Hi again Bazz,

On closer reading of related information (especially David Rutledge's assessment of economic limits to fossil fuel extraction, linked by michael_in_adelaide under Aleklett's piece: http://rutledge.caltech.edu/ ) I'll revise my criticism.

Aleklett's numbers aren't wrong, just excessively optimistic. If his projections of fossil fuel economics are accurate -- and I hope they are -- we *still* risk hitting a major climatic turning point if, for instance, thawing Arctic tundra releases large quantities of methane.

The politically-agreed "safe" level of 450ppm CO2 equivalent or a 2 degree temperature increase (to which Kyoto aspires) is *too high* to prevent catastrophic feedbacks unless we're exceptionally lucky. Aleklett's projection would still have us reach this point (and Kyoto is far too feeble to prevent it). *This* is why further intervention is urgently required, limiting emissions and accelerating the development of non-fossil energy supplies.

Aleklett is a "Polyanna economist": he argues that fossil fuel extraction will stop altogether when it becomes too costly, after the historical precedent that energy resources are never mined out because cheaper supplies displace them.

I am 100% in agreement with Aleklett's case for future fossil-fuel extraction, providing that renewable energy is developed on a grand scale, sufficiently early to compete. This is probable, but not inevitable.

The historical limits on extraction of fuel resources have been imposed not by scarcity but by abundance -- the pattern will continue only if cheaper alternatives continue to become available as each fossil fuel resource becomes more expensive to extract. Otherwise we will pay more for fossil fuels, reopening all those already-abandoned wells and seams. The scope is huge for improved consumption efficiency, so poor EROEI today doesn't limit extraction in the long term.

If, on the other hand, alternatives out-compete fossil fuels before they are mined out, there is not "too little energy". Aleklett's closing words (echoed somewhere by our own relda) are contradictorily pessimistic.

I'm confident a future energy path is possible which has much lower emissions than Aleklett projects. But even Aleklett's scenario would be unrealistic without strong development of low-emission energy technology.

Xoddam;
Thanks for the interesting reply. One thing that seems to be lost in
these global warming discussions is the natural warming effect.
So many statements and discussions seem to assume that it is all the
fault of humanity.
For just what percentage is human activity responsible ?
1934 was the warmest year by some measurements.

I suspect that perhaps human activity effect is quite small.
If our efforts are overwhelmed by nature are we completely wasting our
time worrying about our CO2 emmissions ?

Only a few factors affecting climate are human in origin: changes in greenhouse gas levels and albedo from land clearing, agriculture and industry, and atmospheric particulates.

Each of these factors would have only a minor impact in isolation. But some "natural" factors -- atmospheric humidity (water vapour is a potent greenhouse gas), polar albedo, cloud cover and "wild" biosphere activity -- are sensitive to temperature, mostly *amplifying* any warming due to other causes.

While human land use may have had a small effect on climate for a long time, and we have been burning fossil fuels for several centuries, it is really only since WWII that human activity been on a scale sufficient to make a big difference, and only since the 1970s (when sulfur dioxide emissions were largely eliminated from coal flue gases) that the net human effect has clearly been a warming one.

The 1934 peak (the hottest year to date in the USA, but not globally) was the result of high solar radiation. It has been amply demonstrated (most vocally by those who deny a significant human effect on the climate) that until the 1980s, temperature changes on earth closely tracked the natural variation in solar radiation. Since the 1980s solar radiation has *decreased* as the Earth has warmed up.

While recent (and imminent "committed") climate change is alarmingly rapid, it is nothing compared to the chaotic responses that might develop as a result of delayed positive feedbacks.

The climate has changed before, and it has changed suddenly before. The biosphere has suffered, and (over geological time) recovered. But the climate has never before changed suddenly with 6 billion human beings dependent on agriculture. *That* is why we need to be worried. It is the human cost, more than anything else, that demands that we avoid abrupt climate change.

I completely reject any argument that we should not try to stabilise the climate if anthropogenic factors were not responsible for the observed change. Civilisation depends on a relatively stable climate and we are justified in the attempt to preserve our existence.