Download Relating paleoclimate data and past temperature gradients: Some

Quaternary Science Reviews 19 (2000) 381}390
Relating paleoclimate data and past temperature gradients:
Some suggestive rules
D. Rind*
NASA Goddard Space Flight Center, Institute for Space Studies, 2880 Broadways, New York, NY 10025, USA
Abstract
Understanding tropical sensitivity is perhaps the major concern confronting researchers, for both past and future climate
change issues. Tropical data has been beset by contradictions, and many techniques applicable to the extratropics are either
unavailable or fraught with uncertainty when applied at low latitudes. Paleoclimate data, if interpreted within the context of
the latitudinal temperature gradient data they imply, can be used to estimate what did happen to tropical temperatures in the
past, and provide a "rst guess for what might happen in the future. The approach is made possible by the modeling result that
atmospheric dynamical changes, and the climate impacts they produce, respond primarily to temperature gradient changes. Here we
review some `rulesa obtained from GCM experiments with di!erent sea surface temperature gradients and di!erent forcing, that can
be used to relate paleoclimate reconstructions to the likely temperature gradient changes they suggest. Published by Elsevier Science
Ltd.
1. Introduction
The question of the sensitivity of tropical temperatures
to climate change remains one of the most fundamental
uncertainties in climate change research. The question
really has three components. First, as is well known,
major discrepancies exist between reconstructions of past
tropical temperature changes when di!erent paleo-indicators are used. In general, the tendency is for land
reconstructions to show greater cooling in the past colder
climates than do ocean indicators, although there are
some exceptions (CLIMAP, 1981; Guilderson et al. 1994;
Stute et al., 1995; Colinveaux et al., 1996; Beck et al.,
1997). For past warm climates (Pliocene, Tertiary in
general), the tropical ocean data actually implies some
cooling although the data sets are subject to larger errors
(e.g., Dowsett et al., 1994). In general, then, during the
past 200 million years geologic proxies suggest that in
both warmer and colder climates, the tropics are not very
sensitive, but this result is somewhat dependent upon the
type of proxies being used.
Second, when General Circulation Models (GCMs)
are forced with the CO changes that are known, or
2
* Tel.: 001-212-6785593; fax: 001-212678-552.
E-mail address: [email protected] (D. Rind)
0277-3791/99/$ - see front matter Published by Elsevier Science Ltd.
PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 7 0 - 0
thought, to have accompanied past climate regimes, the
models produce much larger tropical oceanic temperature changes than many paleo-indicators suggest
(Manabe and Broccoli, 1985; Chandler et al., 1994). Conversely, when utilizing the deduced sea surface temperatures for these past climates, models do not reproduce
the degree of temperature change thought to have occurred over tropical land (Rind and Peteet, 1985). Therefore
in addition to lack of consistency between di!erent
paleo-climate reconstruction techniques, we now have to
wonder about the GCM sensitivities.
The question comes to fruition in the third area of
disagreement, predicting the future tropical climate that
will result from anthropogenic trace gas releases. Model
predictions of doubled atmospheric CO show factors of
2
two di!erence in their tropical response even without
considering ocean dynamical changes (Rind, 1987a).
Given that the paleorecord does not provide an unambiguous test of model validity, it is hard to estimate what
the expected magnitude of tropical warming should be.
Estimates of the temperature change for the past century show that tropical warming on the order of 0.53C
has occurred, on a pace with extratropical warming (see
Fig. 1). Nevertheless, these magnitudes are small compared to either the large-scale paleochanges or those
projected for the next century. Extrapolation of current
trends, given all of the nonlinear feedbacks in the system,
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D. Rind / Quaternary Science Reviews 19 (2000) 381}390
Fig. 1. Change in surface air temperature from 1880 through 1995 for the global average (top), tropical region (middle) and extratropics (bottom). As
can be seen, the estimated changes in the tropics have been almost as large as those in the extratropics over this time period. Courtesy James Hansen.
D. Rind / Quaternary Science Reviews 19 (2000) 381}390
would be a risky procedure. How then can we estimate
what has happened in the past, and is likely to happen in
the future?
Obviously the best approach would be to improve our
techniques for determining what happened in the past,
and, with the aid of such observations, improve our
ability to model the past and future climates. As this
general problem has existed for at least the last 20 yr, and
it is not obvious that the solution is any clearer, we might
anticipate that such solutions are not likely to be immediately available. Instead, what is proposed here, is to
rely on the fact that large-scale atmospheric dynamical
responses are primarily a function of the latitudinal (and
longitudinal) temperature gradient, and deduce, from
GCM studies, what would be expected from changes in
these gradients. To the extent that these expectations
may already be testable in the paleorecord, we may be
able to determine what the likely change in latitudinal
gradient was. In combination with our understanding of
midlatitude temperature changes, this may then give us
an aid to evaluating how tropical temperatures varied.
An assumption in this regard is that the models will be
more accurate in delineating the response to latitudinal
gradient changes than they are to past or future climate
forcings. That cannot be proven, per se, but evaluations
of the response of models to recent gradient changes, as
part of the AMIP program for example, do show overall
consistency (Boyle, 1996). In addition, the model's response to gradient changes are more of an atmospheric
dynamics issue, reacting to tendencies inherent in the
Navier}Stokes equations for conservation of momentum,
mass and energy (and moisture). To some extent theoretical expectations are available from solutions of simpli"ed forms of these equations. Such solutions involve fewer
assumptions than do the physical responses of the system, which depend strongly on the intricacies of clouds
and convection that determine the overall and tropical
climate sensitivity. Model results can then be compared
with theoretical results. Nevertheless, the answers given
below are the result of simulations made with the GISS
GCM, and some model-dependency is to be expected.
(Of course, the physical responses will contribute to
the GCM's gradient response to some extent as well, for
example by in#uencing the magnitude of latent heat
release gradient associated with a latitudinal temperature
gradient; it is due to complexities such as this, as well as
many others, that require GCM studies of the nonlinear
system be used to test the theoretical expectations of
gradient changes.)
The procedure then is to change the latitudinal temperature gradients in the system, note the dynamical and
climate responses, and provide a set of guidelines for
what would be expected, from this perspective, when
gradients change. The experiments are of four types: (1)
simple gradient changes imposed via alterations of the
global sea surface temperature "elds, representative of
383
times such as in the Holocene without large global temperature alterations; (2) gradient changes associated with
an overall mean global temperature change, of more
relevance to the larger paleoclimate changes that have
occurred; (3) gradient changes associated with di!erent
types of forcing, altered CO or solar irradiance versus
2
altered ocean heat transports; and (4) gradient changes
that occur in only one ocean, or situations in which the
changes are opposite in di!erent ocean basins.
This paper summarizes the results of the di!erent studies, which have appeared or been submitted previously
(Rind, 1998; Rind et al., 1999a, 1999b). The concentration
is on the generalized rules that can be deduced from such
an approach. Some application of these rules to various
paleoclimate situations has been done in the individual
papers; given here are some comments concerning
paleoclimate implications of the various results.
2. Experiments
Shown in Table 1 is a list of the various experiments
utilizing the GISS GCM, and the category into which
they fall. Results are compared with their respective control runs. Runs other than those with speci"ed sea surface
temperatures (SSTs) use a q-#ux ocean (Miller et al.,
1983), in which ocean heat transports are prescribed; for
the transient forcing experiments, heat di!usion through
the bottom of the mixed layer is also employed. The
speci"ed SST experiments are run for six years, with
results from the last 5 yr; the temperature gradient and
absolute temperature changes employed are given in
Fig. 2. For the experiments with speci"ed SST changes in
particular ocean basins, the gradient changes given for
Experiments 1 and 2 are used in the Atlantic or Paci"c.
The equilibrium experiment is run for 35 yr, with results
from the last 10 yr. The transient experiments are for the
time periods given.
3. Results
3.1. Simple latitudinal gradient changes (Category 1)
In this category, we address what is to be expected
from a simple latitudinal temperature gradient change
that is globally uniform (insofar as the SSTs are concerned), and not associated with any overall global temperature change. This would be applicable to potential
climate changes during the Holocene, or perhaps even
during most of this century, although the magnitude of
the gradient change utilized is greater than has likely
occurred. However, in investigating the combinations of
experiments, the results appear linear (Rind, 1998), so
that the qualitative descriptions given below are likely to
be applicable. Note that as there is no global temperature
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D. Rind / Quaternary Science Reviews 19 (2000) 381}390
Table 1
Experiments used to assess impacts of latitudinal gradient changes
Name
Description
Category
1
Global increase in gradient achieved by specifying warmer tropical SSTs, and cooler polar SSTs. No
global temperature change occurs
1
2
Global decrease in gradient from specifying cooler tropical SSTs and warmer polar SSTs. Again no
global temperature change occurs
1
3
Increase in gradient, as in (1) but in addition a uniform decrease of SSTs superimposed to
produce a globally colder climate
2
4
Decrease in gradient, as in (2) but in addition a uniform increase of SSTs superimposed to
produce a globally warmer climate
2
2]CO
2
Equilibrium response to doubled atmospheric CO
3
Solar
Transient response to estimated solar irradiance variations during the past 500 yr
3
Trans CO
2
Transient response to estimated CO variations during the past 300 yr
2
3
AI
As in (1) but the gradient increase is only in the Atlantic Ocean; all other SSTs are left unchanged
4
AD
As in (2) but the gradient decrease is only in the Atlantic Ocean; all other SSTs are left unchanged
4
PI
As in (1) but the gradient increase is only in the Paci"c Ocean; all other SSTs are left unchanged
4
PD
As in (2) but the gradient decrease is only in the Paci"c Ocean; all other SSTs are left unchanged
4
ADPI
Decreased gradient in the Atlantic combined with increased gradient in the Paci"c
4
AIPD
Increased gradient in the Paci"c combined with decreased gradient in the Atlantic
4
change, an increased gradient implies warmer tropical
temperatures and colder temperatures at high latitudes,
with the reverse true for a decreased gradient (Fig. 2,
Experiments 1 and 2). Some of the results will be associated with the absolute value of the tropical temperature
in addition to or instead of the gradient change; in those
cases, determined by comparison with experiments 3 and
4, they will be discussed in the following Section 3.2, and
a comment to that e!ect provided here.
The following results arise from these experiments
(Rind, 1998). Some obvious paleoclimate or future implications of the results are provided in italics. Unless
otherwise indicated, the conclusion as stated also holds in
an inverse fashion, i.e., if an increased gradient produces
one e!ect, a decreased gradient produces the opposite one.
A1. An increased gradient leads to the atmospheric
temperature anomaly being warmer than the surface
temperature anomaly, as the warmer tropical temperature in#uence extends higher in the atmosphere than
decreased polar temperatures. This will awect isotopes,
such as 18O, whose distribution is associated with temperature at all levels due to Rayleigh fractionation.
A2. With an increased gradient, the global mean lapse
rate decreases, driven by the moist adiabatic response in
the tropics. See also B1. Experiments using simple 1-D or
2
2-D models need to allow for variation in the vertical lapse
rate independent of global mean temperature changes.
A3. Global water vapor loading is larger with an
increased gradient, due to the reduced lapse rate and
warmer atmospheric temperatures (see A1 and A2). This
is true even above 500 mb. See also B2. No evidence exists
in the model for a strong negative water vapor response in
the upper troposphere associated with an intensixed Hadley
Cell, which has been suggested as a negative feedback to
minimize global warming (Lindzen, 1990).
A4. Global rainfall is reduced when the latitudinal
gradient decreases, driven by the equatorial sea surface
temperatures. See also B3. Equatorial temperatures are the
key to global rainfall.
A5. Snow cover is reduced when the latitudinal gradient decreases, a result of both less overall atmospheric
moisture and warmer high latitude temperatures. High
latitude warmth does not aid glacial ice sheet growth
(e.g., the Laurentide during the Last Glacial Maximum) by
promoting more snow cover.
A6. An increased gradient intensi"es the Hadley Circulation, with increases in tropical precipitation and
decreases in subtropical precipitation. Gradients in
precipitation are a key indicator of gradients in tropical and
subtropical SSTs.
D. Rind / Quaternary Science Reviews 19 (2000) 381}390
Fig. 2. Zonal average surface air temperature change between experiments d1}4 and the control run for the annual average. The sea surface
temperature gradient change is applied as a linear function of latitude in
such a way as to conserve the global mean temperature (in Experiments
1 and 2), i.e., the change is about twice as large at high latitudes as at the
equator, and of opposite sign. In Experiments 1 and 2 sea ice cover is
not altered, which a!ects the atmospheric temperature gradients at high
latitudes.
A7. Over land, an increased SST gradient leads to little
moisture change in the tropics, but decreased moisture
availability in the subtropics. The tropical moisture does
not increase noticeably because the peak tropical rainfall
is over the ocean, and that induces subsidence and warming over the land, with added evaporation counteracting
any precipitation increase. See also B7. (The inverse of
this is not true, as indicated in A8). Tropical land soil
moisture changes are not a good indicator of tropical SST
warming.
A8. A decreased SST gradient leads to drying over the
tropics, and moistening of the subtropics, a direct response to the weakened Hadley Circulation. Reduced soil
moisture in the tropics, when not associated with a global
temperature change, is a good indicator of a decreased
gradient.
A9. The poleward extent of the Hadley Circulation
does not appear to be a!ected by the change in latitudinal gradient. Gradient changes should not be expected to
alter the poleward expansion of the subtropical deserts.
A10. An increased gradient leads to greater extratropical eddy energy, due to greater baroclinic energy transformations. See also D3. To the extent that convection
truly mixes momentum, an increased gradient leads
to reduced tropical eddy energy, associated with the
greater convection. Extratropical storms will be stronger
with an increased gradient, although tropical waves may be
weaker.
A11. The ratio of transient to stationary eddy energy
does not change with the gradient, as the transient
response to altered baroclinicity parallels the standing
wave response to altered background winds. Shifting of
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longitudinal climate zones due to variation in the ratio
of stationary to transient energy is not to be expected from
a simple gradient change.
A12. With an increased gradient there is a slight
change to longer wavelengths for planetary-scale Rossby
waves, but in general, gradient changes just a!ect the
amplitude of the waves in their current location. Similarly, shifting of the mean positions of ridges and troughs is
not to be expected from latitudinal gradient changes.
A13. Winds are stronger with a stronger gradient, both
at the surface and at the jet stream level (the latter being
the thermal wind response in conjunction with the
stronger surface winds). Greater ablation, generation and
transport of dust, sea salt, etc. should accompany a stronger
gradient, and transient storm tracks will be more zonal
along with the increased jet stream.
A14. With increased gradients, both eddy and total
atmospheric energy transports increase. A greater atmospheric connection exists between the tropics and extratropics with a stronger gradient.
A15. The implied ocean heat transport changes associated with the altered SST gradients do mostly o!set the
change in atmospheric transports; this is mostly a thermohaline circulation e!ect (although the model used here
does not explicitly contain a thermohaline circulation,
the importance of the overturning circulation in this
regard has been shown by many other models, for
example, Russell and Rind (1999)). Altered SST gradients,
when not accompanied by global temperature changes, are
the province of altered ocean heat transports.
3.2. Latitudinal gradient changes in conjunction with
global mean temperature changes (Category 2)
The bigger climate changes obviously involve both
a possible change in latitudinal gradient and a global
mean temperature response. The canonical view is that as
the global temperature decreases, the latitudinal gradient
increases. This is reasonable, given the snow/ice albedo
feedback at high latitudes which would exaggerate the
temperature response there. Hence the following experiments considered how altering the global mean temperature a!ected the results di!erently from considering
simply the gradient change itself. A uniform temperature
change was superimposed upon the gradient changes
(Experiments 3 and 4 in Fig. 2). The results can be applied
to understanding whether the Little Ice Age was really
associated with a global temperature change, or whether
the Pliocene was really considerably warmer globally.
They are not particularly well-suited to investigate the
Last Glacial Maximum, for which altered ice sheets
would a!ect the extratropical response. Previous experiments, however, indicate that the tropical response is less
a!ected by the presence of land ice in the extratropics,
and so the low latitude results might still have some
applicability (Rind, 1987b).
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D. Rind / Quaternary Science Reviews 19 (2000) 381}390
B1. The global average lapse rate is associated with the
absolute value of the tropical temperatures, which govern
the moist adiabatic value. A distinct diwerence in the lapse
rate which would awect tropical mountain glaciers arises
as tropical temperatures warm or cool, independent of
a gradient change.
B2. A colder climate, even with an increased gradient,
is associated with less atmospheric water vapor, due to
the absolute cooling of the tropics. To dry the global
atmosphere, tropical temperatures must be colder.
B3. As the global rainfall is primarily associated with
the absolute value of the tropical temperatures, it is not
a good indicator of latitudinal gradient changes. Global
rainfall response to increased CO2 will depend on the
magnitude of tropical warming, not the gradient change.
B4. The distribution of rainfall patterns as a function of
latitude is not associated with the global mean temperature, but only the latitudinal gradient. Gradients in rainfall patterns are not indicators of global mean temperature
changes.
B5. Extratropical eddy energy, its ratio of transient to
stationary energy, and its wavelength distribution are not
a!ected by global mean temperature (this result is not
relevant when the global mean temperature change produces new orographic features, such as large ice sheets).
The locations of troughs and ridges, and their regional
ewects, are not a function of the global mean temperature.
B6. Energy transports are not a function of the global
mean temperature except for transports of latent heat
(which is about 25% of the total atmospheric energy
transport). The tropical/extratropical atmospheric interactions depend primarily on the temperature gradients,
except that the ability to advect tropical moisture to higher
latitudes is also a function of the absolute value of the
tropical temperature.
B7. When increased gradients are associated with colder tropical ocean temperatures, the tropical soil moisture clearly increases, and subtropical drying is less
pronounced, as the colder air inhibits evaporation. An
increased gradient during the last ice age probably should
have resulted in wetter conditions in the tropics.
B8. Sea ice increases strongly amplify low level temperature gradients across mid-latitudes, with a resultant
decrease in precipitation and increase in sea level
pressure at polar and subpolar latitudes, especially in the
winter hemisphere. However, despite the gradient increase, eddy energy decreases somewhat, as the cooling
e!ect is primarily at low levels, so atmospheric stability
increases. (The inverse of this is not true, as indicated in
B9). Researchers considering uncertainties in high latitude
sea ice reconstructions should recognize its impact on
storms and precipitation in ice core regions.
B9. Sea ice decreases also result in some reduction in
eddy energy, as the associated high latitude warming
extends high enough into the atmosphere to a!ect
baroclinic eddy generation, regardless of the decrease in
vertical stability. The assumption of weaker polar storms in
a climate without much sea ice would appear to be justixed.
3.3. Global mean temperature changes with little
temperature gradient change (Categories 1 and 2).
We can use the experiments in the reverse sense, by
determining how the climate is changed when the global
mean temperature varies without a!ecting the latitudinal
gradient (i.e., comparing Experiments 1 and 3 or 2 and 4).
Since our knowledge of tropical temperature responses is
somewhat uncertain, and the higher latitude response
often comes from ice core data, these results are applicable to the general question of how do we determine
when an ice-core indicated climate change was only at
high latitudes (hence altering the gradient) or was global
(hence just altering the global mean temperature). If the
result is modi"ed depending on whether the warming
was initially over the ocean, as in these experiments, or
was radiatively forced over the land as well, reference is
made to the appropriate conclusion in the following
section (Section 3.4).
C1. A warmer climate (without a gradient change) has
more precipitation at all latitudes (zonally and annually
averaged). This is a direct result of the Claussius}
Clapeyron relationship in which the water holding capacity of the air is an exponentially increasing function of
temperature. Dynamic responses cannot alter the ability of
the atmosphere to provide more rain annually at all latitudes when a gradient change does not occur; this has
implications for predictions of future rainfall patterns.
From the paleoclimate perspective, increased precipitation
at all latitudes cannot be a response to gradient changes
alone in the absence of overall global warming.
C2. Tropical precipitation over land (as opposed to the
zonal average, discussed in C1) is reduced when the
oceans are warmer. Soil moisture is reduced from 30N to
30S. See also D2. A climate with warmer ocean temperatures but no gradient change will be dry throughout both
the tropics and subtropics.
C3. Increased rainfall associated with the warmer climate in the extratropics leads to soil moisture increases
from 303 to 603 latitude. Midlatitudes are wetter in warmer
climates, as the evaporation increase does not exceed the
precipitation response (local temperatures inyuencing
evaporation are not as far along the exponential curve of
the Claussius}Clapeyron relationship as are the tropical
temperatures which inyuence moisture availability and
rainfall).
C4. The Hadley Cell weakens somewhat, and there is
a poleward expansion of extent in the warmer climate.
Poleward movement of the subtropical desert regions are
therefore more a function of the overall warmth than
a change in latitudinal gradient.
C5. The subpolar lows are a!ected both by gradient
changes and the absolute temperature. Increased gradients
D. Rind / Quaternary Science Reviews 19 (2000) 381}390
strengthen the lows; however, colder temperatures at
high latitudes lead to stronger polar high pressure systems (as noted in B8), and with the presence of more
extensive sea ice, weaken the polar lows. Circulation
changes associated with the Aleutian and Icelandic Lows
will be dizcult to relate to either gradient or mean temperature changes.
3.4. The ewect of diwerent types of climate forcing
(Category 3)
The previous results were obtained by altering the sea
surface temperatures not associated with any global climate forcing. In e!ect they were implicitly associated
with changes in ocean heat transports (OHTs). While
such changes have likely occurred, other types of forcing
undoubtedly have as well, such as changes in atmospheric CO levels, and probably total solar irradiance.
2
Would these other forcings have produced similar
responses? An obvious di!erence is that OHTs directly
a!ect the ocean and not the land, while greenhouse gas
changes and solar forcing a!ect both; induced longitudinal gradients will thus be di!erent. The experiments
performed in this category can address this question,
again within the context of this particular GCM. The
experiments are not useful for assessing the impact of
seasonally varying latitudinal forcing such as that
associated with orbital variations, although latitudinal
gradients in general will a!ect the monsoon intensity;
and the conclusions are untested for more esoteric ocean
circulation changes, such as those associated with the
closing of the isthmus of Panama.
D1. Increased CO forcing reduces the contrast be2
tween ocean and land warming, relative to the ocean heat
transport change experiments. The land/ocean temperature contrast change is therefore a potential discriminator
between increased CO2 and altered ocean heat transports
for time periods such as the Tertiary climates. Also relevant
in this regard is the question of the equitability of continental interiors in past warm climates, when the magnitude of
the high latitude ocean warmth is properly considered.
(Sloan and Barron, 1992; Chandler et al., 1992).
D2. Tropical and subtropical drying is less extreme
when tropical warming is due to increased CO than
2
when it is associated with altered ocean heat transports.
As the land/ocean temperature forcing has less contrast,
there is less subsidence induced over the land. Drying
throughout the tropics and subtropics in a warmer climate
is therefore dependent on the forcing that provided that
warming.
D3. Storms are weaker when the latitudinal temperature gradient is reduced by increased CO -warming,
2
than when associated with increased ocean heat transports. Increased ocean heat transports warm the extratropical oceans, and the contrast with cold air over land
maintains baroclinic contrasts to some extent. Increased
387
CO forcing warms extratropical land more than it
2
warms the ocean, contributing to decreased longitudinal
temperature contrasts in winter, and hence more severe
reduction in baroclinicity * even wave number 1 and the
Aleutian Low are a!ected. Extratropical storm intensity
reduction in a warmer climate will be more noticeable from
increased CO2 forcing than from increased ocean heat
transports.
D4. The latitudinal temperature gradient response
does not di!er when the forcing is by solar irradiance
increases or atmospheric CO increases. The feedbacks
2
(water vapor, sea ice, cloud cover) respond generally
similarly to the two types of forcings, hence the resultant
climate response shows no obvious patterns of di!erence.
Shown in Table 2 are the temperature changes over the
past 244 yr between the transient CO change experi2
ment and the transient solar change experiment at
speci"c latitudes. The CO increase produced greater
2
warming than the solar radiation change over this time
frame; however, within the variability, the warming differential was the same at each latitude, indicating no
change in the latitudinal gradient between these two
forcings (even the slightly higher mean di!erence at 823N
is basically a threshold e!ect on sea ice melting, due to
the greater magnitude of CO warming). While this re2
sult arises from transient experiments (TRANS CO and
2
SOLAR), it was also the conclusion for equilibrium experiments involving 2]CO and 2% solar irradiance
2
increase (Hansen et al., 1984). It would be dizcult to
diwerentiate solar forcing from trace gas forcing in the
paleo-record based solely on the climate response. (Caveat:
the response of the stratosphere and ozone is not included in
this assessment.)
3.5. Latitudinal gradient changes in individual ocean
basins (Category 4)
If climate forcing does not dominate ocean heat transport changes, it is possible that gradient changes will
occur in one ocean basin primarily, or in an opposite
sense in the Paci"c and Atlantic. An example of the
former e!ect occurs during El Ninos (La Ninas), which
increase (decrease) the latitudinal temperature gradient
in the Paci"c. Gradient changes that di!er in direction between ocean basins occur in some coupled
Table 2
* average warming (3C) at speci"c latitudes over the last 244 yr, CO
2
transient change experiment minus solar transient change experiment
Latitude
Mean
STD.DEV.
43N
353N
583N
823N
0.421
0.382
0.477
0.589
0.225
0.266
0.414
0.771
388
D. Rind / Quaternary Science Reviews 19 (2000) 381}390
atmosphere-ocean modeling experiments for the next
century, in which global warming reduces the gradient in
the Paci"c, while reduction in North Atlantic Deep
Water production lead to an increased gradient in the
Atlantic (IPCC, 1995; Russell and Rind, 1999). Larger
gradient increases in the Atlantic are presumed to have
occurred during the Last Glacial Maximum (CLIMAP
(1981) although all such values are no more certain than
the estimates of tropical SSTs for this time period) and in
the Younger Dryas (e.g., Rind et al., 1986). The last six
experiments in Table 1 are used to investigate these
e!ects, with the magnitude of change in each ocean basin
similar to those given for Experiments 1 and 2 (Fig. 2)
(gradients were not changed in the Indian Ocean).
E1. Gradient increases in one ocean basin act in many
ways like decreased gradients in the other ocean basin.
This is the result of longitudinal circulation cells that
develop due to contrasts between the oceans. For
example, warming in the tropical Paci"c leads to subsidence in the tropical Atlantic; similarly, a decreased gradient in the Atlantic also leads to subsidence in the tropical
Atlantic. Opposing gradient changes, as predicted in some
models for the coming century, can have larger impacts
than if the gradient changes were uniform. Areas particularly awected in this regard include the Americas, Australia, and southern Africa.
E2. Altered latitudinal SST gradients in the Paci"c can
simulate many aspects of ENSO phenomena even
though longitudinal gradients have not been changed.
Altered atmospheric convergences a!ect the atmospheric
temperature of the tropical eastern Paci"c, due to its
proximity to a large land mass, more than the tropical
western Paci"c. When considering paleoclimate indications of past El Ninos, altered latitudinal temperature gradients must be considered as well.
E3. Changes in latitudinal gradients in one ocean basin
will in#uence the surface air temperature globally,
through changing the atmospheric water vapor loading
and associated greenhouse capacity. This result emphasizes the possibility that paleoocean circulation changes in
the Atlantic could inyuence the climate globally via their
ewect on the atmosphere.
E4. Gradient changes in the Paci"c have more in#uence on global quantities, although Atlantic changes
in#uence many land areas. The larger the global climate
response, the more likely the Pacixc is heavily involved.
E5. The Americas are signi"cantly a!ected by gradient
changes in both ocean basins. Reconstructions of climate
in North and South America must consider the inyuence
of changes in ocean temperature gradients for both the
Atlantic and Pacixc.
E6. Gradient changes in both ocean basins a!ect the
Southern Oscillation and the North Atlantic Oscillation
pressure patterns in opposing ways. Paleoreconstructions,
and even present analysis, must consider gradient changes
in both basins when correlating with these phenomena.
4. Discussion and conclusions
The overall thrust of the majority of these conclusions
involves the concept that climate change is not (necessarily) a local process. It can be local * for example, the
change in wetness in the western United States between
the Pliocene and today could be the result of orographic
uplift in that region (Thompson, 1991). However, it often
is not, a point which has obviously been recognized from
the standpoint of global climate changes. What needs
greater recognition is that large-scale gradients often play
a role, especially where moisture variables are concerned,
and these can be even more important than the global
average change.
Central to this last point, is the main conclusion that
much of atmospheric dynamics responds primarily to
gradient changes, rather than changes in the global
mean temperature. Therefore, any climatic change impact thought to be associated with changes, for example,
in the Hadley Circulation, storm intensity or storm
tracks, or wind strength, should be viewed as the product
of a gradient change. If paleoclimate indicators suggest
that such a change was of a similar nature at a wide range
of longitudes, it is likely that a global gradient change
was involved; if longitudinal variability in the response
exists, latitudinal gradient changes in a speci"c ocean
basin are more likely. Longitudinal gradients will also be
changed in the latter case, and land/ocean contrast can
well be altered, setting up additional longitudinal circulations. Approaches of this general nature have been employed in interpretations of the e!ects of orbital changes
(e.g., Kutzbach and Street-Perrott, 1985); the suggestion
here is that it can have much wider applicability.
Global climate changes of course will be involved in
alterations of the radiative balance of the planet, and so
a!ect the absolute temperatures. As temperature
changes, so does the moisture holding capacity of the
atmosphere, and its ability to produce both rainfall and
evaporation. Separating the global climate perturbation
from any changes in gradient may be possible by considering the gradient in the changed paleoclimate indicator,
which is suggestive of how the atmospheric dynamics has
varied; the global climate change can then be viewed as
overlaid on this signal.
Separating CO forcing from ocean heat transport
2
changes may be possible by noting that ocean heat transport increases exaggerate the extratropical winter longitudinal gradient, while CO increases diminish it by
2
warming the land more than the ocean. Separating solar
from CO forcing is more di$cult, at least when consid2
ering total solar irradiance. The possibility exists that
solar irradiance variations in the ultraviolet, by a!ecting
stratospheric ozone, might produce patterns of climate
response that di!er from those of well-mixed gases (Shindell et al., 1999); the importance of this e!ect on decadal
or century climate change is still to be determined.
D. Rind / Quaternary Science Reviews 19 (2000) 381}390
Less obvious, but becoming more apparent through
our understanding of the e!ect of processes such as
ENSO, is the concept that changes in the latitudinal
gradient in one ocean basin can a!ect the climate in the
other ocean basin. Therefore, in this general expansion of
consideration of non-local forcing, we should not necessarily limit our deductions to the nearby ocean alone. We
also need to recognize the possibility that via ocean
dynamical changes, gradients can change in di!erent
ways in the di!erent basins, which can sometimes produce greater, superimposed e!ects.
To return to the problem posed in the introduction:
paleoclimate indicators, if understood in terms of their
response to latitudinal gradient changes, can therefore be
employed to determine how the latitudinal gradient has
varied. As extratropical phenomena are often better de"nable (e.g., through tree rings or pollen or ice cores, or
obvious changes in ocean #ora/fauna), it could then be
possible to determine, from the deduced gradient change,
what likely happened to sea surface temperatures in the
tropics. Some examples of this approach are given in
Rind (1998) and Rind et al. (1999b), but much more can
be done.
Caveats in this regard include that the gradient between the tropics and subtropics could change in one
direction, while the gradient between the subtropics and
extratropics vary in the other sense; if the CLIMAP
(1981) reconstruction of the very warm subtropical Paci"c during the Last Glacial Maximum is valid, this would
be such a case. Care then has to be exercised in utilizing
paleodata from the appropriate latitudes, say tropical
and subtropical data to estimate what happened to
gradients equatorward of 303 latitude, and extratropical
data poleward of that. Adding the two changes together
would then indicate the absolute tropical response.
Additional caveats associated with the `rulesa given in
the results section is that these do come from one particular model. As noted earlier, however, AMIP experiments
do seem to imply that atmospheric dynamical changes
responding to altered sea surface temperatures have
strong similarities from model to model, although their
absolute hydrologic responses (governed by the physics
of the parameterizations) may vary (Sengupta and Boyle,
1997). To some extent, many of the dynamical responses
to gradient changes would be expected based on theoretical considerations * eddy energy changes related to
latitudinal gradient changes, for example * but even
here con"rmation in a GCM is useful, for other feedbacks
may come into play that are not included in the theoretical analysis.
To understand the regional impact of the climate
change forecast for the next century, it is incumbent upon
us to gain a better estimate of tropical sensitivity. Paleoclimate conditions are far from perfect analogs for future
warming, but they do suggest what sensitivities have been
displayed previously. Just as latitudinal gradient changes
389
can dominate the pattern of response for paleoclimate
conditions, they will likely have a strong in#uence on
local climate and `winners and losersa for the future
climate change. Motivating action to alleviate adverse
e!ects will only be possible if the outcome for speci"c
nations is more predictable. With the proper interpretation, paleoclimate data can be used to contribute to this
process.
Acknowledgements
Results shown in this paper come from work supported by NOAA grant NA56GP0450, NOAA grant
NA66GP0426, and NASA Earth System Science Grant
NCC5-117. Mark Chandler and two anonymous reviewers provided useful comments in review.
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