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In 2016, we at the Alfred P. Sloan Foundation put out a request for proposals to advance research on the economics of energy efficiency. In response, a group of researchers [...]

The August 2022 passage of the Inflation Reduction Act enables a massive investment in reducing carbon emissions and mitigating their effects. It also focuses attention on strategies for removing carbon [...]

Author(s): Tim Palmer Author Affiliations: Short-term tests validate long-term estimates of climate change How sensitive is climate to atmospheric carbon dioxide levels? For a doubling of CO[sub.2] concentration from pre-industrial [...], Six-hour weather forecasts have been used to validate estimates of climate change hundreds of years from now. Such tests have great potential -- but only if our weather-forecasting and climate-prediction systems are unified. Climate sensitivity to atmospheric CO2 levels is likely to be high.

Even though Northern Hemisphere atmospheric C[O.sub.2] mixing ratios have recently exceeded 400 parts per million (ppm), few investigations have directly explored the effect of changes in well-mixed greenhouse gases on [...], The climatic impact of C[O.sub.2] and other greenhouse gases is usually quantified in terms of radiative forcing (1), calculated as the difference between estimates of the Earth's radiation field from pre-industrial and present-day concentrations of these gases. Radiative transfer models calculate that the increase in C[O.sub.2] since 1750 corresponds to a global annualmean radiative forcing at the tropopause of 1.82 ± 0.19 W [m.sup.-2] (ref. 2). However, despite widespread scientific discussion and modelling of the climate impacts of well-mixed greenhouse gases, there is little direct observational evidence of the radiative impact of increasing atmospheric C[O.sub.2]. Here we present observationally based evidence of clear-sky C[O.sub.2] surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric C[O.sub.2]. The time series of this forcing at the two locations--the Southern Great Plains and the North Slope of Alaska--are derived from Atmospheric Emitted Radiance Interferometer spectra (3) together with ancillary measurements and thoroughly corroborated radiative transfer calculations (4). The time series both show statistically significant trends of 0.2 W [m.sup.-2] per decade (with respective uncertainties of ± 0.06 W [m.sup.- 2] per decade and ± 0.07 W [m.sup.-2] per decade) and have seasonal ranges of 0.1-0.2 W [m.sup.-2]. This is approximately ten per cent of the trend in downwelling longwave radiation (5-7). These results confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions, and provide empirical evidence of how rising C[O.sub.2] levels, mediated by temporal variations due to photosynthesis and respiration, are affecting the surface energy balance.

Author(s): P. K. Patra [sup.1] [sup.2] , M. C. Krol [sup.3] , S. A. Montzka [sup.4] , T. Arnold [sup.5] , E. L. Atlas [sup.6] , B. R. Lintner [sup.7] [...], Observations of methyl chloroform combined with an atmospheric transport model predict a Northern to Southern Hemisphere hydroxyl ratio of slightly less than 1, whereas commonly used atmospheric chemistry models predict ratios 15-45% higher. The north-south distribution of atmospheric OH The hydroxyl radical is an important atmospheric oxidant, but our knowledge of its global distribution remains imprecise, with estimates for the ratio of Northern Hemisphere to Southern Hemisphere hydroxyl radical concentration varying from 0.85 to 1.4. These authors use a three-dimensional chemistry-transport model that has been well validated for interhemispheric transport using sulphur hexafluoride measurements, to obtain an interhemispheric hydroxyl radical ratio of 0.97[plus or minus]0.12. This information can help improve our understanding of the fate of atmospheric pollutants and greenhouse gases. The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere.sup.1,2,3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane.sup.4,5,6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 [plus or minus] 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns.sup.11,12,13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Author(s): V. Jomelli [sup.1] , V. Favier [sup.2] , M. Vuille [sup.3] , R. Braucher [sup.4] , L. Martin [sup.5] , P.-H. Blard [sup.5] , C. Colose [sup.3] , D. [...], A moraine chronology determined by surface exposure dating shows that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas stadial (12,800 to 11,500 years ago), contrary to previous studies; as a result, previous chronologies and climate interpretations from tropical glaciers may need to be revisited. Glaciation in the tropical Andes The Younger Dryas was a cold period immediately prior to the present warm interglacial period. There is clear evidence that it occurred throughout much of the Northern Hemisphere, but its global extent remains debatable. Prior work in the tropical Andes suggested extensive glacial advances during the Younger Dryas. Vincent Favier and colleagues present new cosmogenic dating of glacial deposits -- and a reassessment of existing datasets from other glaciated Andean landscapes -- to show that instead, maximal glacial advances took place in the preceding Antarctic cold reversal. Rather than a major advance, the Younger Dryas now appears to have been associated with modest glacial retreat. The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last deglaciation, is thought to coincide with the last major glacial re-advance in the tropical Andes.sup.1. This interpretation relies mainly on cosmic-ray exposure dating of glacial deposits. Recent studies, however, have established new production rates.sup.2,3,4 for cosmogenic .sup.10Be and .sup.3He, which make it necessary to update all chronologies in this region.sup.1,5,6,7,8,9,10,11,12,13,14,15 and revise our understanding of cryospheric responses to climate variability. Here we present a new .sup.10Be moraine chronology in Colombia showing that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas. On the basis of a homogenized chronology of all .sup.10Be and .sup.3He moraine ages across the tropical Andes, we show that this behaviour was common to the northern and southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlantic meridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere. During the Younger Dryas, however, northern tropical Andes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land-surface feedbacks triggered by a weakened Atlantic meridional overturning circulation. Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence from mid-latitude Andean glaciers.sup.16, our results argue for a common glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.

Author(s): Cawas B. Engineer [sup.1] , Majid Ghassemian [sup.2] , Jeffrey C. Anderson [sup.3] , Scott C. Peck [sup.3] , Honghong Hu [sup.1] [sup.4] , Julian I. Schroeder [sup.1] Author [...], The continuing rise in atmospheric carbon dioxide concentrations suppresses the development of stomatal pores, and thus gas exchange, in plant leaves on a global scale; now, a framework of mechanisms by which carbon dioxide represses development has been identified. Control of plant stomatal development The continuing rise in atmospheric CO.sub.2 levels is suppressing the development of stomatal pores in plant leaves on a global scale. This, combined with the increasing scarcity of water for agriculture, can significantly affect plant carbon assimilation, heat stress and water-use efficiency. Julian Schroeder and colleagues investigate the genes and mechanisms through which CO.sub.2 controls development of the stomatal pores that plants use to regulate gas exchange in leaves. They identify a framework of mechanisms: at high CO.sub.2 levels, extracellular signalling and carbonic anhydrase regulate a novel protease called CRSP and the pro-peptide EPF2; this in turn represses stomatal development. Environmental stimuli, including elevated carbon dioxide levels, regulate stomatal development.sup.1,2,3; however, the key mechanisms mediating the perception and relay of the CO.sub.2 signal to the stomatal development machinery remain elusive. To adapt CO.sub.2 intake to water loss, plants regulate the development of stomatal gas exchange pores in the aerial epidermis. A diverse range of plant species show a decrease in stomatal density in response to the continuing rise in atmospheric CO.sub.2 (ref. 4). To date, one mutant that exhibits deregulation of this CO.sub.2-controlled stomatal development response, hic (which is defective in cell-wall wax biosynthesis, ref. 5), has been identified. Here we show that recently isolated Arabidopsis thaliana [beta]-carbonic anhydrase double mutants (ca1 ca4).sup.6 exhibit an inversion in their response to elevated CO.sub.2, showing increased stomatal development at elevated CO.sub.2 levels. We characterized the mechanisms mediating this response and identified an extracellular signalling pathway involved in the regulation of CO.sub.2-controlled stomatal development by carbonic anhydrases. RNA-seq analyses of transcripts show that the extracellular pro-peptide-encoding gene EPIDERMAL PATTERNING FACTOR 2 (EPF2).sup.7,8, but not EPF1 (ref. 9), is induced in wild-type leaves but not in ca1 ca4 mutant leaves at elevated CO.sub.2 levels. Moreover, EPF2 is essential for CO.sub.2 control of stomatal development. Using cell-wall proteomic analyses and CO.sub.2-dependent transcript analyses, we identified a novel CO.sub.2-induced extracellular protease, CRSP (CO.sub.2 RESPONSE SECRETED PROTEASE), as a mediator of CO.sub.2-controlled stomatal development. Our results identify mechanisms and genes that function in the repression of stomatal development in leaves during atmospheric CO.sub.2 elevation, including the carbonic-anhydrase-encoding genes CA1 and CA4 and the secreted protease CRSP, which cleaves the pro-peptide EPF2, in turn repressing stomatal development. Elucidation of these mechanisms advances the understanding of how plants perceive and relay the elevated CO.sub.2 signal and provides a framework to guide future research into how environmental challenges can modulate gas exchange in plants.

As the primary atmospheric oxidant, the OH radical has a key role in the removal or production of major air pollutants, greenhouse gases and many ozone-depleting substances (1-3). A better [...], The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere (1-3). The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane (4-6). It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4,7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns (11-13). Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 maybe overestimated.

Each year, on average, land and ocean carbon sinks absorb the equivalent of about half of the global fossil fuel emissions, thereby providing a critical service that slows the rise [...], The land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations (1). Although the uptake of carbon by oceanic and terrestrial processes has kept pace with accelerating carbon dioxide emissions until now, atmospheric carbon dioxide concentrations exhibit a large variability on interannual timescales (2), considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests (3). We use a terrestrial biogeochemical model, atmospheric carbon dioxide inversion and global carbon budget accounting methods to investigate the evolution of the terrestrial carbon sink over the past 30 years, with a focus on the underlying mechanisms responsible for the exceptionally large land carbon sink reported in 2011 (ref. 2). Here we show that our three terrestrial carbon sink estimates are in good agreement and support the finding of a 2011 record land carbon sink. Surprisingly, we find that the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60 per cent of carbon uptake attributed to Australian ecosystems, where prevalent La Nina conditions caused up to six consecutive seasons of increased precipitation. In addition, since 1981, a six per cent expansion of vegetation cover over Australia was associated with a fourfold increase in the sensitivity of continental net carbon uptake to precipitation. Our findings suggest that the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability and that tropical rainforests may become less relevant drivers in the future. More research is needed to identify to what extent the carbon stocks accumulated during wet years are vulnerable to rapid decomposition or loss through fire in subsequent years.

A list of authors and their affiliations appears at the end of the paper. The Pacific, as the world's largest ocean, is intricately involved in the prominent changes in the [...], Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

The South Pacific convergence zone (SPCZ) is the Southern Hemisphere's most expansive and persistent rain band, extending from the equatorial western Pacific Ocean southeastward towards French Polynesia (1,2). Owing to [...]

It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state (1). Although different types of uncertainty limit our capacity to assess this risk [...]

Climate change poses a fundamental challenge for natural resource management: Climate patterns are shifting in space and time, but national parks, national forests, and other natural areas remain at fixed [...]

The North Pacific subarctic--subtropical boundary region is an important sink for atmospheric C[O.sub.2], with high annual influx relative to surrounding waters and large seasonal variability. The location of the subarctic--subtropical gyre boundary is relatively stable, but there is a large seasonal migration of the North Pacific Chlorophyll Front, between ~ 30[degrees]N and ~ 45[degrees]N. To determine the role of biological productivity on the seasonal C[O.sub.2] flux, we measured sections of oxygen-argon gas ratios on cruises across the transition zone in November 1997 (autumn) and September 2008 (summer). A simple upper ocean model was used with the [O.sub.2]:Ar data to estimate the net community production (NCP) in the euphotic zone. On both cruises the NCP was highest at the chlorophyll front with average values from 30[degrees]N to 45[degrees]N of 3.4 [+ or -] 2.0 and 8.1 [+ or -] 2.7 mmol C[m.sup.-2] [d.sup.-1] in autumn and summer, respectively. These values are sufficient to make biologically induced carbon export an important component of the C[O.sub.2] drawdown in this region. Processes that control the relatively high NCP ar this boundary are not certain, but it has been demonstrated that horizontal convergence of nitrate plays some role. doi: 10.4319/lo.2010.55.2585

Changes in ocean circulation have been proposed as a trigger mechanism for the large coupled climate and carbon cycle perturbations at the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma). An abrupt warming of oceanic intermediate waters could have initiated the thermal destabilization of sediment-hosted methane gas hydrates and potentially triggered sediment slumps and slides. In an ensemble of fully coupled atmosphere-ocean general circulation model (AOGCM) simulations of the late Paleocene and early Eocene, we identify such a circulation-driven enhanced intermediate-water warming. Critically, we find an approximate twofold amplification of Atlantic intermediate-water warming when C[O.sub.2] levels are doubled from 2x to 4x preindustrial C[O.sub.2] compared to when they are doubled from 1x to 2x. This warming is largely focused on the equatorial and South Atlantic and is driven by a significant reduction in deep-water formation from the Southern Ocean. This scenario is consistent with altered PETM circulation patterns inferred from benthic carbon isotope data and the intensity of deep-sea carbonate dissolution in the South Atlantic. The linkage between intermediate-water warming and gas hydrate destabilization could provide an important feedback in the establishment of peak PETM warmth. doi: 10.1130/G31184.1

In the high-nutrient, low-chlorophyll waters of the Gulf of Alaska, microcosm manipulation experiments were used to assess the effect of C[O.sub.2] on growth and primary production under iron-limited and iron-replete conditions. As expected, iron had a strong effect on growth and photosynthesis. A modest and variable stimulation of growth and biomass production by C[O.sub.2] (high C[O.sub.2]: 77-122 Pa; low C[O.sub.2]: 11-17 Pa) was observed under both iron-replete and iron-limited conditions, though near the limit of precision of our measurements in slow-growing low-iron experiments. Physiological acclimations responsible for the changes in growth were assessed. Under iron-limited conditions, growth stimulation at high CO2 appeared to result from an increase in photosynthetic efficiency, which we attribute to energy savings from down-regulation of the carbon concentrating mechanisms. In some cases, iron-rich photosynthetic proteins (PsbA, PsaC, and cytochrome [b.sub.6]) were down-regulated at elevated C[O.sub.2] in iron-limited controls. Under iron-replete conditions, there was an increase in growth rate and biomass at high C[O.sub.2] in some experiments. This increase was unexpectedly supported by reductions in cellular carbon loss, most likely decreased respiration. We speculate that this effect may be due to acclimation to decreased pH rather than high CO2. The variability in responses to C[O.sub.2] among experiments did not appear to be caused by differences in phytoplankton community structure and may reflect the sensitivity of the net response of phytoplankton to antagonistic effects of the several parameters that co-vary with C[O.sub.2]. doi: 10.4319/lo.2010.55.5.2011

Past glacial-interglacial increases in the concentration of atmospheric carbon dioxide (C[O.sub.2]) are thought to arise from the rapid release of C[O.sub.2] sequestered in the deep sea, primarily via the Southern Ocean. Here, we present radiocarbon evidence from the Atlantic sector of the 50uthern Ocean that strongly supports this hypothesis. We show that during the last glacial period, deep water circulating around Antarctica was more than two times older than today relative to the atmosphere. During deglaciation, the dissipation of this old and presumably C[O.sub.2]-enriched deep water played an important rote in the pulsed rise of atmospheric C[O.sub.2] through its variable influence on the upwelling branch of the Antarctic overturning circulation. doi: 10.1126/science.1183627

Studies of both the suspended and the dissolved loads of the world's largest rivers and of hill-slope denudation have shown a strong link between physical erosion and chemical weathering (8,9). [...], Over geologic timescales, C[O.sub.2] is emitted from the Earth's interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions (1). Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion (1,2) are superimposed on fluctuations in organic carbon burial (3). Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation (1,2,4). Widespread observations of a recent (0-5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them (4-6). Accelerated uplift and global cooling have been given as possible causes (2,4,6,7), but because of the links between rates of erosion and the correlated rate of weathering (8,9), an increase in the drawdown of C[O.sub.2] that is predicted to follow may be the cause of global climate change instead (2). However, globally, rates of uplift cannot increase every where in the way that apparent sedimentation rates do (4,10). Moreover, proxy records of past atmospheric C[O.sub.2] provide no evidence for this large reduction in recent C[O.sub.2] concentrations (11,12). Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record (13). As evidence, we recast the ocean dissolved [sup.10]Be/[sup.9]Be isotope system as a weathering proxy spanning the past ~12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates.

A leading hypothesis explaining Phanerozoic mass extinctions and associated carbon isotopic anomalies is the emission of greenhouse, other gases, and aerosols caused by eruptions of continental flood basalt provinces. However, the necessary serial relationship between these eruptions, isotopic excursions, and extinctions has never been tested in geological sections preserving all three records. The end-Triassic extinction (ETE) at 201.4 Ma is among the largest of these extinctions and is tied to a large negative carbon isotope excursion, reflecting perturbations of the carbon cycle including a transient increase in C[O.sub.2]. The cause of the ETE has been inferred to be the eruption of the giant Central Atlantic magmatic province (CAMP). Here, we show that carbon isotopes of leaf wax derived lipids (n-alkanes), wood, and total organic carbon from two orbitally paced lacustrine sections interbedded with the CAMP in eastern North America show similar excursions to those seen in the mostly marine St. Audrie's Bay section in England. Based on these results, the ETE began synchronously in marine and terrestrial environments slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco, a C[O.sub.2] super greenhouse, and marine biocalcification crisis. Because the temporal relationship between CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same place, this is the strongest case for a volcanic cause of a mass extinction to date. astrochronology | C[O.sub.2] | Jurassic | large igneous provinces | n-alkane doi/ 10.1073/pnas.1001706107

Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric C[O.sub.2]. The oxygen isotopic composition of atmospheric C[O.sub.2] can be used to estimate these fluxes because oxygen isotopic exchange between C[O.sub.2] and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil C[O.sub.2] hydration. Exchange was 10-300 times faster than the uncatalyzed rate, consistent with typical population sizes for CAcontaining soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global CO[sup.18]O budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of C[O.sub.2] gross fluxes obtained by using the [delta][sup.18]O of atmospheric C[O.sub.2] by 50%. carbon cycle | water cycle | carbonic anhydrase | oxygen isotopes | terrestrial biosphere doi/10.1073/pnas.0905210106

The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons. Here we present a 2.1-million-year record of sea surface partial pressure of C[O.sub.2] (PC[O.sub.2]), based on boron isotopes in planktic foraminifer shells, which suggests that the atmospheric partial pressure of C[O.sub.2] (pC[O.sub.2]) was relatively stable before the mid-Pleistocene climate transition. Glacial PC[O.sub.2] was ~31 microatmospheres higher before the transition (more than 1 million years ago), but interglacial PC[O.sub.2] was similar to that of [ate Pleistocene interglacial cycles (

Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric C[O.sub.2] concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [C[O.sub.2]] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated C[O.sub.2] (ambient + 340 ppm) accelerated soil elevation gain by 3.9 mm [yr.sup.-1] in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the C[O.sub.2] effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, C[O.sub.2], may paradoxically aid some coastal wetlands in counter-balancing rising seas. coastal wetlands | nitrogen pollution | tidal marsh loss | root productivity | salinity

Atmospheric aerosol deposition is an important source of nutrients and trace metals to the open ocean that can enhance ocean productivity and carbon sequestration and thus influence atmospheric carbon dioxide concentrations and climate. Using aerosol samples from different back trajectories in incubation experiments with natural communities, we demonstrate that the response of phytoplankton growth to aerosol additions depends on specific components in aerosols and differs across phytoplankton species. Aerosol additions enhanced growth by releasing nitrogen and phosphorus, but not all aerosols stimulated growth. Toxic effects were observed with some aerosols, where the toxicity affected picoeukaryotes and Synechococcus but not Prochlorococcus. We suggest that the toxicity could be due to high copper concentrations in these aerosols and support this by laboratory copper toxicity tests preformed with Synechococcus cultures. However, it is possible that other elements present in the aerosols or unknown synergistic effects between these elements could have also contributed to the toxic effect. Anthropogenic emissions are increasing atmospheric copper deposition sharply, and based on coupled atmosphere-ocean calculations, we show that this deposition can potentially alter patterns of marine primary production and community structure in high aerosol, low chlorophyll areas, particularly in the Bay of Bengal and downwind of South and East Asia.

Wind-driven upwelling in the ocean around Antarctica helps regulate the exchange of carbon dioxide (C[O.sub.2]) between the deep sea and the atmosphere, as well as the supply of dissolved silicon to the euphoric zone of the Southern Ocean. Diatom productivity south of the Antarctic Polar Front and the subsequent burial of biogenic opal in underlying sediments are limited by this silicon supply. We show that opal burial rates, and thus upwelling, were enhanced during the termination of the last ice age in each sector of the Southern Ocean. In the record with the greatest temporal resolution, we find evidence for two intervals of enhanced upwelling concurrent with the two intervals of rising atmospheric C[O.sub.2] during deglaciation. These results directly link increased ventilation of deep water to the deglacial rise in atmospheric C[O.sub.2].

Climate science as we know it today did not exist in the 1960s and 1970s. The integrated enterprise embodied in the Nobel Prizewinning work of the Intergovernmental Panel on Climate Change existed then as separate threads of research pursued by isolated groups of scientists. Atmospheric chemists and modelers grappled with the measurement of changes in carbon dioxide and atmospheric gases, and the changes in climate that might result. Meanwhile, geologists and paleoclimate researchers tried to understand when Earth slipped into and out of ice ages, and why. An enduring popular myth suggests that in the 1970s the climate science community was predicting 'global cooling' and an 'imminent' ice age, an observation frequently used by those who would undermine what climate scientists say today about the prospect of global warming. A review of the literature suggests that, on the contrary, greenhouse warming even then dominated scientists' thinking as being one of the most important forces shaping Earth's climate oil human time scales. More importantly than showing the falsehood of the myth, this review describes how scientists of the time built the foundation on which the cohesive enterprise of modern climate science now rests.

Changing temperature and precipitation pattern and increasing concentrations of atmospheric C[O.sub.2] are likely to drive significant modifications in natural and modified forests. Our review is focused on recent publications that discuss the changes in commercial forestry, excluding the ecosystem functions of forests and nontimber forest products. We concentrate on potential direct and indirect impacts of climate change on forest industry, the projections of future trends in commercial forestry, the possible role of biofuels, and changes in supply and demand. C[O.sub.2] | economics | industrial forestry | biofuels

Growth and distribution of coarse roots in time and space represent a gap in our understanding of belowground ecology. Large roots may play a critical role in carbon sequestration belowground. Using ground-penetrating radar (GPR), we quantified coarseroot biomass from an open-top chamber experiment in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA. GPR propagates electromagnetic waves directly into the soil and reflects a portion of the energy when a buried object is contacted. In our study, we utilized a 1500 MHz antenna to establish correlations between GPR signals and root biomass. A significant relationship was found between GPR signal reflectance and biomass ([R.sup.2]= 0.68). This correlation was applied to multiple GPR scans taken from each open-top chamber (elevated and ambient C[O.sub.2]). Our results showed that plots receiving elevated C[O.sub.2] had significantly (P = 0.049) greater coarse-root biomass compared to ambient plots, suggesting that coarse roots may play a large role in carbon sequestration in scrub-oak ecosystems. This nondestructive method holds much promise for rapid and repeatable quantification of coarse roots, which are currently the most elusive aspect of long-term belowground studies. Key words: carbon dioxide; coarse-root biomass; ground-penetrating radar; roots; scrub oak.

The amplitude of the C[O.sub.2] seasonal cycle at the Mauna Loa Observatory (MLO) increased from the early 1970s to the early 1990s but decreased thereafter despite continued warming over northern continents. Because of its location relative to the large-scale atmospheric circulation, the MLO receives mainly Eurasian air masses in the northern hemisphere (NH) winter but relatively more North American air masses in NH summer. Consistent with this seasonal footprint, our findings indicate that the MLO amplitude registers North American net carbon uptake during the warm season and Eurasian net carbon release as well as anomalies in atmospheric circulation during the cold season. From the early 1970s to the early 1990s, our analysis was consistent with that of Keeling et al. [Keeling CD, Chin JFS, Whorf TP (1996) Nature 382:146-149], suggesting that the increase in the MLO C[O.sub.2] amplitude is dominated by enhanced photosynthetic drawdown in North America and enhanced respiration in Eurasia. In contrast, the recent decline in the C[O.sub.2] amplitude is attributed to reductions in carbon sequestration over North America associated with severe droughts from 1998 to 2003 and changes in atmospheric circulation leading to decreased influence of Eurasian air masses. With the return of rains to the U.S. in 2004, both the normalized difference vegetation index and the MLO amplitude sharply increased, suggesting a return of the North American carbon sink to more normal levels. These findings indicate that atmospheric C[O.sub.2] measurements at remote sites can continue to play an important role in documenting changes in land carbon flux, including those related to widespread drought, which may continue to worsen as a result of global warming. atmospheric circulation | atmospheric C[O.sub.2] seasonal cycle | terrestrial carbon sinks | continental droughts

The isotopic composition of long-lived trace molecules provides a window into atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer, because its abundance remains >100 parts per million by volume (ppmv) in the mesosphere. Here, we successfully reproduce the isotopic composition of C[O.sub.2] in the middle atmosphere, which has not been previously reported. The mass-independent fractionation of oxygen in C[O.sub.2] can be satisfactorily explained by the exchange reaction with O([sup.1]D). in the stratosphere, the major source of O([sup.1]D) is [O.sub.3] photolysis. Higher in the mesosphere, we discover that the photolysis of [sup.16][O.sup.17]O and [sup.16][O.sup.18]O by solar Lyman-[alpha] radiation yields O([sup.1]D) 10-100 times more enriched in [sup.17]O and [sup.18]O than that from ozone photodissociation at lower altitudes. This latter source of heavy O([sup.1]D) has not been considered in atmospheric simulations, yet it may potentially affect the 'anomalous' oxygen signature in tropospheric C[O.sub.2] that should reflect the gross carbon fluxes between the atmosphere and terrestrial biosphere. Additional laboratory and atmospheric measurements are therefore proposed to test our model and validate the use of C[O.sub.2] isotopic fractionation as a tracer of atmospheric chemical and dynamical processes. biogeochemical cycles | C[O.sub.2] | mesosphere | stratosphere

The effects of long-term elevated atmospheric CO2 on root production and microbial activity, biomass, and diversity in a chaparral ecosystem in southern California are examined. The fungal biomass is correlated with cellulose and amylase activities, which indicates that fungi are responsible for the stimulation of enzymatic activity in the free air CO2 enrichment (FACE) treatment.

In a stomatal frequency analysis of leaf remains of Quercus nigra, Acer rubrum, Myrica cerifera, Ilex cassine, and Osmunda regalis that were preserved in precisely dated peat deposits of north-central Florida, the stomatal index decreased as a response to an atmospheric C[O.sub.2] increase from 310 ppmv to 370 ppmv over the past 60 years. The observations indicate that C[O.sub.2] responsiveness may occur in different canopy levels of hardwood-swamp vegetation. Apart from common woody plants, long-lived ferns of the undergrowth appear to be affected by C[O.sub.2] changes. Response rates are most pronounced in M. cerifera, I. cassine, and O. regalis. The potential of these species for quantifying past atmospheric C[O.sub.2] levels is assessed by a combined analysis of the well-dated buried leaf record and herbarium material collected during the past century. Leaf remains of the widely occurring species M. cerifera and I. cassine are concluded to be highly suitable for C[O.sub.2] reconstructions, by which the application range of the stomatal frequency proxy is extended into the warm-temperate to subtropical realm of North America. Key words: atmospheric C[O.sub.2]; Florida: peat deposits: stomatal index: subfossil leaves.

A study of transportation, fuel costs, and environmental pollution in ten European Union accession countries indicates the need for future energy policies.

Issues related to silicate weathering in Iceland are examined, focusing on the role of plants in the removal of carbon dioxide from the atmosphere. Topics include the weathering of calcium and magnesium silicate minerals and the roles trees and vascular plants play in increasing weathering.

The authors researched atmospheric carbon dioxide levels from the Mesozoic through the Oligocene by analyzing poleosol carbonates. Results included the compilation of a new database recording the fluctuations through the eras.

The results of a compound-specific carbon-isotope study of Cenomanian/Turonian (C/T) sediments from the southern part of the North Atlantic Ocean, are presented. The abruptness and magnitude of the atmospheric CO(sub2) concentration change is quantified. The data indicates that plants utilizing the C(sub3)-type photosynthetic pathway were succeeded by plants using the C(sub4)-type pathway.

It is not certain whether coral reefs are sources of or sinks for atmospheric [CO.sub.2]. Air-sea exchange of [CO.sub.2] over reefs has been measured directly and inferred from changes in the seawater carbonate equilibrium. Such measurements have provided conflicting results. We provide community metabolic data that indicate that large changes in [CO.sub.2] concentration can occur in coral reef waters via biogeochemical processes not directly associated with photosynthesis, respiration, calcification, and Ca[CO.sub.3] dissolution. These processes can significantly distort estimates of reef calcification and net productivity and obscure the contribution of coral reefs to global air-sea exchange of [CO.sub.2]. They may, nonetheless, explain apparent anomalies in the metabolic performance of reefs close to land and reconcile the differing experimental findings that have given rise to the [CO.sub.2] debate.

The climate of the boreal zone has warmed significantly in this century and is predicted to warm further in the next century[1]. The seasonally and perennially frozen soils of boreal [...], We used eddy covariance; gas-exchange chambers; radiocarbon analysis; wood, moss, and soil inventories; and laboratory incubations to measure the carbon balance of a 120-year-old black spruce forest in Manitoba, Canada. The site lost 0.3 ± 0.5 metric ton of carbon per hectare per year (ton C [ha.sup.-1] [year.sup.-1]) from 1994 to 1997, with a gain of 0.6 ± 0.2 ton C [ha.sup.-1] [year.sup.-1] in moss and wood offset by a loss of 0.8 ± 0.5 ton C [ha.sup.-1] [year.sup.-1] from the soil. The soil remained frozen most of the year, and the decomposition of organic matter in the soil increased 10-fold upon thawing. The stability of the soil carbon pool (~150 tons C [ha.sup.-1]) appears sensitive to the depth and duration of thaw, and climatic changes that promote thaw are likely to cause a net efflux of carbon dioxide from the site.