Did the earliest agriculture kick-start global warming

Most climate scientists encourage us to believe that planetary warming caused by gas emission from our energy intensive life style is both new and an inevitable context for our future.  Yet, one leading authority on past climates, William Ruddiman of the University of Virginia, reminds us that it isn’t only cars and power stations that release warming gases (Ruddiman, W.F. 2005.  How did humans first alter global climate. Scientific American, v. 292 March 2005, p. 34-41).  New evidence from air bubbles in the Vostok core through Antarctic ice shows a strange deviation of atmospheric CO2 around 8000 years ago, from a downward trend in the early Holocene to one that relentlessly rises to the levels that characterised the recent pre-industrial world.  At around that time early agriculturalists in Europe and China began to chop down forest to make fields, thereby releasing the carbon content of felled trees to the atmosphere as CO2.  By 5000 years before present, rice cultivation in East Asia had begun the release of methane from waterlogged paddy fields, and the methane content of ice bubbles reveals a reversal of methane decline at that time exactly..  Ruddiman’s view is that the release of both “greenhouse” gases reversed a natural cooling trend, and that growing populations sustained growth in atmospheric CO2 (methane is quickly oxidised in the atmosphere). Comparing the rising CO2 of the Holocene with its records in ice-bubble for the previous three interglacials, shows that in each previous case the gas rose to a maximum early in the interglacials and then declined steadily.  The invention of agriculture and its spread from around 11000 years ago in the Near East, he claims, could have staved off the onset of global cooling and the climatic descent into another glacial epoch, by eventually adding 40 parts per million of CO2 to the air.  To support his hypothesis Ruiddiman compares the more recent ice-core records with historic catastrophes, mainly plagues that wiped out substantial proportions of the word population .  Sure enough, there are falls in CO2 at the time of each major plague; that between 540 to 542 AD in Europe, the Black Death of the Middle Ages, and the reduction of the population of the Americas by maybe 90% when “Old World” diseases such as smallpox and measles met no resistance among native peoples.  In many respects Ruddiman’s ideas seem plausible, until we see the data.  The problem with ice core data is that its resolution degrades through time, and before 70000 years ago, no annual layers are preserved in glacial ice.  Moreover, records from different Antarctic cores differ wildly for the historic period and Ruddiman does not show the record from Greenland ice.  Finally, records of ice volume and ice-cap temperatures, derived from marine and glacial oxygen isotope records, show that each previous interglacial involved very different fluctuations in many other climate-related parameters.  If nothing else, Ruddiman’s  ideas will be challenged and the issue will “run and run” until the next “big thing”.


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