laden gusts manage to hit the tiny target of our upper atmosphere, but once there, they can wreak magnetic and electrical havoc.
Solar energy on Earth
Let's assume the front of your body spans about nine square feet, or roughly a square metre. If you went to the outer edge of our atmosphere and faced the Sun head on, you'd intercept the energy equivalent of about 14 one-hundred-watt light bulbs. The precise number – 1368 watts per square metre – has traditionally been called the solar constant. This term isn't quite accurate, however. Thanks to ever more precise observations from satellites, we now know the Suns output rises and falls by as much as 0.1 percent over short periods, as well as during the eleven-year solar cycle. Larger variations occur over centuries and millennia, helping to trigger such climatic events as the Little Ice Age that put post-Renaissance Europe in the deep freeze. And ultraviolet energy can rise and fall much more than the Sun's total output, varying by as much as a factor of 10 across the solar cycle.
Scientists are still grappling with the “faint sun paradox”. From three to four billion years ago, when life began on Earth, the Sun was only 75 percent as radiant as it is now. Yet the best evidence shows that Earth's temperature was comparable to today's. Greenhouse gases were probably more prevalent than today, which may explain why Earth was relatively toasty despite the weaker Sun.
The basic engine of our atmosphere's behaviour is the contrast between sunny and not-so-sunny places. Every spot on the planet gets an equal amount of darkness and daylight, but how it's distributed varies by latitude. On the equator, every day of the year is twelve hours long, while the North and South Poles get six months of daylight and six months of darkness each year. Mid-latitudes get the familiar pattern of shorter days in the winter and longer ones in the summer.
However, the sunlight that hits Siberia can't hold a candle to that of Sumatra. Both places get the same amount of daylight each year, but closer to the poles, the Sun sits lower on the horizon, so its rays strike the planet at a sharper angle. The same amount of sunlight is spread over a much larger area near the poles, and so there's less solar heating per patch of land. The net result is a perpetual imbalance: the tropics are always getting more than their share of solar energy, and the poles are forever shortchanged. The atmosphere responds by moving heat from the equator toward the poles, and voila – we have wind and weather.
A little seasoning
What makes the seasons? Perhaps no other weather question is so misunderstood and incorrectly answered. The most common knee-jerk answer is that Earth is closer to the Sun during summer than during winter. If this were all there was to it, then the whole Earth would experience summer at the same time, but we know that's not the case. The tilting of Earth's axis is what gives us seasonality. If Earth stood straight up within the circle it makes around the Sun, there would be no such thing as summer or winter. Instead, Earth leans at an angle of about 23.5° from celestial north, the top of an imaginary line cutting through our solar system. Earth's tilt allows the Sun to face the North Pole in June and the South Pole in December. As each hemisphere turns to face the Sun, the land and air warm up and summer arrives.
A close look at the calendar reveals that the seasons are not created equal. People in the Northern Hemisphere get something of a break: their winter is only about 89 days long, while their summer lasts roughly 94 days. This is tied to the fact that the Sun doesn't sit squarely in the middle of Earth's orbit. Instead, it's displaced about 3 percent of the way toward one side. Because of the laws of angular momentum, Earth has to husde as it passes closer to the Sun, just as the water in a drain rotates more quickly as it nears the centre. Earth's closest approach to the Sun each year happens to fall on January 4, so the seasons near that date (northern autumn and winter, southern spring and summer) are shorter than those on the other side of the calendar (northern spring and summer, southern autumn and winter). The 28-day brevity of February is an adjustment to this astronomical fact.
An oblique and eccentric story
People change, haircuts change – why not orbits? Earth has spun around the Sun for a long time, but the process hasn't been as routine as you might imagine. Over millions of years, each one of the three major orbital parameters has gone through variations, and those variations have played a gigantic role in the climate on Earth. None of them are short-fuse enough to worry about in our own lifetimes, but they do shed light on how a small change in our climate system can have a huge impact.
- The tilt of Earth – its obliquity – varies from about 21.8° to 24.4° and back over a cycle that lasts about 41,000 years. A greater tilt means warmer summers and colder winters across the globe. When the tilt decreases, It can help bring on an ice age (all else being equal), because the progressively cooler summers can't melt the past winter's snow.
- The offset of the Sun from the centre of Earth's orbit, discussed above – in other words, the eccentricity of the orbit – Is nowon a slow decrease. Currently Earth is about 3 percent closer to the Sun, and receives about 7 percent more solar energy, in early January than in early July. The orbit was about twice as eccentric when the most recent Ice Age began, a little more than 100,000 years ago. Higher eccentricity means more contrast in solar heating between the nearest and farthest annual approaches of Earth to the Sun (perihelion and aphelion, respectively).
- A slight wobble of Earth's rotational axis, and a slight wobble of Earth's orbit around the Sun, combine to produce precession – a slow advance in the date of perihelion. Right now, it occurs on January 4, but each year it moves ahead about 25 minutes. By the year 11,750, it'll fall In June, and people in the Northern Hemisphere will endure a winter about five days longer than It is now. When a hemisphere is closer to the Sun during summer than during winter – as is now the case in the Southern Hemisphere – the seasonal contrast in heating is heightened (although the vast southern oceans more than compensate for this by smoothing out seasonal temperature swings).
What on earth does all this mean for climate? James Croll, a janitor in a Glasgow university, speculated in the 1860s that orbital variations might be behind the onset and departure of ice ages. It took until the 1940s for the Serbian scientist Milutin Milankovitch to complete the precise work that pointed in the same direction. By blending the three orbital parameters above, Milankovitch found that certain combinations could reduce the energy reaching one or the other hemisphere by a few percent. Interestingly, it's the mellower combinations – those that spread the radiation most evenly throughout the year – that apparently lead to ice ages. The idea, in a nutshell, is that relatively mild winters allow for heavier snow to fall at high latitudes, while cool summers help keep the higher-latitude snow from melting.
Computer modelling and other tools have largely supported the Milankovitch thesis. The various orbital parameters coincide for the Northern Hemisphere about every 100,000 years, and that's the rough frequency of the last few ice ages, as deduced from geologic records. However, the book isn't closed on what causes glaciation. When the most recent round of ice ages began about 2.7 million years ago, they occurred about every 41,000 years, corresponding nicely to variations in Earth's obliquity. About a million years ago, the ice-age frequency dropped to once every 100,000 years or so, with short periods between each glaciation. It's not quite clear what is setting the current ice-age frequency – and as best we can tell, there have been stretches of up to a billion years without any major glaciation at all. It's possible that the Milankovitch cycles are able to turn glaciation on and off simply because Earth's continents are now located where they shelter the Arctic Ocean and facilitate its freezing.
Experts disagree strongly on when the next ice age is due. Many believe that orbital cycles won't conspire to bring it on for thousands of years – perhaps as many as 50,000. But William Ruddiman (University of Virginia) argues that human-driven changes in land use have postponed the arrival of an ice age that should already have begun. Perhaps the biggest wild card is global warming. If the more dire scenarios come to pass, ice sheets in Greenland and western Antarctica could disappear in the next few hundred years, which would in turn affect the timing of any subsequent glacial expansion.