1,721,114 research outputs found
Quantifying the potential causes of Neanderthal extinction: Abrupt climate change versus competition and interbreeding
No full text© 2020 The Author(s). Anatomically Modern Humans are the sole survivor of a group of hominins that inhabited our planet during the last ice age and that included, among others, Homo neanderthalensis, Homo denisova, and Homo erectus. Whether previous hominin extinctions were triggered by external factors, such as abrupt climate change, volcanic eruptions or whether competition and interbreeding played major roles in their demise still remains unresolved. Here I present a spatially resolved numerical hominin dispersal model (HDM) with empirically constrained key parameters that simulates the migration and interaction of Anatomically Modern Humans and Neanderthals in the rapidly varying climatic environment of the last ice age. The model simulations document that rapid temperature and vegetation changes associated with Dansgaard-Oeschger events were not major drivers of global Neanderthal extinction between 50 and 35 thousand years ago, but played important roles regionally, in particular over northern Europe. According to a series of parameter sensitivity experiments conducted with the HDM, a realistic extinction of the Neanderthal population can only be simulated when Homo sapiens is chosen to be considerably more effective in exploiting scarce glacial food resources as compared to Neanderthals11sci
Using Late Pleistocene sea surface temperature reconstructions to constrain future greenhouse warming
No full text© 2019 Elsevier B.V.Future greenhouse warming projections conducted with coupled climate models still exhibit a substantial spread in response to a given anthropogenic greenhouse gas concentration scenario. In order to constrain this spread and to provide robust warming projections, our understanding of Earth's global-mean surface temperature response to radiative forcing (referred to as climate sensitivity) needs to be further refined. Here we estimate an averaged glacial/interglacial climate sensitivity using 25 transient Earth system model simulations of the Last Glacial Cycle and a global-mean sea surface temperature (SST) reconstruction derived from 64 globally-distributed paleo-proxies of SST. Our results document that Earth's averaged Late Pleistocene equilibrium climate sensitivity is in the order of ∼4.2 K per CO2 doubling. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, this value translates into a global-mean surface warming of ∼5.0 K by the year 2100 relative to pre-industrial levels. This estimate is in excellent agreement with the ensemble-mean projection of climate simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our uncertainty analysis reveals further that the lack of robust reconstructions of glacial aerosol forcing is a key contributor to the overall uncertainty of paleo-based estimates of climate sensitivity11Nsciescopu
Calibration Uncertainties of Tropical Pacific Climate Reconstructions over the Last Millennium
No full textSeveral climate field reconstruction methods assume stationarity between the leading patterns of variability identified during the instrumental calibration period and the reconstruction period. We examine how and to what extent this restrictive assumption may generate uncertainties in reconstructing past tropical Pacific climate variability. Based on the Last Millennium (850-2005 CE) ensemble simulations conducted with the Community Earth System Model and by developing a series of pseudoproxy reconstructions for different calibration periods, we find that the overall reconstruction skill for global and more regional-scale climate indices depends significantly on the magnitude of externally forced global mean temperature variability during the chosen calibration period. This effect strongly reduces the fidelity of reconstructions of decadal to centennial-scale tropical climate variability, associated with the interdecadal Pacific oscillation (IPO) and centennial-scale temperature shifts between the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). In contrast, our pseudoproxy-based analysis demonstrates that reconstructions of interannual El Nino-Southern Oscillation (ENSO) variability are more robust and less affected by changes in calibration period.11Nsciescopu
Fokker–Planck dynamics of the El Niño-Southern Oscillation
No full text© 2020, The Author(s). The asymmetric nature of the El Niño-Southern Oscillation (ENSO) is explored by using a probabilistic model (PROM) for ENSO. Based on a Fokker–Planck Equation (FPE), PROM describes the dynamics of a nonlinear stochastic ENSO recharge oscillator model for eastern equatorial Pacific temperature anomalies and equatorial Pacific basin-averaged thermocline depth changes. Eigen analyses of PROM provide new insights into the stationary and oscillatory solutions of the stochastic dynamical system. The first probabilistic eigenmode represents a stationary mode, which exhibits the asymmetric features of ENSO, in case deterministic nonlinearities or multiplicative noises are included. The second mode is linked to the oscillatory nature of ENSO and represents a cyclic asymmetric probability distribution, which emerges from the key dynamical processes. Other eigenmodes are associated with the temporal evolution of higher order statistical moments of the ENSO system. The model solutions demonstrate that the deterministic nonlinearity plays a stronger role in establishing the observed asymmetry of ENSO as compared to the multiplicative stochastic part.11Nsciescopu
(Un)predictability of strong El Nino events
No full textThe El Niño-Southern Oscillation (ENSO) is a mode of interannual variability in the coupled equa- torial Pacific coupled atmosphere/ocean system. El Niño describes a state in which sea surface temper- atures in the eastern Pacific increase and upwelling of colder, deep waters diminishes. El Niño events typically peak in boreal winter, but their strength varies irregularly on decadal time scales. There were exceptionally strong El Niño events in 1982–83, 1997–98 and 2015–16 that affected weather on a global scale. Widely publicized forecasts in 2014 predicted that the 2015–16 event would occur a year earlier. Predicting the strength of El Niño is a matter of practical concern due to its effects on hydroclimate and agriculture around the world. This paper discusses the frequency and regularity of strong El Niño events in the context of chaotic dynamical systems. We discover a mechanism that limits their predictability in a conceptual “recharge oscillator” model of ENSO. Weak seasonal forcing or noise in this model can induce irregular switching between an oscillatory state that has strong El Niño events and a chaotic state that lacks strong events, In this regime, the timing of strong El Niño events on decadal time scales is unpredictable. (c) The Author(s) 2017. Published by Oxfold University Press
Drivers of future seasonal cycle changes in oceanic pCO2
Full text linkRecent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO2 (pCO2) has been increasing on average at a rate of 2–3µatm per decade (Landschützer et al. 2018). Future increases in pCO2 seasonality are expected, as marine CO2 concentration ([CO2]) will increase in response to increasing anthropogenic carbon emissions (McNeil and Sasse 2016). Here we use seven different global coupled atmosphere–ocean–carbon cycle–ecosystem model simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to study future projections of the pCO2 annual cycle amplitude and to elucidate the causes of its amplification. We find that for the RCP8.5 emission scenario the seasonal amplitude (climatological maximum minus minimum) of upper ocean pCO2 will increase by a factor of 1.5 to 3 over the next 60–80 years. To understand the drivers and mechanisms that control the pCO2 seasonal amplification we develop a complete analytical Taylor expansion of pCO2 seasonality in terms of its four drivers: dissolved inorganic carbon (DIC), total alkalinity (TA), temperature (T), and salinity (S). Using this linear approximation we show that the DIC and T terms are the dominant contributors to the total change in pCO2 seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean pCO2, which enhances DIC and alters the ocean carbonate chemistry. Regional differences in the projected seasonal cycle amplitude are generated by spatially varying sensitivity terms. The subtropical and equatorial regions (40°S–40°N) will experience a ≈ 30–80µatm increase in seasonal cycle amplitude almost exclusively due to a larger background CO2 concentration that amplifies the T seasonal effect on solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of T as a result of stronger ocean stratification and a projected shoaling of mean mixed layer depths. The Southern Ocean will experience a seasonal cycle amplification of ≈ 90–120µatm in response to the mean pCO2-driven change in the mean DIC contribution and to a lesser extent to the T contribution. However, a decrease in the DIC seasonal cycle amplitude somewhat counteracts this regional amplification mechanism.© Author(s) 2018
Decadal Monsoon-ENSO Relationships Reexamined
No full textThe strength of the El Nino-Southern Oscillation (ENSO)-Indian summer monsoon rainfall (ISMR) relationship shows considerable decadal fluctuations, which have been previously linked to low-frequency climatic processes such as shifts in ENSO's center of action or the Atlantic Multidecadal Oscillation. However, random variability can also cause similar variations in the relationship between climate phenomena. Here we propose a statistical test to determine whether the observed time-evolving correlations between ENSO and ISMR are different from those expected from a simple stochastic null hypothesis model. The analysis focuses on the time evolution of moving correlations, their expected variance, and probabilities for rapid transitions. The results indicate that the time evolution of the observed running correlation between these climate modes is indistinguishable from a system in which the ISMR signal can be expressed as a stochastically perturbed ENSO signal. This challenges previous deterministic interpretations. Our results are further corroborated by the analysis of climate model simulations. ©2018. The Authors
Disentangling Impacts of Dynamic and Thermodynamic Components on Late Summer Rainfall Anomalies in East Asia
No full textThis study has examined relative contributions of dynamic and thermodynamic components to East Asian summer monsoon (EASM), compared to the others over Asia. We have decomposed of moisture budget into the dynamic and thermodynamic components composing of interannual variability of the EASM. As represented by the moisture budget, the Asian monsoon is mostly caused by changes in winds (dynamic component); interestingly, changes in moisture (thermodynamic component) play an important role in monsoon rainfall anomalies only for East Asia (27.09%). In terms of the dynamic component over East Asia, strong continental heating, resulting in enhanced land‐sea contrast, is identified as crucial to a local development of winds toward East Asia, and it ultimately strengthens a meridional wind, which is accompanied by the western North Pacific subtropical high. In addition, the negative winter North Atlantic Oscillation could induce the enhanced moisture advection term of the dynamic component over East Asia through barotropic Rossby wave propagation. The thermodynamic component has a localized effect on net precipitation at midlatitudes, with an enhanced wave train pattern with a zonal wavenumber‐5, which reinforces the Okhotsk high. These distinct large‐scale circulation patterns together create favorable conditions for heavy rainfall over East Asia when the two components are positively in‐phase. Here we have also described that the extreme heavy rainfall is noticeable when the Eurasian blocking occurs. This study is expected to improve the detailed predictability of the EASM by understanding the two components to prevent disaster risks in terms of extreme rainfall
Spurious North Tropical Atlantic precursors to El Niño
No full text© 2021, The Author(s).The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO’s autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise.11Nsciescopu
Local insolation changes enhance Antarctic interglacials: Insights from an 800,000-year ice sheet simulation with transient climate forcing
No full textThe Antarctic ice sheet – storing ∼27 million cubic kilometres of ice – has the potential to contribute greatly to future sea level rise; yet its past evolution and sensitivity to long-term climatic drivers remain poorly understood and constrained. In particular, a long-standing debate questions whether Antarctic climate and ice volume respond mostly to changes in global sea level and atmospheric greenhouse gas concentrations or to local insolation changes. So far, long-term Antarctic simulations have used proxy-based parameterizations of climatic drivers, presuming that external forcings are synchronous and spatially uniform. Here for the first time we use a transient, three-dimensional climate simulation over the last eight glacial cycles to drive an Antarctic ice sheet model. We show that the evolution of the Antarctic ice sheet was mostly driven by CO2 and sea level forcing with a period of about 100,000 yr, synchronizing both hemispheres. However, on precessional time scales, local insolation forcing drives additional mass loss during periods of high sea level and CO2, enhancing the Antarctic interglacial and putting northern and southern ice sheet variability temporarily out of phase. In our simulations, partial collapses of the West Antarctic ice sheet during warm interglacials are only simulated with unrealistically large Southern Ocean subsurface warming exceeding ∼4 °C. Overall, our results further elucidate the complex interplay of global and local forcings in driving Late Quaternary Antarctic ice sheet evolution, and the unique and overlooked role of precession therein. © 2018 Elsevier B.
- …
