What about the volcanoes?

The Ian Plimer saga continues, with yet more articles in The Australian featuring his half-truths and attempts at politicizing climate science in the cause of selling his book.

Let’s just look at one more small sampling:

Some 85 per cent of volcanoes are unseen and unmeasured yet these heat the oceans and add monstrous amounts of CO2 to the oceans. Why have these been ignored? Why have there been five significant ice ages when CO2 was higher than now? Why were warmings in Minoan, Roman and medieval times natural, yet a smaller warming at the end of the 20th century was due to human activities? If climate changed at the end of the Little Ice Age (c.1850), is it unusual for warming to follow?

Computer models using the past 150 years of measurements have been used to predict climate for the next few centuries. Why have these models not been run backwards to validate known climate changes?

I would bet the farm that by running these models backwards, El Nino events and volcanoes such as Krakatoa (1883, 535), Rabaul (536) and Tambora (1815) could not be validated.

Let’s take his assertions one at a time.

• 85 per cent of volcanoes are unseen and unmeasured – so what? If this effect is not changing it is not part of any increase in atmospheric CO₂. There are massive fluxes in CO₂ between the atmosphere and the rest of the environment all the time. These fluxes at a period of stable climate are in balance. By adding an additional flux, we are pushing the climate to a new equilibrium state.
  
• Why have there been five significant ice ages when CO₂ was higher than now? – possibly because the other factors driving the climate at the time were different? How about checking paleoclimate research to investigate this question? The effect of a given level of CO₂ or other greenhouse gases on the climate has to be measured against other influences. No one claims CO₂ is the only influence on climate – except psueudo-sceptics like Plimer.
  
• Why were warmings in Minoan, Roman and medieval times natural, yet a smaller warming at the end of the 20th century was due to human activities? – the claim that these earlier warmings were greater than current warming is controversial but leave that aside because it’s irrelevant. What is relevant is whether something different is causing the warming now and whether that different thing is an ongoing effect that could result in a higher, dangerous level of warming. The other warmings were clearly not caused by anthropogenic greenhouse gas emissions because industry did not exist at those times on a scale that could have caused them. Any natural causes such as increased solar activity does not apply today. Continued CO₂ emissions will not stop of their own accord. This argument only makes sense (assuming he is right about the scale of previous warmings and that is doubtful) if Plimer is able to argue that CO₂-based warming will somehow be limited.
  
• If climate changed at the end of the Little Ice Age (c.1850), is it unusual for warming to follow? – so what? No one is arguing that there is no natural climate variation.
  
• Why have these models not been run backwards to validate known climate changes? They have. To assert otherwise is ignorant or dishonest.
  
• I would bet the farm that by running these models backwards, El Nino events and volcanoes such as (1883, 535), Rabaul (536) and Tambora (1815) could not be validated – what is he asking here? Does he want climate models to predict when volcanoes will happen and how large they are? It has been millions of years since a volcano big enough to cause more than a minor climate shift has happened. Krakatoa had an affect that is still measurable today but not so big as to cause problems with the general trend of climate models.
  

Overall, the points he raises are insubstantial and have no real bearing on the problem of anthropogenic climate change. If he has evidence to overturn this judgment, he should publish in the academic literature. Otherwise his claims are vacuous, just so much hot air.