Volcanic winter of 536

The volcanic winter of 536 was the most severe and protracted episode of climatic cooling in the Northern Hemisphere in the last 2,000 years.^[1] The volcanic winter was caused by at least three simultaneous eruptions of uncertain origin, with several possible locations proposed in various continents. Most contemporary accounts of the volcanic winter are from authors in Constantinople, the capital of the Eastern Roman Empire, although the impact of the cooler temperatures extended beyond Europe. Modern scholarship has determined that in early AD 536 (or possibly late 535), an eruption ejected massive amounts of sulfate aerosols into the atmosphere, which reduced the solar radiation reaching the Earth's surface and cooled the atmosphere for several years. In March 536, Constantinople began experiencing darkened skies and cooler temperatures.

Summer temperatures in 536 fell by as much as 2.5 degrees Celsius (4.5 Fahrenheit degrees) below normal in Europe. The lingering impact of the volcanic winter of 536 was augmented in 539–540, when another volcanic eruption caused summer temperatures to decline as much as 2.7 degrees Celsius (4.9 Fahrenheit degrees) below normal in Europe.^[2] There is evidence of still another volcanic eruption in 547 which would have extended the cooler period. The volcanic eruptions, which began in 541, caused crop failures, and were accompanied by the Plague of Justinian, famine, and millions of deaths and initiated the Late Antique Little Ice Age, which lasted from 536 to 560.^[3]

The medieval scholar Michael McCormick wrote that 536 was the worst year in history to be alive: "It was the beginning of one of the worst periods to be alive, if not the worst year."^[4]

Documentary evidence

The Roman historian Procopius recorded in AD 536 in his report on the wars with the Vandals, "during this year a most dread portent took place. For the sun gave forth its light without brightness... and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear".^[5][6]

In 538, the Roman statesman Cassiodorus described the following to one of his subordinates in letter 25:^[7]

• The sun's rays were weak, and they appeared a "bluish" colour.
• At noon, no shadows from people were visible on the ground.
• The heat from the sun was feeble.
• The moon, even when full, was "empty of splendour"
• "A winter without storms, a spring without mildness, and a summer without heat"
• Prolonged frost and unseasonable drought
• The seasons "seem to be all jumbled up together"
• The sky is described as "blended with alien elements" just like cloudy weather, except prolonged. It was "stretched like a hide across the sky" and prevented the "true colours" of the sun and moon from being seen, along with the sun's warmth.
• Frosts during harvest, which made apples harden and grapes sour.
• The need to use stored food to last through the situation.
• Subsequent letters (no. 26 and 27) discuss plans to relieve a widespread famine.

Michael the Syrian (1126–1199), a patriarch of the Syriac Orthodox Church, reported that during 536–537 the sun shone feebly for a year and a half.^[8]

The Gaelic Irish Annals^[9][10][11] recorded the following:

• "A failure of bread in AD 536 AD" – the Annals of Ulster
• "A failure of bread from AD 536–539" – the Annals of Inisfallen

The mid-10th-century Annales Cambriae record for the year 537:

• "The Battle of Camlann, in which Arthur and Medraut fell, and there was great mortality in Britain and Ireland."^[a][12]

Further phenomena were reported by independent contemporary sources:

• Low temperatures, even snow during the summer (snow reportedly fell in August in China, which caused the harvest there to be delayed)^[13]
• Widespread crop failures^[14]
• "A dense, dry fog" in the Middle East, China and Europe^[13]
• Drought in Peru, which affected the Moche culture^[13][15]

There are other sources of evidence regarding this period.^{[16][17][18][19]}

Scientific evidence

Tree ring analysis by the dendrochronologist Mike Baillie, of the Queen's University of Belfast, shows abnormally little growth in Irish oak in 536 and another sharp drop in 542, after a partial recovery.^[20] Ice cores from Greenland and Antarctica show evidence of substantial sulfate deposits in around 534 ± 2, which is evidence of an extensive acidic dust veil.^[21]

Possible explanations

It was originally theorized that the climatic changes of AD 536 were caused by either volcanic eruptions (a phenomenon known as "volcanic winter") or impact events (meteorite or comet).^[22][23][24]

In 2015, revision of polar ice core chronologies dated a major sulfate deposits and a cryptotephra layer to the exact year AD 536 (previously dated to AD 529 before revision).^[25] This is strong evidence that a large explosive volcanic eruption caused the observed dimming and cooling, removing the need for an extraterrestrial explanation,^[21][25] but an impact event around this time period cannot be ruled out.^[26]

The source of volcanic eruption remains to be found but several proposed volcanoes have been rejected:

• R. B. Stothers postulated the volcano Rabaul in New Britain, in Papua New Guinea.^[27] The eruption is now thought to have occurred in the interval AD 667–699 based on wiggle-match radiocarbon dating.^[28]
• David Keys suggested the volcano Krakatoa by shifting a cataclysm in AD 416 recorded in Javanese Book of Kings to AD 535.^[15] Drilling projects in Sunda Strait ruled out any possibility that an eruption took place during this time period.^[29]
• Robert Dull and colleagues proposed the large VEI-7, Tierra Blanca Joven (TBJ) eruption of the Ilopango caldera.^[30][31] Identification of TBJ tephra in ice cores narrowed the eruption date to AD 429–433.^[32]
• Loveluck and his colleagues proposed Icelandic volcanos based on the shards from Swiss glacier.^[4][33] However, the cryptotephras dated exactly to AD 536 is geochemically distinct from Icelandic tephra,^[34] and shards in Swiss glacier have large age uncertainty.^[33]

Geochemical analysis of AD 536 cryptotephras distinguishes at least three synchronous eruptive events in North America.^[25] Further analysis correlates one of the eruptions to a widespread Mono Craters tephra identified in northeast America.^[25][35] The other two eruptions most likely originated from the eastern Aleutians and Northern Cordilleran volcanic province.^[25][36]

Historic consequences

Main article: Late Antique Little Ice Age

The 536 event and ensuing famine have been suggested as an explanation for the deposition of hoards of gold by Scandinavian elites at the end of the Migration Period. The gold was possibly a sacrifice to appease the gods and get the sunlight back.^[37][38] Mythological events such as the Fimbulwinter and Ragnarök are theorised to be based on the cultural memory of the event.^[39]

A book written by David Keys speculates that the climate changes contributed to various developments, such as the emergence of the Plague of Justinian (541–549), the decline of the Avars, the migration of Mongol tribes towards the west, the end of the Sassanid Empire, the collapse of the Gupta Empire, the rise of Islam, the expansion of Turkic tribes, and the fall of Teotihuacán.^[15] In 2000, a 3BM Television production (for WNET and Channel Four) capitalised upon Keys' book. The documentary, under the name Catastrophe! How the World Changed, was broadcast in the US as part of PBS's Secrets of the Dead series.^[40] However, Keys and Wohletz's ideas lack mainstream acceptance. Reviewing Keys' book, British archaeologist Ken Dark commented that "much of the apparent evidence presented in the book is highly debatable, based on poor sources or simply incorrect. [...] Nonetheless, both the global scope and the emphasis on the 6th century AD as a time of wide-ranging change are notable, and the book contains some obscure information that will be new to many. However, it fails to demonstrate its central thesis and does not offer a convincing explanation for the many changes discussed".^[41]

The philologist Andrew Breeze in a recent book (2020) argues that some King Arthur events, including the Battle of Camlann, are historical by happening in 537 as a consequence of the famine associated with the climate change of the previous year.^[42]

See also

• 1257 Samalas eruption
• 1452/1453 mystery eruption^[43]
• 1815 eruption of Mount Tambora, largest ever recorded
• 946 eruption of Paektu Mountain
• Fimbulwinter
• Great Famine of 1315–1317
• Justinian I, Roman emperor at the time
• Laki
• Minoan eruption
• Tierra Blanca Joven eruption
• Volcanism of Iceland
• Year Without a Summer, 1816

Notes

1. The battle is dated 539 in some editions.

References

1. Abbott, D. H.; Biscaye, P.; Cole-Dai, J.; Breger, D. (December 2008). "Magnetite and Silicate Spherules from the GISP2 Core at the 536 A.D. Horizon". AGU Fall Meeting Abstracts. American Geophysical Union, Fall Meeting 2008. Vol. 41. pp. 41B–1454. Bibcode:2008AGUFMPP41B1454A. Abstract #PP41B-1454.
2. Harper, Kyle (2017). The Fate of Rome. Princeton: Princeton University Press. p. 253. ISBN 9780691166834.
3. Peregrine, Peter (2020). "Climate and social change at the start of the Late Antique Little Ice Age". The Holocene. 30 (11): 1643–1648. Bibcode:2020Holoc..30.1643P. doi:10.1177/0959683620941079. ISSN 0959-6836. S2CID 222179333. Retrieved 18 November 2021.
4. Gibbons, Ann (15 November 2018). "Why 536 was 'the worst year to be alive'". Science. doi:10.1126/science.aaw0632. ISSN 0036-8075. S2CID 189287084.
5. Procopius; Dewing, Henry Bronson, trans. (1916). Procopius. Vol. 2: History of the [Vandalic] Wars, Books III and IV. London, England: William Heinemann. p. 329. ISBN 978-0-674-99054-8.
6. Ochoa, George; Jennifer Hoffman; Tina Tin (2005). Climate: the force that shapes our world and the future of life on earth. Emmaus, PA: Rodale. ISBN 978-1-59486-288-5, gives this quote as "The Sun gave forth its light without brightness, like the moon during this whole year, and it seemed exceedingly like the Sun in eclipse".
7. Cassiodorus; Hodgkin, Thomas, trans. (1886). The Letters of Cassiodorus. London, England: Henry Frowde. pp. 518–520. See: "25. Senator, Praetorian Praefect, to his deputy Ambrosius, an Illustris."
8. Michel le Syrien; Chabot, J.-B., trans. (1901). Chronique de Michel le Syrien, Patriarche Jacobite d'Antoche [Chronicle of Michael the Syrian, Jacobite Patriarch of Syria] (in French). Vol. 2nd vol. Paris, France: Leroux. pp. 220–221. From pp. 220–221: "Or, un peu auparavant, en l'an 848, il y eut un signe dans le soleil..., et le vin avait le goût de celui qui provient de raisins acides." (However, a little earlier, in the year 848 [according to the Greek calendar; AD 536/537 according to the Christian calendar], there was a sign in the sun. One had never seen it [before] and nowhere is it written that such [an event] had happened [previously] in the world. If it were not [true] that we found it recorded in most proven and credible writings, and confirmed by men worthy of belief, we would not have written it [here]; for it's difficult to conceive. So it is said that the sun was darkened and that its eclipse lasted a year and a half, that is, eighteen months. Every day it shone for about four hours and yet this light was only a feeble shadow. Everyone declared that it would not return to the state of its original light. Fruits did not ripen, and wine had the taste of what comes from sour grapes.)
9. Gaelic Irish Annals translations
10. "List of Published Texts at CELT". celt.ucc.ie.
11. "Annals of the Four Masters". celt.ucc.ie.
12. "Camlan | Robbins Library Digital Projects". Retrieved 31 July 2018.
13. Ochoa, George; Jennifer Hoffman; Tina Tin (2005). Climate: the force that shapes our world and the future of life on earth. Emmaus, Pennsylvania: Rodale. p. 71. ISBN 978-1-59486-288-5.
14. Rosen, William (2007). Justinian's flea: Plague, Empire and the Birth of Europe. London: Jonathan Cape. ISBN 978-0-224-07369-1.
15. Keys, David Patrick (2000). Catastrophe: an investigation into the origins of the modern world. New York: Ballantine Pub. ISBN 978-0-345-40876-1.
16. Stothers, R.B.; Rampino, M.R. (1983). "Volcanic eruptions in the Mediterranean before AD 630 from written and archaeological sources". Journal of Geophysical Research. 88 (B8): 6357–6471. Bibcode:1983JGR....88.6357S. doi:10.1029/JB088iB08p06357. ISSN 0148-0227.
17. Stothers, R.B. (16 January 1984). "Mystery cloud of AD 536". Nature. 307 (5949): 344–345. Bibcode:1984Natur.307..344S. doi:10.1038/307344a0. ISSN 0028-0836. S2CID 4233649.
18. Rampino, M.R.; Self, S.; Stothers, R.B. (1988). "Volcanic winters". Annual Review of Earth and Planetary Sciences. 16: 73–99. Bibcode:1988AREPS..16...73R. doi:10.1146/annurev.ea.16.050188.000445. ISSN 0084-6597.
19. Arjava, Antti (2005). "The mystery cloud of 536 CE in the Mediterranean sources". Dumbarton Oaks Papers. 59: 73–94. doi:10.2307/4128751. ISSN 0070-7546. JSTOR 4128751.
20. Baillie, M.G.L. (1994). "Dendrochronology Raises Questions About the Nature of the AD 536 Dust-Veil Event." The Holocene fig. 3 p. 215
21. Larsen, L. B.; Vinther, B. M.; Briffa, K. R.; Melvin, T. M.; Clausen, H. B.; Jones, P. D.; Siggaard-Andersen, M.-L.; Hammer, C. U.; et al. (2008). "New ice core evidence for a volcanic cause of the A.D. 536 dust veil". Geophys. Res. Lett. 35 (4): L04708. Bibcode:2008GeoRL..3504708L. doi:10.1029/2007GL032450. ISSN 0094-8276.
22. Baillie, M. G. L. (1999). Exodus to Arthur: Catastrophic Encounters with Comets. London: B.T. Batsford. ISBN 978-0-7134-8352-9.
23. Rigby, Emma; Symonds, Melissa; Ward-Thompson, Derek (February 2004). "A comet impact in AD 536?". Astronomy and Geophysics. 45 (1): 1.23–1.26. Bibcode:2004A&G....45a..23R. doi:10.1046/j.1468-4004.2003.45123.x. ISSN 1366-8781. S2CID 121589992.
24. MacIntyre, Ferren (2002). "Simultaneous Settlement of Indo-Pacific Extrema?". Rapa Nui Journal. 16 (2): 96–104.
25. Sigl, M.; Winstrup, M.; McConnell, J. R.; Welten, K. C.; Plunkett, G.; Ludlow, F.; Büntgen, U.; Caffee, M.; Chellman, N. (2015). "Timing and climate forcing of volcanic eruptions for the past 2,500 years". Nature. 523 (7562): 543–549. Bibcode:2015Natur.523..543S. doi:10.1038/nature14565. ISSN 0028-0836. PMID 26153860.
26. Abbott, Dallas H.; Breger, Dee; Biscaye, Pierre E.; Barron, John A.; Juhl, Robert A.; McCafferty, Patrick (1 September 2014). "What caused terrestrial dust loading and climate downturns between A.D. 533 and 540?". doi:10.1130/2014.2505(23). {{cite journal}}: Cite journal requires |journal= (help)
27. Stothers R.B. (26 January 1984). "Mystery cloud of AD 536". Nature. 307 (5949): 344–345. Bibcode:1984Natur.307..344S. doi:10.1038/307344a0. ISSN 0028-0836. S2CID 4233649.
28. McKee, Chris O.; Baillie, Michael G.; Reimer, Paula J. (4 July 2015). "A revised age of ad 667–699 for the latest major eruption at Rabaul". Bulletin of Volcanology. 77 (7): 65. doi:10.1007/s00445-015-0954-7. ISSN 1432-0819.
29. Southon, John; Mohtadi, Mahyar; Pol-Holz, Ricardo De (9 February 2013). "Planktonic Foram Dates from the Indonesian Arc: Marine 14C Reservoir Ages and a Mythical AD 535 Eruption of Krakatau". Radiocarbon. 55 (3): 1164–1172. doi:10.1017/S0033822200048074. ISSN 0033-8222.
30. Dull, R.; J.R. Southon; S. Kutterolf; A. Freundt; D. Wahl; P. Sheets (13–17 December 2010). "Did the TBJ Ilopango eruption cause the AD 536 event?". AGU Fall Meeting Abstracts. 13: V13C–2370. Bibcode:2010AGUFM.V13C2370D.
31. Dull, Robert A.; Southon, John R.; Kutterolf, Steffen; Anchukaitis, Kevin J.; Freundt, Armin; Wahl, David B.; Sheets, Payson; Amaroli, Paul; Hernandez, Walter; Wiemann, Michael C.; Oppenheimer, Clive (October 2019). "Radiocarbon and geologic evidence reveal Ilopango volcano as a source of the colossal 'mystery' eruption of 539/40 CE" (PDF). Quaternary Science Reviews. 222: 105855. Bibcode:2019QSRv..22205855D. doi:10.1016/j.quascirev.2019.07.037. ISSN 0277-3791. S2CID 202190161.
32. Smith, Victoria C.; Costa, Antonio; Aguirre-Díaz, Gerardo; Pedrazzi, Dario; Scifo, Andrea; Plunkett, Gill; Poret, Mattieu; Tournigand, Pierre-Yves; Miles, Dan; Dee, Michael W.; McConnell, Joseph R.; Sunyé-Puchol, Ivan; Harris, Pablo Dávila; Sigl, Michael; Pilcher, Jonathan R.; Chellman, Nathan; Gutiérrez, Eduardo (20 October 2020). "The magnitude and impact of the 431 CE Tierra Blanca Joven eruption of Ilopango, El Salvador". Proceedings of the National Academy of Sciences. 117 (42): 26061–26068. Bibcode:2020PNAS..11726061S. doi:10.1073/pnas.2003008117. ISSN 0027-8424. PMC 7584997. PMID 32989145.
33. Loveluck, Christopher P.; McCormick, Michael; Spaulding, Nicole E.; Clifford, Heather; Handley, Michael J.; Hartman, Laura; Hoffmann, Helene; Korotkikh, Elena V.; Kurbatov, Andrei V.; More, Alexander F.; Sneed, Sharon B.; Mayewski, Paul A. (14 November 2018). "Alpine ice-core evidence for the transformation of the European monetary system, AD 640–670". Antiquity. 92 (366): 1571–1585. doi:10.15184/aqy.2018.110. ISSN 0003-598X.
34. Plunkett, Gill; Sigl, Michael; Pilcher, Jonathan R.; McConnell, Joseph R.; Chellman, Nathan; Steffensen, J.P.; Büntgen, Ulf (8 June 2020). "Smoking guns and volcanic ash: the importance of sparse tephras in Greenland ice cores". Polar Research. 39 (0). doi:10.33265/polar.v39.3511. ISSN 0800-0395.
35. Plunkett, Gill; Sigl, Michael; McConnell, Joseph R.; Pilcher, Jonathan R.; Chellman, Nathan J. (1 February 2023). "The significance of volcanic ash in Greenland ice cores during the Common Era". Quaternary Science Reviews. 301: 107936. doi:10.1016/j.quascirev.2022.107936. ISSN 0277-3791.
36. Plunkett, Gill; Sigl, Michael; Schwaiger, Hans F.; Tomlinson, Emma L.; Toohey, Matthew; McConnell, Joseph R.; Pilcher, Jonathan R.; Hasegawa, Takeshi; Siebe, Claus (18 January 2022). "No evidence for tephra in Greenland from the historic eruption of Vesuvius in 79 CE: implications for geochronology and paleoclimatology". Climate of the Past. 18 (1): 45–65. doi:10.5194/cp-18-45-2022. ISSN 1814-9324.
37. Morten Axboe (2001). "Året 536". Skalk (4): 28–32.
38. Morten Axboe (1999). "The year 536 and the Scandinavian gold hoards" (PDF). Medieval Archaeology. 43: 186–188.
39. Ström, Folke: Nordisk Hedendom, Studentlitteratur, Lund 2005, ISBN 91-44-00551-2 (first published 1961) among others, refer to the climate change theory.
40. Gunn, Joel D. (2000). The Years Without Summer: Tracing A.D. 536 and its Aftermath. British Archaeological Reports (BAR) International. Oxford, England: Archaeopress. ISBN 978-1-84171-074-7.
41. Dark, Ken (November 1999). . British Archaeology (49). ISSN 1357-4442. Archived from the original on 25 February 2006. Retrieved 14 July 2020.
42. Breeze, Andrew (2020). British Battles 493-937: Mount Badon to Brunanburh. London: Anthem Press. pp. 13–24. doi:10.2307/j.ctvv4187r. ISBN 9781785272233. JSTOR j.ctvv4187r. S2CID 243164764. {{cite book}}: |access-date= requires |url= (help)
43. Gao, Chaochao; Robock, Alan; Self, Stephen; Witter, Jeffrey B.; Steffenson, J. P.; Clausen, Henrik Brink; Siggaard-Andersen, Marie-Louise; Johnsen, Sigfus; Mayewski, Paul A.; Ammann, Caspar (2006). "The 1452 or 1453 A.D. Kuwae Eruption Signal Derived from Multiple Ice Core Records: Greatest Volcanic Sulfate Event of the Past 700 Years" (PDF). Journal of Geophysical Research. 111 (D12107): 11. Bibcode:2006JGRD..11112107G. doi:10.1029/2005JD006710. ISSN 0148-0227.

Further reading

• Arjava, Antti (2006). "The Mystery Cloud of 536 CE in the Mediterranean Sources". Dumbarton Oaks Papers. Vol. 59. Washington, DC: Dumbarton Oaks Research Library and Collection. pp. 73–94.
• Axboe, Morten (2001). "Amulet Pendants and a Darkened Sun". In Bente Magnus (ed.). Roman Gold and the Development of the Early Germanic Kingdoms: Aspects of Technical, Socio-political, Socio-economic, Artistic and Intellectual Development, A.D. 1–500. Almquiest & Wiksell Intl. p. 51. ISBN 978-91-7402-310-7.
• Baillie, M.G.L. (1994). "Dendrochronology Raises Questions About the Nature of the AD 536 Dust-Veil Event". The Holocene. 4 (2): 212–217. Bibcode:1994Holoc...4..212B. doi:10.1177/095968369400400211. S2CID 140595125.
• Baillie, Michael (1995). A Slice Through Time: Dendrochronology and Precision Dating. London: Batsford. p. 93. ISBN 978-0-7134-7654-5.^{[permanent dead link]}
• Büntgen, Ulf; Myglan, Vladimir S.; Ljungqvist, Fredrik Charpentier; McCormick, Michael; Di Cosmo, Nicola; Sigl, Michael; Jungclaus, Johann; Wagner, Sebastian; Krusic, Paul J.; Esper, Jan; Kaplan, Jed O.; de Vaan, Michiel A. C.; Luterbacher, Jürg; Wacker, Lukas; Tegel, Willy; Kirdyanov, Alexander V. (March 2016). "Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD". Nature Geoscience. 9 (3): 231–236. Bibcode:2016NatGe...9..231B. doi:10.1038/ngeo2652.
• Farhat-Holzman, Laina (23 January 2003). . Good Times. GlobalThink.Net Research Papers. Archived from the original on 27 September 2007.
• Gunn, Joel (2000). The Years Without Summer: Tracing A.D. 536 and its Aftermath. British Archaeological Reports (BAR) International. Oxford, England: Archaeopress. ISBN 978-1-84171-074-7.
• Keys, David Patrick (2000). Catastrophe: An Investigation into the Origins of the Modern World. New York: Ballantine Pub. ISBN 978-0-345-40876-1.
• Levy, David (ed.), The Scientific American Book of the Cosmos, ISBN 0-312-25453-9, 2000, (Google Print, p. 186)^{[dead link]}
• Rosen, William (2007). Justinian's Flea: Plague, Empire and the Birth of Europe. London: Jonathan Cape. ISBN 978-0-224-07369-1.
• Salzer, Matthew W.; Hughes, Malcolm K. (January 2007). "Bristlecone pine tree rings and volcanic eruptions over the last 5000 yr". Quaternary Research. 67 (1): 57–68. Bibcode:2007QuRes..67...57S. doi:10.1016/j.yqres.2006.07.004. S2CID 14654597.
• Sigl, M.; Winstrup, M.; McConnell, J. R.; Welten, K. C.; Plunkett, G.; Ludlow, F.; Büntgen, U.; Caffee, M.; Chellman, N.; Dahl-Jensen, D.; Fischer, H.; Kipfstuhl, S.; Kostick, C.; Maselli, O. J.; Mekhaldi, F.; Mulvaney, R.; Muscheler, R.; Pasteris, D. R.; Pilcher, J. R.; Salzer, M.; Schüpbach, S.; Steffensen, J. P.; Vinther, B. M.; Woodruff, T. E. (July 2015). "Timing and climate forcing of volcanic eruptions for the past 2,500 years" (PDF). Nature. 523 (7562): 543–549. Bibcode:2015Natur.523..543S. doi:10.1038/nature14565. PMID 26153860. S2CID 4462058.
• Winchester, Simon (2003). Krakatoa: The Day the World Exploded, August 27, 1883. New York: Harper-Collins. ISBN 978-0-06-621285-2.

External links

• "536 and all that", from Real Climate, March 2008.