Claim: It can take centuries or thousands of years for temperatures to fully rebound out of an ice age. Global warming may be nothing more than a rebound from the last ice age, or the Little Ice Age.

Why this claim is wrong: The time scale for warming depends in large part, of course, on the magnitude and cause of the forcing. If climates are "relaxing" from the last Ice Age, such that each incremental change in Milankovich insolation forcing means a bit warmer planet, leading to a bit less ice, etc, and if the ice is miles thick and covering vast areas of the northern hemisphere, then yes - it can take many hundreds or even a few thousand years for quasi equilibrium to migrate temperatures slowly upward. That has nothing whatsoever to do with the current situation. The forcing is not the slow migration through equilibriums described above, but a rapid external forcing through strong injection of greenhouse atmospheric gases directly into the fastest responding, lowest thermal mass, lowest thermal capacitance element in the system - the atmosphere. The thermal and CO2 capacitance of the ocean is vastly bigger than that of the atmosphere, due to its mass. But it is into the Earth's thin, low-mass atmosphere that we are directly forcing CO2, with no regard to its equilibrium level. Next, the evidence is strong from both theory and observed correlation that the Ice Ages are primarily controlled by the Milankovitch cycles.

These Milankovich changes in the Earth's orbit and spin axis are straightforward to calculate by the laws of physics for both the past and future. Currently we are finishing a period of decreasing insolation at the Arctic Circle, and global temperatures for the past many centuries have shown a small, slow, net, cooling - until the rapid CO2-induced warming beginning about a hundred years ago. If we've been rebounding from the last Ice Age, why has there been centuries of slow cooling - until now? Makes no sense. Global temperatures rise at a rate depending on the magnitude and direction of the forcing mechanism. A study using a suite of climate models all show that the temperature rise in the early 20th century is mosly due to rising solar activity, while anthropogenic CO2 is responsible for warming since then (Meehl 2004). We're currently adding a strong forcing upward by CO2 and methane emissions, and temperatures are responding rapidly, as calculations show they should - not the glacially slow progression out of an ice age that ended ~19,000 years ago. The atmosphere's response to thermal forcing in general can be much faster than this ad hoc claimed millenial time scale, most obviously by the response to volcanic eruptions, as graphs on this page show - changes to the atmosphere will change global temperatures in months and years, not millenia. We humans are changing the atmosphere directly, not indirectly as during the Ice Ages. Coming out of true Ice Ages can take many hundreds to thousands of years, due to the feedback of melting ice and resulting lowered albedo and release of ocean and permafrost trapped CO2. Anyone with any familiarity with oceanography ought to know that the turnover time scale for the ocean to release millenia-long sequestered CO2 is about 1,000 years. When air temperatures rise due to Milankovich forcing, it can take therefore ~800 years for significant CO2 to be released to the atmosphere - so the paleo record makes perfect sense. What does that have to do with today? Nothing! Forcings directly applied to the atmosphere will be short, of order years, not centuries or millenia. And that's exactly what we're seeing with current GHG-induced global warming.

Showing science-ignorant people changing temperatures or CO2 levels from the past, spanning hundreds of thousands or millions of years, and claiming (or more outrageous still, simply planting the implicit claim - thus allowing plausible deniability) that it's like today, when today we are seeing comparable or larger changes in only 60-100 years, and doing so without emphasizing the time scale issue, is deeply, unconscionably misleading. See the CO2 graphs below, and the temperature "Hockey Stick" for a better appreciation.

Regarding the Little Ice Age - there is little doubt temperatures were lower for a few hundred years centered around the year 1600. ~30 years ago it had been widely assumed, due to the popularizing of this idea by the late John Eddy, that the Maunder Minimum in solar activity may have caused the Little Ice Age. However that was only a tentative assumpition as we had no precision satellite data on solar luminosity vs solar activity, until recently. We now have good satellite data on insolation over the last several solar cycles, and the insolation only varies by less than 0.1% maximum to minimum in the solar cycle, so that a prolonged period of low solar activity such as the Maunder Minimum would not correspond to low enough solar heating to account for the Little Ice Age. This is confirmed by a recent paper by Feulner and Rahmstorf (2010) , (GRL paper here). Instead, the real causes of the Little Ice Age which best fit the data are (1) there were stronger and more frequent volcanic eruptions, which put sulfate aerosols into the atmosphere and cooled the Earth by reflecting sunlight (Robock 1979), and (2) that the Black Plague and Columbian disease epidemics spread to the New World caused a significant (20%) drop in human population during the late middle ages, leading to cleared land undergoing reforestation which took up enough carbon to account for the 10 ppm drop in observed CO2 levels at the time of the Little Ice Age. This brief reference is interesting, though not a peer-reviewed paper.

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