“The rates of such changes, not the changes themselves, should be our biggest concern. For climate, sea level, and ecosystems can reach tipping points. Pushed too far, positive feedback loops can kick in. What normally takes a thousand years could transpire in a decade or two.” Robert Hazen 
In my recent articles, I have been discussing the ongoing rate of the anthropogenic or human-caused changes in Earth’s climate. We are warming the planet at a rate of 2.76 degrees Celsius per century (NASA GISS and NOAA results but also confirmed by the denier study BEST ) which is unprecedented in the last 300 million years of Earth history. The only plausible explanation is the greenhouse gases we are emitting into the atmosphere also at a rate unrivaled in the last 300 million years. In fact scientists once believed that the extinction event 55 million years ago called the Paleocene-Eocene Thermal Maximum (PETM) would be a good analogue for what we are doing today as it was believed to be the most rapid climate disruption in Earth’s history . In a paper published in 2011, however, a team of geologists from Penn State led by Lee Kump and his graduate student and lead author Ying Cui discovered that even this devastating event was triggered by greenhouse gas emissions less than a tenth the rate of what is happening today . Their paper compares model results with actual measurements for two possible causes for the sudden increase in atmospheric carbon dioxide. Their results indicate that the peak rate of carbon addition to the atmosphere during the onset period of PETM was between 0.3 Pg C/yr and 1.7 Pg C/yr. By comparison the current rate of human emissions is 10 Pg C/yr . The changes that took more than a thousand years to occur during the PETM have occurred within the last century. The PETM cannot therefore help us make predictions about the future other than that what we are doing may be ten times worse.
As we’ve seen in the last couple of articles, the natural change in the amount of carbon dioxide in the atmosphere during the Earth’s recovery from the last glacial maximum was never faster than about 0.023 ppmV/year while the current annual change due to humans is about 2.4 ppmV/year or hundred times faster . There is nothing going on in nature today that can account for the rapid accumulation of greenhouse gases in the atmosphere or the rapid increase in global temperature other than human emissions. But more significantly, scientists have discovered that no natural mechanism has ever progressed that quickly, other than perhaps a 6 mile in diameter meteorite striking Chicxulub at about 22,000 miles per hour 65 million years ago terminating the Reign of T Rex.
Before we revisit the transition between the LGM and the Holocene as I was describing in my last article we should review one other bit of earth science. I should have introduced Figure 1 when discussing the polar amplification  as I did use it in a talk I gave a few weeks later. It shows that the spot in the Arctic above Alaska has already warmed about 6 degrees Celsius whereas the rest of the Earth has only warmed a degree or so and in fact some areas of the Earth’s surface haven’t warmed at all. Global temperature is of course the temperature integrated and averaged over the entire globe including both the cool parts and the warm parts. If we only had one thermometer and we placed it in the Arctic we would think the entire earth warmed by 6 degrees Celsius already which it hasn’t done of course.
Carbon dioxide is a well-mixed gas in the earth’s atmosphere. The ratio of carbon dioxide to nitrogen and oxygen is the same at the poles and at the equator and on the surface and at very high altitudes. It is currently 394 parts per million by volume. If you measure it in one place, then you know the concentration everyplace else. This is not true for water vapor, by the way, as there is very little water vapor in the atmosphere over deserts which are too dry and over the poles which are too cold but lots of water vapor over tropical rain forests. Also as water vapor rises in the atmosphere, which it does at the equator, it condenses out in the cold upper troposphere forming clouds, subsequently precipitating out as rain. Water vapor is not a well mixed gas. It depends intimately on the local temperature whereas carbon dioxide does not. This is why carbon dioxide is the principle greenhouse gas or the Earth’s thermostat  while water vapor is simply an amplifying feedback.
What this means is that the ice core temperature data from Antarctica can only reflect the temperature at Antarctica at any specific time. But the carbon dioxide concentration measured in the ice cores can be used to reliably reflect the global concentration at that same time. Does this mean that the temperature measured by appropriate proxies in the ice core does not tell us anything about the global temperature? No not at all. When the global temperature rises, it rises nearly everywhere but just not at the same rate or by the same amount sometimes lagging by many centuries.
Returning to where we left off in my last article, what happened to cause the Earth to progress from a glacial maximum 18,000 years ago to the interglacial climate we enjoy today, by about 10,000 years ago? As I mentioned several recent papers published this year shed light. Figure 2 is from a paper written by a team of climatologists led by Jeremy Shakun of Harvard University published in the journal Nature . The red curve is Antarctic proxy temperature from the core at Dome C. The yellow dots are the rise in atmospheric carbon dioxide. The blue curve is a composite estimate of the global temperature rise from 80 ocean sediment cores taken from various parts of the globe. As expected the Antarctic warmed first as a result of earth’s orbital obliquity. Sea ice around Antarctica melted releasing a stored charge of carbon dioxide from decaying biotic matter stored up over tens of millennia which could not escape the ocean into the atmosphere because of the sea ice that had persisted up to and during the LGM . Andrea Burke of MIT and Laura Robinson of University of Bristol have recently shown that when this sea ice melted back, the stored carbon dioxide was gradually released into the atmosphere. This pulse of carbon rapidly spread over the earth and contributed to warming the rest of the planet. Atmospheric carbon levels followed the Antarctic temperature increase but led the global temperature increase.
Note that the Earth warmed an estimated 3.5 degrees Celsius over about 8000 years. By comparison we are causing the Earth to warm at a rate of 2.76 degrees per century but that rate is expected to increase as we continue to emit carbon dioxide and other green house gases into the atmosphere. While the magnitude of the anthropogenic warming this coming century will be dramatic enough the rate will be something never before experienced on Earth within the last few hundred million years, 100 times more than the recovery from the last ice age and 10 times more than the PETM. When a global warming denier tells us not to worry it will be good for us, would be a good time to apply some skepticism and ask “you know this how?”
How fast can sea level rise? Pierre Deschamps from Marseille University, France and colleagues have determined that during what is called the “melt water pulse 1A” between 14,650 and 14,310 years ago, at the boundary between the Oldest Dryas and the Bølling–Allerød periods shown in Figure 2, sea levels rose greater than 4 meters per century with a peak rate that may have been as much as 10 meters per century . Current estimates call for just 2 meters this century so it is not out of the question that we may be in for a rude surprise.
Figure 1: The global proxy temperature stack (blue) as deviations from the early Holocene (11.5–6.5 kyr ago) mean, an Antarctic ice-core composite temperature record (red), and atmospheric CO2 concentration (yellow dots). The Holocene, Younger Dryas (YD), Bølling–Allerød (B–A), Oldest Dryas (OD) and Last Glacial Maximum (LGM) intervals are indicated. Error bars, 1-sigma; p.p.m.v. = parts per million by volume. Shakun et al. Figure 2a. 
 Robert Hazen, The Story of Earth, Viking, 2012.
 Tony Noerpel, Blue Ridge Leader, http://brleader.com/?p=8276
 Lee Kump, The Last Great Global Warming, Scientific American, July 2011.
 Ying Cui, Lee R. Kump, Andy J. Ridgwell, Adam J. Charles, Christopher K. Junium, Aaron F. Diefendorf, Katherine H. Freeman, Nathan M. Urban and Ian C. Harding, Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum, Nature Geoscience, 5 June 2011 | DOI: 10.1038/NGEO1179. See also, Panchuk, K., Ridgwell, A. & Kump, L. R. Sedimentary response to PaleoceneEocene Thermal Maximum carbon release: A model-data comparison. Geology 36, 315318 (2008).
 PgC/year means Petagram of Carbon per year. 1000 grams = 1 kilogram, 1000 kilograms = 1 metric ton so a Petagram is a billion metric tons or a Gigaton. Also this is a measure of the carbon content of the carbon dioxide molecule which contains two oxygen atoms and one carbon atom.
 Tony Noerpel, The Last Deglacial Transition, Blue Ridge Leader, See also Michael Kaplan, et al., Glacier retreat in New Zealand during the Younger Dryas stadial, Vol 467| 9 September 2010| doi:10.1038/nature09313. ppmV is parts per million by Volume.
 Tony Noerpel, Arctic Amplification, http://brleader.com/?p=8136
 Robert Berner, The Phanerozoic Carbon Cycle, 2004.
 J.D. Shakun, P.U. Clark, F. He, S.A. Marcott, A.C. Mix, Z. Liu, B. Otto-Bliesner, A. Schmittner, and E. Bard, “Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation”, Nature, vol. 484, 2012, pp. 49-54. DOI.
 Andrea Burke and Laura Robinson, The Southern Ocean’s Role in Carbon Exchange During the Last Deglaciation, Science, vol 335, 3 February 2012.
 Deschamps, P. et al., Ice-sheet collapse and sea-level rise at the Bølling warming 14,600 years ago Nature 483, 559–564 (2012).