BookTalk.org

Please use this thread to discuss Ch. 2: Ice, the magic crystal.

Chapter Two is a physics and chemistry lesson about ice. Pretty well every other chemical shrinks in size when it goes from liquid to solid, but water expands when it freezes into ice, with the result that ice floats on water. This is a basic condition for life, since if ice was heavier than water the whole ocean would probably freeze.

There are so many amazing facts about water physics. For example, have you ever wondered why a snowflake is two-dimensional? Apparently the bonds between hydrogen and oxygen in a water molecule mean that ice crystals form a flat sheet. They always grow like tree branches at an angle of 60°, but doing so rather randomly within that constraint of two dimensions.

The maximum density of fresh water is at 4°C. That means in a lake in winter the surface water cools down to 4° and sinks, mixing and overturning until all the water is that temperature, at which point the surface freezes, and then the surface ice gradually gets thicker. But sea water is different. The salt means it freezes at -1.8°, and its maximum density is less than for fresh water. And the ocean has currents with different direction and chemistry at different depths.

Water is the basis of measurement. A gram of water defines a cubic millimetre and millilitre, while a gigatonne of water is a cubic kilometre. A calorie is the heat needed to raise a gram of water by 1°C. But when you melt ice, you need 80 calories to cross the liquid threshold. This is why ice is so stable.

When the Arctic Sea freezes in autumn, it forms sheets of flat crystals like snowflakes in the water, which gradually link together to form a solid barrier between sea and sky. In the first year, Arctic ice gets to about 1.5 metres thick while Antarctic ice reaches 0.5 metres, depending on waves and temperature. Only the ice crystals whose plane is vertical keep growing, while those with a horizontal plane get crowded out. The salt in first year sea ice forms cells. The ice crystals themselves don’t contain salt, so this ice is about one third as salty as the sea water. The salt then gets forced out by gravity and flushing. If the ice survives through the summer, it has much less salt, and is much stronger than first year ice.

So getting rid of all the multiyear ice means the Arctic is far more fragile.

With global warming, the amount of ice melt increases. This is an example of an accelerating feedback loop, a bit like Jimi Hendrix holding his guitar near the amplifier to make a screaming sound, but with less control. More like when a public address system squeals as the sound loops between the amplifier and the microphone and people rush to turn it off, except with emissions we have no easy off switch.

With melting ice, the positive loop gain works to increase the temperature. This week Siberia had its first ever hundred degree Fahrenheit day, caused by these amplification processes driven by CO2 emissions.

As the summer snow on top of the Arctic ice melts and the water flows down to perforate the ice, making it a bit like a Swiss cheese, it leaves the remaining ice surface very dirty. Black absorbs heat while white reflects, so more snow melt means more ice melt. Reflection is known as albedo. Pure snow can reflect 90% of incoming light, while dirty ice might absorb 50% of the light and heat. The draining melt pools decrease the albedo and make the ice fragile and thin, like osteoporosis in bones that lack calcium, often making a thaw hole right through the ice sheet.

Then the draining fresh water under the ice makes a pool that speeds up the bottom melt. More screaming accelerating feedback.

Robert Tulip wrote:Chapter Two is a physics and chemistry lesson about ice.
When the Arctic Sea freezes in autumn, it forms sheets of flat crystals like snowflakes in the water, which gradually link together to form a solid barrier between sea and sky. In the first year, Arctic ice gets to about 1.5 metres thick while Antarctic ice reaches 0.5 metres, depending on waves and temperature. Only the ice crystals whose plane is vertical keep growing, while those with a horizontal plane get crowded out. The salt in first year sea ice forms cells. The ice crystals themselves don’t contain salt, so this ice is about one third as salty as the sea water. The salt then gets forced out by gravity and flushing. If the ice survives through the summer, it has much less salt, and is much stronger than first year ice.

So getting rid of all the multiyear ice means the Arctic is far more fragile.

I'm appreciating learning the science behind these potential tipping point positive feedback mechanisms. However, I find that Wadhams is having to compromise on clarity of explanation to maintain readability. I ran some of the explanations about layers of ice past my son, who teaches high school chem and has a strong science background, and he found them suspect. I am willing to believe the author has them right, but if he is not explaining them in a depth that would convince a chemistry teacher, it makes me think his presentation could be improved. However I will have to leave those judgments to Wadhams and his editors.

Calling ice a magic crystal is an example of popularising and simplifying a scientific explanation. I have been in touch with Peter Wadhams and he would be happy to answer any questions people may have. I find this material about the physics of water fascinating.

So many amazing and complex stories about the strange behaviour of Arctic ice. Apparently the ancient iceberg scours found on the shallow Arctic sea floor proved the existence of ancient icebergs floating in early Martian oceans, where the same patterns are found. The annual recrushing of ice forms an array of bizarre shapes. Antarctica has the katabatic winds, which accelerate as they flow down from the high mountains to the ocean, forming areas of open sea known as polynyas. Polynyas also occur at smaller scale in the Arctic, where Inuit from ancient times hunted in polynyas within nine degrees of the North Pole.