Given still-high levels of coronavirus disease 2019 (COVID-19) susceptibility and inconsistent transmission-containing strategies, outbreaks have continued to emerge across the United States. Until effective vaccines are widely deployed, curbing COVID-19 will require carefully timed nonpharmaceutical interventions (NPIs). A COVID-19 early warning system is vital for this. Here, we evaluate digital data streams as early indicators of state-level COVID-19 activity from 1 March to 30 September 2020. We observe that increases in digital data stream activity anticipate increases in confirmed cases and deaths by 2 to 3 weeks. Confirmed cases and deaths also decrease 2 to 4 weeks after NPI implementation, as measured by anonymized, phone-derived human mobility data. We propose a means of harmonizing these data streams to identify future COVID-19 outbreaks. Our results suggest that combining disparate health and behavioral data may help identify disease activity changes weeks before observation using traditional epidemiological monitoring.

Despite having a large influence on summer insolation, climatic precession is thought to account for little variance in early Pleistocene proxies of ice volume and deep-water temperature. Various mechanisms have been suggested to account for the dearth of precession variability, including meridional insolation gradients, interhemispheric cancellation of ice-volume changes, and antiphasing between the duration and intensity of summer insolation. We employ a method termed Empirical Nonlinear Orbital Fitting (ENOF) to estimate the amplitudes of obliquity and precession forcing in early Pleistocene proxies and their respective leads or lags relative to the timing of orbital variations. Analysis of a high-resolution North Atlantic benthic δ 18 O record, comprising data from IODP sites U1308 and U1313, indicates a significantly larger precession contribution than previously recognized, with an average precession-to-obliquity amplitude ratio of 0.51 (0.30-0.76 95% confidence interval) in the rate-of-change of δ 18 O between 3 and 1 Ma. Averaged when eccentricity exceeds 0.05, this ratio rises to an average of 1.18 (0.84-1.53). Additional support for precession's importance in the early Pleistocene comes from its estimated amplitude covarying with eccentricity, analyses of other benthic δ 18 O records yielding similar orbital amplitude ratios, and use of an orbitally-independent timescale also showing significant precession. Precession in phase with Northern Hemisphere summer intensity steadily intensifies throughout the Pleistocene, in agreement with its more common identification during the late Pleistocene. A Northern Hemisphere ice sheet and energy balance model run over the early Pleistocene predicts orbital amplitudes consistent with observations when a cooling commensurate with North Atlantic sea surface temperatures is imposed. These results provide strong evidence that glaciation is influenced by climatic precession during the late Pliocene and early Pleistocene, and are consistent with hypotheses that glaciation is controlled by Northern Hemisphere summer insolation.

It is established that changes in sea level influence melt production at midocean ridges, but whether changes in melt production influence the pattern of bathymetry flanking midocean ridges has been debated on both theoretical and empirical grounds. To explore the dynamics that may give rise to a sea-level influence on bathymetry, we simulate abyssal hills using a faulting model with periodic variations in melt supply. For 100-ky melt-supply cycles, model results show that faults initiate during periods of amagmatic spreading at half-rates >2.3 cm/y and for 41-ky melt-supply cycles at half-rates >3.8 cm/y. Analysis of bathymetry across 17 midocean ridge regions shows characteristic wavelengths that closely align with the predictions from the faulting model. At intermediate-spreading ridges (half-rates >2.3 cm/y and ≤ 3.8 cm/y) abyssal hill spacing increases with spreading rate at 0.99 km/(cm/y) or 99 ky ( n = 12; 95% CI, 87 to 110 ky), and at fast-spreading ridges (half-rates >3.8 cm/y) spacing increases at 38 ky ( n = 5; 95% CI, 29 to 47 ky). Including previously published analyses of abyssal-hill spacing gives a more precise alignment with the primary periods of Pleistocene sea-level variability. Furthermore, analysis of bathymetry from fast-spreading ridges shows a highly statistically significant spectral peak ( P < 0.01) at the 1/(41-ky) period of Earth’s variations in axial tilt. Faulting models and observations both support a linkage between glacially induced sea-level change and the fabric of the sea floor over the late Pleistocene.

Proxy reconstructions indicate that sea level responded more sensitively to CO2 radiative forcing in the late Pleistocene than in the early Pleistocene, a transition that was proposed to arise from changes in ice-sheet dynamics. In this study we analyse the links between sea level, orbital variations, and CO2 using an energy-balance model having a simple ice sheet. Model parameters, including for age models, are inferred over the late Pleistocene using a hierarchical Bayesian method, and the inferred relationships are used to evaluate CO2 levels over the past 2 My in relation to sea level. Early-Pleistocene model CO2 averages 246 ppm (244 ppm - 249 ppm 95% c.i.) across 2-1 Ma and indicates that sea level was less sensitive to radiative forcing than in the late Pleistocene, consistent with foregoing δ11B-derived estimates. Weaker early-Pleistocene sea-level sensitivity originates from a weaker ice-albedo feedback and the fact that smaller ice sheets are thinner, absent changes over time in model equations or parameters. An alternative scenario involving thin and expansive early-Pleistocene ice sheets, in accord with some lines of geologic evidence, implies 15 ppm lower average CO2 or ~10-15 m higher average sea level during the early Pleistocene relative to the original scenario. Our results do not rule out dynamical transitions during the middle Pleistocene, but indicate that variations in the sea-level response to CO2 forcing over the past 2 My can be explained on the basis of nonlinearities associated with ice-albedo feedbacks and ice-sheet geometry that are consistently present across this interval.

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The first reconstructions of atmospheric composition over multiple glacial-interglacial cycles, derived from the Vostok ice cores (Barnola et al., 1991;Petit et al., 1999), revealed a close relationship between atmospheric CO 2 and global climate over at least the past 400,000 years. Atmospheric CO 2 decreased from ∼280 to ∼180 ppm over order 100 Kyr periods before rising back to interglacial levels in order 10 Kyr, following the same sawtooth-like pattern that preceding studies had identified in the benthic δ 18 O proxy for global ice-volume and deep-ocean temperature (Hays et al., 1976;Imbrie & Imbrie, 1980). The close coupling of CO 2 and ice volume gave crucial insight, albeit over a limited time interval, into the sensitivity of past climate. Importantly, a subsequent extension of the ice-core record to 650 ka (Siegenthaler et al., 2005) revealed that the relationship between CO 2 and δD, an air temperature proxy, remained consistent through at least 650 ka, and a similar finding was made when the ice-core record was eventually extended to 800 ka (Lüthi et al., 2008), the current extent of continuous observations. The apparent stability of the CO 2 -climate relationship might suggest we could predict CO 2 levels in earlier epochs on the basis of similar coupling, but it is unclear how similar the CO 2 -climate relationships are before and after the mid-Pleistocene (

A number of groups attempted to predict atmospheric CO2 concentrations between 420 to 800 ka prior to publication of the Dome C ice-core record by the European Project for Ice Coring in Antarctica, EPICA. The predictions that fared best assumed that the relationships between CO2 and proxies of air temperature remained consistent over the past 800 ky [7]. Here we extend predictions of atmospheric CO2 concentrations over the last 2 Ma under a similar assumption of consistent physical relationships between CO2 and climate over time and test this assumption against existing observations. Our principal approach is to use a recently-developed Bayesian paleoclimate model to infer CO2 values conditional on past sea level. An ensemble of seven different CO2 histories are inferred from an equal number of sea-level reconstructions. Five of the ensemble members give a consensus prediction that CO2 in the early Pleistocene, 2-0.8 Ma, averaged 241 ppm (238 ppm - 245 ppm 95% c.i.) with 95% of CO2 values within 206 ppm and 275 ppm. Uncertainty estimates account for contributions from orbital forcing, the ice-albedo feedback, age uncertainties, and other factors. The other two ensemble members indicate 20-50 meter higher sea level during the early Pleistocene and imply much higher CO2 levels. Our consensus prediction aligns well with a compilation of previously published δ11B-based CO2 reconstructions that, after calibration to late-Pleistocene ice-core CO2 values, average 237 ppm (95% of CO2 values within 195 ppm to 273 ppm). Furthermore, 94% of consensus CO2 predictions fall within the range indicated by 60 early-Pleistocene CO2 measurements from air trapped in discontinuous ice segments from the Allan Hills in East Antarctica. Our consensus prediction can be definitively tested by obtaining continuous ice-core atmospheric CO2 records that extend into the early Pleistocene.

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