Rapid Forcing Research Articles

The Advancement in Spring Vegetation Phenology in the Northern Hemisphere Will Reverse After 2060 Under Future Moderate Warming Scenarios

AbstractGlobal warming has largely advanced spring vegetation phenology, which has subsequently affected terrestrial carbon and water cycles. However, further shifts in vegetation phenology under future climate change remain unclear. We estimated the start of the growing season (SOS) by applying multiple extraction methods based on the NDVI3g data set, and then parameterized and evaluated 11 spring vegetation phenology models that included chilling, forcing, and the photoperiod. Based on scenario data from three Shared Socioeconomic Pathways (SSP126, SSP245, and SSP585) derived from eight climate models, future vegetation phenology was predicted using the phenology models. Results showed that all the phenology models performed better than the NULL model (mean of the SOS), with the performance of one‐phase models broadly matching that of two‐phase models, although the best models varied by vegetation type. The spatial pattern of simulated SOS was similar among the models, and it explained >75% of the variation. Based on the mean predicted SOS, we found that spring vegetation phenology will continue to advance under strong warming conditions (SSP245 and SSP585), but that the trend of advance will reverse at around 2060 under the SSP126 scenario. The continued trend in SOS advance is likely related to rapid forcing fulfillment under stronger warming conditions. However, under moderate warming, chilling might be reduced and it might require longer to compensate for higher forcing, which ultimately would result in SOS delay. Our findings highlight that trends will likely change under different warming conditions, potentially causing widespread impact on species interaction, biodiversity, and ecosystem function.

Mega-city development impact on hourly extreme rainfall over the South China Greater Bay Area under near-future climate warming

The impacts of near-future urban development and global warming forcing on hourly extreme rainfall over the South China Greater Bay Area (GBA) area have been investigated, by dynamically downscaling General Circulation Model (GCM) outputs using the Weather Research and Forecasting Model (WRF) at convection-permitting resolution, coupled with an Urban Canopy Model (UCM). Three downscaling experiments corresponding to different urban land cover (1999 and projected 2030) and climate (1951-to-2000 and 2001-to-2050 GCM simulations) were designed. Effects of near-future climate change and 1999-to-2030 urban development on GBA extreme precipitation were then examined, using boundary conditions derived from GBA extreme rainfall events in the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2M) historical and RCP8.5 simulations. Results show that climate change and rapid urban development forcing have comparable positive effects on the intensity as well as heavy hourly rainfall probability over the GBA urban area. Global warming tends to increase heavy rainfall probability (from 40 to 60 mm/h) by about 1.3 to 1.8 times, but at the same time suppress the probability of light rainfall (from 1 to 10 mm/h) by about 20%. Urban development increases urban rainfall probability within the whole range of intensity, with frequency for very heavy rainfall (> 90 mm/h) almost doubled. It is worth mentioning that impacts due to rapid urban development can be as important as global warming forcing in the near future in exacerbating hourly extreme rainfall over the GBA coastal megacity.

Laurentide Ice Saddle Mergers Drive Rapid Sea Level Drops During Glaciations

AbstractRecent glacial periods have included several periods of rapid sea level drop that are still not well understood. Here we show that rapid sea level drops can occur due to merger of two separate ice sheets without correspondingly rapid climate forcing. Using the Parallel Ice Sheet Model (PISM), we simulate glaciation of the Laurentide Ice Sheet (LIS) by gradually decreasing equilibrium‐line altitude (ELA). Merger of the extended Keewatin sector of the Laurentide Ice Sheet with Labrador sector of the Laurentide Ice Sheet south of Hudson Bay causes a positive feedback between increasing surface elevation and increasing surface mass balance in the merger region, leading to fast ice sheet growth. The simulated saddle merger of LIS lowers sea level by 20 m in less than 20 kyr with periods of sea level fall exceeding 2 m/kyr, similar to those observed in paleo‐sea level records.

Antarctic ice dynamics amplified by Northern Hemisphere sea-level forcing.

Sea-level rise due to ice loss in the Northern Hemisphere in response to insolation and greenhouse gas forcing is thought to have caused grounding-line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS)1-3. Such interhemispheric sea-level forcing may explain the synchronous evolution of global ice sheets over ice-age cycles. Recent studies that indicate that the AIS experienced substantial millennial-scale variability during and after the last deglaciation4-7 (roughly 20,000 to 9,000 years ago) provide further evidence of this sea-level forcing. However, global sea-level change as a result of mass loss from ice sheets is strongly nonuniform, owing to gravitational, deformational and Earth rotational effects8, suggesting that the response of AIS grounding lines to Northern Hemisphere sea-level forcing is more complicated than previously modelled1,2,6. Here, using an ice-sheet model coupled to a global sea-level model, we show that AIS dynamics are amplified by Northern Hemisphere sea-level forcing. As a result of this interhemispheric interaction, a large or rapid Northern Hemisphere sea-level forcing enhances grounding-line advance and associated mass gain of the AIS during glaciation, and grounding-line retreat and mass loss during deglaciation. Relative to models without these interactions, the inclusion of Northern Hemisphere sea-level forcing in our model increases the volume of the AIS during the Last Glacial Maximum (about 26,000 to 20,000 years ago), triggers an earlier retreat of the grounding line and leads to millennial-scale variability throughout the last deglaciation. These findings are consistent with geologic reconstructions of the extent of the AIS during the Last Glacial Maximum and subsequent ice-sheet retreat, and with relative sea-level change in Antarctica3-7,9,10.

Vegetation changes and plant wax biomarkers from an ombrotrophic bog define hydroclimate trends and human-environment interactions during the Holocene in northern Norway

Holocene climate records from northern Europe improve our understanding of important North Atlantic ocean and atmospheric circulation systems to long-term insolation-driven changes, as well as more rapid forcing and feedback mechanisms. Here we assess Holocene climate and environmental changes in northern Norway based on the analysis of pollen, non-pollen palynomorphs, plant macrofossils, and plant wax biomarkers from a high latitude ombrotrophic bog. We define the extent and thickness of Hollabåttjønnen Bog (0.16 km2), which is located 10 km north of Tromsø. Several cores were analyzed, including a 5.16-m core that spans the last 9.5 cal ka BP. Vegetation changes from several sites were reconstructed and the distribution and hydrogen isotopic composition (δD) of n-alkanes (C21–C33) were analyzed. Our data show several distinct climate intervals that primarily indicate changes in bog surface moisture. In the early Holocene (c. 9.5–7.7 cal ka BP), wetter conditions are defined by the presence of wetland sedges and grasses, higher concentrations of mid-chain length n-alkanes, and a similarity in δD values among homologs. A dry mid-Holocene (c. 7.7–3.8 cal ka BP) is inferred from the presence of a heath shrubland, low peat accumulations rates, and significant differences between δD values of mid- and long-chain length n-alkanes. The late Holocene (c. 3.8 cal ka BP-present) is marked by the onset of wetter conditions, lateral bog expansion, and an increase in sedges and grasses. The Hollabåttjønnen Bog record is also significant because its margins were an important location for human settlement. We correlate early Holocene environmental conditions with changes in Stone Age structures recently excavated, and we identify the occurrence of coprophilous fungi, such as Sporormiella and Sordaria, likely associated with reindeer grazing activity beginning c. 1 cal ka BP. This site therefore provides important regional paleoclimate information as well as context for evaluating local prehistoric human-environment interactions.

The Nature of the Stochastic Wind Forcing of ENSO

El Niño–Southern Oscillation (ENSO) is commonly viewed as a low-frequency tropical mode of coupled atmosphere–ocean variability energized by stochastic wind forcing. Despite many studies, however, the nature of this broadband stochastic forcing and the relative roles of its high- and low-frequency components in ENSO development remain unclear. In one view, the high-frequency forcing associated with the subseasonal Madden–Julian oscillation (MJO) and westerly wind events (WWEs) excites oceanic Kelvin waves leading to ENSO. An alternative view emphasizes the role of the low-frequency stochastic wind components in directly forcing the low-frequency ENSO modes. These apparently distinct roles of the wind forcing are clarified here using a recently released high-resolution wind dataset for 1990–2015. A spectral analysis shows that although the high-frequency winds do excite high-frequency Kelvin waves, they are much weaker than their interannual counterparts and are a minor contributor to ENSO development. The analysis also suggests that WWEs should be viewed more as short-correlation events with a flat spectrum at low frequencies that can efficiently excite ENSO modes than as strictly high-frequency events that would be highly inefficient in this regard. Interestingly, the low-frequency power of the rapid wind forcing is found to be higher during El Niño than La Niña events, suggesting a role also for state-dependent (i.e., multiplicative) noise forcing in ENSO dynamics.

Curved Density Fronts: Cyclogeostrophic Adjustment and Frontogenesis

AbstractCurvature can play a significant role in the dynamics of density fronts at small scales and in low-latitude regions of the ocean. Fronts can be displaced from balance by rapid forcing and undergo an adjustment toward a more stable state or be strained and sharpened by surrounding flow in a process known as frontogenesis. This study investigates the role of curvature in adjustment and frontogenesis using the idealized configuration of an axisymmetric eddy and associated circular front. As a result of the curvature, the balanced state of this system is not geostrophic balance, where pressure and Coriolis forces exactly balance, but cyclogeostrophic balance, where pressure and Coriolis forces combine to supply a net inwards centripetal force on fluid parcels. The parameter range for which cyclogeostrophically balanced states exist for a given unbalanced initial condition is determined. This parameter range is smaller for anticyclonic fronts (i.e., fronts curved around a warm core), which have larger angular velocities than comparable straight fronts, implying they are more likely to break down during adjustment. The reverse is true for cyclonic fronts. A model for the sharpening of a curved front in a background strain flow, analogous to the Hoskins and Bretherton (1972) model for a straight front, is developed. Relative to a straight front subject to the same strain rate, vertical velocities are weaker for an anticyclonic front and stronger for a cyclonic front. Anticyclonic fronts collapse to a near discontinuity during frontogenesis far more rapidly than cyclonic fronts for the same strain rate.

The HMGB1 protein induces a metabolic type of tumour cell death by blocking aerobic respiration.

The high-mobility group box 1 (HMGB1) protein has a central role in immunological antitumour defense. Here we show that natural killer cell-derived HMGB1 directly eliminates cancer cells by triggering metabolic cell death. HMGB1 allosterically inhibits the tetrameric pyruvate kinase isoform M2, thus blocking glucose-driven aerobic respiration. This results in a rapid metabolic shift forcing cells to rely solely on glycolysis for the maintenance of energy production. Cancer cells can acquire resistance to HMGB1 by increasing glycolysis using the dimeric form of PKM2, and employing glutaminolysis. Consistently, we observe an increase in the expression of a key enzyme of glutaminolysis, malic enzyme 1, in advanced colon cancer. Moreover, pharmaceutical inhibition of glutaminolysis sensitizes tumour cells to HMGB1 providing a basis for a therapeutic strategy for treating cancer.

Early warning signals: the charted and uncharted territories

The realization that complex systems such as ecological communities can collapse or shift regimes suddenly and without rapid external forcing poses a serious challenge to our understanding and management of the natural world. The potential to identify early warning signals that would allow researchers and managers to predict such events before they happen has therefore been an invaluable discovery that offers a way forward in spite of such seemingly unpredictable behavior. Research into early warning signals has demonstrated that it is possible to define and detect such early warning signals in advance of a transition in certain contexts. Here we describe the pattern emerging as research continues to explore just how far we can generalize these results. A core of examples emerges that shares three properties: the phenomenon of rapid regime shifts, a pattern of 'critical slowing down' that can be used to detect the approaching shift, and a mechanism of bifurcation driving the sudden change. As research has expanded beyond these core examples, it is becoming clear that not all systems that show regime shifts exhibit critical slowing down, or vice versa. Even when systems exhibit critical slowing down, statistical detection is a challenge. We review the literature that explores these edge cases and highlight the need for (a) new early warning behaviors that can be used in cases where rapid shifts do not exhibit critical slowing down, (b) the development of methods to identify which behavior might be an appropriate signal when encountering a novel system; bearing in mind that a positive indication for some systems is a negative indication in others, and (c) statistical methods that can distinguish between signatures of early warning behaviors and noise.

Sinusoidal high‐intensity exercise does not elicit ventilatory limitation in chronic obstructive pulmonary disease

During exercise at critical power (CP) in chronic obstructive pulmonary disease (COPD) patients, ventilation approaches its maximum. As a result of the slow ventilatory dynamics in COPD, ventilatory limitation during supramaximal exercise might be escaped using rapid sinusoidal forcing. Nine COPD patients [age, 60.2 ± 6.9 years; forced expiratory volume in the first second (FEV(1)), 42 ± 17% of predicted; and FEV(1)/FVC, 39 ± 12%] underwent an incremental cycle ergometer test and then four constant work rate cycle ergometer tests; tolerable duration (t(lim)) was recorded. Critical power was determined from constant work rate testing by linear regression of work rate versus 1/t(lim). Patients then completed fast (FS; 60 s period) and slow (SS; 360 s period) sinusoidally fluctuating exercise tests with mean work rate at CP and peak at 120% of peak incremental test work rate, and one additional test at CP; each for a 20 min target. The value of t(lim) did not differ between CP (19.8 ± 0.6 min) and FS (19.0 ± 2.5 min), but was shorter in SS (13.2 ± 4.2 min; P < 0.05). The sinusoidal ventilatory amplitude was minimal (37.4 ± 34.9 ml min(-1) W(-1)) during FS but much larger during SS (189.6 ± 120.4 ml min(-1) W(-1)). The total ventilatory response in SS reached 110 ± 8.0% of the incremental test peak, suggesting ventilatory limitation. Slow components in ventilation during constant work rate and FS exercises were detected in most subjects and contributed appreciably to the total response asymptote. The SS exercise was associated with higher mid-exercise lactate concentrations (5.2 ± 1.7, 7.6 ± 1.7 and 4.5 ± 1.3 mmol l(-1) in FS, SS and CP). Large-amplitude, rapid sinusoidal fluctuation in work rate yields little fluctuation in ventilation despite reaching 120% of the incremental test peak work rate. This high-intensity exercise strategy might be suitable for programmes of rehabilitative exercise training in COPD.

CUDA Programming

The humble graphics card is now an object of fascination and desire to more than just gamers. The raw processing power of these devices is being harnessed for heavy lifting when it comes to a broad range of processing-intensive applications. And in security circles, it is used – sometimes notoriously – for rapid brute forcing,

Response of Atmospheric Biomarkers to NOx-Induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M Dwarf Stars

Understanding whether M dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N(2)), which leads to production of nitrogen oxides (NO(x)) in the planetary atmosphere, hence affecting biomarkers such as ozone (O(3)). We apply a stationary model, that is, without a time dependence; hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. We estimate the NO(x) production using an air shower approach and evaluate the implications using a climate-chemical model of the planetary atmosphere. O(3) formation proceeds via the reaction O+O(2)+M→O(3)+M. At high NO(x) abundances, the O atoms arise mainly from NO(2) photolysis, whereas on Earth this occurs via the photolysis of molecular oxygen (O(2)). For the flaring case, O(3) is mainly destroyed via direct titration, NO+O(3)→NO(2)+O(2), and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O(3), Rayleigh scattering by the main atmospheric gases (O(2), N(2), and CO(2)) became more important for shielding the planetary surface from UV radiation. A major result of this work is that the biomarker O(3) survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide (N(2)O) could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.

Suppression of hysteresis in a forced van der Pol–Duffing oscillator

This paper examines the suppression of hysteresis in a forced nonlinear self-sustained oscillator near the fundamental resonance. The suppression is studied by applying a rapid forcing on the oscillator. Analytical treatment based on perturbation analysis is performed to capture the entrainment zone, the quasiperiodic modulation domain and the hysteresis area as well. The analysis leads to a strategy for the suppression of hysteresis occurring between 1:1 frequency-locked motion and quasiperiodic response. This hysteresis suppression causes the disappearance of nonlinear effects leading to a smooth transition between the quasiperiodic and the frequency-locked responses. Specifically, it appears that a rapid forcing introduces additional apparent nonlinear stiffness which can effectively suppress hysteresis in a certain range of the rapid excitation frequency. This work was motivated by the important issue of controlling and eliminating hysteresis often undesirable in mechanical systems, in general, and in application to microscale devices, especially.

Exponential averaging under rapid quasiperiodic forcing

We derive estimates on the magnitude of the interaction between a wide class of analytic partial differential equations and a high-frequency quasiperiodic oscillator. Assuming high regularity of initial conditions, the equations are transformed to an uncoupled system of an infinite dimensional dynamical system and a linear quasiperiodic flow on a torus; up to coupling terms which are exponentially small in the smallest frequency of the oscillator. The main technique is based on a careful balance of similar results for ordinary differential equations by Simó, Galerkin approximations and high regularity of the initial conditions. Similar finite order estimates assuming less regularity are also provided. Examples include reaction-diffusion and non-linear Schrödinger equations.

Identifying Nonstationarity in Turbulence Series

Because of rapid forcing by varying cloud and sky conditions, turbulence time series collected in the atmospheric surface layer over land may often be nonstationary. The meteorological community, however, has no consensus definition of what nonstationarity is and, thus, no consensus method for how to identify it. This study, therefore, adopts definitions for first-order and second-order stationarity taken from the time series analysis literature and implements new analysis techniques and probabilistic tests to quantify first-order and second-order nonstationarity. First-order nonstationarity manifests as a change in the series mean; second-order nonstationarity, as a change in the variance. The analysis identifies nonstationarity in surface-level turbulent temperature and water vapour series collected during two sample days with solar forcing influenced by cirrus and cirrostratus clouds, but that nonstationarity is not as severe as expected despite the rapid thermal forcing by these clouds. On the other hand, even with negligible cloud forcing, both sample days exhibited severe nonstationarity at night.

Global terrestrial carbon storage and uncertainties in its temperature sensitivity examined with a simple model

AbstractThe future of the land carbon sink is a significant uncertainty in global change projections. Here, key controls on global terrestrial carbon storage are examined using a simple model of vegetation and soil. Equilibrium solutions are derived as a function of atmospheric CO2 and global temperature, these environmental variables are then linked in an idealized global change trajectory, and the lag between the dynamic and equilibrium solutions is derived for different linear rates of increase in atmospheric CO2. Terrestrial carbon storage is departing significantly from equilibrium because CO2 and temperature are increasing on a similar timescale to ecosystem change, and the lag is found to be proportional to the rate of forcing. Thus peak sizes of the land carbon sink, and any future land carbon source, are proportional to the rate of increase of CO2. A switch from a land carbon sink to a source occurs at a higher CO2 and temperature under more rapid forcing. The effects of parameter uncertainty in temperature sensitivities of photosynthesis, plant respiration and soil respiration, and structural uncertainty through the effect of fixing the ratio of plant respiration to photosynthesis are explored. In each case, the CO2 fertilization effect on photosynthesis is constrained to reproduce the 1990 atmospheric CO2 concentration within a closed global model. New literature compilations are presented for the temperature sensitivities of plant and soil respiration. A lower limit, Q10=1.29, for soil respiration significantly increases future land carbon storage. An upper limit, Q10=3.63, for soil respiration underpredicts the increase in carbon storage since the Last Glacial Maximum. Fixing the ratio of plant respiration to photosynthesis (R/P) at 0.5 generates the largest and most persistent land carbon sink, followed by the weakest land carbon source.

Observations of surf beat forcing and dissipation

We used a simple energy balance equation, and estimates of the cross‐shore energy flux carried by progressive surf beat, to calculate the rate of net surf beat forcing (or dissipation) on a beach near Duck, North Carolina. Far inside the surf zone, surf beat dissipation exceeded forcing. Outside the surf zone, surf beat forcing exceeded dissipation. When incident waves were large, surf beat dissipation inside the surf zone and forcing just outside the surf zone were both very strong (the surf beat energy dissipated in the surf zone in a single beat period was of the same order as the total amount of surf beat energy stored in the surf zone). During storms, shoreward propagation of surf beat maintained surf beat energy in the surf zone. Net surf beat dissipation in the surf zone scaled as predicted by a simple bottom stress parameterization. The inferred dissipation factor for surf beat was 0.08, within the range of wave dissipation factors usually observed in the field and 27–80 times larger than drag coefficients appropriate for the mean longshore current. The observed rapid forcing, rapid dissipation, and shoreward propagation of surf beat are not simulated by existing models of surf beat dynamics.

Exponential averaging for Hamiltonian evolution equations

We derive estimates on the magnitude of non-adiabatic interaction between a Hamiltonian partial differential equation and a high-frequency nonlinear oscillator. Assuming spatial analyticity of the initial conditions, we show that the dynamics can be transformed to the uncoupled dynamics of an infinite-dimensional Hamiltonian system and an anharmonic oscillator, up to coupling terms which are exponentially small in a certain power of the frequency of the oscillator. The result is derived from an abstract averaging theorem for infinite-dimensional analytic evolution equations in Gevrey spaces. Refining upon a similar result by Neishtadt for analytic ordinary differential equations, the temporal estimate crucially depends on the spatial regularity of the initial condition. The result shows to what extent the strong resonances between rapid forcing and highly oscillatory spatial modes can be suppressed by the choice of sufficiently smooth initial data. An application is provided by a system of nonlinear Schrödinger equations, coupled to a rapidly forcing single mode, representing small-scale oscillations. We provide an example showing that the estimates for partial differential equations we derive here are necessarily different from those in the context of ordinary differential equations.

Emergent Properties of Scale in Global Environmental Modeling - Are There Any?

This essay argues that much of the concern over issues of scale in the modeling of complex human-environment systems – of which integrated assessment models are a special case – tends to be preoccupied with bottom-up aggregation and top-down disaggregation. Deep analysis of the underlying explanation of scale is missing. One of the intriguing propositions of complex systems theory is the emergence of new structures at a high level of scale that are difficult if not impossible to predict from constituent parts. Emergent properties are not the mysterious creation of ‘‘new material’’ in the system, but rather the placement of the components of the system into their logical contexts (scales) so that the observer=modeler can see structures arise from them for the first time. The stochastic interaction among low-level elements that gives rise to emergent properties may be part of a larger process of self-organization in hierarchical systems. Self-organization and attendant emergent properties constrain low-level elements through a network of downwardly propagating positive feedbacks. Those feedbacks not only tend to hold the system in a temporary stable state, but they also render it vulnerable to radical reorganization by rapid external forcing. The vulnerability of the USA agricultural production system to climate change is given as an example of how a self-organizing, hierarchical system paradoxically may become susceptible to large external shocks as a result of the emergence of high-level structures that seek to protect its low-level components from shortterm variability. Simulations of changes in Honduran maize production in the aftermath of Hurricane Mitch using the CLUE land use model demonstrate the influence of multi-scale complexity on the resilience of land use after disturbance. Finally, it is argued that improved understanding of emergent properties of scale may give fundamental insight into the conditions of surprise.

Time-Averaging under Fast Periodic Forcing of Parabolic Partial Differential Equations: Exponential Estimates

The phases of a large class of parabolic partial differential equations with rapid time-periodic forcing can be separated up to exponential small errors. The originally nonautonomous equation is transformed such that the nonautonomous terms are exponentially small in the period h of the forcing. This is a counterpart for partial differential equations of the theorem by A. Neishtadt (1984, J. Appl. Math. Mech.48, 134–139) for ordinary differential equations. In our case the exponential rate depends on time t and the estimates have the form hexp(−c(t)h−1/3).