Crossomby,

Thanks you for this excellent comment. There has been a dearth of good comments on this thread for a while.

You said:
"While this is a very simplistic summary, the basic point is that the earth was a very different place in the earlier part of Fig 4 and the difference in C02 between the earlier and later periods must have a complex multi-factorial explanation. Can either section of the graph, or the graph as a whole be used to predict future trends? I doubt it."

I suggest there is an even more important point that jumps out from Figure 4. CO2 concentration has been much higher in the past, and life survived. In fact, it thrived in warmer times. Therefore, neither higher CO2 concentration nor higher temperatures are catastrophic. They weren't in the past so there is no reason to suggest they would be in the future.

(cont.)

Understanding the geological, atmospheric and oceanic systems of the world today, what they were like in the past and how they changed over time, and predicting what might happen in the future is probably the most complex scientific challenge we have. It's not surprising the current models don't work very well, but you have to start somewhere.

However I think that predictive models based on the past (especially when there are many different 'pasts' to consider, as described above) have another inherent problem. It is that the current situation is unprecedented.

Much of the coal, oil and gas we are burning comes from biomass buried over the last 450my or so, since the evolution of the land plants (even if oil, gas etc is endogenous from magma etc., this just means that it's been building up even longer). We will have burnt all of this within just a few hundred years, a mere blink in 500my. Nothing like this has happened before.

The nearest we can get to it is the interpretation of massive continent wide fires following past asteroid strikes. Such an event would happen in a matter of days, an minuter time scale by comparison, but only the standing biomass would burn.

While our burning is slightly more controlled - we're sticking tens of thousands of pinpricks or bucket holes into the earth, sucking up the old biomass and burning it a bit more slowly - within a couple of hundred years, hundreds of millions of years worth of biomass will have been burnt. No past data is adequate to model this.

Given this, the apparent plateau in global warming over the last decade or so is probably trivial in the long-term. I don't think we can really have any idea what is going to happen.

To Peter Lang. Yes, life will survive and evolve. The world will be different (but probably not like it was in the Devonian etc.) Catastrophic? Well, that depends on your perspective - whether you are among the survivors or not!

Well, this second part of your comment is more about emotion and belief than rational argument. I find it unpersuasive.

- Climate sensitivity is coming down [1];
- we have next to no understanding of the damage function;
- warming is probably net beneficial up to ~2C;
- we will likely not burn as much fossil fuel as available because we will probably move to largely replace fossil fuels with nuclear over the next 50 years or so;
- the projected fossil fuel use and the resulting projected CO2 concentrations

[1] Recent studies on 2xCO2 ECS:
Lewis (2013): 1.0-3.0
Berntsen (2013): 0.9-3.2
Lindzen (2011): 0.6-1.0
Schmittner (2011):1.4-2.8
van Hateren (2012): 1.5-2.5
Schlesinger (2012): 1.45-2.01
Masters (2013): 1.5-2.9
average: 1.2-2.5

The average range of these recent studies is 1.2°C to 2.5°C, with a mean value of 1.8°C, or well below the earlier model-based predictions you cite.

Peter Lang,

Humans and, particularly, human "civilisation" have thrived in a particular environment.

"Life" may be a Trilobite, a microbe or a giant tree fern - and "life" may thrive with higher CO2 and higher temperatures - that doesn't mean that humanity can carry on regardless, certainly not in the way we've become accustomed.

Sorry, our comments are crossing. I didn't realise you were going to post a second part to your first comment.

>"Yes, life will survive and evolve. The world will be different (but probably not like it was in the Devonian etc.) Catastrophic? Well, that depends on your perspective - whether you are among the survivors or not!"

I don't find that sort of talk helpful or persuasive.

We need to define what time periods we are discussing. Talking about Devonian is not helpful, IMO. Even returning to Pliocene climates would take millenia. Things will be unrecognisably different in 2050 from from what they are now, so projecting beyond that is nonsensical (e.g. by 2050 they will have technologies that address the issues of CO2 concentrations and who knows what else). But even if we allow ourselves to project to 2100, CAGW is not a realistic scenario - even IPCC AR5 WG1 admits it.