Huybers
Peter
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Peter
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ArticleMeridional circulation during the Last Glacial Maximum explored through a combination of South Atlantic δ18O observations and a geostrophic inverse model(American Geophysical Union, 2006-11-15) Gebbie, Geoffrey A. ; Huybers, PeterThe vertical profile of meridional transport in the South Atlantic is examined by combining paleoceanographic observations with a geostrophic circulation model using an inverse method. δ18Ocalcite observations along the margins of the South Atlantic show that upper-ocean cross-basin differences were weaker during the Last Glacial Maximum (LGM) than the Holocene. The δ18Ocalcite observations can be explained by a shift of water-mass properties without any change in the overturning circulation. Alternatively, they may indicate a reduced LGM cross-basin density difference and, via the thermal wind relation, a reduced vertical shear. Model inversions of δ18Ocalcite are found to require meridional transports different from the modern only after three assumptions are made: temperature and salinity distributions are spatially smooth, the relationship between salinity and δ18Owater is linear and spatially invariant, and LGM temperatures are known to within 1°C along the margins. The last assumption is necessary because an independent constraint on temperature or salinity is required to determine density from δ18Ocalcite observations. δ18Ocalcite observations are clearly useful, but before any firm constraints can be placed on LGM meridional transport, it appears necessary to better determine the relationship between δ18Ocalcite and density.
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ArticleConsequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing(American Geophysical Union, 2006-11-10) Tziperman, Eli ; Raymo, Maureen E. ; Huybers, Peter ; Wunsch, CarlThe consequences of the hypothesis that Milankovitch forcing affects the phase (e.g., termination times) of the 100 kyr glacial cycles via a mechanism known as “nonlinear phase locking” are examined. Phase locking provides a mechanism by which Milankovitch forcing can act as the “pacemaker” of the glacial cycles. Nonlinear phase locking can determine the timing of the major deglaciations, nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear. A consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct. Phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing. Nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record. Nonlinear phase locking can also result in the observed “quantization” of the glacial period into multiples of the obliquity or precession periods.
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PreprintOrigin of spatial variation in US East Coast sea-level trends during 1900-2017(Nature Research, 2018-12-18) Piecuch, Christopher G. ; Huybers, Peter ; Hay, Carling C. ; Kemp, Andrew C. ; Little, Christopher M. ; Mitrovica, Jerry X. ; Ponte, Rui M. ; Tingley, Martin P.Identifying the causes of historical trends in relative sea level—the height of the sea surface relative to Earth’s crust—is a prerequisite for predicting future changes. Rates of change along the U.S. East Coast during the last century were spatially variable, and relative sea level rose faster along the Mid-Atlantic Bight than the South Atlantic Bight and Gulf of Maine. Past studies suggest that Earth’s ongoing response to the last deglaciation1–5, surface redistribution of ice and water 5–9, and changes in ocean circulation9–13 contributed importantly to this large-scale spatial pattern. Here we analyze instrumental data14, 15 and proxy reconstructions4, 12 using probabilistic methods16–18 to show that vertical motions of Earth’s crust exerted the dominant control on regional spatial differences in relative sea level trends along the U.S. East Coast during 1900–2017, explaining a majority of the large-scale spatial variance. Rates of coastal subsidence caused by ongoing relaxation of the peripheral forebulge associated with the last deglaciation are strongest near North Carolina,Maryland, and Virginia. Such structure indicates that Earth’s elastic lithosphere is thicker than has been assumed in other models19–22. We also find a significant coastal gradient in relative sea level trends over this period that is unrelated to deglaciation, and suggests contributions from twentieth-century redistribution of ice and water. Our results indicate that the majority of large-scale spatial variation in longterm rates of relative sea level rise on the U.S. East Coast was due to geological processes that will persist at similar rates for centuries into the future.
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ArticleHow is the ocean filled?(American Geophysical Union, 2011-03-23) Gebbie, Geoffrey A. ; Huybers, PeterThe ocean surface rapidly exchanges heat, freshwater, and gases with the atmosphere, but once water sinks into the ocean interior, the inherited properties of seawater are closely conserved. Previous water-mass decompositions have described the oceanic interior as being filled by just a few different property combinations, or water masses. Here we apply a new inversion technique to climatological tracer distributions to find the pathways by which the ocean is filled from over 10,000 surface regions, based on the discretization of the ocean surface at 2° by 2° resolution. The volume of water originating from each surface location is quantified in a global framework, and can be summarized by the estimate that 15% of the surface area fills 85% of the ocean interior volume. Ranked from largest to smallest, the volume contributions scaled by surface area follow a power-law distribution with an exponent of −1.09 ± 0.03 that appears indicative of the advective-diffusive filling characteristics of the ocean circulation, as demonstrated using a simple model. This work quantifies the connection between the surface and interior ocean, allowing insight into ocean composition, atmosphere-ocean interaction, and the transient response of the ocean to a changing climate.
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ArticleDifferences in radiative forcing, not sensitivity, explain differences in summertime land temperature variance change between CMIP5 and CMIP6(American Geophysical Union, 2021-01-31) Chan, Duo ; Rigden, Angela ; Proctor, Jonathan ; Chan, Pak Wah ; Huybers, PeterHow summertime temperature variability will change with warming has important implications for climate adaptation and mitigation. CMIP5 simulations indicate a compound risk of extreme hot temperatures in western Europe from both warming and increasing temperature variance. CMIP6 simulations, however, indicate only a moderate increase in temperature variance that does not covary with warming. To explore this intergenerational discrepancy in CMIP results, we decompose changes in monthly temperature variance into those arising from changes in sensitivity to forcing and changes in forcing variance. Across models, sensitivity increases with local warming in both CMIP5 and CMIP6 at an average rate of 5.7 ([3.7, 7.9]; 95% c.i.) × 10−3°C per W m−2 per °C warming. We use a simple model of moist surface energetics to explain increased sensitivity as a consequence of greater atmospheric demand (∼70%) and drier soil (∼40%) that is partially offset by the Planck feedback (∼−10%). Conversely, forcing variance is stable in CMIP5 but decreases with warming in CMIP6 at an average rate of −21 ([−28, −15]; 95% c.i.) W2 m−4 per °C warming. We examine scaling relationships with mean cloud fraction and find that mean forcing variance decreases with decreasing cloud fraction at twice the rate in CMIP6 than CMIP5. The stability of CMIP6 temperature variance is, thus, a consequence of offsetting changes in sensitivity and forcing variance. Further work to determine which models and generations of CMIP simulations better represent changes in cloud radiative forcing is important for assessing risks associated with increased temperature variance.
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ArticleCan paleoceanographic tracers constrain meridional circulation rates?(American Meteorological Society, 2007-02) Huybers, Peter ; Gebbie, Geoffrey A. ; Marchal, OlivierThe ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.
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PreprintObliquity pacing of the late Pleistocene glacial terminations( 2005-01-24) Huybers, Peter ; Wunsch, CarlThe timing of glacial/interglacial cycles at intervals of about 100,000 yr (100 kyr) is commonly attributed to control by Earth orbital configuration variations. This “pacemaker” hypothesis has inspired many models, variously depending upon Earth obliquity, orbital eccentricity, and precessional fluctuations, with the latter usually emphasized. A contrasting hypothesis is that glacial cycles arise primarily because of random internal climate variability. Progress requires distinguishing between the more than 30 proposed models of the late Pleistocene glacial variations. Here we present a formal test of the pacemaker hypothesis, focusing on the rapid deglaciation events known as terminations. The null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level. In contrast, for eccentricity and precession, the corresponding null-hypotheses cannot be rejected. The simplest inference, consistent with the observations, is that ice-sheets terminate every second (80 kyr) or third (120 kyr) obliquity cycle — at times of high obliquity — and similar to the original Milankovitch assumption. Hypotheses not accounting for the obliquity pacing are unlikely to be correct. Both stochastic and deterministic variants of a simple obliquity-paced model describe the observations.
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PreprintLinks between annual, Milankovitch and continuum temperature variability( 2006-03-23) Huybers, Peter ; Curry, William B.Climatic processes are intimately coupled so that understanding variability at any one time-scale requires some understanding of the whole. Spectra of Earth’s surface temperature exemplify this interdependence, having a continuum of variability following a power-law scaling1–7. But while specific modes of interannual variability are relatively well understood8, 9, the general controls on continuum variability are uncertain and usually described as purely stochastic processes10–13. Here we show that power-laws scale with the size of annual (1 yr) and Milankovitch period (23,000 and 41,000 yr) cycles. The annual cycle corresponds to scaling at monthly to decadal periods, while millennial and longer periods are tied to the Milankovitch cycles. Thus the annual, Milankovitch, and continuum of temperature variability together represent the response to deterministic insolation forcing. The identification of a deterministic control on the continuum provides insight into the mechanisms governing interannual and longer period climate variability.
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ArticleThe mean age of ocean waters inferred from radiocarbon observations : sensitivity to surface sources and accounting for mixing histories(American Meteorological Society, 2012-02) Gebbie, Geoffrey A. ; Huybers, PeterA number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700–1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. The inversion requires decomposing the World Ocean into source waters, which is obtained here using the total matrix intercomparison (TMI) method at up to 2° × 2° horizontal resolution with 11 113 surface sources. The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. Accounting for the broadness of surface regions where waters originate leads to a reservoir-age correction of almost 100 yr smaller than would be estimated with a two or three water-mass decomposition and explains some of the discrepancy with previous observational studies. A best estimate of mean age is also presented using the mixing history along circulation pathways. Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results.
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ArticleGlobal and Regional Discrepancies between Early-Twentieth-Century Coastal Air and Sea Surface Temperature Detected by a Coupled Energy-Balance Analysis(American Meteorological Society, 2022-03-06) Chan, Duo ; Gebbie, Geoffrey ; Huybers, PeterA major uncertainty in reconstructing historical sea surface temperature (SST) before the 1990s involves correcting for systematic offsets associated with bucket and engine-room intake temperature measurements. A recent study used a linear scaling of coastal station-based air temperatures (SATs) to infer nearby SSTs, but the physics in the coupling between SATs and SSTs generally gives rise to more complex regional air–sea temperature differences. In this study, an energy-balance model (EBM) of air–sea thermal coupling is adapted for predicting near-coast SSTs from coastal SATs. The model is shown to be more skillful than linear-scaling approaches through cross-validation analyses using instrumental records after the 1960s and CMIP6 simulations between 1880 and 2020. Improved skill primarily comes from capturing features reflecting air–sea heat fluxes dominating temperature variability at high latitudes, including damping high-frequency wintertime SAT variability and reproducing the phase lag between SSTs and SATs. Inferred near-coast SSTs allow for intercalibrating coastal SAT and SST measurements at a variety of spatial scales. The 1900–40 mean offset between the latest SST estimates available from the Met Office (HadSST4) and SAT-inferred SSTs range between −1.6°C (95% confidence interval: [−1.7°, −1.4°C]) and 1.2°C ([0.8°, 1.6°C]) across 10° × 10° grids. When further averaged along the global coastline, HadSST4 is significantly colder than SAT-inferred SSTs by 0.20°C ([0.07°, 0.35°C]) over 1900–40. These results indicate that historical SATs and SSTs involve substantial inconsistencies at both regional and global scales. Major outstanding questions involve the distribution of errors between our intercalibration model and instrumental records of SAT and SST as well as the degree to which coastal intercalibrations are informative of global trends. Significance Statement To evaluate the consistency of instrumental surface temperature estimates before the 1990s, we develop a coupled energy-balance model to intercalibrate measurements of sea surface temperature (SST) and station-based air temperature (SAT) near global coasts. Our model captures geographically varying physical regimes of air–sea coupling and outperforms existing methods in inferring regional SSTs from SAT measurements. When applied to historical temperature records, the model indicates significant discrepancies between inferred and observed SSTs at both global and regional scales before the 1960s. Our findings suggest remaining data issues in historical temperature archives and opportunities for further improvements.
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ArticleAn improved ensemble of land-surface air temperatures since 1880 using revised pair-wise homogenization algorithms accounting for autocorrelation(American Meteorological Society, 2024-03-12) Chan, Duo ; Gebbie, Geoffrey A. ; Huybers, PeterLand surface air temperatures (LSAT) inferred from weather station data differ among major research groups. The estimate by NOAA’s monthly Global Historical Climatology Network (GHCNm) averages 0.02°C cooler between 1880 and 1940 than Berkeley Earth’s and 0.14°C cooler than the Climate Research Unit estimates. Such systematic offsets can arise from differences in how poorly documented changes in measurement characteristics are detected and adjusted. Building upon an existing pairwise homogenization algorithm used in generating the fourth version of NOAA’s GHCNm(V4), PHA0, we propose two revisions to account for autocorrelation in climate variables. One version, PHA1, makes minimal modification to PHA0 by extending the threshold used in breakpoint detection to be a function of LSAT autocorrelation. The other version, PHA2, uses penalized likelihood to detect breakpoints through optimizing a model-selection problem globally. To facilitate efficient optimization for series with more than 1000 time steps, a multiparent genetic algorithm is proposed for PHA2. Tests on synthetic data generated by adding breakpoints to CMIP6 simulations and realizations from a Gaussian process indicate that PHA1 and PHA2 both similarly outperform PHA0 in recovering accurate climatic trends. Applied to unhomogenized GHCNmV4, both revised algorithms detect breakpoints that correspond with available station metadata. Uncertainties are estimated by perturbing algorithmic parameters, and an ensemble is constructed by pooling 50 PHA1- and 50 PHA2-based members. The continental-mean warming in this new ensemble is consistent with that of Berkeley Earth, despite using different homogenization approaches. Relative to unhomogenized data, our homogenization increases the 1880–2022 trend by 0.16 [0.12, 0.19]°C century−1 (95% confidence interval), leading to continental-mean warming of 1.65 [1.62, 1.69]°C over 2010–22 relative to 1880–1900.