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On page 85 of their excellent book, Taken By Storm: the Troubled Science, Policy, and Politics of Global Warming, Christopher Essex and Ross McKitrick call our attention to an astonishing line in the Intergovernmental Panel on Climate Change's 2001 Assessment Report:
The climate system is a coupled non-linear chaotic system, and therefore the long-term prediction of future climate states is not possible. [bold added]
This statement appeared in the Executive Summary of Chapter 14 of the report produced by Working Group 1. (See the first bullet point on the right, page 3 of this 18-page PDF - or the third bullet point from the bottom here. The quote appearing in the book differs slightly from the IPCC version now on-line and reproduced above.)
• Reverse the decline of observational networks in many parts of the world. Unless networks are significantly improved, it may be difficult or impossible to detect climate change over large parts of the globe.
Improve methods to quantify uncertainties of climate projections and scenarios, including development and exploration of long-term ensemble simulations using complex models. The climate system is a coupled non-linear chaotic system, and therefore the long-term prediction of future climate states is not possible.** Rather the focus must be upon the prediction of the probability distribution of the system's future possible states by the generation of ensembles of model solutions. Addressing adequately the statistical nature of climate is computationally intensive and requires the application of new methods of model diagnosis, but such statistical information is essential.
The prime suspect for these dramatic swings in Earth's climate is carbon dioxide: hot periods over the past half a billion years generally coincided with high levels of CO2 in the atmosphere and vice versa. A few studies, however, suggest that there may been periods when it was cold when CO2 levels were high, or hot when CO2 levels were low. So what was going on at these times? Are we missing part of the climate puzzle?
In reality, all kinds of things change when the planet heats up. When the atmosphere warms, for instance, it holds more water vapour, a potent greenhouse gas, which leads to further warming. Warming also reduces the area covered by snow and sea ice, meaning less energy is reflected back into space, again leading to further warming. Plugging such feedbacks into computer models gives a climate sensitivity of between 2 °C and 4.5 °C, with a best estimate of 3 °C, the last report of the Intergovernmental Panel on Climate Change concluded.
Unfortunately, in some ways this figure is misleading, and not just because of the uncertainties in the models. The deeper problem is that current climate models include only feedbacks that kick in rapidly in response to warming. Feedbacks that kick in only after decades or centuries, such as changes in the extent of ice sheets on land (as opposed to snow and sea ice), are left out."
I thought this article was relevant to your piece so I opted to piggyback it on your thread, I hope you don't mind.
The existence of such inconsistencies no more disproves the idea that CO2 causes warming than your house warming up on a sunny day proves it does not get warm when you turn the heating on - rather, it suggests that some other factor caused the warming. In theory the mid-Miocene anomaly could be due to the sun warming, for instance, except that as far as we know the sun's output, while gradually increasing, otherwise varies very little.
Climate sensitivity refers to how much global temperature rises when the level of CO2 in the atmosphere doubles.
Lee Kump of Pennsylvania State University in University Park says earlier studies missed the dip because they calculated levels at 10-million-year intervals and the ice age lasted only half a million years.
The dip, he says, was triggered by a burst of volcanic activity that deposited new silicate rocks. These draw CO2 out of the air as they erode. As the ice spread, however, it gradually covered the silicate rocks, slowing the erosion and so allowing CO2 to build up in the atmosphere once more. This eventually would have warmed the atmosphere enough to end the ice age, says Kump.
Silicates comprise the majority of the earth's crust, as well as most planets and moons
(From your link again)
To get a more accurate picture, researchers have to look at periods similar to the present. One such time is the early Pliocene about 4.5 million years ago, when CO2 levels were around 400 ppm - only slightly higher than they are now - yet the Earth was more than 3 °C warmer, with smaller permanent ice sheets and sea level up to 25 metres higher.
One recent study of the Pliocene concluded that the Earth system sensitivity at this time was 4.5 °C per CO2 doubling (Nature Geoscience, vol 3, p 60). Another study, by Pagani and colleagues, using a different methodology, found that it could have been as high as 7 °C per CO2 doubling (Nature Geoscience, vol 3, p 27).
The Ordovician period began approximately 490 million years ago, with the end of the Cambrian, and ended around 443 million years ago, with the beginning of the Silurian. At this time, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern super-continent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater.
"We really don't know how high CO2 has been in the geologic past. Thus we don't know how sensitive the surface temperature of the Earth is to CO2," said Don DePaolo, head of the Earth Sciences Division at the Lawrence Berkley National Laboratory in California.