Couplings Between Changes in the Climate System and Biogeochemistry Coordinating Lead Authors: Kenneth L. Denman (Canada), Guy Brasseur (USA, Germany) Lead Authors: Amnat Chidthaisong (Thailand), Philippe Ciais (France), Peter M. Cox (UK), Robert E. Dickinson (USA), Didier Hauglustaine (France), Christoph Heinze (Norway, Germany), Elisabeth Holland (USA), Daniel Jacob (USA, France), Ulrike Lohmann (Switzerland), Srikanthan Ramachandran (India), Pedro Leite da Silva Dias (Brazil), Steven C. Wofsy (USA), Xiaoye Zhang (China) Contributing Authors: D. Archer (USA), V. Arora (Canada), J. Austin (USA), D. Baker (USA), J.A. Berry (USA), R. Betts (UK), G. Bonan (USA), P. Bousquet (France), J. Canadell (Australia), J. Christian (Canada), D.A. Clark (USA), M. Dameris (Germany), F. Dentener (EU), D. Easterling (USA), V. Eyring (Germany), J. Feichter (Germany), P. Friedlingstein (France, Belgium), I. Fung (USA), S. Fuzzi (Italy), S. Gong (Canada), N. Gruber (USA, Switzerland), A. Guenther (USA), K. Gurney (USA), A. Henderson-Sellers (Switzerland), J. House (UK), A. Jones (UK), C. Jones (UK), B. Karcher (Germany), M. Kawamiya (Japan), K. Lassey (New Zealand), C. Le Quere (UK, France, Canada), C. Leck (Sweden), J. Lee-Taylor (USA, UK), Y. Malhi (UK), K. Masarie (USA), G. McFiggans (UK), S. Menon (USA), J.B. Miller (USA), P. Peylin (France), A. Pitman (Australia), J. Quaas (Germany), M. Raupach (Australia), P. Rayner (France), G. Rehder (Germany), U. Riebesell (Germany), C. Rodenbeck (Germany), L. Rotstayn (Australia), N. Roulet (Canada), C. Sabine (USA), M.G. Schultz (Germany), M. Schulz (France, Germany), S.E. Schwartz (USA), W. Steffen (Australia), D. Stevenson (UK), Y. Tian (USA, China), K.E. Trenberth (USA), T. Van Noije (Netherlands), O. Wild (Japan, UK), T. Zhang (USA, China), L. Zhou (USA, China) Review Editors: Kansri Boonpragob (Thailand), Martin Heimann (Germany, Switzerland), Mario Molina (USA, Mexico) This chapter should be cited as: Denman, K.L., G. Brasseur, A. Chidthaisong, P. Ciais, P.M. Cox, R.E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S Ramachandran, P.L. da Silva Dias, S.C. Wofsy and X. Zhang, 2007: Couplings Between Changes in the Climate System and Biogeochemistry. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

/pdf/couplings-between-changes-in-the-climate-system-and-s80hl32kar.pdf

Couplings between changes in the climate system and biogeochemistry

http://ruby.fgcu.edu/courses/twimberley/envirophilo/wanner.pdf

Mid- to Late Holocene climate change: an overview

The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere. A different hypothesis is posed here: anthropogenic emissions of these gases first altered atmospheric concentrations thousands of years ago. This hypothesis is based on three arguments. (1) Cyclic variations in CO2 and CH4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO2 trend began an anomalous increase 8000 years ago, and the CH4 trend did so 5000 years ago. (2) Published explanations for these mid- to late-Holocene gas increases based on natural forcing can be rejected based on paleocli- matic evidence. (3) A wide array of archeological, cultural, historical and geologic evidence points to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia, including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago. In recent millennia, the estimated warming caused by these early gas emissions reached a global-mean value of ∼0.8 ◦ C and roughly 2 ◦ C at high latitudes, large enough to have stopped a glaciation of northeastern Canada predicted by two kinds of climatic models. CO2 oscillations of ∼10 ppm in the last 1000 years are too large to be explained by external (solar-volcanic) forcing, but they can be explained by outbreaks of bubonic plague that caused historically documented farm abandonment in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for the observed CO2 decreases. Plague-driven CO2 changes were also a significant causal factor in temperature changes during the Little Ice Age (1300-1900 AD).

https://courses.washington.edu/holocene/Ruddiman-Holocene_Carbon_Cycle_Anthropocene-ClimChange03.pdf

The Anthropogenic Greenhouse Era Began Thousands of Years Ago

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.

/pdf/abrupt-climate-change-5bwhk8ky17.pdf

Abrupt climate change

A precise relative chronology for Greenland and West Antarctic paleotemperature is extended to 90,000 years ago, based on correlation of atmospheric methane records from the Greenland Ice Sheet Project 2 and Byrd ice cores. Over this period, the onset of seven major millennial-scale warmings in Antarctica preceded the onset of Greenland warmings by 1500 to 3000 years. In general, Antarctic temperatures increased gradually while Greenland temperatures were decreasing or constant, and the termination of Antarctic warming was apparently coincident with the onset of rapid warming in Greenland. This pattern provides further evidence for the operation of a “bipolar see-saw” in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales.

http://science.sciencemag.org/content/sci/291/5501/109.full.pdf

Timing of Millennial-Scale Climate Change in Antarctica and Greenland During the Last Glacial Period

We present high resolution records of atmospheric methane from the GISP2 (Greenland Ice Sheet Project 2) ice core for four rapid climate transitions that occurred during the past 50 ka: the end of the Younger Dryas at 11.8 ka, the beginning of the Bolling-Allerod period at 14.8 ka, the beginning of interstadial 8 at 38.2 ka, and the beginning of interstadial 12 at 45.5 ka. During these events, atmospheric methane concentrations increased by 200–300 ppb over time periods of 100–300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large-scale decomposition of methane hydrates in sediments. With additional results from the Taylor Dome Ice Core (Antarctica) we also reconstruct changes in the interpolar methane gradient (an indicator of the geographical distribution of methane sources) associated with some of the rapid changes in atmospheric methane. The results indicate that the rise in methane at the beginning of the Bolling-Allerod period and the later rise at the end of the Younger Dryas were driven by increases in both tropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold period during the last deglaciation) the relative contribution from boreal sources was reduced relative to the early and middle Holocene periods.

/pdf/on-the-origin-and-timing-of-rapid-changes-in-atmospheric-3cnpxx5irz.pdf

On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period

Atmospheric Methane and Millennial‐Scale Climate Change

[1] We re-examine the link between July 30°N insolation and methane in the Vostok ice core. Based on this link, Ruddiman [2003] suggested that an anthropogenic source of methane must have been present after 5 kyr BP in order to prevent concentrations from declining as insolation decreased through the Holocene. We conclude however, that since precessional forcing does not explain the large glacial-interglacial excursions, the component of methane variability associated with precession is significantly smaller than assumed by Ruddiman [2003]. The implied decrease from 10 kyr BP to the present associated with precession alone is 60 ppbv or less. We argue that increased emissions controlled by northern wetlands and river delta development likely contributed to the observed increase, consistent with similar stable CH4 levels at MIS 11 in the Vostok and Dome C records. Therefore a significant anthropogenic input is not obviously required to explain the late Holocene record.

/pdf/a-note-on-the-relationship-between-ice-core-methane-pyvrrp7639.pdf

A note on the relationship between ice core methane concentrations and insolation

[1] Ice core records show atmospheric methane mixing ratio and interpolar gradient varying with climate. Changes in wetland sources have been implicated as the basis for this observed variation in the record, but more recently, modeling studies suggest that changes in the CH4 sink resulting from changes in sea surface temperature (SST) and emissions of other volatile organic carbon (VOC) compounds by vegetation must also be considered. We use the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) with the GISS Tropospheric Chemistry Model to study the response of the methane mixing ratio to source changes during the Holocene and also to a changing chemical sink during glacial-interglacial transitions. We combine model results with ice core data to demonstrate a method that provides constraints on changes in northern and tropical methane sources. Results show that within the Holocene, changes in the atmospheric methane mixing ratio and latitudinal gradient are not linear with respect to changing methane emissions. Tropical and northern emissions varied from preindustrial levels by as much as 38% and 15%, respectively, within the Holocene. At glacial-interglacial transitions the methane mixing ratio is sensitive to changes in both VOC and tropical methane emissions and the sensitivity also depends strongly on the assumed SST shift. Our findings suggest that changes in the ice core methane record are likely the result of changes in both the source and the sink. Changes in the sink become especially important when changes in methane mixing ratio and/or climate are large.

/pdf/a-global-climate-model-study-of-ch4-emissions-during-the-1770qdani1.pdf

A global climate model study of CH4 emissions during the Holocene and glacial-interglacial transitions constrained by ice core data

[1] Schmidt et al. [2004] pointed out that linear correlations between CH4 and insolation used by Ruddiman [2003] to project CH4 trends over the Holocene were not appropriate. We note that this principal point of our paper is not disputed by Ruddiman [2005b]. Instead, Ruddiman’s comment uses an analogy with the end of Stage 11 to support his hypothesis: an argument that is not made by Ruddiman [2003], although it is made by Ruddiman [2005a] and was briefly addressed by Schmidt et al. [2004]. This analogy depends on one principle issue: the association of uncertain timing of the end of Stage 11 in Vostok, EPICA Dome C and ocean sediment cores [McManus et al., 2003] to the (well-dated) changes in insolation (the implicit link between Figures 1a and 1b by Ruddiman [2005b]). We maintain that dating uncertainties preclude such a strong identification, while Ruddiman does not. We do not dispute the size of CH4 changes at glacial inception, only whether glacial inception was to be expected in the Holocene. Much of the response to Ruddiman’s comment is already clearly covered by Schmidt et al. [2004], but we nevertheless address his four points in turn. [2] Ruddiman appears to overinterpret the quality of the dating for Vostok and EPICA Dome C. Petit et al. [1999] use an orbital control point at 390 kyr and estimate an error of 15 kyr. Delmotte et al. [2004] also suggests an error of at least 10 kyr in Vostok. Similarly the EPICA dating does not claim an absolute accuracy of better than 10 to 12 kyr (J. Jouzel, personal communication). The Bender [2002] timescale also has a control point at 385 kyr which, even if correct, still leaves some uncertainty over the older part. The key point is that given the uncertainties of dating, all that can be said with certainty is that Stage 11 extended over more than one and half precessional cycles [McManus et al., 2003]. That is, for at least one insolation decrease at a period of low eccentricity there was no concomitant change in CH4 or ice volume. This counter-example stands in clear contrast to Ruddiman’s hypothesis. [3] Ruddiman’s point is based on a misreading of the analysis of Chappellaz et al. [1997]. They do not find the interpolar CH4 gradient decreasing during the last 4,000 years. For the entire period 5–2.5 kyr BP, Chappellaz et al. [1997, Figure 4] calculate the average gradient and state that during the Holocene the largest difference between Greenland and Antarctica is observed during this time. They go on to say (p. 15,994), ‘‘Our model suggests, as an explanation, a concomitant decrease of the tropical source and an increase of the boreal source.’’ Only after 2.5 kyr BP can it be argued that the gradient decreases, though they do not calculate and model another gradient until the period spanning 1–0.25 kyr BP. This difference is smaller and does suggest an increase in tropical emissions as the source for the CH4 increase at this later time. [4] River deltas have increased in scope globally in the late Holocene and there is no obvious anomaly associated with any hypothesised (and as yet completely unquantified) anthropogenic contributions to river deltas in Eurasia compared with those elsewhere. [5] We are a little puzzled that Ruddiman appears not to have noticed the multiple statements in our paper where we state explicitly that we do not think that CH4 can be purely linked to precessional insolation. However, it was because Ruddiman [2003, Figure 1] implicitly does that prompted our note in the first place. [6] In summary, we continue to maintain that in the absence of further studies ruling out boreal wetlands, tropical river deltas and peat lands as sources of the late Holocene increase in CH4 emissions, a definitive attribution [Ruddiman, 2005b] of this trend to anthropogenic sources is premature.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005GL022982

Susan Harder

Papers

On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period

Atmospheric Methane and Millennial‐Scale Climate Change

A note on the relationship between ice core methane concentrations and insolation

A global climate model study of CH4 emissions during the Holocene and glacial-interglacial transitions constrained by ice core data

Reply to comment by W. F. Ruddiman on “A note on the relationship between ice core methane concentrations and insolation”