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J. E. Kutzbach
and
E. W. Wahl

Abstract

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J. E. Kutzbach
and
R. A. Bryson

Abstract

The general character of the variance spectrum of temperature fluctuations on time scales from 1 to 10,000 years is estimated from a combination of botanical, chemical, historical and instrumental records from locations in the North Atlantic sector. Variance spectral density increases with decreasing frequency (increasing period) over the entire frequency domain, reflecting the “thermal inertia” of the ocean and cryosphere portion of the climatic system. This is most pronounced for periods longer than about 30 years.

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John E. Kutzbach
and
Peter J. Guetter

Abstract

General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphere summer monsoons were weaker during the same period. In northern midlatitudes, glacial-age features such as the North American ice shed exerted a strong influence on the climate until 9000 yr BP. Much of the climatic change of the period 12 000 to 6000 yr BP can be described as an amplified (weakened) seasonal cycle in response to the larger (smaller) seasonal radiation extremes of the Northern (Southern) Hemisphere. Summers were warmer and winters colder in Northern Hemisphere lands, but there was little change in annual average temperature. However, because of the nonlinear relationship between saturation vapor pressure and temperature, the sensitivity of the hydrologic cycle to orbital parameter changes was larger in summer than in winter (and in the tropics rather than high latitudes); in the northern tropics, this led to a net increase in estimated annual average precipitation and precipitation minus evaporation. Many features of the results are in agreement with geologic evidence.

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J. E. Kutzbach
and
B. L. Otto-Bliesner

Abstract

The earth's orbital parameters, precession, obliquity and eccentricity, produce solar radiation differences (compared to present) of ∼7% at the solstices 9000 years before present (B.P.): more radiation in June-July-August, less in December-January-February. When this amplified seasonal cycle of solar radiation is used to drive a low-resolution general circulation model, an intensified monsoon circulation is simulated for Northern Hemisphere summer. The annual- and global-average land surface temperature and the annual- and global-average precipitation are the same for the simulated 9000 years B.P. climate and the present climate. Certain features of the simulated monsoon climate from this orbital-parameter sensitivity experiment agree with the paleoclimatic evidence.

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Harold C. Fritts
,
Terence J. Blasing
,
Bruce P. Hayden
, and
John E. Kutzbach

Abstract

Ring widths from trees on certain sites reflect climatic variation. Therefore, long time series derived from replicated and precisely dated ring-width chronologies may be utilized to extend climatic records into prehistoric times. Multivariate analyses of tree-ring chronologies from western North America are used to derive response functions from which one can ascertain what climatic information each ring-width chronology contains. In addition, multivariate analyses are utilized to calibrate a large number of ring-width chronologies of diverse response functions and from widely dispersed sites with a large number of regional climatic variables. A series of transfer functions is derived which allows estimates of anomalous climatic variation from tree-ring records. Reconstructions of anomalous variations in atmospheric circulation for portions of the Northern Hemisphere back to 1700 A.D. are obtained by applying the transfer functions to tree-ring data for time periods when ring data are available but climatic data are not.

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Benjamin Felzer
,
Robert J. Oglesby
,
Hong Shao
,
Thompson Webb III
,
Dena E. Hyman
,
Warren L. Prell
, and
John E. Kutzbach

Abstract

Paleoclimatic data and climate model simulations have demonstrated that orbitally forced changes in solar radiation can have a pronounced effect on global climate. Key questions remain, however, about the spatial patterns in the climatic sensitivity to these changes in solar radiation. The authors use GCM simulations of Kutzbach and Guetter and Prell and Kutzbach that were made with the NCAR Community Climate Model (CCM), version CCM0. The results of these simulations are employed to compute linear equilibrium sensitivity coefficients and jackknife uncertainties relating the response of key climate variables to orbitally forced changes in solar radiation. The spatial distributions of the sensitivities and the corresponding uncertainties reveal the synoptic patterns of climate response for these climate variables and identify areas of high and low sensitivity.

The sensitivity of CCM0 to solar radiation changes such as those experienced during the Quaternary is large and predominately linear for many climatic variables. The climatic response is always greatest in the summer hemisphere, because the orbitally induced radiation changes are more pronounced during the summer. The larger landmasses also show a greater climatic response than the smaller ones, due to both the larger heat capacity of the land relative to the oceans, and to the effects of the fixed SSTs. The land surface temperature always increases with increased radiative heating. The surface pressure generally decreases with increasing solar insulation over the landmasses, which were heated, with corresponding increases over the oceans. The net change in moisture (precipitation - evaporation) to increasing solar radiation is greatest over the summer hemisphere Tropics. All three of these variables combine to produce stronger summer monsoons with increasing solar radiation.

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Michael Notaro
,
Zhengyu Liu
,
Robert Gallimore
,
Stephen J. Vavrus
,
John E. Kutzbach
,
I. Colin Prentice
, and
Robert L. Jacob

Abstract

Rising levels of carbon dioxide since the preindustrial era have likely contributed to an observed warming of the global surface, and observations show global greening and an expansion of boreal forests. This study reproduces observed climate and vegetation trends associated with rising CO2 using a fully coupled atmosphere–ocean–land surface GCM with dynamic vegetation and decomposes the effects into physiological and radiative components. The simulated warming trend, strongest at high latitudes, was dominated by the radiative effect, although the physiological effect of CO2 on vegetation (CO2 fertilization) contributed to significant wintertime warming over northern Europe and central and eastern Asia. The net global greening of the model was primarily due to the physiological effect of increasing CO2, while the radiative and physiological effects combined to produce a poleward expansion of the boreal forests. Observed and simulated trends in tree ring width are consistent with the enhancement of vegetation growth by the physiological effect of rising CO2.

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S. P. Harrison
,
D. Jolly
,
F. Laarif
,
A. Abe-Ouchi
,
B. Dong
,
K. Herterich
,
C. Hewitt
,
S. Joussaume
,
J. E. Kutzbach
,
J. Mitchell
,
N. de Noblet
, and
P. Valdes

Abstract

The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditions in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia suggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, and in the northern and southern extratropics. The small differences among models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflect differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.

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Maurice Blackmon
,
Byron Boville
,
Frank Bryan
,
Robert Dickinson
,
Peter Gent
,
Jeffrey Kiehl
,
Richard Moritz
,
David Randall
,
Jagadish Shukla
,
Susan Solomon
,
Gordon Bonan
,
Scott Doney
,
Inez Fung
,
James Hack
,
Elizabeth Hunke
,
James Hurrell
,
John Kutzbach
,
Jerry Meehl
,
Bette Otto-Bliesner
,
R. Saravanan
,
Edwin K. Schneider
,
Lisa Sloan
,
Michael Spall
,
Karl Taylor
,
Joseph Tribbia
, and
Warren Washington

The Community Climate System Model (CCSM) has been created to represent the principal components of the climate system and their interactions. Development and applications of the model are carried out by the U.S. climate research community, thus taking advantage of both wide intellectual participation and computing capabilities beyond those available to most individual U.S. institutions. This article outlines the history of the CCSM, its current capabilities, and plans for its future development and applications, with the goal of providing a summary useful to present and future users.

The initial version of the CCSM included atmosphere and ocean general circulation models, a land surface model that was grafted onto the atmosphere model, a sea-ice model, and a “flux coupler” that facilitates information exchanges among the component models with their differing grids. This version of the model produced a successful 300-yr simulation of the current climate without artificial flux adjustments. The model was then used to perform a coupled simulation in which the atmospheric CO2 concentration increased by 1 % per year.

In this version of the coupled model, the ocean salinity and deep-ocean temperature slowly drifted away from observed values. A subsequent correction to the roughness length used for sea ice significantly reduced these errors. An updated version of the CCSM was used to perform three simulations of the twentieth century's climate, and several projections of the climate of the twenty-first century.

The CCSM's simulation of the tropical ocean circulation has been significantly improved by reducing the background vertical diffusivity and incorporating an anisotropic horizontal viscosity tensor. The meridional resolution of the ocean model was also refined near the equator. These changes have resulted in a greatly improved simulation of both the Pacific equatorial undercurrent and the surface countercurrents. The interannual variability of the sea surface temperature in the central and eastern tropical Pacific is also more realistic in simulations with the updated model.

Scientific challenges to be addressed with future versions of the CCSM include realistic simulation of the whole atmosphere, including the middle and upper atmosphere, as well as the troposphere; simulation of changes in the chemical composition of the atmosphere through the incorporation of an integrated chemistry model; inclusion of global, prognostic biogeochemical components for land, ocean, and atmosphere; simulations of past climates, including times of extensive continental glaciation as well as times with little or no ice; studies of natural climate variability on seasonal-to-centennial timescales; and investigations of anthropogenic climate change. In order to make such studies possible, work is under way to improve all components of the model. Plans call for a new version of the CCSM to be released in 2002. Planned studies with the CCSM will require much more computer power than is currently available.

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