Download Week 9: Geology and Climate

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Week 9: Geology and Climate
This text written by Stuart P. Raeburn, who asserts his rights of copyright April 2, 1999
Introduction
The geologic record tells us that climate was different in the past. During some periods in
earth history, the earth was warmer than it is today, while at other times it was colder. Equally,
some regions that are relatively wet today were arid deserts at times in the geologic past, and
the reverse is also the case for other places. Understanding the earth’s climate history is a
worthy goal in terms of scientific progress alone, but there are additional more practical
benefits. Understanding how and why climate changed in the past may help us plan for, and
limit, the impact of climate change caused by human activity. Human-induced global warming
is the raising of surface temperatures due to increased atmospheric concentrations of CO 2 and
other greenhouse gases. Global warming is just one example of a number of ways human
activity is modifying the global environment (others include deforestation and urbanization).
So what do we mean by climate? Climate is the combined set of average conditions for
phenomena such as temperature, precipitation, winds and storm strength and frequency. There
will be differences in average conditions between seasons, and also in some cases (e.g.,
temperature) between day and night. A complete description of climate would include
information about such seasonal and diurnal differences. The variability of weather is such that
precise prediction of the exact conditions on any given day is not possible, but when averages
are taken over a number of years, the resulting set of typical conditions corresponds to climate.
In simple terms, climate is what you expect, but weather is what you actually get.
The geologic record preserved in rocks, in sediments on the deep ocean floor and in layers
of ice in thick ice sheets can tell us about climate changes in the past. Using this record we can
learn how life on earth was affected, and can also try to determine the probable causes of
climate change and the processes involved.
Natural causes of climate change
Climate change can be the result of a number of different natural causes, some acting over
millions of years and resulting in changes which remain in place for millions of years, while
others occur more rapidly and cause less long-lasting effects. Plate tectonics falls in the former
category. The motions of plates are slow, but over a period of several million years can change
the arrangement of continents and ocean basins. Ocean circulation patterns can be radically
altered by the opening or closing of passageways . An example is the onset of the Antarctic
circumpolar current which cooled Antarctica and created conditions that caused the build-up of
a thick ice sheet there. The current could only begin to circulate following separation of
Antarctica and Australia and the opening of the Drake passage between South America and
Antarctica.
The distribution of landmasses on the earth can also affect climate. Because the earth is a
spherical body, areas at different latitudes receive incident sunlight at different angles. The
amount of radiation reflected back to space is different for land (20%) and sea (5 - 25%). The
sea is a more efficient reflector of radiation when light arrives at low angles (as at the poles)
than it is when light arrives at high angles (as at the equator). An earth with landmasses located
close to the equator, and oceans in high latitude regions will retain less of the incident radiation
than an earth with landmasses in high latitude regions, and oceans at the equator. Lastly,
changes in global rates of seafloor spreading can occur, resulting in changes in the total
volcanic emission rate of CO2 . Such changes can modify the atmospheric concentration of
CO2 , and thereby alter the magnitude of warming caused by the natural greenhouse effect.
Changes in the intensity of volcanic activity can occur over shorter time scales too, and
need not be related to seafloor spreading rates. An example is the generation of flood basalts
which are the result of an outpouring of huge volumes of magma over a few million years at
most. In general increased rates of magma release result in increased CO2 flux to the
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atmosphere, and therefore elevated temperatures. By contrast, individual volcanoes erupting
for days or weeks may cause a very short term cooling lasting a few years as a result of ash and
sulfuric acid being temporarily placed in the upper atmosphere.
Ocean circulation changes are not only the result of plate motions that rearrange the
locations of the continents. Changes in the volume of freshwater entering the oceans or the
location of sea ice can also impact ocean circulation. Ocean circulation at the present day is
dominated by the flow of deep cold water, that is relatively salty, from the north Atlantic
southwards to beyond the southern tip of Africa where it joins the circumpolar current. Deep
cold salty water also flows northwards in the Indian and Pacific oceans starting from north of
Antarctica. These deep flows are replaced by surface flows of warmer water traveling in the
opposite direction. The overall circulation pattern is termed the ocean conveyor, as it transports
heat around the earth. We have seen previously that atmospheric circulation also transports
heat from the equator to higher latitudes. The net effect of the ocean conveyor and atmospheric
circulation is to transfer heat from the equator to higher latitudes, reducing the extremes in
temperature that would otherwise exist because of differential heating of the earth at different
latitudes. Rapid climate change resulting from a temporary shutting off of heat transfer in the
North Atlantic has been suggested as the cause of a dramatic cooling of northern Europe that
occurred about 10,000 years ago during otherwise long term warming following the last peak
in ice coverage during the most recent ice age (which ended about 14,000 years ago). The
sudden, but temporary cooling about 10,000 years ago is termed the Younger Dryas event.
The record of climate variations for the past two million years shows that small fluctuations
in surface temperatures are superimposed on the average climatic conditions determined by
such things as landmass configuration, and seafloor spreading rates. Such fluctuations are the
result of small changes in the amount of solar radiation reaching the earth caused by minor
changes in the earth’s orbit. There is a cyclicity (or predictably repeating character) to these
changes, and there are three types. Eccentricity refers to the deviation of the earth’s orbit from
a circular orbit. Eccentricity changes from zero to slight eccentricity with a 100,000 year cycle.
Tilt (the angle between the earth’s spin axis and the earth’s orbit), without which there would
be no seasons, changes from 21.5 to 24.5 degrees with a cycle of 41,000 years. Lastly the
earth’s spin axis wobbles, giving rise to the precession of the equinoxes with a 23,000 year
cycle. As a result of precession each hemisphere passes from conditions of warmer summers
and colder winters to cooler summers and milder winters every 11,500 years. When the three
orbital changes: tilt, eccentricity and wobble are combined the result is a complex, but
predictable and repeating set of variations in solar heating termed a Milankovitch cycle.
A final source of climate change is modification to the carbon cycle. Changes in volcanic
activity referred to above are an example, but other possible changes include differences in rates
of burial of organic carbon in sediments or differences in rates of chemical weathering of rocks;
both are processes that remove CO2 from the atmosphere. You should refer to the full text for
week 2 for more information.
Geologic Evidence
What sort of information do we look for in rocks to tell us about past climate? Arid
environments result in a characteristic suite of deposit types: dunes, lake sediments containing
salts and preserved mud cracks, and alluvial fan deposits. Dunes can be recognized from a
characteristic oblique-angle internal layering structure, called cross-bedding that lies at a fairly
steep angle to the larger scale horizontal layering that occurs between separate rock layers.
Cross-bedding is created during the downwind migration of sand dunes. Dunes are dynamic
structures in which the constituent sand grains migrate by rolling and jumping up the shallow
dune face pointing into the wind, followed by falling of the grains down the steeper dune face
pointing away from the wind. Ephemeral lakes, that is lakes which periodically dry out, exist
in arid environments. As water evaporates, salts dissolved in the lake waters become
increasingly concentrated until crystals begin to form, resulting in settling out of layers of salts
on the lake bed. When the water dries out completely, any exposed muddy sediments will
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crack as they dry out. Mudcrack structures can be preserved if they are covered by additional
sediment before the lake waters are replenished.
In cold, wet environments winter snowfall may be preserved year-round and turn to ice
until accumulations are thick enough for the ice to flow downhill under the influence of gravity.
A flowing body of ice is termed a glacier. A glacier will leave evidence that it passed by, in the
form of small scale features such as striations (long linear scratches in rocks) and glacial polish
(smoothed rock surfaces). Striations are the result of scratching by rock fragments embedded
in ice at the base of a glacier and glacial polish is the result of abrasion by fine sediment trapped
between the base of the ice and the rock surface, over which the moving ice is sliding. Glaciers
also leave their mark by modifying pre-existing landform features such as mountains, lakes and
valleys. A cirque is a bowl-shaped depression in the face of mountain which marks the former
presence of a cirque glacier (a small glacier formed on the mountain face). Where a mountain
was eroded by cirque glaciers on opposite sides of a shoulder of the mountain, a narrow knifeedged ridge called an arete is produced. A horn is a spiked pyramidal peak formed by glacial
erosion by cirque glaciers on all sides of the mountain.
Glaciers moving along a valley erode the sides of the valley as well as the valley floor.
Valley glaciers change the characteristic V-shape of an upland valley into a rounded U-shape.
In addition erosion rates of the valley floor by a large valley glacier are faster than the rates of
erosion of the floors of tributary valley glaciers. When the glaciers have receded the valley
floors of a main valley and tributary valleys are no longer at the same elevation. Where prior to
glaciation, tributary streams joined a river directly, following glaciation and then ice retreat
tributaries are separated from the main valley floor by waterfalls. The lip of the side valley
stands high above the floor of the main valley and is the side valley is termed a hanging valley.
Glaciers are also responsible for deposition of glacially-derived sediment. The general term
for such deposits is drift. Drift can be of two types: till, which is unsorted and unlayered, and
stratified drift, which is sorted by grain size and layered. Stratified drift is typically the product
of transport and redeposition of till by meltwater. The largest volume of stratified drift is found
as outwash deposits which form downstream from the front of a glacier, where abundant
meltwater flows away from the ice front across a broad outwash plain. Ridge-like deposits of
till are termed moraine. Moraines may be a number of types: terminal moraines are ridges of till
running parallel to the ice front, which represent the line of furthest advance of the glacier;
medial moraines are long ridges of till within a valley glacier, separating ice added to the glacier
by different tributary glaciers. Ground moraine is the deposit of till underneath a glacier, that
becomes visible following retreat of a glacier
Meltwater channels within the ice and at the margins of the ice carry sediment which may be
deposited along the channel walls. Linear features produced by deposition in meltwater
channels are termed eskers. When glaciers recede retreat of the ice front may leave stagnant
blocks of incompletely melted ice in the outwash plain. Water transporting sediment in the
outwash plain flows around the ice obstacles causing a build up of sediment around the
stagnant ice. When the ice eventual melts, an oval-shaped hole surrounded by elevated walls of
glacial sediment may remain. Such a hole is termed a kettle, and if the hole is deep enough,
and/or the water table high enough, the kettle will contain water forming a kettle lake.
The precise oxygen isotope composition of polar ice (as recorded in ice cores extracted
from polar ice sheets) can be used to draw inferences about the air temperature at the time the
snow accumulated at the surface. Oxygen has three different isotopes, which means an atom of
oxygen may contain any of three different numbers of neutrons (8,9 or 10). The atomic mass
of the two more important isotopes are 16 and 18, and these isotopes are referred to as O16 and
O18 . The O16 isotope is by far the most abundant. There is a very small difference in the
energy required to evaporate O18 rich water (H2 O18 ) compared with lighter O16 rich water
(H2 O16 ). As a result, water vapor placed in the atmosphere by evaporation over low latitude
oceans tends to be slightly depleted in H2 O18 . Equally, when water vapor is removed by
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precipitation, H2 O18 is removed more readily than H2 O16 . The net effect is that by the time air
masses reach polar regions and provide snowfall that accumulates as a fresh layer of ice at the
top of existing ice sheets, the snow is depleted in O 18 and enriched in O16 . In addition to this
effect there is a temperature effect that impacts ice composition. When air temperatures above
an ice sheet are cool the snowfall contains less O18 than it does when air temperatures are a little
warmer. It is possible, therefore, to reconstruct the history of air temperatures in polar regions
from ice core compositions. The oldest ice available is the 160,000 year-old ice at the base of
the Antarctic ice sheet. The oldest ice at the base of the Greenland ice sheet is somewhat
younger than this.
Environments that were wetter in the recent geological past than they are today can be
identified from wave cut benches, or terraces. Terraces are flat platforms separated by steeper
slopes into a series of steps carved like a staircase into the land surrounding a lake. Each
terrace is the product of wave action at the shoreline, so each one documents higher water
levels in the lake in the past. Higher lake levels in earlier times are also indicated by walls of
calcium carbonate deposits, termed tufa, that are the product of evaporation and precipitation of
calcium carbonate crystals in a lake. Tufa cliffs that are stranded above the present waterline in
a lake indicate that water levels must have been higher in the past, since the tufa was originally
precipitated from lake waters.
One final source of information, and perhaps the most comprehensive, comes from fossils.
Previously we have seen how fossils have been used to determine a relative age scale for rocks.
Fossils also contain clues to climate, because different organisms occupy different climatic
niches. In addition, some organisms record changing climate through differences in
morphology or composition. Microscopic pollen grains shed by flowering trees and plants are
dispersed by wind and water. The pollen can be trapped in sediment and preserved in rocks.
Identification of the particular plant from its type of pollen can provide information on the
geographical distribution of particular plants in the past. Dead trees record climate changes in
the width of their annual growth rings. Radiocarbon dating can be used to determine the age of
organic carbon back to about 40,000 years ago. During periods of relative drought tree growth
is limited compared with growth during wetter years.
Calcium carbonate shells formed by sub-millimeter sized organisms called foraminifera
(forams for short) are particularly useful climate indicators. Forams making shells in colder
water have shells that have a higher content of O18 than shells made from water of the same
isotopic composition that is warmer. Foram isotopic composition is therefore a thermometer.
It should be noted that some correction is required to allow for the fact that the isotopic
composition of sea water itself changes somewhat in response to changes in the volume of
glacial ice. Ice in large ice sheets has a smaller ratio of O18 / O16 than ocean water and therefore
during cold periods when ice sheets are extensive, the O18 /O16 ratio of ocean water falls.
Foraminifera provide additional temperature information because different forams are suited to
different water temperatures. Forams can be divided into tropical, subtropical, subpolar and
polar types. By mapping out the abundance of different types (distinguished by shell
morphology) the area extent of polar, sub-polar, sub-tropical and tropical waters can be
inferred for different times in the geologic past. In order to obtain climate information from
forams, drilling in the deep ocean basins is required to recover cylindrical sediment cores that
contain undisturbed layers of sediment, with oldest sediment at the base and youngest at the
top.
Earth climate through time
Extensive continental glaciation has not been a permanent feature throughout earth history.
Instead periods of continental glaciation centered on the polar regions are termed ice ages.
There have been at least five ice ages during earth history; the earliest occurred from 2300 to
2100 million years ago. This first ice age is thought to have been result of a lowering of CO2 in
the atmosphere (due to photosynthesis by early plant life) and hence lessening of natural
greenhouse effect warming coupled with the lower luminosity of the younger Sun. . The most
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recent ice age started between one and two million years ago. However, each ice age itself may
be part of a longer period of cold conditions. For example the ice age which began in the past
two million years was the culmination of a cooling trend which began about 35 million years
ago when ice began to accumulate in Antarctica. There are also fluctuations in ice volume
during an ice age, in response to changes in global temperatures on a variety of time scales.
The ice age of the past 1.6 million years is the best known, and at least 16 periods of relative
cold separated by periods of relative warmth have been identified. The cold periods are termed
glacials; the warmer periods are termed interglacials. We are currently in an interglacial, which
began about 14,000 years. The peak of the previous glacial occurred at about 20,000 years
ago.
Other examples of profound climate change that occurred in earth history include a period
of extensive arid conditions in the interior of continental landmasses between 245 and 180
million years ago, when the continents were assembled in a single supercontinent, called
Pangaea. During this time inland areas were very remote from oceans and very little moisture
that evaporated from the oceans was able to travel into the interior and cause precipitation. In
addition, continental interiors experienced more pronounced extremes between summer and
winter temperatures, because of the absence of an ameliorating effect provided by what is
termed the thermal inertia of ocean water. Because water requires a lot of heat energy to warm
it up, or conversely must lose a lot of heat energy to cool it down, nearby oceans dampen
temperature fluctuations in adjacent land areas. The consequence of the dry conditions was the
deposition of sand dune deposits in huge areas (e.g., the western US) at this time. The sand
grains in the rocks are red in color due to a surface coating of iron oxide, hence layers of these
types of sedimentary rock are often called red beds. Evaporites: sedimentary rocks formed
through drying out of lakes, and concentration of dissolved salts were also formed in large
areas during this 60 million year period.
The earth was at its warmest, at least since the first ice age, during a 40 million year period
between 120 and 80 million years ago. Global sea level was as much as 200 meters higher
than today, and shallow seas existed in the interiors of a number of continental regions. The
presence of an interior seaway which covered much of the western US can be inferred from the
marine sedimentary rocks spanning this age found in the area.
One final example of more recent climate change comes from the Mediterranean Sea.
Drilling of sediments on the sea floor recovered thick layers of evaporite minerals (halite and
anhydrite) which formed between 26 and 5 million years ago when water levels in the basin
were very low. During this time inflow of water from the Atlantic ocean to the west through
the narrow Straits of Gibraltar between Europe and Africa was shut off. A global lowering of
sea level could have been responsible as the floor of the passageway between the Atlantic and
the Mediterranean is close to sea level, even though the average depth of the Mediterranean Sea
today is about 1.5 km. Fossils of stromatolites were also found in the sediments drilled from
the Mediterranean Sea floor, indicating shallow, salty water conditions in the past.
Stromatolites are formed when algae growing in colonies trap sediment and build laminated
mound-like structures.
The Last Great Ice Age
The last Great Ice Age which began about 1.6 million years ago was the culmination of a
cooling trend which began about 55 million years ago. The first significant glaciation began
about 35 million years ago following separation of Antarctica and Australia 38 million years
ago, and the opening of the Drake passage. Formation of an ice sheet in Antarctica is
documented in the isotopic composition of forams, which become richer in O18 as a result of
preferential removal of O16 from ocean waters and storage in continental ice. Between 35 and 5
million years ago the Antarctic ice sheet expanded and contracted several times. However, the
northern hemisphere remained largely ice-free until about 3 million years ago. Extensive ice
sheets in both hemispheres were established by 1.6 million years ago, regarded as the start of
the last Great ice age. Mechanisms proposed to explain the cooling trend which occurred after
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55 million years ago include decreased seafloor spreading rates, increases in rates of chemical
weathering and an increasing abundance of microscopic aquatic plants called diatoms. The
production of diatoms has been considered to be a possible positive feedback because increased
diatom production results in greater storage of organic carbon and greater removal of
atmospheric CO2 causing cooling. Such cooling triggers strong surface winds which speed up
ocean circulation, in particular the rate of upwelling of nutrient-rich deep ocean water, which
itself promotes more diatom growth.
The most recent glacial in the last Great Ice Age reached its peak about 20,000 years ago.
The Laurentide ice sheet covered much of Canada and the American midwest as far south as
Illinois and Ohio. The Cordilleran ice sheet in the western US covered western Canada and the
northwestern US. In addition in North America there were smaller ice bodies, called ice caps
in the Rocky Mountains, and the Sierra Nevada. Elsewhere ice caps existed in the Andes in
South America and in the Himalayas in Asia. Sea level was about 150 meters lower than today
and broad expanses of the continental; shelves were exposed along the eastern continental
margin of the US. Pollen data indicate that spruce trees, which prefer cooler climates grew in
what is now Alabama and Georgia during the last glacial maximum, and migrated northwards
to Canada during the present interglacial.
The most recent glacial ended about 14,000 years ago when our present interglacial began.
Although the period 14,000 years to the present has been a period of general warming there
have been occasional reversals to colder conditions. The younger Dryas cold period that
occurred about 10,000 years ago is thought to have been the result of a temporary change in
ocean circulation caused by the large volumes of fresh water arriving in the North Atlantic from
melting of the retreating Laurentide ice sheet. Even more recently, the Little Ice age was a
period of colder temperatures in northern Europe between the 15th and 19th centuries, split into
two segments by a warm period in the 17th century.
The Laurentide ice sheet covered Michigan during the last glacial maximum. The present
surface coverage of drift which blankets the lower peninsular owes its origins to the last glacial
period. Retreat of the ice left behind a number of depositional landforms. There are abundant
kettle lakes, eskers and moraines. Terminal moraines were formed after the glacial maximum
when the ice had retreated from its maximum extent south of Michigan. Retreat was punctuated
by minor readvances, and the large number of terminal moraines allow reconstruction of the
direction of ice flow in Michigan during this time. The Great Lakes themselves were the
product of deepening of pre-existing valleys by glacial action. The lakes only achieved their
present configuration relatively recently. Water levels and the locations of shorelines changed
considerably during the period following ice retreat. Locations of outflows from the lakes
changed as the elevation of the land was modified by a process known as isostatic rebound.
Whenever the lithosphere is loaded with a large load, such as a big ice sheet, it is depressed.
When the load is removed the lithosphere slowly rebounds to its former position. Depression
and rebound are possible because beneath the lithosphere is the weaker asthenosphere which
can flow. In the same way that a heavily laden boat sits lower in the water, so ice-covered
lithosphere sinks and asthenosphere flows out from underneath it to accommodate the sinking.
The asthenosphere flows back underneath it when the load is removed. The effects of the last
glacial are still evident from changes in elevation across the northern hemisphere. Areas
previously loaded with ice such as the Canadian shield (Ontario, Quebec, Manitoba) and
Scandinavia are now rising. Areas to the south (e.g., Holland) which previously were uplifted
by the displaced aesthenosphere flowing into them are now sinking as the asthenosphere
returns northwards. Michigan is now no longer rising, and neither is it sinking. Areas to the
south are sinking somewhat and the Canadian shield is rising somewhat (at a maximum rate of
1 cm/yr.).
Present day global change
We can assess global change in the past century by examining records for mean surface
temperature, and less directly by looking at other evidence such as retreat of mountain glaciers
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or rising sea level. The temperature record has a large amount of variability, but the overall
trend is one of increasing temperature. However, it is not possible to unequivocally state that
there is a clear cause and effect relationship between rising CO2 concentrations and rising
temperature. Certainly CO2 concentrations have risen since the mid-eighteenth century, i.e.,
since the start of the industrial revolution. Data collected for the past few decades in Hawaii
show a trend with a single upward direction (modulated by annual changes: greater CO2
concentration in a given year during the northern hemisphere summer compared with the
northern hemisphere winter the same year) These data have been extended back to 1750 with
data from trapped air recovered from Antarctic ice. The strength of the correlation between
CO2 concentration and temperature on short time scales (decades) is weakened somewhat by the
fact that temperatures declined slightly between 1940 and 1950, and again in the 1960’s despite
the fact that CO2 increased throughout this period. The discrepancy has been attributed to the
increased levels of pollution in the atmosphere which would have reduced incident radiation.
Since the 1960’s legislation has sharply reduced air pollution, perhaps explaining the sharp rise
in temperatures in the 1970’s and 1980’s. Another possibility is that the warming trend is a
natural rebound of the climate system from the cooler temperatures of the 18th and 19th
centuries. Satellite data collected in the past decade to measure atmospheric temperatures do not
show an increase. However, the time period spanned by the data is so short that natural
variability may be obscuring the longer term trend.
We can learn more about the connection between atmospheric CO2 and temperature from ice
cores recovered from the Greenland and Antarctic ice sheets. Trapped gas bubbles within the
ice contain a direct sample of the atmosphere at the time the ice formed. Over the past 160,000
years the record recorded in the Vostok ice core from Antarctica shows that trends in
temperature, and CO2 and methane concentrations in the atmosphere all follow one another.
Still despite this data we can not conclude with absolute certainty that increased temperatures in
the past century are the result of increased CO2 in the atmosphere. The correspondence
between the trends in temperature, CO 2 and methane in the ice cores may reflect the fact that
temperature changes over the past 160,000 years have been triggered by external factors (i.e.,
Milankovitch orbital effects), but that the changes have been amplified by a positive feedback
effect due to an increase in atmospheric CO2 and methane occurring at times of elevated
temperature. A reasonable mechanism exists for this type of feedback. The amount of CO2
that can be dissolved in ocean water declines as temperature rises. Equally considerable
amounts of methane are stored in a solid form in frozen arctic regions of permafrost and under
pressure in deep ocean sediments. Warming of the earth’s surface would release some of this
methane, which, as methane is a greenhouse gas, would cause more warming (i.e., a positive
feedback).
The increase in atmospheric concentrations of CO2 and methane and some other greenhouse
gases (Nitrous oxide, and chlorofluorocarbons - CFCs) as a result of human activity is
however quite different to that which occurred at times in the past 160,000 years as displayed
in the ice core record. In the present case anthropogenic emissions are the forcing factor, and
increases are occurring much more rapidly than as a result of natural processes. So the issue of
global warming is perhaps not so much whether it will happen, but whether we can detect it
yet, and how fast it will happen and how much warming there will be. Our current incomplete
understanding of the climate system means that humanity is really conducting a big experiment,
using the earth as a laboratory, to see the effect on the climate system of doubling the
atmospheric concentration of CO2 in less than a century.
Positive feedbacks will reinforce or amplify a perturbation in the climate system; negative
feedbacks will tend to counteract or reduce a perturbation in the climate system. Changing the
amount of polar ice results in a positive feedback because the removal of ice lowers earth’s
reflectivity (or albedo) causing more solar energy to be retained hence causing more
temperature rise and more melting of polar ice. A negative feedback is provided by cloud
formation. Higher temperatures lead to more water vapor in the atmosphere and more cloud
formation. Clouds tend to reflect incoming solar radiation, hence tending to lower surface
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temperatures and reduce the amount of water vapor in the atmosphere. Water vapor is also a
greenhouse gas, but it is believed that the reflective effect exceeds any greenhouse warming
effect making cloud formation a negative feedback.
Present work with global circulation models (GCMs) is aimed at assessing likely climate
changes resulting from changes in atmospheric CO2 . GCMs are computer models which
explore climate change in a gridded model of the earth, using numerical calculations in
simulations of ocean and atmospheric circulation. The models are limited by the present coarse
size of the grid which prohibits a good description of clouds, and also by the complexity of
cloud processes themselves. In twenty years or so computers may be powerful enough, and
models may be sufficiently detailed to provide accurate predictions of the impact of human
greenhouse gas emissions on climate. In the meantime science can only provide estimates with
attached uncertainty. What to do about the potential for global change requires political and
societal judgment. One can either adopt a precautionary stance or be a gambler and do nothing.
The problem is that by the time a definitive answer is available it may be too late to act to
prevent environmental disaster. Humankind is conducting a dramatic experiment, whose
results will tell us much about the operation of the earth system. The results of the experiment
may however adversely affect the earth’s inhabitants.
Preferred estimates suggest a warming of about 2˚C in the next 100 years and a sea level
rise of 1-2 feet. Much of this sea level rise would come about because of expansion of sea
water as temperature rises. The impact on severe weather and the global distribution of rainfall
may be serious. The way global rainfall will respond is poorly known, and critical because
agricultural systems are organized today for the present climate system. Natural ecosystems
also will probably be unable to adapt to extremely rapid changes in climate. Predictions made
using GCMs include: (a) warmer temperatures and wetter conditions year round for high
latitudes (e.g., Canada and Scandinavia); (b) warmer temperatures and wetter conditions in the
winter, drier conditions in the summer for the US and central Europe; (c) melting of mountain
glaciers; (d) a higher frequency of hurricanes as a result of increased loading of the atmosphere
with moisture. The impact on large ice sheets is uncertain. Temperature increase would be
expected to cause melting leading to thinning and retreat of ice sheets in Greenland and
Antarctica, but if snowfall increases significantly in polar latitudes then thickening of ice sheets
might be a possibility. There is barely any historical data on the mass of ice in the two ice
sheets, so we do not know if the ice sheets have been shrinking or stable over the past decades.
Recently a NASA team reported satellite data that indicate that the Greenland ice sheet has
shrunk in the past decade.
It is reasonable to conclude that global warming is a real threat, although some uncertainty
remains over whether it is detectable yet, and the likely effects. Predictions of CO2 levels at the
end of the 21st century require assumptions about population growth, energy usage, and the
mix of energy sources being used to satisfy demand. If rates of greenhouse gas emission
continue to grow unchecked by any attempt to limit them (the BAU or business as usual
scenario) then by 2100 atmospheric CO2 will be triple its pre-industrial levels. The intergovernmental panel on climate change (IPCC) published a report in 1995 detailing estimates of
CO2 levels resulting from various emission control scenarios. Part of their work was aimed at
establishing what action would be required to ensure that rising CO2 levels level off at double
pre-industrial levels. The measures required to stabilize CO2 levels include energy conservation
and conversion from fossil fuel energy sources to non-pollution sources. Action requires
political will, and much debate centers around questions of economic cost. Economists have
investigated the costs and benefits of early, and gradual emission reductions compared with
later, more drastic reductions. Study of such questions lies beyond science and hence beyond
the boundaries of ISP203.