Certainty about climate change


Okay, so now you know what we’re talking about (climate not weather) and how the greenhouse effect keeps the surface of the planet much warmer that it otherwise would be. So we’re ready to talk about the core physical science of climate change.

The greenhouse gases in the atmosphere trap the energy the planet gets from the sun. That’s to say they make it more difficult for the surface of the planet to lose the energy it is continually receiving as radiation from the sun. To achieve a balance it needs to be about 30oC warmer than it would otherwise be and that’s what we observe.

It’s pretty obvious, therefore, that if the amount of greenhouse gases in the atmosphere increases or decreases then the amount of energy trapped will increase or decrease. In an average sense, temperature is a measure of the amount of energy something contains – that’s physics. The really basic, common sense expectation is therefore that with increasing atmospheric greenhouse gases, the surface of the planet should warm.

It’s clear that too little greenhouse gas in the atmosphere would leave the Earth cold and inhospitable. But why should we worry about having a bit more of our comfortable blanket? Because in a system that is a balancing act, it is possible to have too much of a good thing.

Back to blankets – the more blankets you put on your bed, the warmer you’re going to get. Lovely, until you get too hot and sweaty. And the more greenhouse gases we have in our atmosphere, the more of the sun’s heat we will trap. Being too hot is as much of a problem as being too cold, for a planet as much as a bed – worse, actually, as you can always kick the covers off your bed, or stick out a foot to cool down.

The science is clear – a basic expectation of increasing greenhouse gases in the atmosphere is a warmer planet. This is like throwing a ball in the air – you expect it to come down. And this is based on very very very well understood science.

Does this count as certainty? Well if you throw a ball in the air it might not come down – maybe a bird will catch it and fly away. The basic expectations of climate change are uncertain in just the same way; not 100% certain but you wouldn’t want to bet anything of any value (like perhaps the stability of global society) on it being wrong.

Nevertheless, confidence in the general response doesn’t mean that every year will be warmer than the last one because energy sloshes around the climate system all the time and leads to lots of natural variability. So observations aren’t my basis for being concerned about climate change but I’d like to see that they generally reflect the basic understanding.

But this only provides a connection between greenhouse gas concentrations in the atmosphere and some sort of average surface temperature. Are greenhouse gases in the atmosphere rising?

(Follow the “bit more detail” link for a bit more questioning of the basic argument.)

[Video 2V + maybe a video with throwing balls in the air.]


Going up?


A bit more