Future Atmospheric CO2 Levels Research Articles

Sieving soil before incubation experiments overestimates carbon mineralization but underestimates temperature sensitivity

The sensitivity of soil organic carbon (SOC) mineralization to temperature could affect the future atmospheric CO2 levels under global warming. Sieved soils are widely used to assess SOC mineralization and its temperature sensitivity (Q10) via laboratory incubation. However, sieved soils cause a temporary increase in mineralization due to the destruction of soil structure, which can affect estimates of SOC mineralization, especially in soils managed with no-till (NT). To identify the effects of soil sieving on SOC mineralization and Q10, soil was collected from an 11-year field experiment under a wheat-maize cropping system managed with a combination of tillage [NT and plow tillage (PT)] and residue [residue returning (RR) and residue removal (R0)]. Soil was either sieved or left in an undisturbed state and incubated at 15 °C and 25 °C. SOC mineralization in sieved soils at 25 °C was 47.28 g C kg−1 SOC, 160.1% higher than SOC mineralization in undisturbed soils (P < 0.05). Interestingly, Q10 values in sieved soils were 1.29, 77.6% lower than Q10 in undisturbed soils (P < 0.05). Highly significant correlations (P < 0.01) were observed between sieved and undisturbed soils for SOC mineralization (r = 0.85–0.98) and Q10 (r = 0.78–0.87). Soil macro-aggregates had lower SOC mineralization by 6.1–21.9%, but higher Q10 values by 4.7–6.5% compared with micro-aggregates, contributing to lower mineralization and higher Q10 under NT and RR. Furthermore, structure equation and random forest modelling showed that increased SOC contents in NT and RR could not only reduce SOC mineralization, but also constrained the improvement of Q10 in NT and RR. Overall, these results indicated that although sieved soils overestimated SOC mineralization and underestimated Q10 due to the destruction of macro-aggregates, the patterns between treatments were similar and sieving soil for incubation is considered as a suitable approach to evaluate the relative impacts of NT and RR on SOC mineralization and Q10.

Carbonate chemistry of an in-situ free-ocean CO2 enrichment experiment (antFOCE) in comparison to short term variation in Antarctic coastal waters

Free-ocean CO2 enrichment (FOCE) experiments have been deployed in marine ecosystems to manipulate carbonate system conditions to those predicted in future oceans. We investigated whether the pH/carbonate chemistry of extremely cold polar waters can be manipulated in an ecologically relevant way, to represent conditions under future atmospheric CO2 levels, in an in-situ FOCE experiment in Antarctica. We examined spatial and temporal variation in local ambient carbonate chemistry at hourly intervals at two sites between December and February and compared these with experimental conditions. We successfully maintained a mean pH offset in acidified benthic chambers of −0.38 (±0.07) from ambient for approximately 8 weeks. Local diel and seasonal fluctuations in ambient pH were duplicated in the FOCE system. Large temporal variability in acidified chambers resulted from system stoppages. The mean pH, Ωarag and fCO2 values in the acidified chambers were 7.688 ± 0.079, 0.62 ± 0.13 and 912 ± 150 µatm, respectively. Variation in ambient pH appeared to be mainly driven by salinity and biological production and ranged from 8.019 to 8.192 with significant spatio-temporal variation. This experiment demonstrates the utility of FOCE systems to create conditions expected in future oceans that represent ecologically relevant variation, even under polar conditions.

Causes of uncertainty in China's net primary production over the 21st century projected by theCMIP5Earth system models

ABSTRACTNet primary production is the initial step of the carbon cycle in which atmosphericCO2is fixed by plants. The responses of net primary production (NPP) to climate change andCO2are key processes that have the potential to significantly affect the climate–carbon feedback and future atmosphericCO2levels. Understanding futureNPPchanges is important for China that became the world's largestCO2emitter since 2006. Here, we analysedNPPchanges in China under the four emission scenarios from 11 Earth system models participating in the Coupled Model Intercomparison Project Phase 5. We find a general increase ofNPPover the 21st century under the four emission scenarios, with the large percentage increase in northwestern China and Qinghai–Tibetan Plateau. However, there is a large model spread in the increase ofNPPat both country and local scales. We present a statistical approach to assess various processes to explain this large spread, and find that the large spread at the country level is predominantly attributed to inter‐model difference in parameterization ofCO2fertilization effect within each emission scenario. But the parameterization ofCO2fertilization effect not always dominates over the model spread across China. When it comes to the local scale, the model spread can be significantly contributed by inter‐model difference in parameterization ofNPPresponses to precipitation along with precipitation projection in northwestern China. Our findings provide the reasons for divergent responses of futureNPPthrough process decomposition and are the first to pinpoint that the model process dominating over the uncertainty exhibits regional dependence.

Spatiotemporal patterns of evapotranspiration along the North American east coast as influenced by multiple environmental changes

AbstractThe North American east coast has experienced significant land‐use and climate changes since the beginning of the 20th century. In this study, using the Dynamic Land Ecosystem Model 2.0 driven by time‐series input data of land use, climate and atmospheric CO2, we examined how these driving forces have affected the spatiotemporal trends and variability of evapotranspiration (ET) in this region during 1901–2008. Annual ET in the North American east coast during this period was 648.3 ± 38.6 mm/year and demonstrated an increasing trend. Factorial model simulations indicated that climate variability explained 76% of the inter‐annual ET variability. Although land‐use change only explained 16% of the ET temporal variability, afforestation induced the upward trend of ET and increased annual ET by 12.8 mm/year. Elevated atmospheric CO2 reduced annual ET by 0.84 mm, and its potential impacts under future atmospheric CO2 levels could be much larger than estimates for the historical 1901–2008 period. Climate change determined the spatial pattern of ET changes across the entire study area, whereas land‐use changes dramatically affected ET in watersheds with significant land conversions. In spite of the multiple benefits from afforestation, its impacts on water resources should be considered in future land‐use policy making. Elevated ET may also affect fresh water availability for the increasing social and economic water demands. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Central European hardwood trees in a high‐CO2 future: synthesis of an 8‐year forest canopy CO2 enrichment project

Summary Rapidly increasing atmospheric CO2 is not only changing the climate system but may also affect the biosphere directly through stimulation of plant growth and ecosystem carbon and nutrient cycling. Although forest ecosystems play a critical role in the global carbon cycle, experimental information on forest responses to rising CO2 is scarce, due to the sheer size of trees. Here, we present a synthesis of the only study world‐wide where a diverse set of mature broadleaved trees growing in a natural forest has been exposed to future atmospheric CO2 levels (c. 550 ppm) by free‐air CO2 enrichment (FACE). We show that litter production, leaf traits and radial growth across the studied hardwood species remained unaffected by elevated CO2 over 8 years. CO2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO2 and dissolved inorganic carbon both increased suggesting enhanced below‐ground activity. Carbon release to the rhizosphere and/or higher soil moisture primed nitrification and nitrate leaching under elevated CO2; however, the export of dissolved organic carbon remained unaltered. Synthesis. Our findings provide no evidence for carbon‐limitation in five central European hardwood trees at current ambient CO2 concentrations. The results of this long‐term study challenge the idea of a universal CO2 fertilization effect on forests, as commonly assumed in climate–carbon cycle models.

The non-steady state oceanic CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; signal: its importance, magnitude and a novel way to detect it

Abstract. The role of the ocean has been pivotal in modulating rising atmospheric CO2 levels since the industrial revolution, sequestering nearly half of all fossil-fuel derived CO2 emissions. Net oceanic uptake of CO2 has roughly doubled between the 1960s (~1 Pg C yr−1) and 2000s (~2 Pg C yr−1), with expectations that it will continue to absorb even more CO2 with rising future atmospheric CO2 levels. However, recent CO2 observational analyses along with numerous model predictions suggest the rate of oceanic CO2 uptake is already slowing, largely as a result of a natural decadal-scale outgassing signal. This recent CO2 outgassing signal represents a significant shift in our understanding of the oceans role in modulating atmospheric CO2. Current tracer-based estimates for the ocean storage of anthropogenic CO2 assume the ocean circulation and biology is in steady state, thereby missing the new and potentially important "non-steady state" CO2 outgassing signal. By combining data-based techniques that assume the ocean is in a steady state, with techniques that constrain the net oceanic CO2 uptake signal, we show how to extract the non-steady state CO2 signal from observations. Over the entire industrial era, the non-steady state CO2 outgassing signal (~13 ± 10 Pg C) is estimated to represent about 9% of the total net CO2 inventory change (~142 Pg C). However, between 1989 and 2007, the non-steady state CO2 outgassing signal (~6.3 Pg C) has likely increased to be ~18% of net oceanic CO2 storage over that period (~36 Pg C). The present uncertainty of our data-based techniques for oceanic CO2 uptake limit our capacity to quantify the non-steady state CO2 signal, however with more data and better certainty estimates across a range of diverse methods, this important and growing CO2 signal could be better constrained in the future.

Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

Future atmospheric CO2 levels will most likely have complex consequences for marine organisms, particulary photosynthetic calcifying organisms. Corallina officinalis L. is an erect calcifying macroalga found in the inter- and subtidal regions of temperate rocky coastlines and provides important substrate and refugia for marine meiofauna. The main goal of the current study was to determine the physiological responses of C. officinalis to increased CO2 concentrations expected to occur within the next century and beyond. Our results show that growth and production of inorganic material decreased under high CO2 levels, while carbonic anhydrase activity was stimulated and negatively correlated to algal inorganic content. Photosynthetic efficiency based on oxygen evolution was also negatively affected by increased CO2. The results of this study indicate that C. officinalis may become less competitive under future CO2 levels, which could result in structural changes in future temperate intertidal communities.

Ocean acidification off the south coast of Japan: A result from time series observations of CO2parameters from 1994 to 2008

[1] Ocean acidification resulting from increases in present and future atmospheric CO2 levels could seriously affect diverse coastal and oceanic ecosystems. In this work, we determine that a significant trend in ocean acidification is superposed on the large seasonal and interannual variabilities of acidity in surface waters off the south coast of Honshu, Japan, based on our repeated observations of partial pressure of CO2 (pCO2), total inorganic carbon (TCO2), and pH. Multiple regression analysis of TCO2 as a function of temperature, salinity, and timing of observations shows that TCO2 increased at a rate of +1.23 ± 0.40 μmol kg−1 yr−1 for the period 1994–2008, while no long-term change has been determined for total alkalinity calculated from TCO2 and pCO2 in seawater. These results indicate that pH and the aragonite saturation state (Ωarag) are decreasing at a rate of −0.020 ± 0.007 decade−1 and −0.12 ± 0.05 decade−1, respectively. If future atmospheric CO2 levels keep increasing as predicted by the Intergovernmental Panel on Climate Change emission scenario A1FI, which postulates intensive fossil fuel use associated with very rapid economic growth, a further reduction of −0.8 to −1.0 in Ωarag is likely in the next 50 years. Such a rapid reduction of Ωarag could have negative impacts on a variety of calcareous organisms.

Trends in the sources and sinks of carbon dioxide

Efforts to control climate change require the stabilization of atmospheric CO2 concentrations. This can only be achieved through a drastic reduction of global CO2 emissions. Yet fossil fuel emissions increased by 29% between 2000 and 2008, in conjunction with increased contributions from emerging economies, from the production and international trade of goods and services, and from the use of coal as a fuel source. In contrast, emissions from land-use changes were nearly constant. Between 1959 and 2008, 43% of each year's CO2 emissions remained in the atmosphere on average; the rest was absorbed by carbon sinks on land and in the oceans. In the past 50 years, the fraction of CO2 emissions that remains in the atmosphere each year has likely increased, from about 40% to 45%, and models suggest that this trend was caused by a decrease in the uptake of CO2 by the carbon sinks in response to climate change and variability. Changes in the CO2 sinks are highly uncertain, but they could have a significant influence on future atmospheric CO2 levels. It is therefore crucial to reduce the uncertainties.

Predicted impact of ocean acidification on a marine invertebrate: elevated CO2 alters response to thermal stress in sea urchin larvae

Ocean acidification (OA) and the biological consequences of altered seawater chemistry have emerged as a significant environmental threat to healthy marine ecosystems. Because a more acidic ocean interferes with fixation of calcium carbonate to form shells or calcified skeletons, future ocean chemistry may significantly alter the physiology of calcifying marine organisms. These alterations may manifest themselves directly in the calcification process, or have synergistic effects with other environmental factors such as elevated temperatures. New tools permit us to explore subtle changes in gene expression patterns in response to environmental conditions. We raised sea urchins (Strongylocentrotus franciscanus) under conditions simulating future atmospheric CO2 levels of 540 and 970 ppm. When larvae raised under elevated CO2 conditions were subjected to 1-h acute temperature stress, their ability to mount a physiological response (as measured by expression of the molecular chaperone hsp70) was reduced relative to those raised under ambient CO2 conditions. These results represent the first use of gene expression assays to study the effects of OA on sea urchin development. They highlight the importance of looking at multiple environmental factors simultaneously as this approach may reveal previously unsuspected biological impacts of atmospheric changes.

Climate change feedbacks on future oceanic acidification

Oceanic anthropogenic CO2 uptake will decrease both the pH and the aragonite saturation state (Ωarag) of seawater leading to an oceanic acidification. However, the factors controlling future changes in pH and Ωarag are independent and will respond differently to oceanic climate change feedbacks such as ocean warming, circulation and biological changes.We examine the sensitivity of these two CO2-related parameters to climate change feedbacks within a coupled atmosphere-ocean model. The ocean warming feedback was found to dominate the climate change responses in the surface ocean. Although surface pH is projected to decrease relatively uniformly by about 0.3 by the year 2100, we find pH to be insensitive to climate change feedbacks, where as Ωarag is buffered by˜15%. Ocean carbonate chemistry creates a situation whereby the direct pH changes due to ocean warming are almost cancelled by the pH changes associated with dissolved inorganic carbon concentrations changes via a reduction in CO2 solubility from ocean warming. We show that the small climate change feedback on future surface ocean pH is independent to the amount of ocean warming. Our analysis therefore implies that future projections of surface ocean acidification only need to consider future atmospheric CO2 levels, not climate change induced modifications in the ocean.

In situ Effects of Elevated Atmospheric CO2on Leaf Freezing Resistance and Carbohydrates in a Native Temperate Grassland

The objectives of this study were to quantify changes in leaf freezing resistance and carbohydrate concentrations caused by long-term (6 years) exposure to elevated CO2(ambient: 360μll−1, elevated: 600μll−1) in five dominant plant species growing in situ in a native temperate grassland. Across all five species tested from three functional groups, the mean temperature at which all leaves were damaged (T100) significantly (P=0.016) increased from −9.6 to −8.5°C under elevated CO2, and a similar marginally significant (P=0.079) reduction was observed for the mean temperature that caused 50% leaf damage (T50), from −6.7 to −6.0°C. The mean temperature at which initial leaf damage was observed (T0) was not significantly influenced by elevated CO2. Although concentrations of soluble sugars (+25%,P=0.042), starch (+53%, P<0.001), and total non-structural carbohydrates (TNC, +40%, P<0.001) were significantly higher under elevated CO2, leaf freezing resistance actually decreased under elevated CO2. Concentrations of soluble sugars were positively correlated with freezing resistance when viewed across all five community dominants, but within any individual species, no such relationships were found. We also found no evidence for our original hypothesis that increased concentrations of soluble sugars increase freezing resistance. Thus, future atmospheric CO2levels may instead increase the risk of late spring freezing damage. Furthermore, the strong differences in freezing resistance observed among the species, along with decreased freezing resistance, may increase the risk of losing species that have inherently weak freezing resistances from the plant community.

The Importance of Feedback Processes and Vegetation Transition in the Terrestrial Carbon Cycle

The Integrated Model to Assess the Greenhouse Effect (IMAGE 2) is developed to simulate dynamically the global society-biosphere-climate system. The terrestrial C cycle model is an important component of this model. It is implemented on a grid, runs on a annual time step and simulates the major C fluxes between the atmosphere and the biosphere and controls the storage capacity of C in different compartments. The C fluxes are influenced by the direct and indirect response of ecosystems to changing atmospheric CO2 concentrations and climates. Each ecosystem is characterized by its NPP, which depends on environmental conditions. Incorporation of grid cell specific feedback processes is an innovation in this model. Implemented feedback processes are CO2 fertilization, effects of climate change on photosynthesis, plant respiration and decomposition and shifts in vegetation patterns due to climate change and changes in water use efficiency. This paper presents an evaluation of the importance of feedback processes on global and regional scales. Tempera- ture feedback on plant growth is the most important feedback process at the global scale, while CO2 fertilization is of lesser importance. However, CO2 fertilization is the most significant feedback in low latitudes, while temperature change most strongly influences the C cycle in the high latitudes. The C dynamics of land cover change, including feedbacks, depend on characteristics of the former and actual land cover type. The shifts in land cover are parameterized so that they mimic succession. The course and duration of the transitional phase strongly affect the C fluxes. We applied both an equilibrium approach with instantaneous transitions and dy- namic approaches with gradual type-dependent transitions. Transitions in natural vegetation between 1990 and 2050 involve about one-third of all grid cells, especially in the higher latitudes. The impact of vegetation transitions on C fluxes differs depending on the instantaneous or gradual tran- sition strategy. The latter results in higher and delayed NEP fluxes, resulting in significantly different atmospheric CO2 concentrations. The dynamic approach is more realistic and should be included in integrated assessment models that project future atmospheric CO2 levels.

The Global Carbon Dioxide Budget

The increase in atmospheric CO 2 levels during the last deglaciation was comparable in magnitude to the recent historical increase. However, global CO 2 budgets for these changes reflect fundamental differences in rates and in sources and sinks. The modern oceans are a rapid net CO 2 sink, whereas the oceans were a gradual source during the deglaciation. Unidentified terrestrial CO 2 sinks are important uncertainties in both the deglacial and recent CO 2 budgets. The deglacial CO 2 budget represents a complexity of long-term dynamic behavior that is not adequately addressed by current models used to forecast future atmospheric CO 2 levels.

Ocean flux studies: A status report

Marine biogenic cycles play a key role in controlling atmospheric CO2 concentrations. To predict future atmospheric CO2 levels and to interpret past changes that may have been associated with global climatic events, it is necessary to determine on a global scale the rates at which carbon is cycled through the ocean and the factors that may alter the rates of transfer. Because of uncertainties and limitations in existing measurement techniques and the extreme spatial and temporal variability of ocean properties and processes, studies have not yet produced an accurate, comprehensive description of the marine carbon cycle. While some advances in the measurement of specific components of the carbon cycle have been made, the fundamental problem of undersampling an extremely heterogeneous ocean has remained. Recently, satellite‐deployed sensors have provided large‐scale, nearly synoptic images of ocean surface water properties, such as temperature and color, which have revealed mesoscale features not observed previously. In addition, moored and free‐drifting observation systems are being developed to determine directly the temporal variability of surface and subsurface water characteristics. With these remotely sensed properties to provide the space and time framework by which individual measurements of ocean processes and material fluxes can be extrapolated throughout ocean basins and over decadal time scales, we are poised to make a fundamental improvement in our understanding of ocean elemental cycles.