Beyond equilibrium climate sensitivity

ISSN:1752-0908ISSN:1752-089

To Tune or not to Tune: Detecting Orbital Variability in Oligo-Miocene Climate Records

We address the problem of detecting quasi-periodic variability at orbital frequencies within pre-Pleistocene climate records using depth-derived and orbitally tuned chronologies. Many studies describing orbital variability in pre-Pleistocene sediment hosted isotope records employ climatic records that are set on orbitally tuned chronologies, without accounting for the bias in spectral power estimates introduced by orbital tuning. In this study we develop a method to quantify the effects of tuning upon spectral estimates and, in particular, to more properly determine the statistical significance of spectral peaks associated with orbital frequencies. We apply this method to two marine sediment δ18O records spanning the Oligo-Miocene, from ODP cores 1090 and 1218. We find that using linear age–depth relationships reveals statistically significant spectral peaks matching eccentricity in core 1090, and obliquity and precession in core 1218, where the last appears most significant. Tuning the chronologies to the orbital solutions of Laskar et al. (2004) increases the statistical significance of the precession peak, whereas the obliquity and eccentricity peaks become indistinguishable from those expected from tuning noise. This result can be understood in that tuning records with high signal to noise ratios tends to lead to more significant spectral peaks, whereas a linear age–depth relationship is better suited for detecting peaks when signal to noise ratios are low. We also demonstrate this concept using synthetic records.Earth and Planetary Science

Estimating the timing of geophysical commitment to 1.5 and 2.0 °C of global warming

Following abrupt cessation of anthropogenic emissions, decreases in short-lived aerosols would lead to a warming peak within a decade, followed by slow cooling as GHG concentrations decline. This implies a geophysical commitment to temporarily crossing warming levels before reaching them. Here we use an emissions-based climate model (FaIR) to estimate temperature change following cessation of emissions in 2021 and in every year thereafter until 2080 following eight Shared Socioeconomic Pathways (SSPs). Assuming a medium-emissions trajectory (SSP2–4.5), we find that we are already committed to peak warming greater than 1.5 °C with 42% probability, increasing to 66% by 2029 (340 GtCO2 relative to 2021). Probability of peak warming greater than 2.0 °C is currently 2%, increasing to 66% by 2057 (1,550 GtCO2 relative to 2021). Because climate will cool from peak warming as GHG concentrations decline, committed warming of 1.5 °C in 2100 will not occur with at least 66% probability until 2055

Influence of late Pleistocene sea-level variations on midocean ridge spacing in faulting simulations and a global analysis of bathymetry

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

Comment on “Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply”

Olive et al. (Reports, 16 October 2015, p. 310) argue that ~10% fluctuations in melt supply do not produce appreciable changes in ocean ridge bathymetry on time scales less than 100,000 years and thus cannot reflect sea level forcing. Spectral analysis of bathymetry in a region they highlight as being fault controlled, however, shows strong evidence for a signal from sea level variation

Biased Estimates of Equilibrium Climate Sensitivity and Transient Climate Response Derived From Historical CMIP6 Simulations

This study assesses the effective climate sensitivity (EffCS) and transient climate response (TCR) derived from global energy budget constraints within historical simulations of eight CMIP6 global climate models (GCMs). These calculations are enabled by use of the Radiative Forcing Model Intercomparison Project (RFMIP) simulations, which permit accurate quantification of the radiative forcing. Long-term historical energy budget constraints generally underestimate EffCS from CO2 quadrupling and TCR from CO2 ramping, owing to changes in radiative feedbacks and changes in ocean heat uptake efficiency. Atmospheric GCMs forced by observed warming patterns produce lower values of EffCS that are more in line with those inferred from observed historical energy budget changes. The differences in the EffCS estimates from historical energy budget constraints of models and observations are traced to discrepancies between modeled and observed historical surface warming patterns

Radiative feedbacks from stochastic variability in surface temperature and radiative imbalance

Estimates of radiative feedbacks obtained by regressing fluctuations in top-of-atmosphere (TOA) energy imbalance and surface temperature depend critically on assumptions about the nature of the stochastic forcing and on the sampling interval. Here we develop an energy-balance framework that allows us to model the different contributions of stochastic atmospheric and oceanic forcing on feed- back estimates. The contribution of different forcing components are parsed based on their impacts on the covariance structure of temperature and TOA energy fluxes, and the framework is validated in a hierarchy of climate model simulations that span a range of oceanic configurations and reproduce the key features seen in observations. We find that at least three distinct forcing sources, feedbacks, and time scales are needed to explain the full covariance structure. Atmospheric and oceanic forc- ings drive modes of variability with distinct relationships between near-surface air temperature and TOA radiation, and the net regression-based feedback estimate is found to be a weighted average of the distinct feedbacks associated with each mode. Moreover, the estimated feedback depends on whether surface temperature and TOA energy fluxes are sampled at monthly or annual timescales. The results suggest that regression-based feedback estimates reflect contributions from a combina- tion of stochastic forcings, and should not be interpreted as providing an estimate of the radiative feedback governing the climate response to greenhouse gas forcing