The global warming debate - a personal perspective

Making decisions under uncertainty

In both our personal and collective lives we are forced to make decisions under uncertainty. What's worse, even after the fact we cannot be sure we made the right choice.

The Reserve Bank raises interest rates to avert an increase in inflation. In so doing it inflicts hardship on people with mortgages and business loans. The concomitant rise in the Aussie dollar makes life harder for exporters. However inflation remains under control.

But would inflation have remained low anyway? Was the pain the Reserve Bank inflicted necessary? There is no way of being certain.

Your doctor advises you to take a class of medications known as a "statin" to control your cholesterol levels. He tells you that will reduce the risk of suffering a heart attack. Fearing some of the well-known adverse effects of statins you do not take his advice. Two years later you suffer a heart attack.

Could you have avoided a heart attack if you had taken the medication? Again, there is no way of knowing. Many people who suffer heart attacks are taking statins? The most that can be said is that taking statins has been shown to reduce the probability of heart attack in certain circumstances. They are risk reducers, not preventers.

Decisions about climate are no different to the ones I've just described. We all have to make decisions under uncertainty and even after the fact we may not know whether we've made the right call. That is the real world. That is the human condition. Let's not kid ourselves that it can be any different.

Is there a decisions to be made?

The forgoing begs a question. Is there any decision that needs to be made about climate? Are there any actually risks in continuing to pump CO2 into the atmosphere?

Here, in outline, are the facts.

• A group of wild-eyed scientists did not dream up the theory of anthropogenic global warming (AGW) out of nothing. Well-established laws of basic physics suggest a significant impact on climate if we continue pumping CO2 and other greenhouse gases into the atmosphere. The theory of AGW has a firm basis in the laws of physics.

• Yes there are many uncertainties and anomalies but, taken as a whole, the evidence tends to support the physics. The case for anthropogenic global warms (AGW) has not been made beyond all reasonable doubt; but the preponderance of evidence points in the direction of human induced global warming. That is the best we can ever hope to do. To expect more is unrealistic.

• The average lifetime of a CO2 molecule in the atmosphere is 60 - 100 years. A planet-wide climate system responds slowly to changes in atmospheric content. Thus if we do find that pumping CO2 into the atmosphere produces adverse consequences we shall be stuck with them for a long time even if we stop emitting.

• The definite albeit difficult to quantify risks coupled with the difficulty of reversing the effects of extra CO2 in the short term suggests that the only prudent response is to begin to take steps to reduce CO2 emissions. We may not know everything but we know enough to say that the risks of continuing with business as usual are high.

• However this has to be a global effort. Australia on its own can do nothing. We should follow, not attempt to lead.

Scientists behaving badly

Certain scientists have behaved badly. Some have fudged their data. The shocking behaviour of some scientists does not alter the laws of physics. I hope readers will bear this truism in mind in what follows.

The physics explained - greenhouse effect in a nutshell

Incoming radiation from the sun

Energy from the sun reaches us in the form of electromagnetic (EM) radiation. 43% of the energy is in the form of visible light. About 8% is in the form of ultraviolet (short-wave) radiation. The rest is in the form of near-infrared (long wave) radiation.

97% of the ultraviolet (UV) radiation is blocked by the Earth's ozone layer.

All major atmospheric gases are transparent to visible light. On a clear day most of the visible light makes it through the atmosphere to the surface. Some of the light is reflected from the surface. Most is absorbed and serves to heat up the surface of the Earth. That is, mostly it heats up the surface of the oceans.

The picture for infrared (IR) is more complicated. The atmosphere is opaque to some wavelengths and partially transparent at others. For some IR wavelengths, the so-called atmospheric window, the atmosphere is completely transparent.

I can best summarise the situation as follows. If the atmosphere were completely clear and transparent at all wavelengths an average of 341 watts / square metre of EM energy would reach Earth surface from the sun. The actual amount would vary by time of day, season and latitude but 341 watts / square metre would be the average.

The actual average taking into account scattering by clouds and dust and absorption of same wavelengths by the atmosphere is 184 watts / square metre according to satellite measurements.

If that were all there was Earth surface would be frozen. Average temperatures at the surface would below freezing instead of around 15 degrees Celsius.

Upward radiation from Earth surface

Every body with a temperature above absolute zero radiates EM energy. Earth surface is no exception. The average upward radiation from Earth surface is around 396 watts / square metre. Most of this is absorbed by greenhouse gases which then heat the atmosphere.

Note that that the upward radiation from the surface is well above the direct incoming EM from the sun. How can that be?

Back radiation from the atmosphere

As the atmosphere heats up it too begins radiating EM energy. Some of this is radiated out into space and some back to Earth surface. Counter-intuitively the "back radiation" radiation from the atmosphere to Earth surface is almost twice as intense as the direct radiation from the sun. It is this back radiation that keeps average temperature at Earth surface above freezing.

The earth's energy budget

For surface temperature stability the average incoming energy from all sources - which includes direct energy from the sun plus back radiation from the atmosphere - must equal outgoing energy. If incoming energy rises then, EVERYTHING ELSE BEING EQUAL, surface temperatures will rise until a new equilibrium is established at a higher temperature.

Anthropogenic global warming (AGW)

The theory is simple. Add a greenhouse gas, in this case CO2, to the atmosphere and the amount of back radiation increases. Everything else being equal this should result in a rise in average surface temperatures.

There is a simple approximation formula that can be used to calculate the extra back radiation from adding CO2 to the atmosphere:

Extra radiation (watts / sq. metre) = 5.35 ln(C / C0)

Where ln = natural logarithm

C0 = atmospheric CO2 level in 1750 = 280 parts per million (ppm)

C = current CO2 level = 385 ppm.

Plugging in the numbers gives additional back radiation of 1.7 watts / sq.metre. This translates into an average temperature rise at Earth surface of about 0.3 degrees Celsius. If this was the sum total of AGW there would be nothing to worry about.

What is not equal? Positive and negative feedbacks

Positive feedbacks amplify the effects of adding CO2 to the atmosphere. Negative feedbacks mitigate the effect of additional CO2.

Unfortunately most feedbacks that have been investigated appear to be positive. I'll confine myself to discussing four.

Water vapour

Water vapour is by far the most important greenhouse gas. Physics theory says that adding CO2 should lead to an increase in the water vapour content of the atmosphere. This appears to be happening. Additional water vapour magnifies the effects of additional CO2.

Albedo

Albedo is a measure of the amount of sunlight Earth surface reflects. The more that is reflected the less heating at Earth surface. Higher temperatures, mainly by shortening the "ice and snow" periods in the northern hemisphere, decrease the Earth's albedo leading to further rises in temperature.

Clouds

These are the big unknown. There is no simple physics theory to guide us here as there is in the case of water vapour and albedo. Attempts to prove that clouds provide a negative feedback mechanism have all failed. What evidence there is suggests that temperature induced changes in the nature of cloud cover will result in a small net positive feedback effect.

Stratospheric water vapour

There is some evidence that the concentration of water vapour in the stratosphere declined in the past decade. No one knows why. This by itself could explain the pause in the upward movement of temperature we have observed since 2000, a pause which appears to be coming to an end.

Conclusion

Physics theory implies that adding CO2 to the atmosphere will alter climate. A reasonable inference from the available evidence indicates that this is most likely happening. Therefore there is a decision to be made.

Because CO2 molecules persist in the atmosphere for such a long period prudence suggests that we should start taking steps to reduce CO2 emissions. I shall not put it more strongly than that. Nor am I going to make cheery forecasts about "green jobs". Frankly this is going to be an expensive pain the backside.

It's like spending money on submarines and jet fighters. You don't do it because it's a good thing; you do it because sometimes you have to.