Friday, 11 January 2013

Astronomy and Global Warming

An Astronomical society friend has commented that there seem to be fewer clear skies since he bought his telescope.
When I heard about global warming, I got an impression (shared, I suspect, by many) of rising sea levels and the weather becoming more prone to storminess. The main message, apart from “stop making CO2” was: “live away from low ground”. And there was the bit about the conveyor belt turning off and causing a mini-ice age around the British Isles.

The increasing propensity for flooding has shown (to me) an unsuspected aspect: that of higher humidity and rainfall due to the warmer atmosphere.
When the ice melted after the ice age, the air temperatures were relatively low, so the extra liquid water stayed in the sea, raising sea levels. At the relatively warmer air temperatures of today, the glacial  melt water is evaporating, raising humidity levels, increasing cloudiness and producing more rain; hence the flooding. Also, I suspect, this process is maintaining the salinity of the oceans and, with it, the operation of the conveyor belt.

So the message is not so much “stay away from low level” as “stay away from rivers and valleys”. And councils need to make sure there is good road drainage, especially on hilly roads. Last winter, when returning to Sussex on a wet evening, I was appalled at the large puddles in the right hand lane of the M4: a recipe for disaster.
I wonder how many people are aware of the latent heat of fusion of ice. Ice doesn’t automatically turn into liquid water at 0 degrees Celsius. The ice requires a further dose of heat to change state from solid to liquid. That’s called the latent heat of fusion. It is possible for ice to be raised to 0 degrees just before winter, then, effectively store heat until the following spring, when it finally melts. This might help to explain why unexpectedly large pieces break off from polar glaciers. The recent BBC TV programmes Operation Iceberg showed that there is still a lot being learnt about glaciers and icebergs

Longer term, the situation is more complicated, goes further.
With the increased cloud coverage we will presumably see a greater greenhouse effect, which implies accelerated global warming. Our “evil twin” neighbour, Venus, is completely engulfed in cloud, and its surface temperature is 560 degrees Celsius; not a comfortable place to be.

Venus is about two thirds of our distance from the Sun, which means that, under the inverse square law, it gets more than double the Sun’s radiant energy, compared with the Earth. I suspect that Venus has much more internal heat than Earth: the extra radiation it gets from the Sun may have delayed the radiation of its own heat. Not only that, but its clouds are somewhat darker, (being sulphur dioxide-based) than our white water vapour clouds. That means that a fully cloud-shrouded Earth could display a higher albedo (the property of reflecting radiation) than Venus does. Currently Earth’s is much lower than Venus’s. The higher albedo of a cloudier Earth would deflect more of the Sun’s radiation, than currently, and could potentially allow the planet to cool itself down again. Once the planet has cooled, the polar ice could reform at the expense of the humidity and cloud cover, taking us back to “normal”.  So perhaps the Earth is self-regulating in that respect.
Clever planet is the Earth. There’s more to the Goldilocks zone than mere distance from the Sun!

Don’t hold your breath, though, it’s a very slow process.
Meanwhile, from an observational Astronomical point of view, and in the short term, I see a continuing reduction in the number of clear-sky nights for observing, and an increase in the need for dew management. Reputedly, a hazy sky yields improved seeing, so, at least, we should get clearer views when we can see the stars.

Conclusion: treat every clear night as if it’s the last!

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