Open letter to Pat Michaels request for information

To talk about global cooling at the end of the hottest decade the planet has experienced in many thousands of years is ridiculous.”  Ken Caldeira, Climate Scientist [1]

You’ve all seen articles saying that global warming stopped in 1998.  With all due respect, that’s being a little bit unfair to the data.”  Pat Michaels, Climate Scientist [2]

Dear Dr. Pat Michaels

I write and edit a column called “Sustainable Planet” [3] for a small local internet news paper in Northern Virginia called the Blue Ridge Leader.  With this open letter, I’m asking if you wouldn’t mind describing your view on anthropogenic global warming (AGW) for my readers.  We are specifically interested in the science and not policy or economics.

The science in support of Svante Arrhenius’ theory of AGW is assessable, unambiguous and coherent.  We can begin with the four IPCC reports but can add to that record a host of text books related to earth sciences and climate physics [see fore example 4 and 5].  There are several really good review papers and one by Stefan Rahmstorf is especially helpful [6].  The video of Richard Alley’s invited lecture at last year’s American Geophysical Union conference summarizes the paleoclimate record and the impact of atmospheric carbon dioxide on Earth’s climate [7].  The peer-reviewed literature is compelling and overwhelmingly in support of the theory as several studies have demonstrated, the latest being Anderegg et al. [8].  Furthermore Arrhenius’ theory is consistent will all science from microbiology [9] to astrobiology [10].

By contrast it is hard to find any science supporting the denialist [11] view.  Not only is this science sparse as evident by reference [8], but it is obscured by the noisy and obfuscating nature of denialist arguments, most of which ignore data, and contradict each other as well as the laws of physics, or simply are outrageous attacks on individuals such as James Hansen, Michael Mann or Al Gore.  The vast sea of arguments on policy or economics is an attempt to put the cart before the horse while the horse has already galloped off in the other direction.  The denialist canard that global warming stopped in 1998 is typical of arguments which ignore data and contradict physical laws. 

Each year Heartland Institute hosts a global warming denier conference.  On March 2, 2008, you were their keynote speaker.  The focus of your talk was the disingenuousness of this particular global warming denialist argument.  You said, addressing the room full of deniers: “You’ve all seen articles saying that global warming stopped in 1998.  With all due respect, that’s being a little bit unfair to the data.”  You then went on to describe why.  Peter Sinclair captured your candid admission in this informative youtube video [2].  While you are more charitable, you are in complete agreement with Ken Caldiera.  That puts you in good company.

You opine “Make an argument that you can get killed on and you kill us all.”  Your meaning, I presume, is that if many denialists make arguments that are easily debunked all global warming denialists, including yourself, will lose their credibility.  You conclude: “Global warming is real and the warming in the second half of the twentieth century, people had something to do with it.”  In the Cato Institute handbook for policy makers [12] you repeat this sentiment: “Global warming is indeed real, and human activity has been a contributor since 1975.

Physics teaches us that a doubling of atmospheric carbon dioxide will result in a radiation imbalance of 4 Watts per meter squared (W/m2) which will directly cause the Earth to warm about 1 degree C.  Your article in the Cato Handbook aligns to this view.  The difference between your view and the consensus view is related to the strength of feedbacks in the Earth’s climate system.  As you point out water vapor is a greenhouse gas and as the Earth temperature climbs as a result in increased atmospheric carbon dioxide, more water evaporates off the oceans.  This additional water vapor reinforces the warming.  You don’t mention but I’m sure you agree that as the temperature climbs, snow and ice at the poles melts.  The exposed darker dirt and water absorb more of the short wave solar energy than the white ice and snow once did, further reinforcing the warming.  In addition, warm ocean water holds less carbon dioxide than cold water, thus as the oceans warm the equilibrium point between the atmosphere and ocean changes.  These are positive feedbacks.   Most identified carbon cycle feedbacks are positive.  The consensus view, the view defended in the IPCC reports, is that including these feedbacks the equilibrium climate sensitivity, the amount the Earth’s surface will warm as a result of a doubling of atmospheric carbon dioxide is between 1.5 and 4.5 degrees C. 

James Hansen’s view as described in [13] is that equilibrium climate sensitivity may be as high as 6 degrees C.  This high value does not contradict the consensus view which, as you know, does not rule out the possibility of higher values. 

In contrast, your view, if I’m interpreting your policy paper correctly, is that climate sensitivity is very low, 1 degree C or less.  This view is outside the bounds of the consensus view.  It means that all of these positive feedbacks must be counterbalanced by some unspecified negative feedbacks.  Pointedly, you do not describe any of these possible negative feedbacks.  In other words, your paper does not address the physics.  Your paper is an attempt to defend a policy based on conservative ideology, and not a defense of your scientific view.  This paper ignores the fact that policy that is not based on credible science or reality can’t help but be bad policy.

What would be helpful instead is a high quality paper defending your opinion that equilibrium climate sensitivity is indeed dominated by unidentified negative feedbacks and therefore that though the Earth’s surface will warm as a result of human emissions of carbon dioxide, the warming will not be very great.  Your policy paper does not do this.  Your logic is based on one peer-reviewed reference, from a May 2008 article in Nature by Noel Keenlyside et al. [14].  Figure 4 from Keenlyside’s paper (see below) shows that they are forecasting temperature (the green curve) to end up in exactly the same place as the IPCC scenarios which you cite (the black curve).  The measured temperature is shown in red and falls in between. 

Keenlyside is forecasting a hot climate than hotter.  It does not support your hypothesis.  I recommend Joe Romm’s blog, including interviews with the authors, in order to better understand Keenlyside’s results [15].  Keenlyside’s forecasts are somewhat controversial and already underestimating warming that is happening, so it is not clear that even if you had interpreted it correctly that this is the best reference to be using.  A paper by Rind and Lean should also be considered [16].

My request by this open letter is if you wouldn’t mind describing for us what your scientific view is on this important issue including references.  I am not looking for a paper of comparable high quality and completeness as the Hansen paper.  I am assuming that perhaps such a paper or papers may already exist in the peer-reviewed literature.  My concern is that science supporting denialist point of view is obscured by the ludicrous nature of most denier argument.  This makes it a difficult and tedious exercise to uncover.  If you could summarize where in the scientific literature possible negative feedbacks are described and verified in the paleoclimate record or by analysis, this would be much appreciated.

At Sustainable Loudoun, we are skeptics and appreciate good references and then validate them.  But we do not discriminate.  We hold everybody’s feet to the fire, especially our own. 

Best regards and thank you kindly

Tony Noerpel

[1] Ken Caldeira

[2] Sinclair


[4] Kump, L. R., Kastings, J. F., and Crane, R. G., The Earth System, 2004.

[5] Lunine, J. I., Earth, Evolution of a Habitable World, 2000. 

[6] Rahmstorf, S., 2008: Anthropogenic Climate Change: Revisiting the Facts. In: Global Warming: Looking Beyond Kyoto., E. Zedillo, Ed., Brookings Institution Press, Washington, pp. 34-53

[7] R. Alley, 2009,

[8] Anderegg, W.,  Prall, J., Harold, J., and Schneider, S., Expert credibility in climate change, Proceedings of the National Academy of Science, 2010.

[9] Gaines, S., Eglinton, G. and J. Rullkotter, Echoes of Life, Oxford, 2009.

[10] Plaxco, K., and Gross, M., Astrobiology, Johns Hopkins University Press, 2006.

[11] The science journal Nature referred to AGW skeptics as denialist in an editorial on so called climate-gate.   “denialists use every means at their disposal to undermine trust in scientists and science.” Nature Editorial Staff, Vol 462 | Issue no. 7273 | 3 December 2009

 [12] Michaels, 2009,

[13] Hansen, J., Sato, M., Kharecha1, P., Beerling. D., Robert Berner, R., Masson-Delmotte, V., Pagani. M., Raymo, M., Royer, D. and Zachos, J., “Target Atmospheric CO2: Where Should Humanity Aim?” The Open Atmospheric Science Journal, 2008, 2, 217-231.

 [14] Keenlyside, N., Latif, M., Jungclaus, J., Kornblueh, L., and Roeckner, E., Advancing decadal-scale climate prediction in the North Atlantic sector, Vol 453| 1 May 2008| doi:10.1038/nature06921.

 [15] see and and

 [16] Lean, J., and Rind, D., “How will Earth’s surface temperature change in future decades?”, GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L15708, 5 PP., 2009 doi:10.1029/2009GL038932


 “What distinguishes science from pseudoscience is not whether your theory originated with some particular conviction about how the world works, or whether you feel an emotional attachment to it. What matters is the evidence you find to support it, and whether you are ultimately prepared to accept that it could be wrong.”  Gabrielle Walker, Snowball Earth.

 When, on July 25, 1997, the United States Senate voted 95 to nothing to oppose the Kyoto Protocols under the illusion that doing so would destroy the United States economy, it occurred to me that our reaction to global warming was not going to be nearly as rational as our reaction to the threat posed to the ozone layer by the continued use of chlorofluorocarbons.  Putting aside the science for the moment every positive quarter of economic growth is traditionally attributed to increases in productivity and of course every increase in productivity is due to improvements in efficiency.  Use resources more efficiently and the economy grows.  In 1997 the entire senate was gripped by the irrational fear that further improvements in efficiency would suddenly cause the economy to fall into a tailspin.  The Senate voted 95-0 for Senator Robert Byrd’s 1997 resolution number 98 keeping the United States out of the Kyoto process.  If we all drove Priuses instead of Hummers somehow that would be an economic disaster.  Their logic escapes me, too.  As we all now know, these fine folks did the complete opposite, just in case, and caused the worst economic crises since the Great Depression by voting 90-8 in favor of the Gramm-Leach-Bliley Act in 1999 which in fact did destroy our economy.

 Seeing this coming, I felt compelled to learn the science behind the anthropogenic global warming theory for myself.  I approached this task the way any scientist or engineer would, by reading the peer-reviewed science and climate physics text books. 

 Any discipline of real science is exciting, alive, compelling and leads one to exponentially increasing discovery of even more knowledge and scientific understanding.  Following the scientific trail of AGW led me to the historic papers of Joseph Fourier, Svante Arrhenius and Louis Agassiz and that lead me to the study of the ice ages which inevitably leads one to the greatest ice age catastrophe of all time: Snowball Earth.  Thus, I had already read many of the papers by Paul Hoffman, the hero of Gabrielle Walker’s book as well as those of many other researchers such as Joe Kirchvink and Ray Pierrehumbert.  Knowing the story though did not preclude me from learning a great deal more. 

 Walker is an exceptional writer and her book Snowball Earth is a fascinating account of the development of the snowball Earth theory by the remarkable geologist Hoffman.  In fact the book is full of personal stories not only about Hoffman but many of the geologists who contributed to the development of the theory. 

Brian Harland first proposed that at one time the Earth might have frozen over solid about 600 million years ago because he found evidence of drop stone sediments and glacial scratches in pre-Cambrian rock from all over the world.  But he could not prove it and, if the Earth had in fact frozen over solid, he could not explain how the Earth could have possibly thawed out.  This is because ice is white and reflects all of the short-wave solar radiation back into space.  In other words, once the Earth froze, the sun could no longer warm it up.  Harland published his ideas in 1963.

 Joe Kirschvink first proposed the solution to that problem in a two page paper written in 1992.  Volcanoes emit carbon dioxide at a rate of about 60 million tonnes of carbon a year.  However over geologic time it does not accumulate in the atmosphere because silicate rock weathering, which extracts carbon and buries it as deep ocean sediment, proceeds just as fast.  However, if rocks are covered in ice and snow and if the Earth is so cold that very little water evaporates into the atmosphere, then rock weathering stops and the carbon dioxide accumulates.  Our planet was spared a lifeless fate because of the green house effect of carbon dioxide.  In other words, if Senator Inhofe was right, he would not exist. 

 Paul Hoffman’s place in all this was that he proved the snowball theory.  He was not without adversaries.  The story is full of egos and personalities.  The development and acceptance of the snowball theory is science at its most entertaining and the book reads like a thriller.

There were two episodes of snowball Earth.  The first occurred 2350 million years ago at the boundary between the Archaean and Proterozoic Eons.  Prior to the Earth freezing over solid this first time, atmospheric oxygen is thought to have been just a few hundred parts per million by volume and after the snowball, it had shot up to 1 or 2% of the atmosphere.  Between 750 and 590 million years ago, the Earth froze over again and again oxygen shot up, this time to about the current 20% of the atmosphere by volume.  Thus the most recent episode may have been responsible for the explosion of complex life on Earth, leading ultimately to the evolution of a species which has the mental capacity to actually work it all out and deny it all in one go.  You will enjoy Gabrielle Walker’s book.  It is simply brilliant.

 Tony Noerpel


Even if expense were no object, none of these [biosphere] services could be performed at such scales and with such efficacy by any anthropogenic means.  Our dependence on biosphere services is literally a matter of survival, and that’s why the integrity of the biosphere matters.” Vaclav Smil.

[T]he accumulation of atmospheric oxygen paved the way for significant leaps in biological evolution in the Paleoproterozoic with the rise of macroscopic oxygen-breathing organisms and in the Neoproterozoic-Cambrian with the emergence of animals.” Dominic Papineau

When economists try to put a value on the biosphere, they are kidding themselves and us.  As Vaclav Smil points out without a healthy biosphere, humans cannot survive [1].  Our dependence is existential.  Nor is this dependence limited to the current biosphere.  We owe our existence to the biosphere extending back through deep time.

 The Earth is 4.55 billion years old and its history is divided into four eons.  The earliest the Hadean Eon ended 3800 million years ago.  The Hadean Earth was dominated by the kinetic energy of constant collisions as it swept up debris scattered along its orbit in the young solar system.  A magma ocean bubbled on its surface, a frightening uninhabitable place.  The Archean Eon lasted from 3800 million years ago until 2500 million years ago.  The Archean Earth climate was temperate despite the faint young sun we’ve discussed in a previous article [2].  James Kasting proposed that it was moderated by carbon dioxide and the methane produced by methanogenic Archaea [3].  These methane-producing microorganisms kept the Earth from freezing solid, while the sun’s fusion reactor gradually intensified through the Archean, giving the rest of life a chance.

 The Proterozoic Eon began where the Archean left off and ran until the Cambrian Explosion 544 million years ago, the start of the present Eon, the Phanerozic.  But we are interested today in two remarkably similar events which bookend the Proterozoic.  These events have in common the breakup of a supercontinent, several snowball Earth episodes, where the Earth’s oceans may have frozen to the equator, interspersed between hothouse climates, a rise in atmospheric oxygen and a leap in biological evolution as described by Papineau [4].

 As the Archean Eon gave way to the Paleoproterozoic nickel isotope sedimentary deposits suggest that the productivity of the methanogens were winding down [5].  Methanogens use nickel in their metabolism to produce the atmospheric methane which along with the principle greenhouse gas carbon dioxide was keeping the Earth warm.  At the same time the supercontinent Kenorland was breaking up.  Rifting of supercontinents is accompanied by increased weathering of the newly exposed surfaces.  When the most recent supercontinent Pangaea broke up 200 million years ago the rift valley forming between South America and Africa became the Atlantic Ocean which is still spreading.  The rifting of the supercontinent Rodinia during the Neoproterozoic, about 700 million years ago gave rise to the Iapetus Ocean.

 Increased weathering released phosphorus into the seas.  Phosphorus is the most limiting element in the biosphere presently, as discussed by Dave Vaccari in a recent Sustainable Planet article [6, see also 4 and 11].  Even today plants concentrate phosphorus and can contain up to seven times the concentrations in the surrounding soils.  At the same time methanogens were becoming less productive at the end of the Archean, ancestors to present day cyanobacteria bloomed as a consequence of increased phosphorus which these microbes need for oxygen photosynthesis.  From Susan Gaines remarkable textbook on molecular fossils Echoes of Life [7] and from Plaxco and Gross’ Astrobiology [8] text book we learn that these bacteria may have been around for several hundred million years waiting for this opportunity. 

Oxygen photosynthesis increased the level of atmospheric oxygen after the breakup of Kenorland from essentially zero to about 2% of the atmosphere.  Oxygen is poisonous to methanogens so this turn of events created the opportunity for an entirely new biological regime.  But it also drew down the atmospheric carbon dioxide.  As a consequence of the loss of methane and carbon dioxide the Earth froze over.  Since the ice and snow which now blanketed the planet reflects most incoming short wave solar radiation rather than absorbing it, there is no known way which the Earth could have recovered except for volcanic activity and the release of carbon dioxide.  Enough carbon dioxide accumulated in the atmosphere over millions of years to melt back the ice and snow by trapping long wave heat radiation from the Earth surface.  Once the ice melted away completely, the huge quantity of carbon dioxide necessary to melt it in the first place now created a superheated greenhouse effect.  The subsequent increased weathering of silicate rocks [9] and additional bacterial blooms kick started the process all over again, drawing down the carbon dioxide leading to yet another snowball earth episode.  Each cycle may have pumped more oxygen into the atmosphere.

Essentially two chemical reactions take place which draw down carbon dioxide in a hot house climate.  The first is inorganic and involves the weathering of silicate rocks.  This is the Earth’s thermostat [9] and is given by the following simplified equation.

CO2 + CaSiO3 -> CaCO3 + SiO2

 In a hot house climate more water evaporates off the oceans and forms carbonic acid with the carbon dioxide in the atmosphere.  This weak acid rains out onto rocks weathering them.  Note that in this equation carbon dioxide is drawn down when the calcium carbonate and silica are “buried in marine sediments and eventually into the geological record [9].”  This process extracts excess carbon dioxide from the atmosphere but does not create free oxygen.

 However, oxygenic photosynthesis performed by the bacterial blooms, encouraged by the newly releases phosphorus performs the following reaction.

 CO2 + H2O -> CH2O + O2

 When organic compounds, here represented by CH2O, are buried as sediment without being oxidized and consumed by other organisms there is a net draw down of carbon dioxide and atmospheric oxygen is created.  Note that while rock weathering can act as the Earth’s thermostat by controlling the amount of carbon dioxide in the atmosphere, it cannot create atmospheric oxygen.  We need life for that.  The free oxygen created an opportunity for heterotrophic bacteria and eukaryotes to exploit and they did.  It also relegated methanogens, the heroes of the Archean, to anoxic hideouts such as deep ocean sediment, swamps and cow stomachs. 

 The Neoproterozic rifting resulting in the breakup of the supercontinent Rodinia about 750 million years ago had the same effect.  This rifting forming the Iapetus Ocean is recorded in the geological record of Loudoun County [10].  The sun was much warmer now, about 94% of today’s sun and the Earth was kept warm by its blanket of carbon dioxide.  With the breakup of Rodinia, the events of the Paleoproterozic were repeated.  Increased weathering of the continents increased burial of both inorganic and organic carbon with a subsequent rise in atmospheric oxygen, this time from about 2% to 20% of the atmosphere.   Again the Earth’s climate oscillated between a snowball and a hothouse several times between 750 and 580 million years ago [4].  While Eukaryotes were certainly already around, it was this rise in oxygen, due to photosynthesis, which allowed the evolution and radiation of metazoans; complex life. 

So we are alive today, not just because of the other inhabitants of our biosphere, the Earth’s environment but we also owe a debt of gratitude to the biospheres in Earth’s past.

 Tony Noerpel

 [1] Vaclav Smil, Global Catastrophes and Trends, the next fifty years, 2008


[3] Kastings, J., “When Methane made Climate”, Scientific American, 2004.

[4] Papineau, D., “Global Biogeochemical Changes at Both Ends of the Proterozoic: Insights from Phosphorites,” Astrobiology, Vol 10, Number2, 2010.

[5] Konhauser, O., et al.  “Ocean nickel depletion and a methaogen famine before the Great Oxidation Event,” Nature vol 458, April 9, 2009.


[7] Gaines, S., Eglinton, G. and J. Rullkotter, Echoes of Life, Oxford, 2009.

[8] Plaxco, K., and Gross, M., Astrobiology, Johns Hopkins University Press, 2006.

 [9] Berner, R., The Phanerozic Carbon Cycle, Oxford University Press, 2004.

 [10] Southworth, S. et al.  Geologic map of Loudoun County, Virginia, U.S. Department of the Interior, to accompany map OF-99-150 U. S. Geological Survey.

 [11] Filippelli, G., “The global phosphorus cycle: past, present and future,” Elements, Vol. 4, pp 97-104, April, 2008.


The warmer climate facilitates hurricane activity. This amounts to a positive feedback, which can potentially lead to multiple climate states – one with permanent El Nino-like conditions and strong hurricane activity and the other corresponding to modern climate with a cold equatorial Pacific.” Fedorov, et al., 2010.
The Pliocene epoch spans 3 million years of Earth’s history between 5.4 and 2.4 million years ago. In the early Pliocene our earliest pre-human ancestors discovered so far Ardipithecus ramidus appeared. And the first humans Homo habilis and possibly even Homo erectus were around at the end of the Pliocene for the transition into the ice ages of the Pleistocene. While Homo habilis died out long before the appearance of Homo sapiens sapiens (us) Homo erectus was still around by the time we evolved.
At the beginning of this epoch, the Mediterranean Sea evaporated and remained dry for about 170,000 years. The African continental plate had been colliding with the European continental plate since about 85 million year ago when the ancient Mediterranean was still the Tethys Ocean. Five million years ago the African continental plate slid under Spain uplifting it and causing the Mediterranean to be cut off from the Atlantic Ocean [Govers, 2009]. Since the Mediterranean Sea loses more water to evaporation than is supplied by all the rivers which feed into it, in a few tens of years it had virtually dried up forming a deep hole some 3 miles below sea level at its deepest point. The average depth of the Mediterranean is about 1 mile. Imagine some pre-human following the edge of the Mediterranean Sea and all of its bounty a mile or so below sea level, perhaps a successful strategy for tens of thousands of years and thousands of generations. When the Atlantic finally breached the Gibraltar dam the flooding must have been dramatic catching millions of animals unaware.
But the most interesting thing about the Pliocene is its climate. It turns out that the Pliocene epoch is the best analog for the current Earth climate of all the 4.55 billion year history of our planet. The sun’s luminosity was nearly the same as it is today, the atmospheric carbon dioxide level was between 300 and 400 parts per million by Volume (ppmV), and the continents were in approximately the same location. We have discussed the faint young sun in a previous article [climate factors]. And if you recall, our sun has been steadily increasing in luminosity as the original hydrogen in its core has been fusing into helium. Because of this, during the Pliocene the solar forcing may have been about 0.2 W/m2 less than today or about the same as during the little ice age [Wang, 2005, Krivova, 2007]. Despite the slightly cooler sun, the early Pliocene was 4oC warmer than today and the mid Pliocene was about 2oC warmer. The carbon dioxide forcing had to account for the warm climate and the slightly cooler sun. This is an enigma since the current estimate for the equilibrium climate sensitivity, or the amount that the temperature would increase with a doubling of atmospheric carbon dioxide, i.e, to about 560 ppmV is about 3oC. Atmospheric carbon dioxide levels today are about 390 ppmV, or at the upper end of the Pliocene values, yet the Pliocene climate was hotter than the accepted equilibrium climate sensitivity would predict.
While the continents were nearly in the positions they are in today, there were some differences. The difference which probably affected the climate the most is that the Isthmus of Panama between North and South America did not close the connection between the Pacific and the Atlantic Oceans, the Central American Seaway, until about 3 million years ago [Murdock, 1997]. This closure impacted ocean circulation of heat.
Recently a new paper suggests another feedback mechanism associated with increased warmth in the early Pliocene. Fedorov et al propose that increased hurricane activity contributed to the warm climate as part of a positive feedback mechanism that maintained the warmth with permanent El Nino-like conditions [Fedorov, 2010]. In a review article, Ryan Sriver writes “These results may provide clues to understanding not only the climate of the early Pliocene, but also the nature of future climate change in a greenhouse world.” [Sriver, 2010]
Perhaps the most important difference, not discussed in the Fedorov paper, is that the Earth’s climate had been gradually cooling since the hot house Eocene 50 million years ago whereas our climate is recovering from an ice age. The last glacial maximum was only 20,000 years ago. I asked Kerry Emanuel of MIT and a co-author of the Fedorov paper about this and he replied to me: “Although CO2 levels were similar then to today’s, that climate had plenty of time to equilibrate to the forcing whereas ours clearly has not. It is also plausible that the climate exhibits hysteresis and multiple equilibria, so that approaching 370 ppm of CO2 from a warmer state may yield a different climate than approaching it from a colder state.”
James Hansen of NASA GISS, has suggested that the accepted value for the sensitivity of the Earth’s climate only accounts for fast feedbacks such as increasing water vapor and not very slow feedbacks. Hansen suggests that equilibrium climate sensitivity might be closer to 6oC [Hansen, 2008] when slow feedbacks are accounted for. A related aspect is that most of the trapped energy is currently warming the oceans as shown in figure 1 and it takes a very long time for these bodies of water to warm up or cool down [Murphy, 2009].

"ocean heat content"

Figure 1: Total Earth Heat Content anomaly from 1950 (Murphy 2009). Ocean data taken from Domingues et al 2008. Land + Atmosphere includes the heat absorbed to melt ice.

What we can appreciate from a study of the Pliocene climate is that equilibrium climate sensitivity may be higher than the consensus view and we may see an unexpected increase once the oceans warm up or equilibrate to the new higher level of carbon dioxide and further that the climate may change states from the current state where we experience an El Nino event every 3-8 years to a permanent El Nino state which may be self sustaining.
To view maps of the locations of continents in the Earth’s past see Chirstopher R. Scotese’s fascinating web site

Tony Noerpel

Fedorov, A. V., Brierley, C. M., and Emanuel, K., Tropical cyclones and permanent El Nino in the early Pliocene epoch, Nature, Vol. 463, February 25, 2010, 1066-1070.
Govers et al. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, 2009; 37 (2): 167 DOI: 10.1130/G25141A.1
Murdock, T. Q., A. J. Weaver, and A. F. Fanning (1997), Paleoclimatic response of the closing of the Isthmus of Panama in a coupled ocean-atmosphere model, Geophys. Res. Lett., 24(3), 253–256.
Wang, Y.-M., J. L. Lean, J. L., and Sheeley, N. R. Jr , Modeling the sun’s magnetic field and irradiance since 1713, The Astrophysical Journal, 625:522–538, May 20, 2005
Krivova, N. A., Balmaceda, L., and Solanki, S. K., Reconstruction of solar total irradiance since 1700 from the surface magnetic flux, Astronomy and Astrophysics, Volume 467, Number 1, May III 2007, 335 – 346.
Hansen, J., Sato, M., Kharechal, P., Beerling, D., Berner, R., Masson-Delmotte, V., Pagani, M., Raymo, M., Royer, D. L., and Zachos, J. C., Target Atmospheric CO2: Where Should Humanity Aim?, The Open Atmospheric Science Journal, 2008, 2, 217-231.
Murphy, D. M., S. Solomon, R. W. Portmann, K. H. Rosenlof, P. M. Forster, and T. Wong (2009), An observationally based energy balance for the Earth since 1950, J. Geophys. Res., 114, D17107, doi:10.1029/2009JD012105.
Sriver, R., Tropical cyclones in the mix,” Nature, Vol 463, 25 Februrary, 2010, 1032-1033.
When the Mediterranean Sea dried up lat Miocene
early hominids

Climate Overview

Climate Factors

Here is a brief summary of those physical factors which influence a planet’s climate and in the case of Earth, make life possible. These are included in the anthropogenic global warming theory presented in the Intergovernmental Panel on Climate Change (IPCC) reports [IPCC, 2007].

  • Solar luminosity
  • Atmospheric greenhouse effect
    • Carbon dioxide
    • Methane
    • Ozone
    • Water vapor
    • Nitrous Oxide
    • Others…
  • Earth orbital variation
    • Croll-Milankovic cycles
    • The fortuitous circumstance of our large moon which stabilizes Earth’s orbit.
  • Earth’s oceans
  • Plate tectonics
  • Position of continents and oceans
  • High mountains (long term weathering and winds)
  • Ocean circulation
  • Subduction and regeneration of CO2
  • Volcanism
    • contributes CO2 (carbon cycle)
    • contributes aerosols and dust
  • Plant and bacterial life via photosynthesis
    • Consumption of CO2
    • Creation of oxygen
    • Amplification of rock weathering
  • Burial of organic matter in oceans
  • Mountain weathering and deposition as carbonate layers in oceans
  • Surface and cloud Albedo
  • Glaciers and polar ice sheets
  • Geothermal heating from radioactive decay
  • Land use
  • Air currents

This is not necessarily a complete list. These factors are not in any special order and are interrelated. Plate tectonics would not be possible without Earth’s oceans for example.

Faint Young Sun Paradox

Having observed that the Earth climate system is complex, I want to focus on the two principle components, the solar luminosity and the atmosphere. Joseph Fourier published the first energy balance for the Earth back in 1826. He calculated that in order for incoming solar radiation and outgoing heat radiation to balance, the Earth would only be about -18oC assuming the Earth is a black body radiator and absorbs all of the incoming energy. In fact, even then some of the solar energy would be reflected back out into space without warming the Earth and it would be even colder as shown in Figure 1. Fourier hypothesized in his paper that the atmosphere must have some effect which is keeping the planet warm. In 1860 John Tyndall discovered that carbon dioxide, water vapor, methane and ozone were the greenhouse gases responsible for the warming the planet. This was surprising at the time because all of these gases have very low concentrations in the atmosphere the bulk of which is nitrogen and oxygen.

Climate Factors

Figure 1 – Solar and atmospheric forcing overview

The red curve in Figure 1 illustrates the “faint young Sun” paradox, [from Koch, 2008]. Our sun is a G2 star on the main sequence; it was only 70% as luminous then as now. This is because the sun was mostly low density hydrogen. As fusion takes place in the solar core, four hydrogen protons combine to form one helium nucleus releasing energy by a complex process of collisions and reactions. As hydrogen is converted to helium, the core density increases as does the temperature and pressure resulting in a higher probability of fusion reactions and more radiation. Yet since about 4 billion years ago the Earth has always had a temperature compatible with liquid surface water and life. Earth’s surface temperature is thought to have varied between about 10 and 25 degrees C throughout its history, except for the Hadean Eon as shown by the grey band in Figure 1.

The lower of the two brown curves in Figure 1 is the Earth’s temperature without an atmosphere and the upper curve shows what the temperature would have been with today’s atmospheric concentrations. We first observe the profound impact of the greenhouse gases on the Earth temperature today, warming our planet from about -18oC to about +15oC.

What mechanism kept the Earth warm before present time? And how did this mechanism constrain the Earth’s temperature to such a narrow window despite the solar luminosity changes? Why didn’t the Earth freeze and what would have happened if it did?

Possible excursions below this range are thought to have occurred in the Proterozoic about 2.25 billion years ago and again between 750 and 590 million years ago. These snowball Earth events are shown by the two grey arrows in this figure. Earth’s temperature plummeted and the oceans froze to the equator. These events are contemporaneous with the two step-wise increases in atmospheric oxygen, the first from practically no oxygen to about 2% of the atmosphere by volume and the second to the present level of about 20% of the atmosphere by volume.

Figure 2 shows the atmospheric carbon dioxide partial pressure needed to maintain a temperate climate throughout Earth’s history. Since atmospheric oxygen was low during the Archean Eon, before 2.35 billion years ago, both methane and carbon dioxide could have been dominant greenhouse gases. A possible moderating feedback mechanism, involving both these gases is described by [Kastings, 2000]. However, during the Proterozoic and Phanerozoic Eons, carbon dioxide alone would have had to keep the Earth warm and balance the increasing solar radiation. This feedback process is described by [Berner, 2004]. It is called the carbonate-silicate cycle or the long term carbon cycle. Briefly, when the Earth warms, water evaporates off the oceans increasing rainfall. The water vapor combines with carbon dioxide in the atmosphere creating carbonic acid. These acids rain onto silicate rocks increasing the rate of weathering and carbonate sediment formation, effectively leaching the carbon dioxide out of the atmosphere. As a result the Earth cools. A cooler Earth results in less evaporation and less rainfall and subsequently less weathering and burial. This negative feedback cycle operates over millions of years.

Faint Young Sun

Figure 2 – Solar luminosity and CO2 partial pressure

Berner makes the point that the principle greenhouse gas is. We read that methane as a greenhouse gas is stronger than by a factor of between 21 and 33 depending on how it is measured but in all cases this only applies for up to one hundred years, a period which is entirely relevant to the current human condition but not important over geologic time. Methane reacts with atmospheric oxygen to become and water fairly quickly. The residence time of is only about ten years. During the Archean Eon when there was little to no atmospheric oxygen, methane may have been the dominant greenhouse gas. Although vapor is the strongest greenhouse gas, “it is buffered by evaporation and condensation that is driven by external factors such as solar radiation and the greenhouse effect.” [Berner, 2004]

Figure 3 [Royer, 2006] shows the combined solar forcing and the carbon dioxide forcing. Hot house climates experienced only 4 to 6 W/m2 radiative forcing above pre-industrial values. In other words, despite solar radiation which was increasing by about 5% over the phanerozoic, or about 12 W/m2, reduced levels of atmospheric carbon dioxide maintained the Earth’s temperate climate. This figure also illustrates the correlation between global temperature and ice ages over the entire Phanerozoic. Note that all ice ages occur when the combined solar- forcing is relatively the same as pre-industrial Holocene demonstrating a high correlation between atmospheric and global temperature including the Ordovician-Silurian boundary glaciation as described by [Young, 2009].

Combined radiative forcing relative to pre-industrial 280 ppmV carbon dioxide

Figure 3 Combined radiative forcing relative to pre-industrial 280 ppmV carbon dioxide [from Royer, 2006]

According to [NASA, 2009] and [Trenberth, 2009] the total increase in radiative forcing including all factors since 1850 is about 1.8 W/m2. This increase is shown by the red line in Figure 3. We can see that the Earth’s climate is potentially being forced into a hothouse regime from the current ice house climate in a geologically short time. Often, such excursions are associated with extinction events especially when combined with other factors such as in the present case over fishing, deforestation and mountaintop removal mining [Hallam, 2004].

— Tony Noerpel

Discuss this topic at this Sustainable Loudoun forum thread




[Young, 2009] Young, Saltzman, Foland, Linder and Kump, “A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate?” Geology, October, 2009.

[Hallam, 2004] Hallam, Catastrophes and Lesser Calamities, Oxford University Press.

[Royer, 2006] Royer, “CO2-forced climate thresholds during the Phanerozoic”, Geochimica et Cosmochimica Acta 70 (2006) 5665–5675

[Berner, 2004], Robert, The Phanerozoic Carbon Cycle, Oxford University Press, 2004.

[Trenberth, 2009] Trenberth, K. E., 2009: An imperative for adapting to climate change: Tracking Earth’s global energy. Current Opinion in Environmental Sustainability, 1, 19-27. DOI 10.1016/j.cosust.2009.06.001.

[NASA, 2009]

[Koch, 2008]

[Kasting, 2000] Kasting, Pavlov, Brown, Rages, Freedman, “Greenhouse warming in the atmosphere of early Earth,” Journal of Geophysical research, v 105, no. E5, 11,981-11990, May 25, 2000.