William T. Hyde

Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model

Ice sheets may have reached the Equator in the late Proterozoic era (600–800 Myr ago), according to geological and palaeomagnetic studies, possibly resulting in a ‘snowball Earth’. But this period was a critical time in the evolution of multicellular animals, posing the question of how early life survived under such environmental stress. Here we present computer simulations of this unusual climate stage with a coupled climate/ice-sheet model. To simulate a snowball Earth, we use only a reduction in the solar constant compared to present-day conditions and we keep atmospheric CO2 concentrations near present levels. We find rapid transitions into and out of full glaciation that are consistent with the geological evidence. When we combine these results with a general circulation model, some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals.

Climate sensitivity constrained by temperature reconstructions over the past seven centuries.

There is a Brief Communications Arising (01 March 2007) associated with this documentThe magnitude and impact of future global warming depends on the sensitivity of the climate system to changes in greenhouse gas concentrations. The commonly accepted range for the equilibrium global mean temperature change in response to a doubling of the atmospheric carbon dioxide concentration, termed climate sensitivity, is 1.5–4.5 K (ref. 2). A number of observational studies, however, find a substantial probability of significantly higher sensitivities, yielding upper limits on climate sensitivity of 7.7 K to above 9 K (refs 3–8). Here we demonstrate that such observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. We use large-ensemble energy balance modelling and simulate the temperature response to past solar, volcanic and greenhouse gas forcing to determine which climate sensitivities yield simulations that are in agreement with proxy reconstructions. After accounting for the uncertainty in reconstructions and estimates of past external forcing, we find an independent estimate of climate sensitivity that is very similar to those from instrumental data. If the latter are combined with the result from all proxy reconstructions, then the 5–95 per cent range shrinks to 1.5–6.2 K, thus substantially reducing the probability of very high climate sensitivity.

Filtering of milankovitch cycles by earth's geography

Abstract Earths land-sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation. We examine this modification in the frequency domain by generating 800,000-yr time series of maximum summer temperature in selected regions with a linear, two-dimensional, seasonal energy balance climate model. Previous studies have demonstrated that this model has a sensitivity comparable to general circulation models for the seasonal temperature response to orbital forcing on land. Although the observed response in the geologic record is sometimes significantly different than modeled here (differences attributable to model limitations and feedbacks involving the ocean-atmosphere-cryosphere system), there are several results of significance: (1) in mid-latitude land areas the orbital signal is translated linearly into a large (>10°C) seasonal temperature response; (2) although the modeled seasonal response to orbital forcing on Antarctica is 6°C, the annual mean temperature effect (

Detection of Human Influence on a New, Validated 1500-Year Temperature Reconstruction

Abstract Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas–induced climate change, since a substantial fraction of the variance in these reconstructions from the ...

Modeling ocean heat content changes during the last millennium

[1] Observational studies show a significant increase in ocean heat content over the last half century. Herein we estimate heat content changes during the last millennium with a climate model whose forcing terms have been best-fit to surface proxy data. The model simulates the observed late 20th century ocean heat content increase and a comparable Little Ice Age minimum. When glacial advances are factored in, these results imply a sea level fall after the Middle Ages that is consistent with some geologic data. The present ocean heat content increase can be traced back to the mid-19th century, with a near-linear rate of change during the 20th century.

Validation of coral temperature calibrations

Geochemical analyses of coral skeletons are increasingly used to estimate past sea surface temperatures (SSTs). In this paper we suggest that the standard method of calibrating geochemical time series against a (usually short) local time series requires modification. In order to draw large-scale inferences about climate from coral proxy data it is also necessary to (1) calibrate against larger fields such as the local gridded data sets and (2) validate results against an independent data set (e.g., early 20th century). This approach has been applied in a pilot study to a coral record from New Caledonia. Despite a high δ18O correlation (r=−0.88) with the in situ and gridded SST data sets, estimated early 20th-century temperatures are more than 1.5°C colder than observed if the standard seasonal calibration is used. Regression against mean annual temperatures, which has a different slope relation, yields better estimates of early 20th-century SSTs. However, testing of a Sr/Ca record from New Caledonia yields better agreement with early 20th century SSTs. Routine validation exercises for other coral sites are necessary to clarify the robustness of geochemical coral proxies as estimators of past environmental change.

CO2 levels required for deglaciation of a “near‐snowball” Earth

Geologic evidence suggests that in the Late Neoproterozoic (-600 Ma) almost all land masses were glaciated, with sea-level glaciation existing even at the equator. A recent modeling study has shown that it is possible to simulate an ice-covered Earth glaciation with a coupled climate/ice-sheet model. However, separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics. Although 0.1 to 0.3 of an atmosphere of CO 2 (-300 to 1000 X) is required for deglaciation of a Snowball Earth, the exit CO 2 levels for an open water solution could be significantly less. In this paper we utilize a coupled climate/ice sheet model to demonstrate four points: (1) the open water solution can be simulated in the coupled model if the sea ice parameter is adjusted slightly; (2) a major reduction in ice volume from the open water/equatorial ice solution occurs at a CO, level of about 4X present values -about two orders of magnitude less than required for exit from the hard snowball initial state; (3) additional CO 2 increases are required to get fuller meltback of the ice; and (4) the open water solution exhibits hysteresis properties, such that climates with the same level of CO 2 may evolve into either the snowball, open water, or a warmer world solution, with the trajectory depending on initial conditions. These results set useful targets for geochemical calculations of CO 2 changes associated with the open-water solution.

Comparison of GCM and Energy Balance Model Simulations of Seasonal Temperature Changes over the Past 18 000 Years

Abstract The Sensitivity of a linear two dimensional Energy Balance Model (EBM) to altered surface albedo and insolation over the last 18 000 years is compared to simulators made with the NCAR Community Climate Model (CCM). The two-dimensional EBM is a more general form of that described in North et al. and allows for regionally varying albedos of ice sheets and sea ice. It is shown that the EBMs hemispherically averaged land and sea seasonal temperature departures agree excellently with the CCMs in the Northern Hemisphere. In the Southern Hemisphere the seasonal comparisons are legs favorable, although the annual-averaged oceanic temperature departures at glacial maximum agree to within 0.3°C. Since the CCM used prescribed SSTs (from CLIMAP), whereas the ERMs are calculated, our results suggest that the hemispherically averaged glacial- interglacial SST change estimated by CLIMAP is consistent with the altered energy balance requirements of the earth-atmosphere system. Results also suggest that on the ...

Transient nature of late Pleistocene climate variability

Climate in the early Pleistocene varied with a period of 41 kyr and was related to variations in Earths obliquity. About 900 kyr ago, variability increased and oscillated primarily at a period of ∼100 kyr, suggesting that the link was then with the eccentricity of Earths orbit. This transition has often been attributed to a nonlinear response to small changes in external boundary conditions. Here we propose that increasing variablility within the past million years may indicate that the climate system was approaching a second climate bifurcation point, after which it would transition again to a new stable state characterized by permanent mid-latitude Northern Hemisphere glaciation. From this perspective the past million years can be viewed as a transient interval in the evolution of Earths climate. We support our hypothesis using a coupled energy-balance/ice-sheet model, which furthermore predicts that the future transition would involve a large expansion of the Eurasian ice sheet. The process responsible for the abrupt change seems to be the albedo discontinuity at the snow–ice edge. The best-fit model run, which explains almost 60% of the variance in global ice volume during the past 400 kyr, predicts a rapid transition in the geologically near future to the proposed glacial state. Should it be attained, this state would be more ‘symmetric’ than the present climate, with comparable areas of ice/sea-ice cover in each hemisphere, and would represent the culmination of 50 million years of evolution from bipolar nonglacial climates to bipolar glacial climates.

Climate model comparison of Gondwanan and Laurentide glaciations

Geologic studies indicate that the Carboniferous glaciation on Gondwanaland was approximately as extensive as the ice sheets during the Pleistocene. However, there is one major difference between the climate boundary conditions for the two ice sheets: the Gondwanan ice sheet was located on a supercontinent. Three different levels of sensitivity experiments were conducted to examine the effect of the large landmass on the magnitude of summer warming over the ice sheets: (1) simulations with present solar luminosity and present orbital forcing resulted in summer temperatures over the Gondwanan Ice Sheet 12°–17°C greater than over the Pleistocene Laurentide Ice Sheet; (2) lowering the solar constant 3% or modifying the seasonal insolation cycle to a “cold summer orbit” reduced the warming but still led to significant differences between Gondwanan and Pleistocene simulations; and (3) the combined effect of lowering the solar constant and modifying the seasonal insolation cycle to a cold summer orbit resulted in temperature patterns over the Gondwanan Ice Sheet similar to the Pleistocene. Model simulations also predict tropical sea surface temperatures about 4°C less than at present as a result of reduced solar luminosity. These results suggest that conditions necessary to explain Gondwanan ice sheet stability may be known, but required boundary conditions would be more extreme than in the Pleistocene. Although a number of uncertainties remain in these calculations, they help to better define critical conditions for glaciation for one of the most prolonged periods of continuous glaciation in Earth history.