710 research outputs found
Orbital tuning, eccentricity, and the frequency modulation of climatic precession
The accuracy of geologic chronologies can, in principle, be improved through orbital tuning, the systematic adjustment of a chronology to bring the associated record into greater alignment with an orbitally derived signal. It would be useful to have a general test for the success of orbital tuning, and one proposal has been that eccentricity ought to covary with the amplitude envelope associated with precession variability recorded in tuned geologic records. A common procedure is to filter a tuned geologic record so as to pass precession period variability and compare the amplitude modulation of the resulting signal against eccentricity. There is a reasonable expectation for such a relationship to be found in paleoclimate records because the amplitude of precession forcing depends upon eccentricity. However, there also exists a relationship between eccentricity and the frequency of precession such that orbital tuning generates eccentricity-like amplitude modulation in filtered signals, regardless of the accuracy of the chronology or the actual presence of precession. This relationship results from the celestial mechanics governing eccentricity and precession and from the interaction between frequency modulation and amplitude modulation caused by filtering. When the eccentricity of Earth's orbit is small, the frequency of climatic precession undergoes large variations and less precession energy is passed through a narrow-band filter. Furthermore, eccentricity-like amplitude modulation is routinely obtained from pure noise records that are orbitally tuned to precession and then filtered. We conclude that the presence of eccentricity-like amplitude modulation in precession-filtered records does not support the accuracy of orbitally tuned time scales
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Pleistocene glacial variability as a chaotic response to obliquity forcing
The mid-Pleistocene Transition from 40 ky to ~100 ky glacial cycles is generally characterized as a singular transition attributable to scouring of continental regolith or a long-term decrease in atmospheric CO<sub>2</sub> concentrations. Here an alternative hypothesis is suggested, that Pleistocene glacial variability is chaotic and that transitions from 40 ky to ~100 ky modes of variability occur spontaneously. This alternate view is consistent with the presence of ~80 ky glacial cycles during the early Pleistocene and the lack of evidence for a change in climate forcing during the mid-Pleistocene. A simple model illustrates this chaotic scenario. When forced at a 40 ky period the model chaotically transitions between small 40 ky glacial cycles and larger 80 and 120 ky cycles which, on average, give the ~100 ky variability
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Peculiarly pleasant weather for US maize.
Continuation of historical trends in crop yield are critical to meeting the demands of a growing and more affluent world population. Climate change may compromise our ability to meet these demands, but estimates vary widely, highlighting the importance of understanding historical interactions between yield and climate trends. The relationship between temperature and yield is nuanced, involving differential yield outcomes to warm ([Formula: see text]C) and hot ([Formula: see text]C) temperatures and differing sensitivity across growth phases. Here, we use a crop model that resolves temperature responses according to magnitude and growth phase to show that US maize has benefited from weather shifts since 1981. Improvements are related to lengthening of the growing season and cooling of the hottest temperatures. Furthermore, current farmer cropping schedules are more beneficial in the climate of the last decade than they would have been in earlier decades, indicating statistically significant adaptation to a changing climate of 13 kgĀ·ha-1Ā· decade-1 All together, the better weather experienced by US maize accounts for 28% of the yield trends since 1981. Sustaining positive trends in yield depends on whether improvements in agricultural climate continue and the degree to which farmers adapt to future climates
Glacial Variability Over the Last Two Million Years: An Extended Depth-Derived Agemodel, Continuous Obliquity Pacing, and the Pleistocene Progression
An agemodel not relying upon orbital assumptions is estimated over the last 2 Ma using depth in marine sediment cores as a proxy for time. Agemodel uncertainty averages +/- 10 Ka in the early Pleistocene (similar to 2-1 Ma) and +/- 7 Ka in the late Pleistocene (similar to 1 Ma to the present). Twelve benthic and five planktic delta O-18 records are pinned to the agemodel and averaged together to provide a record of glacial variability. Major deglaciation features are identified over the last 2 Ma and a remarkable 33 out of 36 occur when Earth's obliquity is anomalously large. During the early Pleistocene deglaciations occur nearly every obliquity cycle giving a 40 Ka timescale, while late Pleistocene deglaciations more often skip one or two obliquity beats, corresponding to 80 or 120 Ka glacial cycles which, on average, give the similar to 100 Ka variability. This continuous obliquity pacing indicates that the glacial theory can be simplified. An explanation for the similar to 100 Ka glacial cycles only requires a change in the likelihood of skipping an obliquity cycle, rather than new sources of long-period variability. Furthermore, changes in glacial variability are not marked by any single transition so much as they exhibit a steady progression over the entire Pleistocene. The mean, variance, skewness, and timescale associated with the glacial cycles all exhibit an approximately linear trend over the last 2 Ma. A simple model having an obliquity modulated threshold and only three adjustable parameters is shown to reproduce the trends, timing, and spectral evolution associated with the Pleistocene glacial variability.Earth and Planetary Science
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Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing
Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the
precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, matching the period of changes in Earthās obliquity. The resolution of this 40,000-year problem is
that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated summer insolation is primarily controlled by obliquity and not precession because, by Keplerās second
law, the duration of the summer is inversely proportional to Earthās distance from the Sun.Earth and Planetary Science
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Compensation between Model Feedbacks and Curtailment of Climate Sensitivity
The spread in climate sensitivity obtained from 12 general circulation model runs used in the Fourth Assessment of the Intergovernmental Panel on Climate Change indicates a 95% confidence interval of , but this reflects compensation between model feedbacks. In particular, cloud feedback strength negatively covaries with the albedo feedback as well as with the combined water vapor plus lapse rate feedback. If the compensation between feedbacks is removed, the 95% confidence interval for climate sen- sitivity expands to ,. Neither of the quoted 95% intervals adequately reflects the understanding of climate sensitivity, but their differences illustrate that model interdependencies must be understood before model spread can be correctly interpreted.
The degree of negative covariance between feedbacks is unlikely to result from chance alone. It may, however, result from the method by which the feedbacks were estimated, physical relationships represented in the models, or from conditioning the models upon some combination of observations and expectations. This compensation between model feedbacks when taken together with indications that variations in radiative forcing and the rate of ocean heat uptake play a similar compensatory role in models suggests that conditioning of the models acts to curtail the intermodel spread in climate sensitivity. Observations used to condition the models ought to be explicitly stated, or there is the risk of doubly calling on data for purposes of both calibration and evaluation. Conditioning the models upon individual expectation (e.g., anchoring to the Charney range of , to the extent that it exists, greatly complicates statistical interpretation of the intermodel spread.Earth and Planetary Science
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Tropical Cooling and the Onset of North American Glaciation
We offer a test of the idea that gradual cooling in the eastern tropical Pacific led to cooling of North America and the initiation of glaciation ~3 Myr ago. Using modern climate data we estimate how warming of the eastern tropical Pacific affects North American temperature and ice-ablation. Assuming that the modern relationship holds over the past millions of years, a ~4Ā°C warmer eastern tropical Pacific between 3ā5 Ma would increase ablation in northern North America by approximately two meters per year. By comparison, a similar estimate of the ablation response to variations in Earth's obliquity gives less than half the magnitude of the tropically-induced change. Considering that variations in Earth's obliquity appear sufficient to initiate glaciations between ~1ā3 Ma, we infer that the warmer eastern equatorial Pacific prior to 3 Ma suffices to preclude glaciation.Earth and Planetary Science
Reconciling discrepancies between Uk37 and Mg/Ca reconstructions of Holocene marine temperature variability
Significant discrepancies exist between the detrended variability of late-Holocene marine temperatures inferred from Mg/Ca and Uk37 proxies, with the former showing substantially more centennial-scale variation than the latter. Discrepancies exceed that attributable to differences in location and persist across various calibrations, indicating that they are intrinsic to the proxy measurement. We demonstrate that these discrepancies can be reconciled using a statistical model that accounts for the effects of bioturbation, sampling and measurement noise, and aliasing of seasonal variability. The smaller number of individual samples incorporated into Mg/Ca measurements relative to Uk37 measurements leads to greater aliasing and generally accounts for the differences in the magnitude and distribution of variability. An inverse application of the statistical model is also developed and applied in order to estimate the spectrum of marine temperature variability after correcting for proxy distortions. The correction method is tested on surrogate data and shown to reliably estimate the spectrum of temperature variance when using high-resolution records. Applying this inverse method to the actual Mg/Ca and Uk37 data results in estimates of the spectrum of temperature variance that are consistent. This approach provides a basis by which to accurately estimate the distribution of intrinsic marine temperature variability from marine proxy records
Optimizing significance testing of astronomical forcing in cyclostratigraphy
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