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Despite recent empirical and modeling advances, atmospheric CO2 concentrations during the early Pleistocene remain uncertain. Using a recently‐developed Bayesian paleoclimate model, an ensemble of seven different CO2 records is inferred conditional on reconstructions of past sea level. Five ensemble members give a consensus prediction that CO2 concentrations averaged 241 ppm (238–245 ppm 95% CI) between 2 and 0.8 Ma. Uncertainty estimates account for contributions from orbital forcing, age uncertainties, and other factors. Our consensus prediction aligns well with a compilation of previously published δ11B‐based CO2 reconstructions after calibration to late‐Pleistocene ice‐core CO2 values, as well as with 60 early‐Pleistocene CO2 measurements 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. Plain Language Summary: A complete record of past atmospheric CO2 levels is important for understanding the sensitivity of the climate system to ongoing changes earth's energy balance. Although the current continuous record of CO2 levels ends ∼800,000 years ago (800 Ka), multiple reconstructions of sea level—which is closely correlated with CO2—extend several million years deeper into the past. We use a previously‐published method to derive a statistical‐physical relationship between CO2 and sea level over the past 800 Kyr and, then, to predict CO2 levels over the past 2 Myr from a collection of seven sea‐level histories. Five sea‐level histories closely agree with each other and yield CO2 levels that also agree with sparse CO2 data that exist prior to 800 Ka, especially after correcting those data for possible sources of bias. These results provide a complete prediction of CO2 levels between 800 Kyr and 2 Myr ago that can be tested as the continuous ice‐core record is extended deeper in time. Key Points: Atmospheric CO2 levels in the early Pleistocene remain unclear due to a paucity of observational constraintsA Bayesian paleoclimate model yields CO2 histories averaging 241 ppm over 0.8–2 Ma, with a 95% of all realized CO2 values remaining within 206–275 ppmAssuming unchanging relationships among CO2, orbital variations, and sea level in the past 2 Myr yields model CO2 agreeing with observations [ABSTRACT FROM AUTHOR]

Despite recent empirical and modeling advances, atmospheric CO2 concentrations during the early Pleistocene remain uncertain. Using a recently‐developed Bayesian paleoclimate model, an ensemble of seven different CO2 records is inferred conditional on reconstructions of past sea level. Five ensemble members give a consensus prediction that CO2 concentrations averaged 241 ppm (238–245 ppm 95% CI) between 2 and 0.8 Ma. Uncertainty estimates account for contributions from orbital forcing, age uncertainties, and other factors. Our consensus prediction aligns well with a compilation of previously published δ11B‐based CO2 reconstructions after calibration to late‐Pleistocene ice‐core CO2 values, as well as with 60 early‐Pleistocene CO2 measurements 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. Plain Language Summary: A complete record of past atmospheric CO2 levels is important for understanding the sensitivity of the climate system to ongoing changes earth's energy balance. Although the current continuous record of CO2 levels ends ∼800,000 years ago (800 Ka), multiple reconstructions of sea level—which is closely correlated with CO2—extend several million years deeper into the past. We use a previously‐published method to derive a statistical‐physical relationship between CO2 and sea level over the past 800 Kyr and, then, to predict CO2 levels over the past 2 Myr from a collection of seven sea‐level histories. Five sea‐level histories closely agree with each other and yield CO2 levels that also agree with sparse CO2 data that exist prior to 800 Ka, especially after correcting those data for possible sources of bias. These results provide a complete prediction of CO2 levels between 800 Kyr and 2 Myr ago that can be tested as the continuous ice‐core record is extended deeper in time. Key Points: Atmospheric CO2 levels in the early Pleistocene remain unclear due to a paucity of observational constraintsA Bayesian paleoclimate model yields CO2 histories averaging 241 ppm over 0.8–2 Ma, with a 95% of all realized CO2 values remaining within 206–275 ppmAssuming unchanging relationships among CO2, orbital variations, and sea level in the past 2 Myr yields model CO2 agreeing with observations [ABSTRACT FROM AUTHOR]

High temperatures are associated with reduced crop yields (1,2), and predictions for future warming (3) have raised concerns regarding future productivity and food security (4-8). However, the extent to which adaptation can mitigate such heat-related losses remains unclear (9-13). Here we empirically demonstrate how maize is locally adapted to hot temperatures across US counties. Using this spatial adaptation as a surrogate for future adaptation, we find that losses to average US maize yields from a 2°C warming would be reduced from 14% to only 6% and that loss in net production is wholly averted. This result does not account for possible changes in temperature variability or water resources, nor does it account for all possible forms of adaptation (14-18), but it does show that adaptation is of first-order importance for predicting future changes in yield. Further research should be undertaken regarding the ability to adapt to a changing climate, including analysis of other crops and regions, the application of more sophisticated models of crop development, and field trials employing artificially increased temperature., Global maize yields are forecast to decline in response to increasing temperature, particularly as the upper range of growing season temperatures become hotter (1,2,4-7,19). The sensitivity of crop yields to [...]

Recently observed extreme temperatures at high northern latitudes (1-7) are rare by definition, making the longer time span afforded by climate proxies important for assessing how the frequency of such [...]

A new statistical test shows that precession as well as obliquity contributes to the pacing of deglaciations in the late Pleistocene. How Pleistocene deglaciations gathered pace The glacial cycles that characterize Earth's climate history are paced by variations in Earth's orbital orientation relative to the Sun. Into the middle Pleistocene about a million years ago, glacial cycles were paced by ~40,000-year cycles in Earth's tilt, or obliquity. Since then 100,000-year variations in orbital eccentricity seem to have dominated. Although it has been speculated that a combination of obliquity and precession (orbital axis orientation) might conspire to pace deglaciations, firm quantitative evidence has not emerged. Peter Huybers now uses advances in timing and statistics to show that indeed, obliquity and precession simultaneously pace deglaciations. Milankovitch.sup.1 proposed that Earth resides in an interglacial state when its spin axis both tilts to a high obliquity and precesses to align the Northern Hemisphere summer with Earth's nearest approach to the Sun. This general concept has been elaborated into hypotheses that precession.sup.2, obliquity.sup.3,4 or combinations of both.sup.5,6,7,8 could pace deglaciations during the late Pleistocene.sup.9,10. Earlier tests have shown that obliquity paces the late Pleistocene glacial cycles.sup.4,11 but have been inconclusive with regard to precession, whose shorter period of about 20,000 years makes phasing more sensitive to timing errors.sup.4,11,12. No quantitative test has provided firm evidence for a dual effect. Here I show that both obliquity and precession pace late Pleistocene glacial cycles. Deficiencies in time control that have long stymied efforts to establish orbital effects on deglaciation are overcome using a new statistical test that focuses on maxima in orbital forcing. The results are fully consistent with Milankovitch's proposal but also admit the possibility that long Southern Hemisphere summers contribute to deglaciation., Author(s): Peter Huybers [sup.1] Author Affiliations: (1) Department of Earth and Planetary Sciences, Harvard University, Cambridge, USA Main During the late Pleistocene--roughly over the past million years--Northern Hemisphere continental ice [...]

Do the polar layered deposits on Mars reflect orbital control or stochastic variability? It is first useful to determine whether an orbital signal would be detected, even if present. An estimate of the uncertainty in the time-depth relationship of the polar stratigraphy shows that nonlinearities in this relationship and noise in the signal will hamper or preclude detection of orbital forcing, even if layer composition is directly proportional to insolation. Indeed stratigraphic sections of the north polar layered deposits reconstructed from spacecraft images yield no clear evidence of orbital control and are largely consistent with an autoregressive, stochastic formation process. There is, however, a broad rise in spectral power centered on a wavelength of roughly 1.6 m that appears in many of the stratigraphic sections. This bedding may record a time scale associated with processes internal to Mars' climate system, perhaps related to dust storms. Alternatively, if formed in response to variations in Mars' obliquity or orbital precession, the 1.6 m bedding implies that the ~1-km-thick upper north polar layered deposits formed over 30-70 Myr.

Author(s): Peter Huybers (corresponding author) [1]; William Curry [1] Climate variability exists at all timescales--and climatic processes are intimately coupled, so that understanding variability at any one timescale requires some [...]

Author(s): Peter Huybers (corresponding author) [1]; Carl Wunsch [2] The 100,000-year timescale in the glacial/interglacial cycles of the late Pleistocene epoch (the past [similar]700,000 years) is commonly attributed to control [...]

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 analyze 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 Bayesian method, and the inferred relationships are used to evaluate CO2 levels over the past 2 million years in relation to sea level. Early Pleistocene model CO2 averages 244 ppm (241–246 ppm 95% confidence interval) across 2 to 1 million years ago 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 million years can be explained on the basis of nonlinearities associated with ice-albedo feedbacks and ice-sheet geometry that are consistently present across this interval. Significance Statement: Insight into the consequences of atmospheric CO2 variations for sea level is possible from studying past changes in these two variables. Previous studies showed, in particular, that sea level was more sensitive to CO2 forcing in the late Pleistocene [~0–1 million years ago (Ma)] than in the early Pleistocene (~2–1 Ma). Using a Bayesian methodology, we condition a physical model on late Pleistocene sea level and CO2 observations and show that the inferred relationships also predict the lower early Pleistocene sensitivity. Temporal changes in sensitivity come from nonlinearities involving the ice-albedo feedback and ice-sheet geometry that intensified when ice sheets grew larger after 1 Ma. An alternative scenario involving changes in conditions at the base of the ice sheet also satisfies observations but such changes are not required for explaining sensitivity changes. [ABSTRACT FROM AUTHOR]