Thomas Crowley - Profile on Academia.edu (original) (raw)
Personal by Thomas Crowley
Obituary in Guardian, in 2014, by John Abraham and Dana Nutticelli
Papers by Thomas Crowley
Abstract Stable global temperatures of the last 10–15 years have been a topic of considerable dis... more Abstract Stable global temperatures of the last 10–15 years have been a topic of considerable discussion.
A new proxy extension of the global temperature record enables better placement of this feature in
a longer historical perspective. The fixed-grid composite covers the interval 1801–1984, with an extension
to 1782, and anchors the global temperature record in the last major cold interval of the Little Ice
Age, when carbon dioxide concentration was at preanthropogenic levels. Except for greater and longer
cooling (approximately twice the length of Pinatubo) associated with the Tambora eruption, the proxy
agrees with the most widely cited previous assessment of global temperature over this interval, lending
more confidence to a centennial extension of the global temperature record. The proxy correlation is as
high as 0.83 for the interval 1907–1984 (df=8, p=0.001), with the 21st century 1.0∘C±0.2∘C warmer
than the nonvolcanic base state. This remarkable linearity requires a clear theoretical understanding as to
how an exceedingly complex system can, on the global average, behave in such a simple way. Removal of
the linear radiatively forced component from the global temperature record yields an estimate of natural
variability for the last 230 years and indicates no unusual natural variability during the recent 10–15 years.
Based on the estimate of unforced variability over the last 170 years, there is about a 40% chance of continued
“natural cooling” over the next few years, with about a 10% chance of cooling persisting into the
next decade
Comparison of GCM and Energy Balance Model Simulations of Seasonal Temperature Changes over the Past 18 000 Years
Journal of Climate, 1989
The sensitivity of a linear two dimensional Energy Balance Model (EBM) to altered surface albedo ... more 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 simulations made with the NCAR Community Climate Model (CCM). The two-dimensional EBM is a more general form of that described in North {ital et} {ital al}. and allows for regionally varying albedos of ice
Journal of Geophysical Research, 1990
The seasonal cycle of surface temperature is largely controlled by the land-sea distribution. Pre... more The seasonal cycle of surface temperature is largely controlled by the land-sea distribution. Previous studies with a two-dimensional, seasonal energy balance model (EBM) suggested that large annual cycles on supercontinents could produce sufficiently high summer temperatures to melt summer snow, even when the continents were located in polar regions. The above calculations were done with a linear model. In this paper we test the sensitivity of these conclusions to seasonally varying snow albedo feedback by developing a new nonlinear two-dimensional, seasonal EBM. The model satisfactorily reproduces the present annual and semi-annual cycles, plus snow and sea ice margins; its sensitivity is • 25% greater than that of the linear model and comparable to a number of general drculation model (GCM) studies for altered CO2 levels. Experiments are described for a series of idealized supercontinent configurations; in addition we examine the effect of changing land-sea distributions on the climate of Greenland and Antarctica. Supercontinent model simulations are similar to the previously published linear model results and provide further support for the hypothesis that ice-free states could occur on polar supercontinents. However, some of our results for smaller land masses differ from the earlier study. Although temperatures are systematically lower for the nonlinear run, we simulate > 15øC decrease in summer temperatures over Greenland during the last !00 million years in both the llnear and nonlinear models. However, there is only a weak suggestion of a long-term trend in the southern hemisphere. Revised reconstructions of southern hemisphere geography further obscure any long-term trend in this region. Overall, model simulations provide further support for the proposition that changes in seasonality have played an important role in climate change over the past few hundred million years. 18,653 18,654 HYDE •',T AL.: POLAR. CONTIN•',NTALITY AND CLIMATE STUDIES 18,656 HYDE I•T AL.: POLAI• CONTINI•NTALITY AND CLIMATE STUDIES
Paleoceanography, 1999
AbstractGeochemical analyses of coral skeletons are increasingly used to estimate past sea surfac... more AbstractGeochemical 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.
Nature, 2006
The magnitude and impact of future global warming depends on the sensitivity of the climate syste... more The 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 1 , termed climate sensitivity, is 1. 5-4.5 K (ref. 2). A number of observational studies 3-10 , 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.
Journal of Climate, 2007
Climate records over the last millennium place the twentieth-century warming in a longer historic... more 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-twentiethcentury 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.
Geophysical Research Letters, 2003
Abstract[1] Observational studies show a significant increase in ocean heat content over the last... more Abstract[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.
Climate Model Comparison of Gondwanan and Laurentide Glaciations
Journal of Geophysical Research, 1991
Three different levels of sensitivity experiments were conducted to examine the effect of Gondwan... more Three different levels of sensitivity experiments were conducted to examine the effect of Gondwanaland landmass on the magnitude of summer warming over 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 percent 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.
Probability of Future Climatically Significant Volcanic Eruptions
Journal of Climate, 2000
... Corresponding author address: Dr. William T. Hyde, Department of Oceanography, Texas A&M ... more ... Corresponding author address: Dr. William T. Hyde, Department of Oceanography, Texas A&M University, College Station, TX 77843-3146. ... Self 1982) provides volcanologic estimates of eruptions from ejecta data; for example, Pinatubo (1991) has a VEI of 6, Agung (1963) one ...
Geophysical Research Letters, 1992
Observational and modeling studies support the concept of significant Milankovitch fluctuations i... more Observational and modeling studies support the concept of significant Milankovitch fluctuations in the pre-Pleistocene. Many of these studies have focused on the mid-Cretaceous, a time of high sea level and moderately dispersed land-sea distribution. In this paper we utilize a two-dimensional energy balance climate model to examine the potential effects of Milankovitch fluctuations for a supercontinent configuration (Early Jurassic, 195 Ma). We calculate that Milankovitch variations can modulate the magnitude of summer warming by as much as 14-16øC on Pangaea, with large changes occurring in both the northern and southern hemispheres. These values are comparable to or slightly larger than the calculated range for the present Eurasian landmass for the Pleistocene. In addition, mean monthly summer temperatures of 25øC reach 65 ø paleo!afitude for "hot orbit" configurations. This latter result suggests that some organisms may have been able to migrate into higher latitudes during precession half-cycles of 10,000 years duration, and a potential bias should be considered when evaluating evidence for past high4atitude warmth. Our results indicate that general circulation model (GCM) runs for past time periods which use only one orbital configuration may be missing a substantial amount of the potential variance on large land masses. grant ATM90-02808 to T.J.C. References Berger, A. L., Long-term variations of caloric insolation resuiting from the earth's orbital elements, Quat. Res., 9, 139-167, 1978. Berger, A. L., M. F. Loutre, and V. Dehant, Influence of the changing lunar orbit on the astronomical frequencies of pre-Quaternary insolation patterns, Paleoceanog., 4, 555-564, 1989. Berner, R. A., A model for atmospheric CO2 over Phanerozoic time, Amer. J. Sci., 291,339-376, 1991. Cerling, T.E., Carbon dioxide in the atmosphere: Evidence from Cenozoic and Mesozoic paleosols, Amer. J. Sci., 291,377-400, 1991. COHMAP Members, Climatic chafiges of the last 18,000 years: Observations and model simulations, Science, 241, 1043-!052, 1988. Crowley, T. J., and S. K. Baum, Toward reconciliation of Late Ordovician (-440 Ma) glaciation with very high CO2 levels, J. Geophys. Res. 96, 22597-22610, 1991. Crowley, T. J., W. T. Hyde, and D. A. Short, Seasonal cycle variations on the supercontinent of Pangaea, Geology, 17, 457-460, 1989. Vakhrameev, V. A., Jurassic and Early Cretaceous floras of Eurasia and the paleofloristic provinces of this period,
Geophysical Research Letters, 2001
Geologic evidence suggests that in the Late Neoproterozoic,(-600 Ma) almost all land masses were ... more 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 2 level of about 4X present valuesabout 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. Res., 104, 9517-9545, 1999.
SCIENCE 280, 728-730, May 1, 1998
During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. L... more During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. Large local changes in precession forcing could have produced this pattern if there were a rectified response in sea ice cover. Results from a coupled sea ice–ocean general circulation model supported this hypothesis when it was tested for three intervals around the last interglacial. Such a mechanism may play an important role in contributing to phase offsets between Northern and Southern Hemisphere climate change for other time intervals.
Nature 405, 425-429, May 25, 2000
Ice sheets may have reached the Equator in the late Proterozoic era (600±800 Myr ago), according ... more 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 ®nd 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.
Science 289,270-277, 2000
Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 1000... more Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 1000 years allow the warming of the 20th century to be placed within a historical context and various mechanisms of climate change to be tested. Comparisons of observations with simulations from an energy balance climate model indicate that as much as 41 to 64% of preanthropogenic ( pre-1850) decadal-scale temperature variations was due to changes in solar irradiance and volcanism. Removal of the forced response from reconstructed temperature time series yields residuals that show similar variability to those of control runs of coupled models, thereby lending support to the models’ value as estimates of low-frequency variability in the climate system. Removal of all forcing except greenhouse gases from the #1000-year time series results in a residual with a very large late-20th-century warming that closely agrees with the response predicted from greenhouse gas forcing. The combination of a unique level of temperature increase in the late 20th century and improved constraints on the role of natural variability provides further evidence that the greenhouse effect has already established itself above the level of natural variability in the climate system. A 21st-century global warming projection far exceeds the natural variability of the past 1000 years and is greater than the best estimate of global temperature change for the last interglacial.
Geology; v. 34; no. 6; p. 421–424; doi: 10.1130/G22108.1, Jun 2006
Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modelin or proxy d... more Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modelin or proxy data. Because the uncertainty inherent in such reconstructions is large
enough to be climatically significant, inverse climate modeling may help to constrain paleo- CO2 estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for CO2—the value beneath which Siberia acquires ‘‘excess’’ ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360–340 Ma), but model results imply that paleo-CO2 levels had to be greater than 2–4! modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil CO2 proxies and the timing of Gondwanan deglaciation, thus providing support for a significant CO2 increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation.
Geophys. Res. Lett., 33, L18404, doi:10.1029/ 2006GL027445, 2006
Researchers and coastal managers are pondering how accelerated sea-level rise and possibly intens... more Researchers and coastal managers are pondering how accelerated sea-level rise and possibly intensified storms will affect shorelines. These issues are most often investigated in a cross-shore profile framework, fostering the implicit assumption that coastline responses will be approximately uniform in the alongshore direction. However, experiments with a recently developed numerical model of coastline change on a large spatial domain suggest that the shoreline responses to climate change could be highly variable. As storm and wave climates change, large-scale coastline shapes are likely to shift—causing areas of greatly accelerated coastal erosion to alternate with areas of considerable shoreline accretion. On complex-shaped coastlines, including cuspate-cape and spit coastlines, the alongshore variation in shoreline retreat rates could be an order of
magnitude higher than the baseline retreat rate expected from sea-level rise alone.
nature 45, 226-230; doi:10.1038/nature07365, Nov 13, 2008
Climate in the early Pleistocene varied with a period of 41 kyr and was related to variations in ... more Climate in the early Pleistocene varied with a period of 41 kyr and was related to variations in Earth’s 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 Earth’s 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 permanentmid-latitude Northern Hemisphere glaciation. From this perspective the pastmillion years can be viewed as a transient interval in the evolution of Earth’s climate. We support our hypothesis using a coupled energybalance/ice-sheetmodel, 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.
Nature, 2006
The magnitude and impact of future global warming depends on the sensitivity of the climate syste... more The 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 1 , termed climate sensitivity, is 1. 5-4.5 K (ref. 2). A number of observational studies 3-10 , 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.
Ice-age methane variations
Nature, 1991
Obituary in Guardian, in 2014, by John Abraham and Dana Nutticelli
Abstract Stable global temperatures of the last 10–15 years have been a topic of considerable dis... more Abstract Stable global temperatures of the last 10–15 years have been a topic of considerable discussion.
A new proxy extension of the global temperature record enables better placement of this feature in
a longer historical perspective. The fixed-grid composite covers the interval 1801–1984, with an extension
to 1782, and anchors the global temperature record in the last major cold interval of the Little Ice
Age, when carbon dioxide concentration was at preanthropogenic levels. Except for greater and longer
cooling (approximately twice the length of Pinatubo) associated with the Tambora eruption, the proxy
agrees with the most widely cited previous assessment of global temperature over this interval, lending
more confidence to a centennial extension of the global temperature record. The proxy correlation is as
high as 0.83 for the interval 1907–1984 (df=8, p=0.001), with the 21st century 1.0∘C±0.2∘C warmer
than the nonvolcanic base state. This remarkable linearity requires a clear theoretical understanding as to
how an exceedingly complex system can, on the global average, behave in such a simple way. Removal of
the linear radiatively forced component from the global temperature record yields an estimate of natural
variability for the last 230 years and indicates no unusual natural variability during the recent 10–15 years.
Based on the estimate of unforced variability over the last 170 years, there is about a 40% chance of continued
“natural cooling” over the next few years, with about a 10% chance of cooling persisting into the
next decade
Comparison of GCM and Energy Balance Model Simulations of Seasonal Temperature Changes over the Past 18 000 Years
Journal of Climate, 1989
The sensitivity of a linear two dimensional Energy Balance Model (EBM) to altered surface albedo ... more 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 simulations made with the NCAR Community Climate Model (CCM). The two-dimensional EBM is a more general form of that described in North {ital et} {ital al}. and allows for regionally varying albedos of ice
Journal of Geophysical Research, 1990
The seasonal cycle of surface temperature is largely controlled by the land-sea distribution. Pre... more The seasonal cycle of surface temperature is largely controlled by the land-sea distribution. Previous studies with a two-dimensional, seasonal energy balance model (EBM) suggested that large annual cycles on supercontinents could produce sufficiently high summer temperatures to melt summer snow, even when the continents were located in polar regions. The above calculations were done with a linear model. In this paper we test the sensitivity of these conclusions to seasonally varying snow albedo feedback by developing a new nonlinear two-dimensional, seasonal EBM. The model satisfactorily reproduces the present annual and semi-annual cycles, plus snow and sea ice margins; its sensitivity is • 25% greater than that of the linear model and comparable to a number of general drculation model (GCM) studies for altered CO2 levels. Experiments are described for a series of idealized supercontinent configurations; in addition we examine the effect of changing land-sea distributions on the climate of Greenland and Antarctica. Supercontinent model simulations are similar to the previously published linear model results and provide further support for the hypothesis that ice-free states could occur on polar supercontinents. However, some of our results for smaller land masses differ from the earlier study. Although temperatures are systematically lower for the nonlinear run, we simulate > 15øC decrease in summer temperatures over Greenland during the last !00 million years in both the llnear and nonlinear models. However, there is only a weak suggestion of a long-term trend in the southern hemisphere. Revised reconstructions of southern hemisphere geography further obscure any long-term trend in this region. Overall, model simulations provide further support for the proposition that changes in seasonality have played an important role in climate change over the past few hundred million years. 18,653 18,654 HYDE •',T AL.: POLAR. CONTIN•',NTALITY AND CLIMATE STUDIES 18,656 HYDE I•T AL.: POLAI• CONTINI•NTALITY AND CLIMATE STUDIES
Paleoceanography, 1999
AbstractGeochemical analyses of coral skeletons are increasingly used to estimate past sea surfac... more AbstractGeochemical 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.
Nature, 2006
The magnitude and impact of future global warming depends on the sensitivity of the climate syste... more The 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 1 , termed climate sensitivity, is 1. 5-4.5 K (ref. 2). A number of observational studies 3-10 , 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.
Journal of Climate, 2007
Climate records over the last millennium place the twentieth-century warming in a longer historic... more 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-twentiethcentury 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.
Geophysical Research Letters, 2003
Abstract[1] Observational studies show a significant increase in ocean heat content over the last... more Abstract[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.
Climate Model Comparison of Gondwanan and Laurentide Glaciations
Journal of Geophysical Research, 1991
Three different levels of sensitivity experiments were conducted to examine the effect of Gondwan... more Three different levels of sensitivity experiments were conducted to examine the effect of Gondwanaland landmass on the magnitude of summer warming over 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 percent 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.
Probability of Future Climatically Significant Volcanic Eruptions
Journal of Climate, 2000
... Corresponding author address: Dr. William T. Hyde, Department of Oceanography, Texas A&M ... more ... Corresponding author address: Dr. William T. Hyde, Department of Oceanography, Texas A&M University, College Station, TX 77843-3146. ... Self 1982) provides volcanologic estimates of eruptions from ejecta data; for example, Pinatubo (1991) has a VEI of 6, Agung (1963) one ...
Geophysical Research Letters, 1992
Observational and modeling studies support the concept of significant Milankovitch fluctuations i... more Observational and modeling studies support the concept of significant Milankovitch fluctuations in the pre-Pleistocene. Many of these studies have focused on the mid-Cretaceous, a time of high sea level and moderately dispersed land-sea distribution. In this paper we utilize a two-dimensional energy balance climate model to examine the potential effects of Milankovitch fluctuations for a supercontinent configuration (Early Jurassic, 195 Ma). We calculate that Milankovitch variations can modulate the magnitude of summer warming by as much as 14-16øC on Pangaea, with large changes occurring in both the northern and southern hemispheres. These values are comparable to or slightly larger than the calculated range for the present Eurasian landmass for the Pleistocene. In addition, mean monthly summer temperatures of 25øC reach 65 ø paleo!afitude for "hot orbit" configurations. This latter result suggests that some organisms may have been able to migrate into higher latitudes during precession half-cycles of 10,000 years duration, and a potential bias should be considered when evaluating evidence for past high4atitude warmth. Our results indicate that general circulation model (GCM) runs for past time periods which use only one orbital configuration may be missing a substantial amount of the potential variance on large land masses. grant ATM90-02808 to T.J.C. References Berger, A. L., Long-term variations of caloric insolation resuiting from the earth's orbital elements, Quat. Res., 9, 139-167, 1978. Berger, A. L., M. F. Loutre, and V. Dehant, Influence of the changing lunar orbit on the astronomical frequencies of pre-Quaternary insolation patterns, Paleoceanog., 4, 555-564, 1989. Berner, R. A., A model for atmospheric CO2 over Phanerozoic time, Amer. J. Sci., 291,339-376, 1991. Cerling, T.E., Carbon dioxide in the atmosphere: Evidence from Cenozoic and Mesozoic paleosols, Amer. J. Sci., 291,377-400, 1991. COHMAP Members, Climatic chafiges of the last 18,000 years: Observations and model simulations, Science, 241, 1043-!052, 1988. Crowley, T. J., and S. K. Baum, Toward reconciliation of Late Ordovician (-440 Ma) glaciation with very high CO2 levels, J. Geophys. Res. 96, 22597-22610, 1991. Crowley, T. J., W. T. Hyde, and D. A. Short, Seasonal cycle variations on the supercontinent of Pangaea, Geology, 17, 457-460, 1989. Vakhrameev, V. A., Jurassic and Early Cretaceous floras of Eurasia and the paleofloristic provinces of this period,
Geophysical Research Letters, 2001
Geologic evidence suggests that in the Late Neoproterozoic,(-600 Ma) almost all land masses were ... more 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 2 level of about 4X present valuesabout 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. Res., 104, 9517-9545, 1999.
SCIENCE 280, 728-730, May 1, 1998
During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. L... more During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. Large local changes in precession forcing could have produced this pattern if there were a rectified response in sea ice cover. Results from a coupled sea ice–ocean general circulation model supported this hypothesis when it was tested for three intervals around the last interglacial. Such a mechanism may play an important role in contributing to phase offsets between Northern and Southern Hemisphere climate change for other time intervals.
Nature 405, 425-429, May 25, 2000
Ice sheets may have reached the Equator in the late Proterozoic era (600±800 Myr ago), according ... more 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 ®nd 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.
Science 289,270-277, 2000
Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 1000... more Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 1000 years allow the warming of the 20th century to be placed within a historical context and various mechanisms of climate change to be tested. Comparisons of observations with simulations from an energy balance climate model indicate that as much as 41 to 64% of preanthropogenic ( pre-1850) decadal-scale temperature variations was due to changes in solar irradiance and volcanism. Removal of the forced response from reconstructed temperature time series yields residuals that show similar variability to those of control runs of coupled models, thereby lending support to the models’ value as estimates of low-frequency variability in the climate system. Removal of all forcing except greenhouse gases from the #1000-year time series results in a residual with a very large late-20th-century warming that closely agrees with the response predicted from greenhouse gas forcing. The combination of a unique level of temperature increase in the late 20th century and improved constraints on the role of natural variability provides further evidence that the greenhouse effect has already established itself above the level of natural variability in the climate system. A 21st-century global warming projection far exceeds the natural variability of the past 1000 years and is greater than the best estimate of global temperature change for the last interglacial.
Geology; v. 34; no. 6; p. 421–424; doi: 10.1130/G22108.1, Jun 2006
Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modelin or proxy d... more Reconstructions of Phanerozoic CO2 levels have generally relied on geochemical modelin or proxy data. Because the uncertainty inherent in such reconstructions is large
enough to be climatically significant, inverse climate modeling may help to constrain paleo- CO2 estimates. In particular, we test the plausibility of this technique by focusing on the climate from 360 to 260 Ma, a time in which the Siberian landmass was in middle to high latitudes, yet had little or no permanent land ice. Our climate model simulations predict a lower limit for CO2—the value beneath which Siberia acquires ‘‘excess’’ ice. Simulations provide little new information for the period in which Siberia was at a relatively low paleoaltitude (360–340 Ma), but model results imply that paleo-CO2 levels had to be greater than 2–4! modern values to be consistent with an apparently ice-free Siberia in the late Permian. These results for the later period in general agree with soil CO2 proxies and the timing of Gondwanan deglaciation, thus providing support for a significant CO2 increase before the end-Permian boundary event. Our technique may be applicable to other time intervals of unipolar glaciation.
Geophys. Res. Lett., 33, L18404, doi:10.1029/ 2006GL027445, 2006
Researchers and coastal managers are pondering how accelerated sea-level rise and possibly intens... more Researchers and coastal managers are pondering how accelerated sea-level rise and possibly intensified storms will affect shorelines. These issues are most often investigated in a cross-shore profile framework, fostering the implicit assumption that coastline responses will be approximately uniform in the alongshore direction. However, experiments with a recently developed numerical model of coastline change on a large spatial domain suggest that the shoreline responses to climate change could be highly variable. As storm and wave climates change, large-scale coastline shapes are likely to shift—causing areas of greatly accelerated coastal erosion to alternate with areas of considerable shoreline accretion. On complex-shaped coastlines, including cuspate-cape and spit coastlines, the alongshore variation in shoreline retreat rates could be an order of
magnitude higher than the baseline retreat rate expected from sea-level rise alone.
nature 45, 226-230; doi:10.1038/nature07365, Nov 13, 2008
Climate in the early Pleistocene varied with a period of 41 kyr and was related to variations in ... more Climate in the early Pleistocene varied with a period of 41 kyr and was related to variations in Earth’s 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 Earth’s 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 permanentmid-latitude Northern Hemisphere glaciation. From this perspective the pastmillion years can be viewed as a transient interval in the evolution of Earth’s climate. We support our hypothesis using a coupled energybalance/ice-sheetmodel, 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.
Nature, 2006
The magnitude and impact of future global warming depends on the sensitivity of the climate syste... more The 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 1 , termed climate sensitivity, is 1. 5-4.5 K (ref. 2). A number of observational studies 3-10 , 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.
Ice-age methane variations
Nature, 1991
Journal of Geophysical Research, 1993
The latest Precambrian (-0.57 Ga) was marked by extensive glaciation on a supercontinent. Ice cov... more The latest Precambrian (-0.57 Ga) was marked by extensive glaciation on a supercontinent. Ice cover may have been in lower latitudes than during the Pleistocene. Deglaciation and breakup of the supercontinent were followed by the first appearance/expansion of metazoans. Herein we report results from a seasonal climate model that clarify some of the processes operating during this important time interval. We demonstrate that, because solar luminosity was about 6% less than present, the modeled snowline was approximately 15 ø equatorward of its modeled Pleistocene limit. The significance of this response depends on choice of paleogeographic reconstruction. If the supercontinent was located entirely in low latitudes, the freezing line changes would not be enough to trigger glaciation on land. However, the luminosity changes are much more important if the supercontinent extended into midlatitudes (-50 ø paleolatitude). Such a configuration has extensive summer snowcover and would provide a "seed" area for ice growth into lower latitudes. We postulate that if the large snowline changes we simulate were coupled to an ice sheet model, the ice margin could have reached to within 25 ø of the equator. Such a response could reconcile models and geologic data, but the reconciliation would critically depend on a more precise definition of low latitude glaciation, that is, whether the ice was at 25-30 ø or 0 ø latitude.