panic attacks #2

Mark Jones jones118 at SPAMlineone.net
Tue Jun 6 07:58:10 MDT 2000


"The mid-range scenarios employed in IPCC projections for the end of the
next century would result in global mean surface temperatures that exceed
any well-documented warming in the last million years... The net impression
of this evaluation of "things past" is that the future climate promises to
look very different than the present and, perhaps more disconcertingly,
possibly unlike anything known before..."


Remembrance of Things Past: Greenhouse Lessons from the Geologic Record
by
Thomas J. Crowley
Global climate--like local weather--is ever changing, on all time scales, in
response to natural variations. For this reason, projections of global
warming, resulting from increasing levels of atmospheric greenhouse gases,
need to be set in the context of what has happened in the past. A knowledge
of what has changed, and when and where, provides insights into why past
changes have occurred. Such information can also help in evaluating the
reliability of global warming forecasts.
Predictions of climate change are based on numerical models of the
atmosphere and ocean. The inherent limitations in these models have led
those who develop and refine them to seek ways in which projections can be
tested and sharpened, including a comparison with times when conditions on
the Earth were much different from the present day. We should like to know,
for example, how concentrations of greenhouse gases--or other factors that
fix the climate of the Earth--have varied in the past. How has surface
temperature responded? Is there evidence for natural controls that limit the
range through which climate naturally varies?
To answer these and other questions, geologists and climatologists have
literally unearthed a vast amount of information regarding climate and how
it has varied. Natural records that tell of past changes are scattered over
the globe. They can be recovered only piece by piece, and reassembled like a
jigsaw puzzle, through the efforts of many people over many years. What
emerges is not only a record of climate change, but a clearer picture of the
climate system itself.
Can we hope to find in the past a time when temperatures rose by the amount
that some models now project for next 100 years, to ascertain how the
natural world reacts to such conditions? As we shall see, although the Earth
has seen times as warm as are now projected, there are no exact parallels.
But a great deal can be learned by examining what we know of times when
conditions were something like what we now might expect, and even more from
a knowledge of how the climate system has responded to induced changes of
any sort.
In this review we look at some of the findings that allow us to place
modeled projections of greenhouse warming in the context of past, or
paleoclimates. In doing so we take as a baseline for comparison the climate
of the present era and the range of global surface temperature increase that
has been projected by the most recent assessment of the Intergovernmental
Panel on Climate Change (IPCC): a "mid-range" rise of about 1 to 3°
Centigrade (about 2 to 5° Fahrenheit) for the end of the next century.
First, we review what is known of how the climate has varied in the past,
and the sources from which this information is obtained. We then cite some
of the lessons that can be drawn from a closer look at the paleo record.
SOURCES OF CLIMATE HISTORY
Records of local weather, made with the help of meteorological instruments,
cover at most about two hundred years, and an even shorter span if a truly
global picture is desired. Other, historical accounts exist for individual
places--most notably in China, where for certain sites they extend back two
thousand years. But although these records are useful, more extensive
information is required to understand the full range of natural climate
variability.
Laboratory analyses of geologic sediments and other layered materials help
meet this need, extending what is known of surface temperature,
precipitation, and other meteorological parameters many thousands and even
millions of years into the past. Many of these tools rely on the fact that
plants and other forms of life respond in distinctive ways to changes in the
local environment, thus preserving an indirect, or proxy record, of climatic
conditions.
Annual growth rings in trees, for example, can be read much like a diary:
tree-ring widths tell of seasonal variations of local air and water
conditions--as does the chemical composition of the wood within each ring.
The presence of forests and other vegetation, which are indicators of
climatic conditions, can be reconstructed from the analysis of pollen in
lake sediments. Cores extracted from the floor of the ocean allow us to
examine in fossil form the microscopic life that once lived near the ocean
surface, and through this analysis, to recover information about the
temperature of the ocean many millions of years ago. The extent and
composition of coral reefs are indicators of tropical ocean temperatures
and, through changes in ocean salinity, of local precipitation.
A particularly powerful technique of recent years has been the recovery and
analysis of ice cores, about ten cm (four inches) in diameter and as much as
two miles long, drawn from the permanent glaciers on Greenland and
Antarctica. Similar samples have been retrieved from high mountain glaciers
in South America and Asia. As in the case of trees, the ice is composed of
annual layers, although the temporal resolution degrades systematically from
top to bottom in the deepest cores.
The analysis of the hydrogen and oxygen in the extracted ice core provides a
continuous index of temperature from as far back as 200,000 years ago,
sampling conditions in the air above the ice sheet and in the nearby oceans
from which the water was evaporated, to later fall as snow. As the snow
accumulates, over the years, the underlying layers are compressed into ice.
Windblown dust and the residue of ancient volcanic eruptions can also be
analyzed. Bubbles entrapped in the ice during the process of compaction
preserve samples of fossil air, affording an opportunity for precise
measurement of the amount of carbon dioxide (CO2), methane (CH4), and other
greenhouse gases in the global atmosphere of long ago.
These techniques are calibrated against similar samples from the present
day, for which temperature and other climatic variables are measured
directly. What is learned is often limited, as are modern weather station
records, to conditions at one region. However, measurements of CO2 and CH4
taken from isolated cores give a global picture, since these long-lived
gases are uniformly distributed in the global atmosphere.
When combined, these various forms of paleo-data allow us to reconstruct an
imperfect but ever-clearer picture of the climate of the past. All of them
clearly indicate that climate varies, due to natural causes, on all time
scales, from decades to millions of years.
A CAPSULE HISTORY OF CLIMATE CHANGE
A picture of what is known of climate changes through the last 100 million
years is shown in terms of estimated surface temperature in Figures 1-6, and
described briefly below.
The last 1000 years (Fig. 1)
A prominent feature found in some regions during the first centuries of the
present millennium is a time of particularly mild temperatures, reaching
maximum warmth in the 12th to 13th centuries. In some locations at that
time, surface conditions may have been similar to today. However, it is not
at all clear whether this climatic feature occurred at the same time in all
places--an important distinction from the somewhat more uniform pattern in
recent decades.
This so-called Medieval Warm Period was followed by a longer span of
considerably colder climate, often termed the "Little Ice Age"
(approximately 1450-1890), when the global mean temperature may have been
0.5- 1.0°C lower than today. At this time alpine glaciers moved into lower
elevations, and rivers that rarely freeze today were often ice- covered in
winter. Precipitation patterns also changed in many regions.
Several explanations have been proposed for climate oscillations of
decadal-to-millennial scale. Changes in volcanism seem to play an important
role in year-to-year climate variability, although there is less evidence
that prolonged clusters of eruptions can cause cooling trends of decades or
longer. Changes in the output of the Sun may also be important--as noted in
an accompanying article in this issue--and solar-induced changes of decadal
scale could alter the pace of projected warming during the next few decades.
It is important to realize that future impacts of the Sun on otherwise-
rising temperatures could be either negative or positive; for example, solar
changes could also accelerate, for a time, an underlying warming trend.
Other explanations for decadal-to-millennial scale variations relate to the
currently popular concept of "chaotic" interactions within the climate
system. This process involves complex and inherently unpredictable
interactions between, for example, the oceans and the atmosphere. Such
interactions could cause the climate system to change or drift with time,
resulting, quite by chance, in a period of cooling or warming that may
persist for decades to centuries. The continued action of these
non-deterministic effects might then restore conditions to something
resembling the earlier state.
Some ocean-atmosphere interactions are more predictable. One example is the
well-known El Niño phenomenon: a surface warming of equatorial Pacific
waters, persisting for a year or more, that has a widespread effect on
climate and human affairs.
There are indications of decadal-scale warming trends in the Pacific that
may bear some resemblance to the shorter and better-known, El Niño
phenomena. For example, one of these longer-term periods began in 1976-1977,
and another may have started in 1988-1989. Although we lack a full
understanding of the relative importance of these and other decadal
fluctuations, there is hope that more can be learned in coming decades, in
time to refine the accuracy of global warming projections.
An application of our understanding of ten to 100 year variability may be
found in the present debate over several features of the temperature history
of the last 100 years, and the fact that surface temperature has not
consistently risen during this time at the rate predicted by simple
greenhouse warming. The period from the 1950s through the early 1970s
exhibited a cooling trend; and while global temperatures have been high over
the last fifteen years, they have also been relatively stable.
Those who challenge greenhouse warming predictions point to such patterns as
indications that the model predictions are seriously in error. However, most
climatologists point out that natural oscillations of a decadal-scale can
modify a greenhouse warming signal during the present, relatively early
stage of the perturbation. This is but one example where an historical
perspective can give insight into what at first appears as a troubling
discrepancy between models and observations.
Were "natural" climatic variations of the sort that have characterized the
last 1000 years to recur in the next 100 years or so, they could modify the
expected effects of increased greenhouse gases: either masking an underlying
upward trend during the early stages of a greenhouse warming or accelerating
the rate at which it occurs. From what we know, however, the effect--either
way--might not be great: only the extreme 1.0° C cooling estimate for the
Little Ice Age approaches in magnitude the smallest temperature perturbation
that is now projected for the end of the next century.
The last 15 thousand years (Fig. 2)
The Medieval Warm Period and the Little Ice Age appear as minor, short-term
fluctuations when we take a longer view. In Figure 2 we recognize these
features as little more than ripples on the longer and more significant
global warming of 4 to 5° C of the last 15 thousand years, that marks the
recovery of the Earth from the grip of the last major glaciation, or Ice
Age. The present warm epoch, beginning roughly 10 thousand years ago, is
known geologically as the Holocene Interglacial. The "inter" is a reminder
that such time intervals are relatively infrequent on time scales of a
million years or so, and have lasted on average about 10 thousand years
before a return to colder climates.
The onset and recovery from Ice Age conditions is now attributed to slow
changes in the Earth's orbit--the so-called Milankovitch effect--that modify
the seasonal cycle of solar radiation at the Earth's surface. These
modifications bring about regional changes in both temperature and
precipitation.
Especially affected in terms of rainfall are areas that fall under the
influence of the African-Asian monsoon, which have experienced a long-term
decrease in precipitation since the beginning of the Holocene, and
particularly in Northern Africa and the Middle East. For example, several
thousand years ago Neolithic or late Stone-Age man occupied the Tibesti
Massif in what is now the driest part of the Sahara Desert. The development
of agriculture in Mesopotamia and in the Indus Valley of present-day
Pakistan and India benefited from the increased moisture that characterized
the area during the end of the period of higher temperature: the so-called
"Holocene Maximum."
Although regional temperatures during the Holocene Maximum were on the order
of 1° C warmer than present, the warming did not occur at the same time in
all places; thus the global average temperature may not have been
significantly different from now. For this reason, we cannot turn to such
times for reliable models, or analogs, of what is now anticipated, or cite
them as evidence that humankind has in the past experienced increases in
global temperature equivalent to those projected for even the early stages
of enhanced greenhouse warming.
The last 150 thousand years (Fig. 3)
Seen in the earlier portion of this more extended span of time is the onset
and end of the last interglacial period, lasting about 10,000 years, as well
as the much longer Ice Age that separated it from the present Holocene. The
interglacial period that began about 130,000 years BP (before present) is
often called the "Eemian." Regional temperatures were sometimes 1 to 2° C
higher than those of the Holocene interglacial. However, there is less
evidence that the temperature changes were globally synchronous, so in terms
of global temperature change, conditions in the Eemian--once again--may not
have been much different from the present.
At the time of maximum cold during the last Ice Age--about 15,000 to 23,000
years BP--ice sheets more than 2 kilometers, or well over a mile, in
thickness, extended to about 40° latitude in North America, as far south as
New York City and St. Louis. The massive amount of water in the ice sheets
required evaporation of almost 50 million cubic kilometers of water from the
oceans. As a result, sea level dropped about 105 meters, or about 350 feet,
below present levels. The seas' retreat exposed most of the continental
shelves, and allowed early man to migrate across the Bering Strait,
eventually to populate all of North and South America.
The last Ice Age was also marked by an equatorward expansion of sea ice in
both hemispheres. Ocean currents were displaced toward lower latitudes.
Paleodata also suggest a significant reduction in the area of tropical rain
forests, and an expansion of savanna vegetation that is typical of drier
climates. There was also a nearly worldwide increase in the amount of dust
in the atmosphere.
What is found in ice cores regarding CO2, CH4, and nitrous oxide (N2O) is
particularly relevant to present concerns of anticipated greenhouse warming,
for these are the same atmospheric trace gases that are now of such concern.
We now know that each of them varied, systematically, with the coming and
going of the Ice Ages (Fig. 4). During times of glacial cold their
concentrations in the atmosphere dropped dramatically; during the Eemian and
with the onset of the Holocene interglacial they rose to values typical of
the air of pre-industrial times. The generally parallel behavior of surface
temperature and atmospheric greenhouse gases supports the positive
relationship between greenhouse gases and climate change that has long been
discussed.
The reasons behind some of the changes in trace gases during a glacial cycle
are not fully understood, but it is likely that greenhouse gases have acted
as a positive feedback to amplify smaller variations in surface temperature.
For example, the amount of CO2 in the atmosphere probably varied in response
to changes in ocean chemistry, for the ocean stores much more carbon than
the land or the atmosphere. Changes in methane were most probably related to
temperature-related changes in the global extent of wetlands, for wetlands
(such as swamps and peat bogs) are today the primary source of this trace
gas. Ice Age variations in N2O may have been tied to changes in marine
biological activity, but the details of the process are as yet not well
understood.
Since greenhouse gases appear to have played so important a role in
modifying the climate of the last million years, one might ask to what
degree "natural" changes in their concentrations could alter--or perhaps
nullify--the projected impact of what we ourselves are adding to the
atmosphere. To answer this question it is necessary to understand that the
amount we have added to the atmosphere since the Industrial Revolution is
already equal to the changes in concentration that accompanied the climatic
variations of the Ice Ages. Since about 95 percent of the fossil fuel
reservoir remains to be processed (see below), future increases are likely
to overwhelm any naturally induced perturbations in the climate system.
The last million years (Fig. 5)
The jagged valleys in this reconstruction of temperature in the Pleistocene,
or glacial epoch, are repeated Ice Ages. The warm peaks are interglacial
periods--including the Eemian and at far right, the present Holocene. What
is immediately obvious is how rare are times as warm as now. Ancestral Homo
sapiens did not appear until the middle part of the period that is shown,
and "modern" Homo sapiens (Cro-Magnon Man) not until the last 100,000 years.
The temperatures that are ascribed in this representation of the last
800,000 years were not obtained directly, but are based on fluctuations of
global ice volume, and scaled to what is known of conditions during the last
glacial maximum. They are meant to represent estimates of the mean surface
temperature of the Earth.
As noted earlier, there is good evidence that seasonal changes in the way
that sunlight is distributed--driven by periodic changes in the Earth's
orbit--trigger both the waxing and waning of glacial ice. At the same time,
climate model simulations suggest that these slow variations are not
sufficient, in themselves, to account for all that is known of the
glacial-interglacial fluctuations of the last million years. Resulting
changes in greenhouse gas concentrations are now thought to amplify the
effects of changes in solar radiation, through processes that are not
entirely understood.
The last 100 million years (Figure 6)
Paleodata of various kinds indicate that the Earth's climate prior to the
last million years was considerably warmer. For almost all of the time from
about 2 to at least 200 million years ago (Ma) the surface temperature
exceeded that of today. The greatest warmth is found in what geologists call
the Cretaceous Period, about 100 Ma, when the mean global surface
temperature may have been as much as 6 to 8° C above that of today. This was
followed by a fairly steady cooling, sometimes with abruptly stepped
transitions, towards the unique glacial oscillations of the last few million
years.
During most of this long period of time, and certainly from about 150 to 50
Ma, there is little evidence for ice sheets of continental scale, and
subtropical plants and animals lived far poleward (almost 55-60° latitude)
of their present limit of about 30°--the latitude of northern Florida. The
Age of Dinosaurs, ending about 65 Ma, overlaps most of this warm interval,
and fossilized remains of these large reptiles have been found on the North
Slope of Alaska. Later, during the warmest part of the Age of Mammals (55
Ma), large trees grew in Arctic Canada (78°N), in regions that today are
covered by tundra. Alligators and primates, also indicative of warm climate,
have been found on nearby Ellesmere Island. Fossils of warm-water mollusks
have also been recovered on the Antarctic Peninsula.
Climate model simulations suggest that large increases in CO2 are needed to
explain the high temperatures of the Cretaceous. These results are in
agreement with geochemical models that simulate the exchange of carbon and
other elements among the ocean and air and land reservoirs, which imply that
atmospheric CO2 levels have also varied on time scales of millions of years.
There is growing evidence from the geologic record to support these
conclusions. The geochemical models indicate that the concentration of this
gas can be affected by long-term changes in several natural sources and
reservoirs: CO2 emissions from volcanoes, the amount of carbon stored in the
terrestrial biosphere, and the effect of water erosion of the land surface.
The latter process affects the chemical weathering of surface rocks,
altering their ability to remove CO2 from the atmosphere, and it can vary
with changes in the amount of elevated terrain. The average height of each
of the continents has changed over time, due to slow oscillations in the
rate at which mountains are built; thus the weathering rates and the
consequent effect on CO2 have also probably changed on time scales of
millions of years.
Although the estimated levels of CO2 in the atmosphere for these very
ancient times are uncertain to within a factor of three to four, it is
interesting to note that the amount thought to be present in the air of the
Cretaceous is comparable to estimates of what now remains in the fossil fuel
reservoir: the yet unused amount of coal and the hydrocarbon fuels that
include crude oil, natural gas, oil shales, and oil sands. Coal is far and
away the major constituent, and it will supply most of the carbon for future
increases in greenhouse gases. Coal is about ten times more plentiful than
all the sources of hydrocarbon, and about thirty times more abundant than
crude oil in particular. Utilization of all known or likely reserves of
crude oil would add only about 25 percent to the carbon dioxide now present
in the atmosphere. For reference, about 5 percent of the available fossil
fuel reservoir has thus far been utilized, and a doubling of CO2 levels will
consume only about 20 percent.
Barring a radical change in the manner in which energy is utilized in the
future, continued depletion of the fossil fuel reservoir in the next few
centuries could result in levels of atmospheric greenhouse gases that are
comparable to the warm time period of the Cretaceous. The warming that is
calculated to result from nearly full utilization of the fossil fuel
reservoir is also consistent with independent estimates of temperatures in
the Cretaceous. Thus the geologic record yields the rather startling
conclusion that the climate of AD 2400-2700 could be comparable to that
experienced during the Age of Dinosaurs, which was as warm as any time in
the last billion years.
LESSONS FROM THE PAST
As we have seen, the geologic record leaves no doubt that the Earth's
climate system is capable of some very large changes. This almost trivial
conclusion challenges any presumption that the surface temperature will,
through natural checks and balances, remain stable in the face of
perturbations that we now impose. The geologic record also provides insight
into specific characteristics of the climate system that relate to future
greenhouse warming. Some of these are outlined below.
Times of similar temperature
Of obvious interest are comparisons of the surface temperatures now foreseen
by climate models with what the Earth has experienced in the past. The
projected warming of several degrees C has occasionally prompted comparisons
with the last, or Eemian, interglacial period of about 125,000 years ago
(Fig. 3), when regional temperatures were at times and places sometimes
warmer than the present. As noted earlier, the qualification is important,
for further study has revealed that these times of warming pertain only to
specific regions, and not to a simultaneous, worldwide change. As an analog,
the Eemian is thus of little help.
To find times when global-averaged surface temperatures significantly
exceeded those of the present day--reaching, for example, the 2.0-2.5°C
mid-range of IPCC projections--we need look several million years into the
past (Fig. 6). To evaluate the full potential of future greenhouse gas
increases, including what will ensue when much of the available fossil fuel
reservoir has been consumed, we must look much further back in time.
Assessments of fossil fuel use and deforestation suggest that concentrations
of atmospheric CO2 could eventually increase to six or seven times the
pre-industrial level. Were severe conservation methods imposed, greenhouse
gases would still rise to about two and a half times the present
concentrations--or slightly more than three times the pre-industrial
level--within the next few centuries. Such concentrations would likely raise
global temperatures to a range that has not been experienced since the early
part of the Age of Mammals, about 55 million years ago.
Still, when major climatic changes are involved, it is hazardous to take any
period in the distant past as a reliable analog for how the climate system
will respond in the future. One reason is the difference in geography: the
continents drift with time, and during the Cretaceous their placement was
quite different from today, as were the contours of mountains and other
features of surface relief. In addition, the very rapid change in surface
temperature that is now projected will result in unstable or non-equilibrium
climates. The reason is that neither the deep oceans nor ice sheets--which
can respond only slowly--will be able to keep pace with changes in air
temperature.
One implication of the rapid changes that are projected is that the
Greenland and Antarctic ice caps will persist in the presence of global air
temperatures that are normally associated with an ice-free Earth. I know of
no time period in Earth history with a combination of very high CO2 levels,
polar ice caps, and a non-equilibrium climate.
Potential for abrupt transitions
It has long been thought that the great Ice Ages came and went on time
scales measured in thousands of years, and less momentous changes--such as
the Holocene Maximum or the Little Ice Age--over the span of several
centuries. Current studies and more recent paleodata have revealed quite
another face of the climate system, called "abrupt transitions," in which
major shifts in some components of the Earth's climate are accomplished on
time scales of decades or less.
Initially proposed, and later verified, was the revolutionary notion that
the large-scale circulation in the North Atlantic could persist in one of
two patterns, or states, both of which were quite stable, with the
possibility of abrupt switching between the two. In the first, the warm Gulf
Stream that flows along the eastern coast of the U.S. continues northward,
reaching beyond the British Isles to the Norwegian Sea, ameliorating the
climate of northwest Europe. James Joyce aptly referred to this condition in
Ulysses, when he wrote that "All Ireland is washed by the Gulf Stream."
In the other possible mode, the northward extension of the Gulf Stream is
weakened by a reduction in the salinity of surface waters in high latitude
regions of the North Atlantic. With less salt, seawater is not as dense, and
is less able to sink during normal wintertime cooling. Restricting the
ability of the North Atlantic to circulate water downward limits the amount
flowing in from the warm Gulf Stream. The result of this "short-circuit" in
ocean circulation is a much cooler climate for all who live downstream,
including Northern Europe.
The surprising evidence from the paleoclimate record is how quickly the
switch between warm and cold states can be accomplished. Evidence from
ice-age portions of recent Greenland ice cores suggests that changes of this
sort may have taken place in the past in the span of five to ten years.
These abrupt transitions are most likely linked to an increase in the
release of icebergs from continental glaciers, which on melting contribute
large volumes of freshwater into the ocean, systematically reducing the
local salinity.
Whatever the cause, we now know that in at least the North Atlantic the
climate system can change very rapidly. Might ocean circulation change as
rapidly in the future, perhaps as a consequence of other significant changes
in the system? The answer is "maybe." There are no permanent ice sheets
today on the North American continent, as was the case in the past, but
melting of Arctic sea ice or the extensive Greenland ice cap could well
influence ocean salinities. Increased precipitation over the North Atlantic,
induced by warmer temperatures, could also reduce the saltiness of seawater,
short- circuiting the ocean circulation in a manner similar to what occurred
during the ice ages. In fact, greenhouse models call for such a change in
precipitation, and the present rate of warming in the subpolar North
Atlantic--less than what is recorded in the rest of the world--is also in
agreement with what should happen as a result of an altered state of ocean
circulation. A test of the models is whether the slower warming of the
subpolar North Atlantic will persist.
Checking climate model results against paleodata
Even if there were no period in the past that could serve as an analog for
future climate, the geologic record can still provide valuable insight into
the modeling of specific processes in the climate system. Examples of such
processes include the response of climate to changes in freshwater input to
the ocean, or to changes in the contour of the land. These changes have
occurred in the past and have left their traces in the geologic record.
Incorporating more accurate predictions of specific processes will improve
the overall reliability of global climate models.
Testing general circulation models against what is known of past climatic
changes provides powerful insights, particularly when applied to regional,
as opposed to global, changes. In many cases these reality checks have
bolstered our confidence in numerical simulations of the atmosphere and
oceans and other parts of the climate system. Some examples are the temporal
evolution of the African-Asian monsoon; the wet conditions in the U.S.
Southwest during the last Ice Age; the expansion of Antarctic sea ice during
the last glacial maximum; changes in deep water circulation both during the
Pleistocene and on longer time scales; regional climate trends due to
changes in mountain heights over the last few million years; formation and
fluctuations of glaciers during the ice ages at 300 and 440 million years
ago; and the formation of coal deposits that are now found in the eastern
U.S. These and other results have been obtained with the same climate models
that are now used to project impending greenhouse warming.
Surface temperatures derived from paleodata reveal that when the global
temperature warms, changes in polar regions are systematically larger than
nearer the equator. Climate models predict a similar distinction between
polar and lower latitude regions with future warming, but with differences
that are less than what have been found in available data from the past. It
may be that this discrepancy reflects an inherent limitation in the use of
paleodata for precise validation of climate models. Or, the models may not
adequately simulate the manner in which ocean circulation transports heat
from the equator to the poles. If this is so, some of the regional
predictions of climate models may be in error in regions strongly influenced
by ocean heat transport.
We can better understand the differing degrees of success with which models
replicate climate changes of the past by distinguishing between global and
regional temperatures. In general, climate models do a better job of
estimating changes in global temperatures, because the energy budget of the
entire planet is affected. In contrast, regional changes are a step removed:
they reflect the response of the atmosphere and ocean circulation to changes
in the total energy budget, and as a result, are more difficult to predict.
In some cases the models have still yielded valid first-order predictions of
what is found in paleo records. In others, the modeled projections do not
agree as well. Some of these discrepancies may tell not so much about model
deficiencies as about our inability to read what Nature has written in the
record of the past.
SUMMARY AND CONCLUSIONS
The paleo record provides a wealth of information relevant to current
concerns of enhanced greenhouse warming, including the underlying truth that
the Earth's climate has experienced substantial changes in the past.
Major, known changes of surface temperature in the past correlate well with
variations in atmospheric greenhouse gases and appear to be caused or
amplified by them. The mid-range scenarios employed in IPCC projections for
the end of the next century would result in global mean surface temperatures
that exceed any well-documented warming in the last million years. The mid-
to high-end "out-century" estimates for temperatures are as high as any
known for the last one billion years. The combination of high air
temperatures, polar ice caps, and non-equilibrium climate defines a climatic
condition that may be unprecedented in Earth history. Paleodata also support
earlier suggestions that the subpolar North Atlantic region may be
susceptible to abrupt climatic transitions as a result of changes in the
salinity of surface waters.
The sulfate aerosols that are also added to the atmosphere when fossil fuels
are burned act to cool the climate, and the degree to which these emissions
might reduce the impact of greenhouse gases is now under intensive study.
Could such effects ameliorate some of the more extreme projections that are
discussed above?
While this is indeed possible we should not forget that other possible
mechanisms could act in the opposite direction, to amplify climate change.
Among such "positive feedbacks" are significant increases in atmospheric
concentrations of trace gases other than carbon dioxide, such as methane,
nitrous oxides, and chlorofluorocarbons. In fact, recent IPCC estimates
suggest that the global climate forcing from the other greenhouse gases is
comparable in magnitude, but approximately opposite in sign, to that of
sulfate aerosols.
Global warming could very well accelerate the release of carbon from soils
to the atmosphere. Possible changes in the ocean circulation and ocean
productivity could also increase the amount of carbon that enters the
atmosphere. Both positive and negative feedbacks therefore need to be
considered in evaluating the uncertainty of current climate projections.
The conclusions given above, despite uncertainties, represent in my opinion
a reasonable assessment of the significance of the greenhouse perturbation
when viewed from the perspective of the geologic record. The net impression
of this evaluation of "things past" is that the future climate promises to
look very different than the present and, perhaps more disconcertingly,
possibly unlike anything known before.
FOR FURTHER READING
"Atmospheric carbon dioxide over Phanerozoic time," by R. A. Berner.
Science, vol. 249, pp 1382-1386, 1990.
Climate Change: The IPCC Scientific Assessment, Edited by J. T. Houghton, G.
J. Jenkins, and J. J. Ephraums. Cambridge University Press, 364 pp, 1990.
"Global climatic change," by R. A. Houghton and G. M. Woodwell, Scientific
American, vol. 260, no. 4, pp 36-44, 1989.
"How sensitive is the world's climate?" by J. Hansen, A. Lacis, R. Ruedy, M.
Sata, and H. Wilson. National Geographic Research and Exploration, vol. 9,
pp 143-158, 1993.
Paleoclimatology by T. J. Crowley and G. R. North. New York, Oxford
University Press, New York, 339 pp, 1991.
"Unpleasant surprises in the greenhouse?" by W. S. Broecker. Nature, vol.
328, pp 123-126, 1987.
Reviewers
Anthony Broccoli is a meteorologist and climate modeler at the Geophysical
Fluid Dynamics Laboratory of the National Oceanic and Atmospheric
Administration, located at Princeton University in New Jersey. He is
interested in the study of climate change using numerical models,
particularly the simulation of past climates.
Dr. William Ruddiman is a marine geologist and currently chair of the
interdisciplinary Department of Earth Sciences at the University of Virginia
in Charlottesville. His main research interests are the generation and
interpretation of paleodata from many periods of Earth history, with
particular emphasis on the effects of plateau uplift on climate.
Dr. Lisa C. Sloan is a geologist and paleoclimate modeler, and an assistant
professor in Earth Sciences at the University of California, Santa Cruz. Her
primary interests are in the modeling of warm intervals of Earth history and
in comparisons of geologic data with climate model
results. -------------------------------------------------------------------
-------- Scientific reviewers provide technical advice to the authors and
Editor, who bear ultimate responsibility for the accuracy and balance of any
opinions that are expressed.







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