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.

Milankovich cycles: changes in the orientation, tilt and eccentricity of the Earth and its orbit combine to change the amount of sunlight hitting the Arctic lands, leading to increasing or decreasing glaciation - the Ice Ages

The great Ice Ages of the past 600,000 years, and CO2 levels. CO2 was sequestered by the ocean and by permafrost during Milankovitch-driven cooling periods, and liberated and therefore added positive feedback heating during the Milankovitch-driven interglacial periods. With the well-known ~1000 year time scale for complete ocean turnover and contact with the atmosphere, the ~800 year lag in CO2 vs. temperature makes perfect sense - perfect sense, that is, during Milkankovich-driven Ice Ages.

Green: temperatures coming out of the depths of the last great Ice Age. Blue: Holocene temperatures, with gradual decline due to Milankovic cycle effects, ending in the sharp spike upward due to man-made greenhouse gas emissions. Does that spike look like a rebound from either the Little Ice Age or the last great Ice Age? No.

Compare to today, and new anthropogenic greenhouse gas forcings (green curve), and look at the rapidity of the change in forcings, and the change in CO2 levels (to make the changes on the graph of comparable horizontal scale required magnifying the time axis by a factor of 500(!) to show the past century). Said another way, changes are happening of order 500 times faster than during the Ice Age cycles.... clearly NOT a mere rebound temperature rise.

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.

1.2 trillion tons of fossil fuel CO2 has been released into the atmosphere in the past 250 years. Fully half of that has happened just since 1975

From the NASA website. This graph is meant to show just how rapid is the current rise in CO2 compared to the geologic time scale. The cycles of Ice Ages show CO2 levels peaking reasonably consistently at 300 parts per million. From today's rapid fossil fuel burning we've now crossed 400 parts per million.

CO2 levels for the past 150 years. Note the levelling off from the mid '30's to 1950, as World War II served as a distraction from fossil fuel burning by the great majority of people. This, and the reflective tropospheric sulfate air pollution which came with post-WWII industrialization, counterbalanced greenhouse warming till ~1970, in agreement with observed temperatures and climate models.

Note that Earth had been slowly cooling, not "rebound heating" after the last great Ice Age ended over 10,000 years ago. And even after the so-called "Little Ice Age" (a term most climatologists regret as it invites over-exagerated images of temperature), we had a resumption of slow cooling throughout the 19th century, until the rapid heating we are seeing in the 20th and 21st century. Does this above look like a rebound from either Ice Age? No.

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.

In Short: Such Ice Age "rebound"'s happen through a series of semi-equilbrium states, and therefore slowly, run by very slow orbital changes. Today we are FORCING CO2 into our atmosphere and doing it extremely rapidly, not coaxing it slowly out of the ocean as in the distant past. The argument is either a deliberate red herring, or at a bare minimum, extremely naive. Today's rate of rise of temperature bears no resemblence to that associated before, during, or after the Little Ice Age.


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