Tides, Why Tide Charts Don’t Average to Zero, and Agendas

Tides are an important part of life on earth. Earthly tides are primarily governed by the moon, a result of Lunar gravity tugging on the planet as the Earth spins along side of our over-sized neighbor.

A few days after the 2013 "supermoon"

A few days after the 2013 “supermoon”

Tides affect both the earth’s crust (raising it enough so that large particle accelerators must be designed with the geo-tide in mind) and, more familiarly, the oceans. Technically speaking, all bodies of water are affected by the tides, but the large tides experienced by coastal dwellers is a result not only of the Earth-Moon-Sun gravitational dynamic but of resonance, coastline shape, and of characteristics of the ocean floor.

Resonance is simply the self-reinforcing effect of synchronization. The most familiar form of resonance for most people may be pumping your legs on a swing. If your timing is right the small amount of energy added to the pendulum motion by pumping your legs will lift you higher and higher. If your timing is off you can kill your speed and come to a stop. You can do the same with your hand and a basin of water, a small amount of hand motion will quickly wind up sloshing water out of a bathtub. Swings, tides, and lasers work on this principle of resonance.

The shape of the coastline and the depth of the ocean floor can concentrate or diffuse tides as well, focusing or dispersing the vast energies at play. This is why the tides in places with fjords like British Columbia and Norway can be so dangerous.

I grew up near the ocean and spent a lot of time watching the ocean and exploring tidepools and the rocky beaches of the California coast.

Mussels anchored on exposed rocks in the intertidal zone.

Mussels anchored on exposed rocks in the intertidal zone.

The interesting part of the coast was not the sandy beaches, but the craggy high surface areas that trapped pools of water. All sorts of creatures live in these pools. Strange and wonderful creatures like the Gumboot Chiton, Cryptochiton stelleri, large molluscs that crawl out of the water at low tide to feed on exposed seaweed.

Cryptochiton stelleri, or Stellars's Hidden Chiton, so named because the characteristic eight bony plates are hidden under rugose red skin

Cryptochiton stelleri, or Stellars’s Hidden Chiton, so named because the characteristic eight bony plates are hidden under rugose red skin

In tidepools I have found and caught octopus, eels, and fish, but two of the most common residents are Shore Crabs

Shore Crab, a common California coast resident

Shore Crab missing an arm, a common California coast resident

and anemones, both hovering between the scavenger and hunter niches.

Anemones: close-up of arms

Anemone: close-up of arms

These and many other creatures live in what is known as the intertidal zone, the region of the coast that is above low tide and below high tide. This is an area of tremendous free energy. Energy is a two edged sword as any scientist or engineer can attest to. Energy makes all things possible, and most things can be broken down into either growth or destruction. Creatures that live in the intertidal zone reap the benefit of straddling two environments, feasting on the windfall of both, but the fierce waves and tides also expose them to the dangers of being left to suffocate in water, desiccate in the sun, be torn from holdfasts, and fall prey to other adaptable creatures. Excellent potential for great growth or swift destruction.

In the intertidal zone there are bands of life that roughly conform to the amount of time spent out of water. Mussels and barnacles occupy the upper reaches and are the most familiar.

Mussels and Gooseneck Barnacles on exposed rocks.  Both are tasty to eat.  Mussels are commonly eaten in the US and Gooseneck Barnacles fetch high prices in Spain, where people risk their lives to collect them.

Mussels and Gooseneck Barnacles on exposed rocks. Both are tasty to eat. Mussels are commonly eaten in the US and Gooseneck Barnacles fetch high prices in Spain, where people risk their lives to collect them.

Lower down, in some places, Sea Palm grows in dense stands, often damaged by the waves.

Sea Palm (Postelsia palmaeformis), the only species of this kelp and one of the few that can live for extended times out of water

Sea Palm (Postelsia palmaeformis), the only species of this kelp and one of the few that can live for extended times out of water

At each level a different selection of species dominates creating a many-layered composite of ecotones, much like a mountain in miniature. The vertical ranges for these species collections is so narrow that it can be used to track sea level changes.

If you want to see nudibranchs, octopus, and sandcastle worm, then when do you go to the coast?

sandcastle worm (Phragmatopoma californica) colony

Sandcastle Worm (Phragmatopoma californica) colony

Clearly you go at low tide… but what does this mean?

One of the things that bothered me about tide-charts (like the one below) is that if you average the high and low tides you do not get zero.

Graphical tide chart made with the TideTrac app

Graphical tide chart made with the TideTrac app

I went out a while back to experiment with day-time long exposure photos. This is the tide chart for the day. Notice that the lowest tide of the day is still +0.4 feet (12cm)? The average tide for the day is +2.9 feet, nearly a meter. What is that all about?

The answer is that tide charts were designed for sailors, not tide-poolers or surfers. Tide charts are set by a datum, one of 17 potential datums, that all are based off of the LOWEST tides, not the average tide. For a sailor who wants his boat to stay in the water and not be slammed against rocks, this is important.

Even a small bump carries a lot of energy

Even a small bump carries a lot of energy

For the rest of us, it is a little non-intuitive. We have picked the lowest tide as the datum because that is what was important to the people making the charts at the time. If it had been house contractors instead, the datum would have been the highest tides. Tide-poolers would probably have picked the average tide as the datum, making it easy to determine when the most species would be exposed at any given moment.

There is a lesson here; be wary of maps and charts. Maps and charts are made to tell a specific story, a story with a perspective, a message, and an agenda. Like an advertisement on TV or a political speech, it is important to be aware of both the audience and the proponents of the product. And in truth, there is little that is more political than maps and charts.

Oh, the long exposure photos turned out great.

Ten seconds of waves washing in and out during the day

Ten seconds of waves washing in and out during the day


Note: for those of you who look at the full-sized photos, several of these have been pulled from my archives and were taken with my first digital camera; a Canon ELPH from the early 2000s. A great little camera, but of a disappointingly low resolution.

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.


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.