Preindustrial Holocene Research Articles

Reconstruction of past climate conditions over central Europe from groundwater data

Here we present a 30 000 years low-resolution climate record reconstructed from groundwater data. The investigated site is located in the Bohemian Cretaceous Basin, in the corridor between the Scandinavian ice sheet and the Alpine ice field. Noble gas temperatures (NGT), obtained from groundwater data, preserved multicentennial temperature variability and indicated a cooling of at least 5–7 °C during the last glacial maximum (LGM). This is further confirmed by the depleted δ 18O and δ 2H values at the LGM. High excess air (ΔNe) at the end of the Pleistocene is possibly related to abrupt changes in recharge dynamics due to progression and retreat of ice covers and permafrost. These results agree with the fact that during the LGM permafrost and small glaciers developed in the inner valleys of the Giant Mountains (located in the watershed of the aquifers). A temporal decrease of deuterium excess from the pre-industrial Holocene to present days is linked to an increase of the air temperatures, and probably also to an increase of water pressure at the source region of precipitation over the past few hundred years.

Multidecadal variability of atmospheric methane, 1000–1800 C.E.

[1] We present a new high-precision, high-resolution record of atmospheric methane from the West Antarctic Ice Sheet (WAIS) Divide ice core covering 1000–1800 C.E., a time period known as the late preindustrial Holocene (LPIH). The results are consistent with previous measurements from the Law Dome ice core, the only other high-resolution record of methane for this time period, and confirm most of the observed variability. Multidecadal variability in methane concentrations throughout the LPIH is weakly correlated or uncorrelated with reconstructions of temperature and precipitation from a variety of geographic regions. Correlations with temperature are dominated by changes in Northern Hemisphere high latitude temperatures between 1400 and 1600 C.E. during the onset of the Little Ice Age. Times of war and plague when large population losses could have reduced anthropogenic emissions are coincident with short periods of decreasing global methane concentrations.

Global vegetation and terrestrial carbon cycle changes after the last ice age

• In current models, the ecophysiological effects of CO₂ create both woody thickening and terrestrial carbon uptake, as observed now, and forest cover and terrestrial carbon storage increases that took place after the last glacial maximum (LGM). Here, we aimed to assess the realism of modelled vegetation and carbon storage changes between LGM and the pre-industrial Holocene (PIH). • We applied Land Processes and eXchanges (LPX), a dynamic global vegetation model (DGVM), with lowered CO₂ and LGM climate anomalies from the Palaeoclimate Modelling Intercomparison Project (PMIP II), and compared the model results with palaeodata. • Modelled global gross primary production was reduced by 27-36% and carbon storage by 550-694 Pg C compared with PIH. Comparable reductions have been estimated from stable isotopes. The modelled areal reduction of forests is broadly consistent with pollen records. Despite reduced productivity and biomass, tropical forests accounted for a greater proportion of modelled land carbon storage at LGM (28-32%) than at PIH (25%). • The agreement between palaeodata and model results for LGM is consistent with the hypothesis that the ecophysiological effects of CO₂ influence tree-grass competition and vegetation productivity, and suggests that these effects are also at work today.

Carbon cycling in tropical ecosystems

The diverse assemblage of ecosystems in tropical regions of the Earth holds a large fraction of the terrestrial biosphere’s carbon (C) stock (Bonan, 2008), and the annual exchange between tropical ecosystems (plants and soils) and the atmosphere is a critical controller of the CO2 concentration of the atmosphere, and hence of climate. Large-scale changes in the structure and function of tropical ecosystems, whether from the pressures of development or the impacts of drought (Meir & Woodward, 2010), can alter the balance in the annual exchange of carbon with far-reaching implications for the pace of climate change. Global models that couple the Earth’s climate system to the C cycle must, therefore, characterize well the biogeochemical and ecophysiological processes of tropical ecosystems and their sensitivity to atmospheric and climatic change. However, major uncertainties about fundamental C cycle processes in tropical ecosystems continue to hinder our progress (Dolman et al., 2010). Diverse aspects of C cycling in tropical ecosystems – from plant physiology and plant–soil interactions to human interactions and regional and global analyses – were discussed at the 23rd New Phytologist Symposium, ‘Carbon cycling in tropical ecosystems’, convened in Guangzhou, China, in November 2009, and summarized by Douglas Schaefer and Matthew Warren (http://www.newphytologist.org/carbon/23rdreport.pdf). In this issue of New Phytologist, we present four papers that emerged from that symposium. They cover the full range of topics discussed – plant physiology, soil processes, human interactions and global analysis – and, from their different perspectives, they all present analyses of C uptake, storage and release in tropical ecosystems. Hättenschwiler et al. (this issue, pp. 950–965) review the literature on leaf litter chemistry in a nutrient-poor Amazonian rainforest. They start with the recognition that, although several large-scale decomposition experiments have been invaluable for parameterizing biogeochemical and global C models, they generally do not include consideration of the influence of soil meso- and macrofauna, which may be particularly important in wet tropical ecosystems, and they often do not incorporate local variation in leaf traits, which are especially important in highly diverse tropical rainforests. Their analysis challenges the commonly held view that litter decomposition in the tropics is fast – decomposition of native litter at their site in French Guiana, and elsewhere in the tropics as shown in a literature survey, was comparatively slow. The authors speculate that natural selection favoured a leaf litter trait syndrome that leads to starvation and inhibition of decomposers, thereby altering competitive dynamics for limiting nutrients, particularly phosphorus. Salinas et al. (this issue, pp. 967–977) carried out an ambitious and comprehensive litter translocation experiment to evaluate the influence of environmental factors on decay rates of multiple species. They conducted their experiment over an elevational gradient from the tropical Andes in Peru to the adjacent Amazon lowlands in what was probably the largest-scale tropical leaf decomposition study to date. Despite the logistical challenges, this gradient had the advantages of the consistently high annual rainfall and the absence of substantial seasonality in temperature and a dormant season that would confound interpretation. They found that soil temperature explained most of the variation in decomposition of a variety of species, and concluded that recent warming in the region may have increased decomposition and nutrient mineralization by c. 10%. This increase in litter mineralization, coupled with rising atmospheric CO2, may partially explain accelerated growth and increased biomass in lowland Amazon forests. Ruiz-Jaen & Potvin (this issue, pp. 978–987) explored whether functional trait diversity or species diversity inform analyses of tree C storage in a mixed-species plantation and a natural forest in Panama. They were motivated by the potential value of using information on functional traits from different natural forests to design tree plantations for C sequestration. They concluded that the C storage capacity of natural forests could not be predicted using data from experimental plantations and, conversely, that forest managers need to be cautious when applying functional traits measured in natural populations when they design plantations for C management. Prentice et al. (this issue, pp. 988–998) approached C cycling questions from a very different temporal and spatial perspective. They assessed the realism of models of the changes in vegetation distribution and C cycling between the last glacial maximum (LGM, c. 21 000 yr ago) to the pre-industrial Holocene (PIH) using stable isotope and pollen data. They concluded that tropical forests accounted for a greater proportion of land C storage at LGM than PIH. Both model results and palaeodata supported the hypothesis that ecophysiological effects of atmospheric CO2 concentration influence tree–grass competition and plant productivity – effects that are assumed still to be in play today.

Holocene carbon emissions as a result of anthropogenic land cover change

Humans have altered the Earth’s land surface since the Paleolithic mainly by clearing woody vegetation first to improve hunting and gathering opportunities, and later to provide agricultural cropland. In the Holocene, agriculture was established on nearly all continents and led to widespread modification of terrestrial ecosystems. To quantify the role that humans played in the global carbon cycle over the Holocene, we developed a new, annually resolved inventory of anthropogenic land cover change from 8000 years ago to the beginning of large-scale industrialization (ad 1850). This inventory is based on a simple relationship between population and land use observed in several European countries over preindustrial time. Using this data set, and an alternative scenario based on the HYDE 3.1 land use data base, we forced the LPJ dynamic global vegetation model in a series of continuous simulations to evaluate the impacts of humans on terrestrial carbon storage during the preindustrial Holocene. Our model setup allowed us to quantify the importance of land degradation caused by repeated episodes of land use followed by abandonment. By 3 ka BP, cumulative carbon emissions caused by anthropogenic land cover change in our new scenario ranged between 84 and 102 Pg, translating to c. 7 ppm of atmospheric CO2. By ad 1850, emissions were 325–357 Pg in the new scenario, in contrast to 137–189 Pg when driven by HYDE. Regional events that resulted in local emissions or uptake of carbon were often balanced by contrasting patterns in other parts of the world. While we cannot close the carbon budget in the current study, simulated cumulative anthropogenic emissions over the preindustrial Holocene are consistent with the ice core record of atmospheric δ13CO2 and support the hypothesis that anthropogenic activities led to the stabilization of atmospheric CO2 concentrations at a level that made the world substantially warmer than it otherwise would be.

Changes in atmospheric CO 2 and its carbon isotopic ratio during the penultimate deglaciation

The largest natural increases in atmospheric CO 2 concentration as recorded in ice cores occur when the Earth climate abruptly shifts from a glacial to an interglacial state. Open questions remain regarding the processes at play, the sequences of events and their similarities along different glacial–interglacial transitions. Here we provide new combined data of atmospheric CO 2 and its carbon isotopic ratio (δ 13CO 2) for the penultimate glacial–interglacial transition (Termination II) from the Antarctic EPICA Dome C ice core. Together with the strongest Antarctic warming, this transition bears the largest CO 2 increase (104 ppmv) of the last nine Terminations, ending with an overshoot of 21 ppmv occurring within ∼300 y and leading to higher levels than those of the late pre-industrial Holocene. The full CO 2 rise is accompanied by an overall decrease of the δ 13CO 2 minimum values, on which three positive excursions are superimposed. Peak-to-peak δ 13CO 2 changes in our record can reach ∼1‰. The ice core atmospheric δ 13CO 2 appears more depleted by ∼0.2‰ during Termination II compared to Termination I, paralleling a similar carbon isotopic depletion recorded in marine data. During both terminations, most of CO 2 and δ 13CO 2 variations are attributed to southern ocean stratification breakdown and decreased efficiency of the biological pump. Compared to Termination I, Termination II ice core data point to different timings of decrease in iron supply and sea-ice extent, suggesting that they could account for distinct patterns of the carbon cycle.

Evaluating source, chemistry and climate change based upon the isotopic composition of nitrate in ice cores

A clear negative trend is found in the nitrogen isotopic ratios (15N/14N) of nitrate over the industrial period, based on a 100-meter ice core from Summit, Greenland. This record indicates that isotopic composition of ice core nitrate reflects changes in nitrogen oxide (NOx) source emissions and that post-industrial emissions of NOx have resulted in a 12‰ (vs. air N2) decline in δ15N of atmospheric nitrate over the last ~300 years. Interestingly, over the last glacial period (as recorded in the Greenland GISP2 ice core), the δ15N of nitrate changes by ~20‰ from a typical pre-industrial Holocene value of 10‰ to a mean glacial value of 28‰, despite the lack of a significant change in nitrate concentration. The more recent ice core record clearly indicates an influence of NOx emission sources, therefore suggesting that the glacial-interglacial change in δ15N may be a record of significant variations in the contribution of NOx sources, such as lightning, biomass burning, biogenic soil emissions and oxidation of nitrous oxide in the stratosphere. In contrast to the source changes recorded by the nitrogen isotopes, the oxygen isotopic record of atmospheric nitrate (18O/16O, 17O/16O) in ice cores has implications for reconstruction of past atmospheric oxidant levels. This is because the oxygen isotopic composition of nitrate reflects oxidation of NOx to nitrate in the atmosphere by oxidants such as ozone and hydroxy and peroxy radicals. Measurements of variations in the isotopic composition of atmospheric nitrate inferred from ice cores could be used to improve interpretation of other records of δ15N such as those from tree rings and/or sediments.

Glacial–interglacial and millennial-scale variations in the atmospheric nitrous oxide concentration during the last 800,000 years

We present records of atmospheric nitrous oxide obtained from the ice cores of the European Project for Ice Coring in Antarctica (EPICA) Dome C and Dronning Maud Land sites shedding light on the concentration of this greenhouse gas on glacial–interglacial and millennial time scales. The extended EPICA Dome C record covers now all interglacials of the last 800,000 years and reveals nitrous oxide variations in concert with climate. Highest mean interglacial nitrous oxide concentrations of 280 parts per billion by volume are observed during the interglacial corresponding to Marine Isotope Stage 11 around 400,000 years before present, at the same time when carbon dioxide and methane reach maximum mean interglacial concentrations. The temperature reconstruction at Dome C indicates colder interglacials between 800,000 and 440,000 years before present compared to the interglacials of the last 440,000 years. In contrast to carbon dioxide and methane, which both respond with lower concentrations at lower temperatures, nitrous oxide shows mean interglacial concentrations of 4–19 parts per billion by volume higher than the preindustrial Holocene value during the interglacials corresponding to Marine Isotope Stage 9–19. At the end of most interglacials, nitrous oxide remains substantially longer on interglacial levels than methane. Nevertheless, nitrous oxide shows millennial-scale variations at the same time as methane throughout the last 800,000 years. We suggest that these millennial-scale variations have been driven by a similar mechanism as the Dansgaard/Oeschger events known from the last glacial. Our data lead to the hypothesis that emissions from the low latitudes drive past variations of the atmospheric nitrous oxide concentration.

Potential glacial‐interglacial changes in stable carbon isotope ratios of methane sources and sink fractionation

Past atmospheric methane emissions can be constrained by δ13CH4 records from ice cores only if changes to source δ13CH4 signatures and sink isotope effects with varying environmental and climatic conditions are accurately known. We present reconstructions of such changes based on paleodata and recent systems observations. The results are specific for budget scenarios and are reported here for two alternative types of budgets, one including aerobic methane emissions (AMP) from plants and the other type without AMP. Shifting atmospheric δ13CO2 potentially led to 13CH4 enrichment by 0.8‰ in the preindustrial Holocene (PIH) (∼150–11,000 years (a) B.P.) and ∼0.3–0.6‰ at the Last Glacial Maximum (LGM) (∼18,000 a B.P.) relative to today. Differing distribution of C3 and C4 plant precursor material may account for 13CH4 enrichment of ∼0.4‰ (PIH) and ∼0.6–1.1‰ (LGM). Temperature‐dependent fractionation and varying methanogenic pathways in wetlands may lead to atmospheric 13CH4 depletion by ∼0.1–1.2‰. Sink fractionation today (7.4‰) is higher than during the PIH (∼7.0‰) and the LGM (∼5.7‰). The cumulative effect of all processes is ∼0.8‰ 13CH4 enrichment in the PIH and ∼1–1.2‰ 13CH4 depletion at the LGM. Budget reconstructions will be inaccurate if these changes are not included.

Holocene organic carbon burial rates in the southeastern Swedish Baltic Sea

To estimate Holocene changes in organic carbon mass accumulation rates (Corg MARs) in the southeastern Swedish Baltic Sea, two sediment cores were studied in combination with biogeochemical modelling. Prior to 11 300 cal. BP, Corg MARs were extremely low, indicating low organic matter production on the catchment of the Baltic Ice Lake. Following a brief regression, the Ancylus Lake stage occurred between 11 100 and 9800 cal. BP. Corg MARs increased substantially during this period because of enhanced washing in of terrestrial organic matter, when boreal forests were initially established. The prominent marine stage, known as the Littorina transgression between 8500 and 3000 cal. BP, is marked by a minor increase in Corg MARs. Our modelling reveals a changing terrestrial organic carbon input between 100 and 1000 g/m2 per yr that accounts for 30—80% of total organic carbon in sediments of the southeastern Swedish Baltic Sea, with maximum values ( c. 20 × 103g/m2 per yr) occurring during the Bronze/Iron Age transition at about 3000 cal. BP. Corg MAR in the entire Baltic basin is estimated at 3.01 × 106 t/yr during the pre-industrial Holocene, comparable with other large inland water bodies. Regardless of the source of carbon, our data indicate that the Baltic basin is an important sedimentary reservoir for organic carbon storage and thus should be included in accounting for global terrestrial carbon cycling during the pre-industrial Holocene.

Constraining past global tropospheric methane budgets with carbon and hydrogen isotope ratios in ice

Upon closer inspection, the classical view of the synchronous relationship between tropospheric methane mixing ratio and Greenland temperature observed in ice samples reveals clearly discernable variations in the magnitude of this response during the Late Pleistocene (<50kyr BP). During the Holocene this relationship appears to decouple, indicating that other factors have modulated the methane budget in the past 10kyr BP. The delta13CH4 and deltaD-CH4 of tropospheric methane recorded in ice samples provide a useful constraint on the palaeomethane budget estimations. Anticipated changes in palaeoenvironmental conditions are recorded as changes in the isotope signals of the methane precursors, which are then translated into past global delta13CH4 and deltaD-CH4 signatures. We present the first methane budgets for the late glacial period that are constrained by dual stable isotopes. The overall isotope variations indicate that the Younger Dryas (YD) and Preindustrial Holocene have methane that is 13C- and 2H-enriched, relative to Modern. The shift is small for delta13CH4 (approx. 1 per thousand) but greater for deltaD-CH4 (approx. 9 per thousand). The YD delta13CH4-deltaD-CH4 record shows a remarkable relationship between them from 12.15 to 11.52kyr BP. The corresponding C- and H-isotope mass balances possibly indicate fluctuating emissions of thermogenic gas. This delta13CH4-deltaD-CH4 relationship breaks down during the YD-Preboreal transition. In both age cases, catastrophic releases of hydrates with Archaeal isotope signatures can be ruled out. Thermogenic clathrate releases are possible during the YD period, but so are conventional natural gas seepages.

Methane and nitrous oxide in the ice core record

Polar ice cores contain, in trapped air bubbles, an archive of the concentrations of stable atmospheric gases. Of the major non-CO2 greenhouse gases, methane is measured quite routinely, while nitrous oxide is more challenging, with some artefacts occurring in the ice and so far limited interpretation. In the recent past, the ice cores provide the only direct measure of the changes that have occurred during the industrial period; they show that the current concentration of methane in the atmosphere is far outside the range experienced in the last 650,000 years; nitrous oxide is also elevated above its natural levels. There is controversy about whether changes in the pre-industrial Holocene are natural or anthropogenic in origin. Changes in wetland emissions are generally cited as the main cause of the large glacial-interglacial change in methane. However, changing sinks must also be considered, and the impact of possible newly described sources evaluated. Recent isotopic data appear to finally rule out any major impact of clathrate releases on methane at these time-scales. Any explanation must take into account that, at the rapid Dansgaard-Oeschger warmings of the last glacial period, methane rose by around half its glacial-interglacial range in only a few decades. The recent EPICA Dome C (Antarctica) record shows that methane tracked climate over the last 650,000 years, with lower methane concentrations in glacials than interglacials, and lower concentrations in cooler interglacials than in warmer ones. Nitrous oxide also shows Dansgaard-Oeschger and glacial-interglacial periodicity, but the pattern is less clear.

Role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations

Recent analyses of ice core methane concentrations suggested that methane emissions from wetlands were the primary driver for prehistoric changes in atmospheric methane. However, these interpretations conflict as to the location of wetlands, magnitude of emissions, and the environmental controls on methane oxidation. The flux of other reactive trace gases to the atmosphere also controls apparent atmospheric methane concentrations because these compounds compete for the hydroxyl radical (OH), which is the primary atmospheric sink for methane. In a series of linked biosphere‐atmosphere chemistry‐climate modeling experiments, we simulate the methane and biogenic volatile organic compound emissions from the terrestrial biosphere from the Last Glacial Maximum (LGM) to the present. Using a state‐of‐the‐art chemistry‐climate model, we simulate the atmospheric concentrations of methane, OH, and other reactive trace gas species. Over the past 21,000 years, methane emissions from wetlands increased slightly to the end of the Pleistocene but then decreased again, reaching levels at the preindustrial Holocene that were similar to the LGM. Global wetland area decreased by 14% from LGM to the preindustrial time. Emissions of biogenic volatile organic compounds (BVOCs), however, nearly doubled over the same period of time. Atmospheric OH burdens and methane concentrations were affected by this major change in BVOC emissions, with methane lifetimes increasing by more than 2 years from LGM to the present. We simulate a change in methane concentration of ∼385 ppb, accounting for 88% of the ∼440 ppb increase in methane concentrations observed in ice cores. Thus glacial‐interglacial changes in atmospheric methane concentrations would have been modulated by BVOC emissions. In addition, the increase in atmospheric methane concentrations since the mid‐Holocene is partly caused in our results by the increases in anthropogenic methane emissions over this period. While the interplay between BVOC and wetland methane emissions since the LGM cannot explain the entire record of ice core methane concentrations, consideration of BVOC source dynamics is central to understanding ice core methane. Rapid changes in atmospheric methane concentrations, also observed in ice cores, require further study.

Unexpected Changes to the Global Methane Budget over the Past 2000 Years

We report a 2000-year Antarctic ice-core record of stable carbon isotope measurements in atmospheric methane (delta13CH4). Large delta13CH4 variations indicate that the methane budget varied unexpectedly during the late preindustrial Holocene (circa 0 to 1700 A.D.). During the first thousand years (0 to 1000 A.D.), delta13CH4 was at least 2 per mil enriched compared to expected values, and during the following 700 years, an about 2 per mil depletion occurred. Our modeled methane source partitioning implies that biomass burning emissions were high from 0 to 1000 A.D. but reduced by almost approximately 40% over the next 700 years. We suggest that both human activities and natural climate change influenced preindustrial biomass burning emissions and that these emissions have been previously understated in late preindustrial Holocene methane budget research.

East Antarctic ice core sulfur isotope measurements over a complete glacial‐interglacial cycle

Both sulfur and oxygen isotopes of sulfate preserved in ice cores from Greenland and Antarctica have provided information on the relative sources of sulfate in the ice and their chemical transformation pathways in the atmosphere over various time periods. The mass‐independent fractionation in the oxygen isotopes of sulfate from the Vostok ice core from east Antarctica suggests that gas‐phase oxidation by the hydroxyl radical (OH) was relatively greater than aqueous‐phase oxidation by O3 and H2O2 during the last glacial period than during the Eemian and preindustrial Holocene. The complete sulfur isotopic composition (δ33S, δ34S, δ36S) from the same Vostok ice core samples along with δ34S measurements from the Dome C, east Antarctic ice core from this study lend support to these conclusions and reveal significant isotopic fractionation of δ34S during chemical transformation and transport to east Antarctica. These findings reveal that conservation of sulfur isotopic signatures upon transport cannot be assumed for the East Antarctic plateau over the time periods considered.

Mineral aerosols: a comparison of the last glacial maximum and preindustrial Holocene

A passive mineral dust aerosol model based on source strengths deduced from polar ice core dust concentrations is introduced into the Canadian Centre for Climate Modelling and Analysis (CCCma) second-generation atmospheric general circulation model (GCMII) and used to compare features of the fine particle mineral dust aerosol in a last glacial maximum (LGM) simulation to those of a preindustrial Holocene (MOD) dust simulation. The resulting dust optical thickness is 8-16 times greater over most of the globe during the LGM. The model displays several seasonal characteristics observed in present-day satellite observations of dust, such as the summer maximum over the Arabian Sea and the seasonal north-south shift of the Sahara-Sahel plume. Both of these features are also present in the LGM simulation, though there are some noticeable differences in seasonal variation of dust between the last glacial maximum and the preindustrial Holocene. Since the simulated dust lifetimes are very similar for the MOD and LGM climates, it seems that increased LGM dust lifetime is not the major reason for the observed increase in dust concentration in polar ice cores during the LGM relative to the present.

Formaldehyde and peroxide concentrations in Law Dome (Antarctica) firn and ice cores

AbstractPeroxide speciation and formaldehyde measurements have been made on ice cores retrieved from Law Dome, Antarctica. Measurements were made for ice deposited during four different periods: modern, pre-industrial Holocene, early Holocene and Last Glacial Maximum (LGM). The data show modern peroxide levels &gt;50% above pre-industrial levels (at ∼1.6 μmol L−1) and an absence of methyl hydroperoxide (down to a detection threshold of 0.003 μmol L−1). Formaldehyde levels show a 40% increase from pre-industrial to modern times (rising from ∼0.07 μmol L−1 to ∼0.10 μmol L−1), with a further increase and possible seasonality near the surface which we associate with post-depositional processes. Peroxide levels in LGM ice are low, but formaldehyde concentrations are high (at ∼0.13 μmol L−1) relative to modern levels. Similar high levels of formaldehyde are seen in early Holocene ice (∼6900 years BP).

The mineral dust aerosol cycle during the Last Glacial Maximum

The Canadian Centre for Climate Modelling and Analysis general circulation model (GCM) is used to investigate the factors influencing the mineral dust distribution at the time of the last glacial maximum (LGM). Simulated deposition rates are used in conjunction with existing paleoclimate data to infer likely characteristics of the mineral dust conditions at that time and identify contributions to the pattern of enhanced “dustiness” of the LGM relative to the present, indicated by ice core and loess data. Maximal consistency between the GCM, the Climate: Long Range Investigation, Mapping and Prediction (CLIMAP) data set, and the ice core data requires approximately a 15‐ to 30‐fold increase in dust production globally during the last glacial maximum relative to the preindustrial Holocene. In particular, regional 10‐ to 50‐fold and 20‐fold LGM increases are indicated in eastern Asia and South America, respectively. These implied source enhancements are quite insensitive to the details of the dust model. Analysis of the contributing factors suggests an increase in fine‐particle availability in the source regions.

The chemical composition of ancient atmospheres: A model study constrained by ice core data

A coupled chemistry radiation transport two‐dimensional model of the lower and middle atmosphere was adapted to study the chemical composition of the atmosphere at preindustrial time and last glacial maximum (LGM). The model was constrained by trace gas concentrations (CO2, CH4, and N2O) inferred from polar ice core records. The formulation of tropospheric dynamics and chemistry was improved in order to more accurately simulate the transport and the oxidation processes below the tropopause. Our objectives are to infer the changes in middle‐atmosphere temperature, ozone layer, and oxidation capacity of the atmosphere (e.g., methane lifetime) over the last 18,000 years. A middle‐atmosphere cooling was obtained between LGM and preindustrial Holocene (PIH) as well as between PIH and present time. This is mainly due to changes in the CO2 and chlorofluorocarbon (CFC) concentrations, respectively. CFCs are also the main contributors to the middle‐atmosphere ozone decrease since PIH. Between LGM and PIH the compensating effects of CO2 and N2O lead to little variation in stratospheric ozone. A 17% decrease in tropospheric OH was obtained between LGM and PIH, whereas the model provides a 6% OH increase since PIH. The corresponding changes in the methane sink are too small to have played a dominant role in the past methane concentration changes. Our model derived methane emissions for LGM, PIH, and present time are in good agreement with methane sources evaluated during these three periods.

The atmospheric CH4 increase since the Last Glacial Maximum. (1). Source estimates

An estimate of the distribution of wetland area and associated CH4 emission is presented for the Last Glacial Maximum (LGM, 18 kyr BP, kiloyear Before Present) and the Pre-Industrial Holocene (PIH, 9000-200 years BP). The wetland source, combined with estimates of the other biogenic sources and sink, yields total source strengths of 120 and 180 Tg CH4/yr for LGM and PIH respectively. These source strengths are shown to be consistent with source estimates inferred from a photochemical model, and point to changes in wetland CH4 source as a major factor driving the atmospheric CH4 increase from LGM to PIH.