Chris Mardon has been a scientist with the CSIRO, an author and an informed commentator on Environmental matters. He is a scientific advisor to the Greens.
It has been revealed in the journal Nature that oil's tipping point has definitely been passed. I don't have a subscription with that journal, but I have found some information about the article from the University of Washington, where one of the authors (Professor James Murray) works:
Attached is an enlarged version of the graphs in that article so that you can see what they are talking about. The Association for the Study of Peak Oil (ASPO) has been saying for years that the global production of conventional crude oil peaked in 2005. Now we have the proof. As you can see from the graphs, oil prices have become highly volatile since 2004, reaching a massive peak in mid-2008. This volatility was no doubt enhanced by the hedge funds, but the sudden change in the price elasticity from 2004 is obvious. This reflects the fact that global production had levelled out. Total oil production has increased somewhat as the production of new marginal sources of hydrocarbons has boosted supply somewhat, but the potential for growth in the total supply is limited because all of the new sources are more expensive than conventional crude oil and the oil price feeds back into the cost of new production infrastructure. The attached figure from "The End of Growth" by Richard Heinberg shows three graphs, the first of which is from the IEA. It shows that production from currently producing oil fields is now on the point of permanent decline. New fields yet to be developed or yet to be discovered may allow the continuation of production close to the peak level for some time to come, but there is no scope for growth, and some people consider even these estimates to be very optimistic. There is some scope for temporary growth from natural gas condensate (a light liquid hydrocarbon unsuitable for heavier petroleum products such as diesel fuel, jet fuel, shipping fuels or lubricants) and unconventional oil such as deepwater oil, heavy oil and tar sands), but it is highly capital intensive (which means that it contains substantial embedded energy), and it takes more direct energy to produce. e.g. Venezuelan heavy oil cannot be pumped out of the ground, so steam is blown down the well and an oil/water emulsion is pumped out. This emulsion is quite stable, but it is not suitable as a fuel, so the Chinese are taking tanker loads of this emulsion back to China for further processing. Similarly, the oil from tar sands is not directly suitable for refinery feedstock, so it too needs some further processing first. Even the extraction of the oil from tar sands requires large amounts of natural gas and water, so its production is also energy-intensive and expensive, not to mention very damaging to the environment. As you can see from the lower graphs in the attached figure, world oil discoveries have been in decline since the 1960s, and the marginal cost of oil from new oil fields is rising. There was a sharp drop in oil prices in 2009 as a result of the GFC, but prices have since risen again. The cost of unconventional oil is particularly expensive because the capital costs and energy used to extract them is much higher. The high oil costs are feeding back into the construction costs of new infrastructure and they are making other sources of energy such as coal and LNG more expensive as well. The upshot of all this is that the Energy Return On Energy Invested (EROEI) for new sources of oil is dropping rapidly and approaching the point of diminishing returns (EROEI = 1 means that no net energy is produced, so energy is just converted from one form to another). Shale oil, oil from coal and tar sands are all close to that figure. That means that in terms of the net benefit to the economy, these marginal sources of energy are not very useful, and the Energy Profit Ratio (EPR) that Brian Fleay likes to use (cf. EROEI) is already worse than some forms of renewable energy. We only use them because our industrial and transport systems are built to take conventional oil.