Happy Aphelion! Wait, what’s that and how does it relate to the climate?

Well, today is one of the most interesting days of the year for me.  This year, 2012, July 5th is aphelion, the day that Earth is furthest from the sun in its orbit.

It is rare (in the tens of thousands of years scale) that the northern hemisphere summer solstice are so close to aphelion.

The time during the year that aphelion and perihelion (when we are closet to the sun) changes over a roughly 100,000 year cycle, known as the Milankovitch Cycle.  Our orbit around the sun is not a circle, it is an ellipse with an eccentricity of about 0.0167.  This orbit both changes shape and rotates around the sun much like a spirogram tracing out a flower-like shape.

Ellipse with 0.5 eccentricity. Earth has a much smaller eccentricity, making the orbit more nearly, but not exactly a circle. Aphelion and perihelion are the two ends of the egg shape.

Perihelion precession

 

It is summer in the northern hemisphere, a time when people often say things like, “We are closer to the sun than we are in winter.”  This is not really true.  Summer is a product of the angle at which Earth is tilted, right now Earth is tilted so that the northern regions lean toward the sun.  In terms of orbit we are actually at the furthest point Earth gets from the sun.

This has interesting implications in terms of the global climate.  This means that right now winters tend to be warm (the planet is closer to the sun) and summers cool (the planet further from the sun).  In the big picture this places us in the midst of a global cool cycle, the type of situation that tends to lead to ice ages, like the one we are emerging from.

The climate picture is not so simple, though.  Even in terms of celestial mechanics there are other factors that play into the climate picture.  Two large, cyclical factors are the precession and wobble of the Earth. One of these is the obliquity, or the angle at which Earth is tilted.  This is our Axial Tilt, and it is currently 23.4º, but the Axial Tilt changes between approximately 22º and 24º, over a 41,000 year cycle.  This affects where the tropics lie and how much solar energy different regions of Earth receive.

Range of Axial tilt – Wikipedia indicates 22.1º to 24.5º , other sources vary on the exact outer limits of the range, but they are all near 22º and 24º of tilt.

The final large scale cycle that comes into play is Axial Precession.  Spin a top, or a gyroscope; the handle by which it was spun precesses, or rotates around the axis of spin at a fixed angle.  In the case of Earth this cycle varies between 19,000 and 23,000 years.

Precession – just like a top or gyroscope, Earth’s axis of spin makes a slow circle

No single one of these factors leads to an ice age or global tropical forests, they must combine in just the right way to set the conditions.  These cycles are like waves in water, sometimes they cancel each other out, other times they reinforce each other.  Past climate records show clear evidence of the effects of these cycles.

NASA – Orbital Time Series showing the cyclic nature of the three major orbital cycles

NASA – Solar Insolation (energy received from the sun) and O18 (an heavy isotope of oxygen) records in sediment indicating temperatures at the time the O18 precipitated.  O18 is heavier, so it takes more energy to keep it aloft, and water made from this isotope tends to precipitate first when the temperature drops.

Once the orbital conditions are right then, other Earth surface factors come into play.  Where mountain ranges are, whether oceans are polar or tropical, how saline the oceans are, the arrangement and distribution of the continents , and volcanic activity, such as the Deccan and Siberian Trap.  These are immense lave flows in India and Siberia that pumped immense amount of greenhouse gasses into the atmosphere and seem to have strongly affected the climate.  Even short duration, one-off events such as the 1815 eruption of Tambora in the Dutch East Indies (now Indonesia), can have global climate effects.  Tambora is implicated in what is commonly known as The Year Without Summer when the global temperatures dropped by 0.4–0.7 °C , leading to massive famines across the northern hemisphere as  result of immense crop shortages.  This may have been a period of low solar activity as well, adding to the poor growing conditions.

Impacting bodies also can play a major role in the global climate.

The upshot of this is that we really should be paying close attention to the current rapid warming trend as it is happening at a time when it appears, from the large scale cycles, that conditions are not right for a warming trend of the magnitude we are experiencing.  This is a warning sign, one that we need to pay attention to.

The climate always changes, and that is as it should be.  The problem we now face is that our physical and social infrastructure has been built around the idea that the changes are small and seasonal rather than global and systemic.  Our cities, roads, fields, power generating systems, and economies are based on things pretty much staying as they are, which will not happen.  Even without us tipping the scale the climate would not stay the same.  We need to be aware of this and act accordingly, looking at the much larger picture.

If we don’t, well, life on Earth will be just fine in a few tens of millions of years, but we may not be around to see it.  If we continue on the path we are now taking, we many not even be around to see the northern summer occur at perihelion.

This is our planet.  Right now it is the only one we have.  Understand it, enjoy it, and please don’t break it.

The good folks at NASA have put together a great, short paleoclimate primer at the link below, which is where the last two charts come from.

http://earthobservatory.nasa.gov/Features/Paleoclimatology_Evidence/

The first three images from from the Wikipedia article on Milankovitch Cycles – they had the best diagrams.

I will return to the usual, smaller scale aspects of nature in the next post, which will be about Dogbane.

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Meteor Impacts and Ourselves

I am fascinated and enthralled by things that fall from space and the marks they leave behind.  It’s not just my love of space, it’s is something far more profound, it is in part what those things signify.

Go to a museum, one that has meteorites.  Often there will be at least one display of a metallic body that you can touch.  Lay your hands on it, press your palms against it, feel the soft curves, the slightly nubby surface, the coolness of the blackened metal.  You are touching the core of an extinct planet.  That should give you pause and send a small shiver up your spine.

On Earth there less than 200 known, confirmed, impact structures.  Just looking at the map it is clear that the distribution is skewed to areas where there are many people (North America & Europe), exposed bedrock (Canada & Scandinavia), or regions where weathering is slow (Australia & North Africa).

Confirmed impact structures on Earth from: www.meteorimpactonearth.com

Every other rocky body in the solar system is liberally coated in the scars left by impacts.  The Earth bears the history of its impacts in a different way.  Weathering, plate tectonics, and the oceans have served to hide the marks of the numerous past impacts.  Except…

The global ocean, that covers 70% of the surface of the planet to a depth of 7 miles in some places, this, the single largest surface feature of the planet, is impact derived.  It is believed that ALL the water on the planet arrived by cometary impacts soon after the planet formed.  The Moon is another large impact structure, a relict left over from  the collision of the proto-Earth and another roughly Mars sized body.

The frequency of large impacts has, thankfully, fallen over time, but they still happen.  Some of you, I hope all of you, may remember the comet Shoemaker-Levy 9, the comet that crashed into Jupiter in 1994 after being torn apart by Jupiter’s immense gravitational field.  The fireballs in Jupiter’s atmosphere were larger than the entire Earth, and there were multiple fireballs.

Shoemaker-Levy 9 impact on Jupiter

The energy released by each of the Shoemaker-Levy impacts was on a par with the Chicxulub impact in the northern Yucatan 65 million years ago that is implicated in the demist of all terrestrial animals larger than a piece of carry-on luggage.

On Earth impacts are still frequent, but most are small and do not survive passage through the atmosphere.  Think shooting stars, grains of sand and dust traveling at orbital speeds, around 20km/second.  Several months ago, on the last day of February, I was treated to a something more dramatic than one of these little grains of dust.  A little after 10pm on the 28th I was driving under a clear sky and the snow covered landscape lit-up with a bright blue flash.  I later found out that the flash of light had been seen from New Jersey to Quebec.  This was just one of the many fireballs that flash in the sky each year, probably something small only a few meters in diameter, an explosion not more than a few kilotons.

In a few places the scars left on the ground from large impacts are still visible.  One of my favorite ones is in NE Canada.  Canada is an excellent place for finding impact structures as much of the Canadian Shield is ancient, exposed bedrock.

Manicoaguan impact crater turned into a reservoir

The Manicoaguan impact is about 215 million years old and approximately 60 miles across.  It has been dammed and the island in the middle is now one of the largest fresh-water islands in the world.  Big impacts like this are rare, but they leave dramatic remains behind.

Small impacts are surprisingly common, the frequency rapidly trailing off the larger the impact.  This is good news, but the picture is very incomplete as we have only been able to watch carefully for a short period of time.

Impact frequency Table from geology.com

We are struggling to understand how the universe fits together and have tremendous difficulty comprehending the scales and energy involved.  We are too used to thinking on our small scales, our bodies, our houses, maybe our planet, for a few our solar system or galaxy.  Our solar system is huge, our galaxy immense, yet in the lager context of our body of knowledge and what we can see even the Milky Way galaxy is barely a microscopic speck.

Look at the ocean, lay back and watch the trails left by falling meteors, look at the background of stars, go to a museum and touch the heart of a planet, if you live near an impact crater go visit.

We often say, “We are all connected,” and this is true, and that web of connection is far greater, wider, and deeper than most of us realize.