Fluctuations In Atmospheric CO2 Research Articles

East Asian winter monsoon intensification over the Northwest Pacific Ocean driven by late Miocene atmospheric CO2 decline.

East Asian winter monsoon (EAWM) activity has had profound effects on environmental change throughout East Asia and the western Pacific. Much attention has been paid to Quaternary EAWM evolution, while long-term EAWM fluctuation characteristics and drivers remain unclear, particularly during the late Miocene when marked global climate and Asian paleogeographic changes occurred. To clarify understanding of late Miocene EAWM evolution, we developed a high-precision 9-million-year-long stacked EAWM record from Northwest Pacific Ocean abyssal sediments based on environmental magnetism, sedimentology, and geochemistry, which reveals a strengthened late Miocene EAWM. Our paleoclimate simulations also indicate that atmospheric CO2 decline played a vital role in this EAWM intensification over the Northwest Pacific Ocean compared to other factors, including central Asian orogenic belt and northeastern Tibetan Plateau uplift and Antarctic ice-sheet expansion. Our results expand understanding of EAWM evolution from inland areas to the open ocean and indicate the importance of atmospheric CO2 fluctuations on past EAWM variability over large spatial scales.

Paleoproductivity and deep-sea oxygenation in Cosmonaut Sea since the last glacial maximum: impact on atmospheric CO2

The paleoproductivity in the Southern Ocean plays a crucial role in controlling the atmospheric CO2 concentration. Here, we present the sediment record of gravity core ANT37-C5/6-07, which was retrieved from the Cosmonaut Sea (CS), Indian Ocean sector of the Southern Ocean. We found that the change in the oxygen concentration in the CS bottom water is strongly correlated with the atmospheric CO2 fluctuations since the Last Glacial Maximum (LGM). Based on the change in the export production, we reconstructed the evolution history of the deep-water ventilation/upwelling in the study area. During the LGM, a large amount of respiratory carbon was stored in the deep Southern Ocean due to the effect of the low export productivity and restricted ventilation. The oxygen concentration was also low at this time. Despite the increase in paleoproductivity, the biological pump efficiency remained at a low level during the Last Deglaciation. Vast quantities of CO2 were released into the atmosphere through enhanced upwelling. The recovery of ventilation during this period facilitated the supply of oxygen-rich surface water to the deep ocean. Moreover, signals were identified during the transitions between the Heinrich Stage 1 (HS1), Antarctic Cold Reverse (ACR), and Younger Drays (YD) periods. During the Holocene, the productivity increased overall, and the oxygen in the bottom water was consumed but still remained at a high level. This may have been caused by the enhanced ventilation and/or the prevalence of East Cosmonaut Polynya (ECP) near Cape Ann.

Southern ocean response to glacial and interglacial forcing

The Southern Ocean is particularly vulnerable to climate change. Atmospheric CO2 fluctuations have been highly correlated with climate on million years time scales or caused or at least amplified, the climatic change. Here we used two simulations with the UVic ESCM 2.9 model for the Last Glacial Maximum (LGM) and Marine Isotope Stage 11c (MIS11c) climate using orbital forcing and CO2 prescribed for the ages. We have clearly identified the differences between the decrease and increase inf CO2 concentration in the Southern Ocean. The LGM simulation produces a colder, saltier, and more alkaline Southern Ocean than MIS11c. The MIS11c simulations show a reduction in O2 content with respect to LGM, which may be related to less primary productivity and more CaCO3 dissolution. According to the MIS11c simulation, the surface ocean carbonate factory is expected to be more sensitive to carbon oscillations due to an increase in CO2 dissolved in sea water. As expected, it will decrease the pH due to decreased alkalinity and ion carbonate concentrations. In this scenario, the aragonite concentration in the deep sediments would be more vulnerable to dissolution as a response of shoaling of the depth of aragonite saturation. The increase in CO2 dissolved in sea water in MIS11c, decreases the pH and saturation depth of CaCO3 becomes more than 1000 m shallower than found in the LGM simulation. This implies in a negative feedback of CO2 absorption with the saturation horizon becoming shallower, less Southern Ocean capacity has to uptake atmospheric CO2. Since the CaCO3 formation in the surface ocean and the dissolution in the deeper both affect TCO2 and TA as also the pCO2. Our results indicated that, under high CO2 conditions, the surface of the Southern Ocean appears to be more sensitive to a carbon perturbation, generally due to a decrease in ion carbonate concentration that leads to changes in the carbonate buffer system that will result in a deepening more than 1000 m the aragonite depth saturation in relation to MIS11c results.

Glacial‐Interglacial Controls on Ocean Circulation and Temperature During the Permo‐Carboniferous

AbstractThe apex of Earth's penultimate icehouse during the Permo‐Carboniferous coincided with dramatic glacial‐interglacial fluctuations in atmospheric CO2, sea level, and high‐latitude ice. Global transformations in marine fauna also occurred during this interval, including a rise to peak foraminiferal diversity, suggesting that glacial‐interglacial climate change impacted marine ecosystems. Nevertheless, changes in ocean circulation and temperature over the Permo‐Carboniferous and their influence on marine ecosystem change are largely unknown. Here, we present simulations of glacial and interglacial phases of the latest Carboniferous‐early Permian (∼305‐295 Ma) using the Community Earth System Model version 1.2 to provide estimates of global ocean circulation and temperature during this interval. We characterize general patterns of glacial and interglacial surface ocean currents, temperature, and salinity, and compare them to the documented abundance and distribution of Permo‐Carboniferous marine fauna as well as a preindustrial climate simulation. We then explore how glacial‐interglacial changes in atmospheric CO2, sea level, and high‐latitude ice extent impact thermohaline circulation. We find that glacial‐interglacial changes in equatorial surface temperatures are consistently ∼3–6°C. Ocean circulation is stronger overall in the glacial simulation, particularly as lower atmospheric CO2 enables deep convection in the Northern Hemisphere. Wind‐driven circulation, heat transport, and upwelling intensity are stronger overall in the Permo‐Carboniferous superocean relative to the preindustrial oceans at the same level of atmospheric CO2. We also find that CO2‐induced glacial conditions of the early Permian may have promoted foraminiferal diversity through increased thermal gradients and suppressed riverine input in marine shelf environments.

CO<sub>2</sub> surface variability: from the stratosphere or not?

Abstract. Fluctuations in atmospheric CO2 can be measured with great precision and are used to identify human-driven sources as well as natural cycles of ocean and land carbon. One source of variability is the stratosphere, where the influx of aged CO2-depleted air can produce fluctuations at the surface. This process has been speculated to be a potential source of interannual variability (IAV) in CO2 that might obscure the quantification of other sources of IAV. Given the recent success in demonstrating that the stratospheric influx of N2O- and chlorofluorocarbon-depleted air is a dominant source of their surface IAV in the Southern Hemisphere, I apply the same model and measurement analysis here to CO2. Using chemistry-transport modeling or scaling of the observed N2O variability, I find that the stratosphere-driven surface variability in CO2 is at most 10 % of the observed IAV and is not an important source. Diagnosing the amplitude of the CO2 annual cycle and its increase from 1985 to 2021 through the annual variance gives rates similar to traditional methods in the Northern Hemisphere (BRW, MLO) but can identify the emergence of small trends (0.08 ppm per decade) in the Southern Hemisphere (SMO, CGO).

A Modified Vegetation Photosynthesis and Respiration Model (VPRM) for the Eastern USA and Canada, Evaluated With Comparison to Atmospheric Observations and Other Biospheric Models

AbstractAtmospheric CO2 measurements from a dense surface network can help to evaluate terrestrial biosphere model (TBM) simulations of Net Ecosystem Exchange (NEE) with two key benefits. First, gridded CO2 flux estimates can be evaluated over regional scales, not possible using flux tower observations at discrete locations for model evaluation. Second, TBM ability to explain atmospheric CO2 fluctuations due to the biosphere can be directly tested, an important objective for anthropogenic emissions monitoring using atmospheric observations. Here, we customize the Vegetation Photosynthesis and Respiration Model (VPRM) for an eastern North American domain with strong biological activity upwind of urban areas. Parameters are optimized using flux tower observations from a historical database with sites in (and near) the domain. In addition, the respiration model (originally a linear function of temperature) is modified to account for impacts of changing foliage, non‐linear temperature, and water stress. Flux estimates from VPRM, the Carnegie‐Ames‐Stanford Approach (CASA) model and the Simple Biosphere Model v4 (SiB4), are convolved with footprints from atmospheric transport models for evaluation with CO2 observations at 21 towers in the domain, with roughly half of the towers used here for the first time. Results show that the new respiration model in VPRM helps to correct a growing season sink bias in the atmosphere associated with underestimated summertime respiration using the original model with annual parameters. The new VPRM also better explains fine‐scale atmospheric CO2 variability compared to other TBMs, due to higher resolution diagnostic phenology, the new respiration model, domain‐specific parameters, and high‐quality input data sets.

Stability of Atmospheric Redox States of Early Mars Inferred from Time Response of the Regulation of H and O Losses

Abstract The ratio of total losses of H and O from the atmosphere is crucial for determining the Martian atmospheric redox state. The H and O escapes are shown to be regulated in a stoichiometric 2:1 ratio in a converged model of present-day Mars over a timescale of ∼105 yr, which is called self-regulation. Self-regulation timescales under different atmospheric conditions on early Mars are not well understood. Here we use a 1D photochemical model to calculate the timescales of self-regulation for denser CO2 atmospheres with various surface temperatures as benchmark cases for early Mars. Self-regulation is driven by variations in the amount of O2 or CO in the atmosphere, depending on the atmospheric redox state. Self-regulation timescales are likely to be controlled by the net redox balance. A 1 bar CO2 atmosphere with a surface temperature of 240 K has a self-regulation timescale of a few million years. Denser atmospheres of early Mars have a longer regulation timescale and are less redox-stable than the atmosphere of present-day Mars. Obliquity variations cause atmospheric CO2 fluctuations, producing a difference in the self-regulation timescale between high and low obliquity. Because an increase in CO2 suppresses H escape, the net effect of the obliquity cycle could have driven the atmospheric redox states to be more reducing. Our results also suggest the possibility of a CO-dominated atmosphere of 10–100 mbars at 3 Ga. The redox state of ancient Mars might have fluctuated more easily than that of the present.

Productivity-climate coupling recorded in Pleistocene sediments off Prydz Bay (East Antarctica)

Export-production in the Southern Ocean is thought to be tightly correlated with atmospheric CO2 fluctuations over geological times. Relevant studies are, however, mainly confined to the Atlantic and Pacific sectors of the Southern Ocean and often only cover the last glacial-interglacial cycle. Here, we present a sediment record retrieved off Prydz Bay (East Antarctica) spanning the last ~520kyrs. We show that the distribution pattern of local export production is closely coupled with global climate change on orbital time scales. We propose that the dominant factors controlling local export production include the amount of nutrients supply from the ocean interior and the extent of sea ice cover, thus representing the combined history of deep water exposure/ventilation and the sea ice coverage in the studied area. Sharp increases in the local export production coincide with abrupt increase in atmospheric CO2 concentrations during deglacial periods, indicating drastic outgassing of the upwelled deep water in the Southern Ocean. Gradually decrease in the export production following interglacial peaks concurs with decrease in deep water exposure/ventilation and increase in sea ice cover, explaining the early decline in atmospheric CO2 over a glacial-interglacial cycle.

A biomarker record of temperature and phytoplankton community structure in the Okinawa Trough since the last glacial maximum

AbstractA variety of biomarkers were examined from Ocean Drilling Program Core 1202B to reconstruct temperature and phytoplankton community structures in the southern Okinawa Trough since 20 ka. Two molecular temperature proxies ( $${\rm U}_{{37}}^{{{\rm K}\prime}} $$ and TEX86) show 5°C to ~6°C warming during the glacial-interglacial transition. Prior to the Holocene, the $${\rm U}_{{37}}^{{{\rm K}\prime}} $$ -derived temperature was generally 1°C to 4°C higher than TEX86-derived temperature. This difference, however, was reduced to <1°C in the Holocene. Correspondingly, the phytoplankton biomarkers (e.g., C37:2 alkenone, brassicasterol, C30 1,15 diol, and dinosterol) indicate a shift of planktonic community structures, with coccolithophorids becoming more abundant in the Holocene at the expense of diatoms/dinoflagellates. This shift is related to the variability of nutrients, temperature, and salinity in the Okinawa Trough, likely controlled by the sea level and the intensity of the Kuroshio Current. The phytoplankton community change may have had profound implications for atmospheric CO2 fluctuations during glacial-interglacial cycles since diatoms and dinoflagellates have a higher efficiency of the biological pump than coccolithophorids.

Early hydrothermal carbon uptake by the upper oceanic crust: Insight from in situ U-Pb dating

It is widely thought that continental chemical weathering provides the key feedback that prevents large fluctuations in atmospheric CO2, and hence surface temperature, on geological time scales. However, low-temperature alteration of the upper oceanic crust in off-axis hydrothermal systems provides an alternative feedback mechanism. Testing the latter hypothesis requires understanding the timing of carbonate mineral formation within the oceanic crust. Here we report the first radiometric age determinations for calcite formed in the upper oceanic crust in eight locations globally via in-situ U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis. Carbonate formation occurs soon after crustal accretion, indicating that changes in global environmental conditions will be recorded in changing alteration characteristics of the upper oceanic crust. This adds support to the interpretation that large differences between the hydrothermal carbonate content of late Mesozoic and late Cenozoic oceanic crust record changes in global environmental conditions. In turn, this supports a model in which alteration of the upper oceanic crust in off-axis hydrothermal systems plays an important role in controlling ocean chemistry and the long-term carbon cycle.

Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation.

Atmospheric CO2 fluctuations over glacial-interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial-interglacial atmospheric CO2 variations invoke changes in deep-ocean carbon storage, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO2 rise observed in ice-core records. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial-interglacial ocean-atmosphere exchange of carbon.

Borate as a Synergistic Anion for Marinobacter algicola Ferric Binding Protein, FbpA: A Role for Boron in Iron Transport in Marine Life

Boron in the ocean is generally considered a nonbiological element due to its relatively high concentration (0.4 mM) and depth independent concentration profile. Here we report an unexpected role for boron in the iron transport system of the marine bacterium Marinobacter algicola. Proteome analysis under varying boron concentrations revealed that the periplasmic ferric binding protein (Mb-FbpA) was among the proteins whose expression was most affected, strongly implicating the involvement of boron in iron utilization. Here we show that boron facilitates Fe(3+) sequestration by Mb-FbpA at pH 8 (oceanic pH) by acting as a synergistic anion (B(OH)4(1-)). Fe(3+) sequestration does not occur at pH 6.5 where boric acid (B(OH)3; pK(a) = 8.55) is the predominant species. Borate anion is also shown to bind to apo-Mb-FbpA with mM affinity at pH 8, consistent with the biological relevance implied from boron's oceanic concentration (0.4 mM). Borate is among those synergistic anions tested which support the strongest Fe(3+) binding to Mb-FbpA, where the range of anion dependent affinity constants is log K'(eff) = 21-22. Since the pKa of boric acid (8.55) lies near the pH of ocean water, changes in oceanic pH, as a consequence of fluctuations in atmospheric CO2, may perturb iron uptake in many marine heterotrophic bacteria due to a decrease in oceanic borate anion concentration.

Controls on the spatial distribution of oceanic δ<sup>13</sup>C<sub>DIC</sub>

Abstract. We describe the design and evaluation of a large ensemble of coupled climate–carbon cycle simulations with the Earth system model of intermediate complexity GENIE. This ensemble has been designed for application to a range of carbon cycle questions, including the causes of late-Quaternary fluctuations in atmospheric CO2. Here we evaluate the ensemble by applying it to a transient experiment over the recent industrial era (1858 to 2008 AD). We employ singular vector decomposition and principal component emulation to investigate the spatial modes of ensemble variability of oceanic dissolved inorganic carbon (DIC) δ13C, considering both the spun-up pre-industrial state and the transient change. These analyses allow us to separate the natural (pre-industrial) and anthropogenic controls on the δ13CDIC distribution. We apply the same dimensionally-reduced emulation techniques to consider the drivers of the spatial uncertainty in anthropogenic DIC. We show that the sources of uncertainty related to the uptake of anthropogenic δ13CDIC and DIC are quite distinct. Uncertainty in anthropogenic δ13C uptake is controlled by air–sea gas exchange, which explains 63% of modelled variance. This mode of variability is largely absent from the ensemble variability in CO2 uptake, which is rather driven by uncertainties in thermocline ventilation rates. Although the need to account for air–sea gas exchange is well known, these results suggest that, to leading order, uncertainties in the ocean uptake of anthropogenic 13C and CO2 are governed by very different processes. This illustrates the difficulties in reconstructing one from the other, and furthermore highlights the need for careful targeting of both δ13CDIC and DIC observations to better constrain the ocean sink of anthropogenic CO2.

Too early to infer a global NPP decline since 2000

The global terrestrial carbon cycle plays a pivotal role in regulating the atmospheric composition of greenhouse gases. It has recently been suggested that the upward trend in net primary production (NPP) seen during the 1980's and 90's has been replaced by a negative trend since 2000 induced by severe droughts mainly on the southern hemisphere. Here we compare results from an individual‐based global vegetation model to satellite‐based estimates of NPP and top‐down reconstructions of net biome production (NBP) based on inverse modelling of observed CO2 concentrations and CO2 growth rates. We find that simulated NBP exhibits considerable covariation on a global scale with interannual fluctuations in atmospheric CO2. Our simulations also suggest that droughts in the southern hemisphere may have been a major driver of NPP variations during the past decade. The results, however, do not support conjecture that global terrestrial NPP has entered a period of drought‐induced decline.

Sea surface temperatures in cooler climate stages bear more similarity with atmospheric CO2 forcing

The interglacial Marine Isotope Stage (MIS) 11 received special attention due to its remarkable resemblance with present‐day climate. Based on synchronicity of marine, ice sheet and terrestrial proxy responses, warm episodes with intervening cool phase(s) at MIS 11 are qualitatively established. Here we quantitatively evaluate 15 climate proxies during 368–552 kyr intervals adopting a novel long‐range cross‐correlation approach and information theory based similarity measures. We also estimate the information flow rate and dominant flow direction between these climate variables using transfer entropy and the related directionality index. Our results unequivocally establish that atmospheric CO2 (pCO2) is the driving signal while all other proxies used in this study are the responses. The climate forcing greenhouse gas, the atmospheric CO2 (pCO2) and the response signals like sea surface temperature (SST) and carbon isotope composition of total organic carbon (δ13CTOC) proxies are strongly correlated (∼1 or −1) without significant observable time lag (less than 1 kyr). Various substages of MIS 11 are recognizable in the SST data alone based on normalized similarity measures. Additionally, eight more proxies from lacustrine sediments are identified as primary. During the cooler substages these proxies bear more similarity with ambient atmospheric pCO2. Thus, the information theory–based similarity measures suggest that atmospheric CO2 fluctuations are best captured by at least 9 climate proxies during cooler interglacial events. Based on the results related to interglacial MIS 11 and 13 obtained in this study, an important implication relevant to anthropogenic CO2input to the present‐day atmosphere can be distilled. It is that sensitive and better‐coupled response proxies such as SST and MAT, which already show an increasing trend, are likely to behave in a more dissimilar manner in future. That is, they tend to behave more independently in the near future (∼0.75 kyr).

Wetlands, temperature, and atmospheric CO2and CH4coupling over the past two millennia

[1] High-resolution records of greenhouse gas concentrations from the Law Dome Antarctic ice core show small fluctuations in atmospheric CO2 and CH4, including a 10 ppmv decrease in CO2 and a 50 ppbv increase in CH4 during the Little Ice Age. Explanations for the biosphere-atmosphere carbon exchanges that caused these fluctuations, and their impacts on global climate, are lacking. We produce here a wetland index derived from the North American Pollen Database and show that for most of the past two millennia, this index is negatively correlated (antiphase) with atmospheric CO2 levels measured in the Law Dome ice core. Our analysis indicates the potential for global wetlands to act as carbon sinks, contributing to small fluctuations in atmospheric greenhouse gas concentrations, and thereby influencing submillennial-scale climatic change. Although our wetland index–CO2 relationship is statistically significant, the relationship between our wetland index–CH4 curve is not, possibly due to the confounding influence of wildfires on CH4.

Ship‐based measurement of air‐sea CO2 exchange by eddy covariance

A system for the shipboard measurement of air‐sea CO2 fluxes by eddy covariance was developed and tested. The system was designed to reduce two major sources of experimental uncertainty previously reported. First, the correction for in situ water vapor fluctuations (the “Webb” correction) was reduced by 97% by drying the air sample stream. Second, motion sensitivity of the gas analyzer was reduced by using an open‐path type sensor that was converted to a closed‐path configuration to facilitate drying of the air stream. High‐quality CO2 fluxes were obtained during 429 14 min flux intervals during two cruises in the North Atlantic. The results suggest that the gas analyzer resolved atmospheric CO2 fluctuations well below its RMS noise level. This noise was uncorrelated with the vertical wind and therefore filtered out by the flux calculation. Using climatological data, we estimate that the techniques reported here could enable high‐quality measurements of air‐sea CO2 flux over much of the world oceans.

Phosphorus burial in the ocean over glacial-interglacial time scales

Abstract. The role of nutrients, such as phosphorus (P), and their impact on primary productivity and the fluctuations in atmospheric CO2 over glacial-interglacial periods are intensely debated. Suggestions as to the importance of P evolved from an earlier proposal that P actively participated in changing productivity rates and therefore climate change, to most recent ones that changes in the glacial ocean inventory of phosphorus were important but not influential if compared to other macronutrients, such as nitrate. Using new data coming from a selection of ODP sites, we analyzed the distribution of oceanic P sedimentary phases and calculate reactive P burial fluxes, and we show how P burial fluxes changed over the last glacial-interglacial period at these sites. Concentrations of reactive P are generally lower during glacial times, while mass accumulation rates (MAR) of reactive P show higher variability. If we extrapolate for the analyzed sites, we may assume that in general glacial burial fluxes of reactive P are lower than those during interglacial periods by about 8%, because the lack of burial of reactive P on the glacial shelf reduced in size, was apparently not compensated by burial in other regions of the ocean. Using the calculated changes in P burial, we evaluate their possible impact on the phosphate inventory in the world oceans. Using a simple mathematical approach, we find that these changes alone could have increased the phosphate inventory of glacial ocean waters by 17–40% compared to interglacial stages. Variations in the distribution of sedimentary P phases at the investigated sites seem to indicate that at the onset of interglacial stages, shallower sites experienced an increase in reactive P concentrations, which seems to point to P-richer waters at glacial terminations. All these findings would support the Shelf-Nutrient Hypothesis, which assumes that during glacial low stands nutrients are transferred from shallow sites to deep sea with possible feedback on the carbon cycle.

Multiple temporal scale variability during the twentieth century in global carbon dynamics simulated by a coupled climate–terrestrial carbon cycle model

A coupled climate–carbon cycle model composed of a process-based terrestrial carbon cycle model, Sim-CYCLE, and the CCSR/NIES/FRCGC atmospheric general circulation model was developed. We examined the multiple temporal scale functions of terrestrial ecosystem carbon dynamics induced by human activities and natural processes and evaluated their contribution to fluctuations in the global carbon budget during the twentieth century. Global annual net primary production (NPP) and heterotrophic respiration (HR) increased gradually by 6.7 and 4.7%, respectively, from the 1900s to the 1990s. The difference between NPP and HR was the net carbon uptake by natural ecosystems, which was 0.6 Pg C year−1 in the 1980s, whereas the carbon emission induced by human land-use changes was 0.5 Pg C year−1, largely offsetting the natural terrestrial carbon sequestration. Our results indicate that monthly to interannual variation in atmospheric CO2 growth rate anomalies show 2- and 6-month time lags behind anomalies in temperature and the NiNO3 index, respectively. The simulated anomaly amplitude in monthly net carbon flux from terrestrial ecosystems to the atmosphere was much larger than in the prescribed air-to-sea carbon flux. Fluctuations in the global atmospheric CO2 time series were dominated by the activity of terrestrial vegetation. These results suggest that terrestrial ecosystems have acted as a net neutral reservoir for atmospheric CO2 concentrations during the twentieth century on an interdecadal timescale, but as the dominant driver for atmospheric CO2 fluctuations on a monthly to interannual timescale.

Climate and vegetational regime shifts in the late Paleozoic ice age earth

The late Paleozoic earth experienced alternation between glacial and non-glacial climates at multiple temporal scales, accompanied by atmospheric CO2 fluctuations and global warming intervals, often attended by significant vegetational changes in equatorial latitudes of Pangaea. We assess the nature of climate-vegetation interaction during two time intervals: middle-late Pennsylvanian transition and Pennsylvanian-Permian transition, each marked by tropical warming and drying. In case study 1, there is a catastrophic intra-biomic reorganization of dominance and diversity in wetland, evergreen vegetation growing under humid climates. This represents a threshold-type change, possibly a regime shift to an alternative stable state. Case study 2 is an inter-biome dominance change in western and central Pangaea from humid wetland and seasonally dry to semi-arid vegetation. Shifts between these vegetation types had been occurring in Euramerican portions of the equatorial region throughout the late middle and late Pennsylvanian, the drier vegetation reaching persistent dominance by Early Permian. The oscillatory transition between humid and seasonally dry vegetation appears to demonstrate a threshold-like behavior but probably not repeated transitions between alternative stable states. Rather, changes in dominance in lowland equatorial regions were driven by long-term, repetitive climatic oscillations, occurring with increasing intensity, within overall shift to seasonal dryness through time. In neither case study are there clear biotic or abiotic warning signs of looming changes in vegetational composition or geographic distribution, nor is it clear that there are specific, absolute values or rates of environmental change in temperature, rainfall distribution and amount, or atmospheric composition, approach to which might indicate proximity to a terrestrial biotic-change threshold.