Ruddiman's hypothesis

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According to Ruddiman's hypothesis, the global temperature course of the last 7,000 years deviates from the expected natural course due to early human greenhouse gas emissions

The Ruddiman hypothesis (often English early-anthropogenic hypothesis 'hypothesis of early human influence on the climate' , occasionally also English early anthropocene hypothesis 'hypothesis of an early Anthropocene' ) is named after the paleoclimatologist William Ruddiman , who wrote the pre-7000 resp. The slight increase in carbon dioxide and methane concentrations in the earth's atmosphere with human influence , which began 5000 years ago, was explained by early agriculture. In Ruddiman's view, the higher concentrations of these greenhouse gases caused a slight warming of the earth - even before their steep rise with the onset of industrialization and the current global warming - which was sufficient to prevent the onset of the next glacial or, in the context of natural climate change, to delay it significantly .

Greenhouse gas concentrations in the Holocene

CO 2 and CH 4 concentrations in the Holocene
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Ghgs-epcia-holocene-CH4-earlyanthropogenic-de.svg

Measurements in ice cores show in Holozän , the present interglacial (interglacial), for the greenhouse gas carbon dioxide (CO 2 ) and methane (CH 4 ) initially slightly decreasing concentrations until about 7000 years ago (CO 2 to about 20 ppm ) or 5000 years ago (CH 4 , around 100 ppb ) began to rise again slightly.

Various natural processes have been proposed to explain the increased CO 2 concentrations, including the carbonate compensation hypothesis and the coral reef hypothesis . Both explanations are based on a delayed feedback, which would have led to lower concentrations of carbonate ions (CO 3 2− ) in the oceans via an increased deposit of calcium carbonate (CaCO 3 ) . In chemical equilibrium , this feedback is associated with lower CO 2 concentrations in the oceans and higher ones in the atmosphere.

According to William Ruddiman, the concentration and temperature profile since the middle Holocene is unusual compared to earlier interglacials and speaks against natural causes. In 2003 he presented the hypothesis of an early human climate impact through agricultural development . The then increasing deforestation would have released significant amounts of CO 2 , wet rice cultivation and livestock farming would have led to methane emissions. The resulting increase in greenhouse gas concentrations would have led to the deviating temperature profile.

A corollary of Ruddiman's hypothesis is that declining human land use and the associated reforestation significantly reduce CO 2 concentrations. The dramatic population decline in America between 1492 and 1700 by an estimated 90%, as well as the decline triggered by medieval and early modern pandemics in Europe, could explain some of the cooling during the Little Ice Age .

Comparison with previous ice age cycles

Uniform fluctuations in solar radiation, temperatures as well as methane and carbon dioxide concentrations over the last 400,000 years

The Earth's orbit and axis are subject to cyclical fluctuations over a period of millennia, which cause changes in solar radiation and are likely to trigger the change between glacials and interglacials (→ Milanković cycles ).

Ice cores can be used to reconstruct carbon dioxide and methane concentrations over the past several hundred thousand years. In addition, they provide hydrogen isotope ratios δD ( deuterium depletion), which provide information about past temperatures. Oxygen isotope ratios δ18O can be obtained from the sea floor, which can also be used to reconstruct temperatures. In past Ice Age cycles, the concentrations of the two greenhouse gases and temperatures decreased with solar radiation. The course of the curves obtained for the Holocene from an Antarctic ice core ( Dome C of the EPICA project) and worldwide from ocean sediments begins to rise again from the middle Holocene and thus deviates from the curves of the past interglacials.

Interglacials, in which the orbital parameters are particularly close to those of the Holocene, are of particular interest for a comparison. Originally it was assumed that the interglacial of the marine oxygen isotope level 11 (MIS 11) would be most similar to the Holocene. However, it differs significantly in the inclination of the earth's axis to the solar orbit. The MIS 19 level is now considered to be the best equivalent to be found in the last 800,000 years. The climate development in this interglacial period, which was not subject to human influence, led to a cold period with the lower greenhouse gas concentrations at that time. According to some climate simulations , if the greenhouse gas concentrations in the late Holocene had been at a similar level, this would have meant the beginning of the next glacial about 1000 years ago. According to the hypothesis of the early anthropogenic climate impact, this indicates that human activity and not natural factors influenced greenhouse gas concentrations and the course of the climate.

Emissions from early agriculture

methane

Wet rice cultivation, which produces large amounts of methane, began to spread around 5000 years ago

In earlier Ice Age cycles, methane concentrations fluctuated roughly in line with changes in the earth's orbit and axis or in solar radiation. A natural explanation for this are changes in the summer monsoon : when the solar radiative forcing is higher in the north, the land masses of the northern hemisphere warm more, which increases the intensity of the monsoon. In wetlands flooded by monsoon rains, organic material then rots in the absence of oxygen and more methane is released. With the decline in orbital radiative forcing, methane concentrations fell again. The methane concentration developed similarly in the Holocene, until it began to rise again to around 100 ppb above the expected value around 5000 years ago. Around this time wet rice cultivation began, spreading from areas on the Yangtze to around 1000 years ago across all of Southeast Asia . Early wet rice cultivation was likely to be very inefficient and, for the same amount of production, flooded larger areas longer than in the recent past.

In 2011, the archaeologist Dorian Fuller developed a model of the spread of rice cultivation from archaeological and archaeobotanical data. He estimates that this could explain a 70 ppb increase up to 1000 years ago. Other sources of methane are the spreading livestock farming in Asia, Africa and Europe and the burning of weeds and harvested areas. A natural explanation for the methane concentrations could be a stronger monsoons in the Amazon .

The fifth assessment report of the Intergovernmental Panel on Climate Change ( IPCC) concluded in 2013 that human methane emissions “as likely as not” caused a concentration increase of 100 ppb by 1750.

carbon dioxide

Distribution of domesticated plants in the Holocene

The carbon dioxide concentrations, which rose again about 7000 years ago, explain the hypothesis primarily with deforestation as a result of the spreading agriculture in Southwest Asia, China, South Asia, Sub-Saharan Africa, Mexico and in the tropical regions of South America. Sufficient archaeological and paleoecological evidence is available from two regions, Europe and China, to estimate the extent of early deforestation, but this is not the case for other regions. Further uncertainties result from not exactly known natural influences on the carbon cycle , such as fluctuations in the monsoons, the uptake of carbon in permafrost or the contribution from the floodplains and the deltas of rivers. In addition, there are feedbacks in the deforested areas, such as changes in the albedo (the reflectivity of surfaces) and evapotranspiration (water evaporation). A more recent work points to significant influences also in land use that does not change the type of land cover - for example in the grazing of savannas. Substantial amounts of organic and inorganic carbon are stored in the soil. When used, carbon can be released in the same order of magnitude as is the case with loss of vegetation. There is also a need for further research on this.

It is questionable how tens of millions of people who farmed around 7000 years ago could have cultivated a comparatively large area of ​​land and why the emissions did not increase any further later, with further population growth. Assuming constant area per capita, deforestation and carbon emissions are far too low to support Ruddiman's hypothesis. An intensification of agriculture could be the key here: While it was initially very inefficient, the per capita land requirement may later be reduced to a quarter.

Another problem for the hypothesis of early human climate influence is the ratio of the carbon isotopes 13 C and 12 C ( δ13C ) obtained from ice cores . Vegetation is relatively poor at 13 C, so with large-scale deforestation a relatively large amount of 12 C is released and the δ13C value in the atmosphere drops. In fact, however, the δ13C values ​​only decreased slightly in the last 7000 years and indicate low emissions from the vegetation. If one assumes, however, that much larger amounts of the light 12 C carbon isotope than originally assumed were stored in bogs, the δ13C data would be consistent with a rise in CO 2 concentration of 23 ppm due to human emissions.

In its fifth assessment report, the Intergovernmental Panel on Climate Change stated that human land use is probably not the sole cause of the increase in CO 2 concentrations. A more recent analysis from 2017 combined updated values ​​of the amounts of carbon stored in bogs and total amounts of carbon in terrestrial reservoirs with various models of human land use. Your budget considerations for the period 7,000–5,000 years before today suggested the ocean as the primary source of the increase in CO 2 concentration during this period, with human activity probably contributing less than half. In the period 5000–3000 before today, human emissions do not play a major role. The authors suspect that u. a. Loss of vegetation in the drier northern Africa and in high latitudes were decisive. In the period beginning 3,000 years ago and extending until 1,000 years ago, human emissions from land use change will be the dominant source. For the declining CO 2 concentrations after 1500, reforestation due to population declines is probably not the only factor.

Partial hypothesis of the overdue glacial period

The transition from interglacial to cold periods is usually attributed to the reduced summer solar radiation in boreal latitudes of the northern hemisphere due to changes in the Earth's orbit parameters. However, although the current summer solar radiation is close to its minimum, unlike in earlier ice age cycles, there is no sign of the beginning of a cold period. Ruddiman attached the partial hypothesis that without them the earth would be on the way to a glacial path to the human cause of early increases in greenhouse gas concentrations. In the first model calculations, he tested this partial hypothesis and mathematically removed the presumed anthropogenic contribution from the atmosphere. The results suggested that, under these conditions, the climate today would have approximated about one-third glacial conditions.

The fact that the particularly similar Interglacial Level 19 was on its way into the glacial, when it had reached a similar age as now the Holocene, seems to support the hypothesis. According to recent model calculations, the threshold to a cold period before the start of industrialization was barely exceeded. This could be due to the relatively high CO 2 concentrations of the late Holocene, combined with a comparatively low eccentricity of the earth.

Proposal for an early Anthropocene

In 2000, atmospheric chemist Paul Crutzen and biologist Eugene F. Stoermer argued that humanity had evolved into a significant geological force. It is appropriate to speak of a new geological epoch , the " Anthropocene ", which follows the Holocene. The end of the 18th century could be seen as the beginning of the Anthropocene, the time for which the rise in global carbon dioxide and methane concentrations can be detected in ice cores.

Ruddiman's 2003 article The Anthropogenic Greenhouse Era Began Thousands of Years Ago , in which he first formulated his hypothesis, was a response to this proposal. She challenged the assumption that human greenhouse gas emissions only had a permanently identifiable geological impact at the beginning of industrialization. With this in mind, the early trend changes in methane or carbon dioxide concentrations were considered to mark the new era. However, because its anthropogenic cause has not been clarified and there is no clearly identifiable turning point in the case of CO 2 , the proposal is considered unsuitable.

The International Union of Geological Sciences' Anthropocene Working Group concluded that human activity is indeed making its ubiquitous and permanent stamp on the earth. However, it locates the beginning of the new epoch in the middle of the 20th century in the time of the Great Acceleration , the extraordinary acceleration of economic activity and resource consumption. The beginning will be clearly visible u. a. in the rates of modern rise in CO 2 concentrations, which are probably the highest since the beginning of the Cenozoic era 66 million years ago. Methane concentrations have risen steeply since the 18th century and in 2004 they reached more than double what could be reconstructed as the previous maximum from ice cores for the period since the Middle Pleistocene . The marked rise in temperature, mainly caused by fossil fuels, led the climate of the northern hemisphere to the edge of the fluctuating range of the Holocene, in the tropics and in the southern hemisphere probably even beyond.

In August 2016, the working group decided to propose the formalization of the Anthropocene as a separate epoch beginning in the middle of the 20th century. Appropriate markings and type localities are currently being evaluated. Primary and secondary markings based on the radionuclides 239 Pu and 14 C, which were released during atmospheric nuclear weapons tests with hydrogen bombs , of carbon isotopes, the occurrence of “technofossils” or microplastics , nitrogen isotopes , fly ash, heavy metals and various organic compounds are taken into account. On this basis, a formal proposal is to be submitted to the International Commission on Stratigraphy for determination. Ruddiman himself later expressed his disapproval of a formal definition of the Anthropocene. In May 2019, the Working Group on the 'Anthropocene' voted with a clear majority in favor of submitting a draft for the introduction of the Anthropocene to the International Commission on Stratigraphy by 2021 , including a final, geologically defined starting point for the new epoch.

literature

  • William Ruddiman: The Early Anthropogenic Hypothesis . In: Oxford Research Encyclopedia Environmental Science . December 2016, doi : 10.1093 / acrefore / 9780199389414.013.192 .
  • Ruddiman's hypothesis . In: Stephen H. Schneider, Terry L. Root and Michael Mastrandrea (Eds.): Encyclopedia of Climate and Weather . Oxford University Press, 2011, ISBN 978-0-19-976532-4 .
  • Dieter Kasang, Lina Teckentrup and Markus Adloff: Early forest destruction, biodiversity and climate . In: José L. Lozán u. a. (Ed.): Warning signal climate: The biodiversity . 2016 ( warnsignal-klima.de ).
  • William Ruddiman: Plows, Plagues, and Petroleum - How Humans Took Control of Climate . Princeton University Press, 2005, ISBN 978-1-4008-3473-0 (with a popular science account of its hypothesis).

Web links

References and comments

  1. ↑ a b c d e f g h i j William F. Ruddiman, Dorian Q. Fuller, John E. Kutzbach, PC Tzedakis, Jed O. Kaplan, Erle C. Ellis, Stephen J. Vavrus, CN Roberts, R. Fyfe , F. He, C. Lemmen, J. Woodbridge: Late Holocene climate: Natural or anthropogenic? In: Reviews of Geophysics . February 2016, doi : 10.1002 / 2015RG000503 .
  2. ^ Wallace S. Broecker et al. a .: Evidence for a reduction in the carbonate ion content of the deep sea during the course of the Holocene . In: Paleoceanography . October 1999, doi : 10.1029 / 1999PA900038 .
  3. ↑ Andy J. Ridgwell, Andrew J. Watson, Mark A. Maslin and Jed O. Kaplan: Implications of coral reef buildup for the controls on atmospheric CO2 since the Last Glacial Maximum . In: Paleoceanography and Paleoclimatology . October 2003, doi : 10.1029 / 2003PA000893 .
  4. ^ A b William Ruddiman: The Anthropogenic Greenhouse Era Began Thousands of Years Ago . In: Climatic Change . December 2003, doi : 10.1023 / B: CLIM.0000004577.17928.fa .
  5. ↑ On early anthropogenic methane emissions: William F. Ruddiman and Jonathan S. Thomson: The case for human causes of increased atmospheric CH 4 over the last 5000 years . In: Quaternary Science Reviews . December 2001, doi : 10.1016 / S0277-3791 (01) 00067-1 .
  6. ↑ Ruddiman's hypothesis . In: Stephen H. Schneider , Terry L. Root and Michael Mastrandrea (Eds.): Encyclopedia of Climate and Weather . Oxford University Press, 2011, ISBN 978-0-19-976532-4 .
  7. ↑ a b c Stephen J. Vavrus et al. a .: Glacial Inception in Marine Isotope Stage 19: An Orbital Analog for a Natural Holocene Climate . In: Scientific Reports . July 5, 2018, doi : 10.1038 / s41598-018-28419-5 .
  8. ↑ Yin and Berger (2015) came to a less clear result in an earlier work with a medium-resolution climate model. In transient climate simulations of level 19 (MIS 19) with pre-industrial greenhouse gas concentrations of a constant 280 ppm - i.e. only with orbital radiative forcing - towards the end of MIS 19, they received a global temperature slightly below the temperature of the Holocene (MIS 1). Taking into account the reconstructed course of the greenhouse gas concentrations of MIS 19 (which at an optimum of 263 ppm corresponds to that of MIS 1 before the suspected human influence, then sinks below), its simulated global temperature was up to approx. 0.5 ° C below that of the MIS 1. Qiuzhen Yin and André Berger: Interglacial analogues of the Holocene and its natural near future . In: Quaternary Science Reviews . July 2015, p. 42-45 , doi : 10.1016 / j.quascirev.2015.04.008 .
  9. ↑ a b Philippe Ciais, Christopher Sabine a. a .: 6 Carbon and Other Biogeochemical Cycles . In: TF Stocker u. a. (Ed.): Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change . 6.2.2 Greenhouse Gas Changes over the Holocene, p. 468, 483-485 ( ipcc.ch ).
  10. ^ A b Benjamin D. Stocker, Z. Yu and F. Joos: Contrasting CO 2 emissions from different Holocene land-use reconstructions: does the carbon budget add up? In: PAGES Magazine . June 2018, doi : 10.22498 / pages.26.1.6 .
  11. ↑ Karl-Heinz Erb et al: Unexpectedly large impact of forest management and grazing on global vegetation biomass . In: Nature . January 2018, doi : 10.1038 / nature25138 . See also the message: Human impacts on forests and grasslands much larger and older than previously assumed. In: ScienceDaily. December 21, 2017. Retrieved November 8, 2018 .
  12. ↑ Erle C. Ellis: Anthropocene: A Very Short Introduction (=  Very Short Introductions . Volume 558 ). Oxford University Press, 2018, ISBN 978-0-19-879298-7 , pp. 90-94 .
  13. ↑ Jeff Tollefson: The 8,000-year-old climate puzzle . In: Nature . 2011, doi : 10.1038 / news.2011.184 .
  14. ↑ Benjamin David Stocker, Zicheng Yu, Charly Massa and Fortunat Joos: Holocene peatland and ice-core data constraints on the timing and magnitude of CO 2 emissions from past land use . In: Proceedings of the National Academy of Sciences . January 2017, doi : 10.1073 / pnas.1613889114 .
  15. ^ William F. Ruddiman, Stephen J. Vavrus and John E. Kutzbach: A test of the overdue-glaciation hypothesis . In: Quaternary Science Reviews . January 2005, doi : 10.1016 / j.quascirev.2004.07.010 .
  16. ^ Andrey Ganopolski, Ricarda Winkelmann and Hans Joachim Schellnhuber : Critical insolation - CO 2 relation for diagnosing past and future glacial inception . In: Nature . January 2016, doi : 10.1038 / nature16494 .
  17. ^ Paul J. Crutzen and Eugene F. Stoermer: The “Anthropocene” . In: IGBP Global Change Newsletter . No. 41 , May 2000, pp. 17–18 ( igbp.net [PDF; 741 kB ]).
  18. ^ Paul J. Crutzen: Geology of mankind . In: Nature . January 2002, doi : 10.1038 / 415023a .
  19. ↑ Simon L. Lewis and Mark A. Maslin: Defining the Anthropocene . In: Nature . March 2015, p. 174 , doi : 10.1038 / nature14258 .
  20. ↑ Colin N. Waters et al. a .: The Anthropocene is functionally and stratigraphically distinct from the Holocene . In: Science . January 2016, doi : 10.1126 / science.aad2622 .
  21. ↑ Helen C. Bostock and David J. Lowe: Update on the Formalization of the Anthropocene . In: Quaternary Australasia . July 2018 ( org.au ).
  22. ^ William F. Ruddiman: The Early Anthropocene Hypothesis: An Update - Comment # 27. March 24, 2016, accessed on November 2, 2018 : “I am not in favor of a formal geologic definition of the 'anthropocene', which I see as being transgressive both in time and space, building from a slow start many millennia ago. "
  23. ^ William F. Ruddiman: Three flaws in defining a formal 'Anthropocene' . In: Progress in Physical Geography: Earth and Environment . July 13, 2018, doi : 10.1177 / 0309133318783142 .
  24. ↑ Meera Subramanian: Anthropocene now: influential panel votes to recognize Earth's new epoch . In: Nature . May 2019. doi : 10.1038 / d41586-019-01641-5 .
  25. ^ Subcommission on Quaternary Stratigraphy: Working Group on the 'Anthropocene' - Results of binding vote by AWG.