DEFINITION OF CLIMATE CHANGE

Earth’s climate changes on many different

timescales, ranging from tens of millions of years to decadal

and even shorter timescale variations. In the last 2.5 billion

years, several periods of glaciation have been identified, separated

by periods of mild climate similar to that of today.

Other periods are marked by global hothouse type conditions,

when the Earth had a very hot and wet climate,

approaching that of Venus. These dramatic climate changes

are caused by a number of different factors that exert their

influence on different timescales. One of the variables is the

amount of incoming solar radiation, and this changes in

response to several astronomical effects such as orbital tilt,

eccentricity, and wobble. Changes in the incoming solar radiation

in response to changes in orbital variations produce

cyclical variations known as Milankovitch cycles. Another

variable is the amount of heat that is retained by the atmosphere

and ocean, or the balance between the incoming and

outgoing heat. A third variable is the distribution of landmasses

on the planet. Shifting continents can influence the

patterns of ocean circulation and heat distribution, and placing

a large continent on one of the poles can cause ice to

build up on that continent, increasing the amount of heat

reflected back to space and lowering global temperatures in a

positive feedback mechanism.

Shorter term climate variations include those that operate

on periods of thousands of years, and shorter, less regular

decadal scale variations. Both of these relatively short-period

variations are of most concern to humans, and considerable

effort is being expended to understand their causes and to

estimate the consequences of the current climate changes the

planet is experiencing. Great research efforts are being

expended to understand the climate history of the last million

years and to help predict the future.

Variations in formation and circulation of ocean currents

may be traced some thousands of years to decadal scale

variations in climate. Cold water forms in the Arctic and

Weddell Seas. This cold salty water is denser than other

water in the ocean, so it sinks to the bottom and gets ponded

behind seafloor topographic ridges, periodically spilling over

into other parts of the oceans. The formation and redistribution

of North Atlantic cold bottom water accounts for about

30 percent of the solar energy budget input to the Arctic

Ocean every year. Eventually, this cold bottom water works

its way to the Indian and Pacific Oceans where it upwells,

gets heated, and returns to the North Atlantic. This cycle of

water circulation on the globe is known as thermohaline circulation.

Recent research on the thermohaline circulation

system has shown a correlation between changes in this system

and climate change. Presently, the age of bottom water

in the equatorial Pacific is 1,600 years, and in the Atlantic it

is 350 years. Glacial stages in the North Atlantic have been

correlated with the presence of older cold bottom waters,

approximately twice the age of the water today. This suggests

that the thermohaline circulation system was only half

as effective at recycling water during recent glacial stages,

with less cold bottom water being produced during the

glacial periods. These changes in production of cold bottom

water may in turn be driven by changes in the North Ameri-

can ice sheet, perhaps itself driven by 23,000-year orbital

(Milankovitch) cycles. It is thought that a growth in the ice

sheet would cause the polar front to shift southward,

decreasing the inflow of cold saline surface water into the

system required for efficient thermohaline circulation. Several

periods of glaciation in the past 14,500 years (known

as the Dryas) are thought to have been caused by sudden,

even catastrophic injections of glacial meltwater into the

North Atlantic, which would decrease the salinity and hence

density of the surface water. This in turn would prohibit the

surface water from sinking to the deep ocean, inducing

another glacial interval.

Shorter term decadal variations in climate in the past

million years are indicated by so-called Heinrich Events,

defined as specific intervals in the sedimentary record showing

ice-rafted debris in the North Atlantic. These periods of

exceptionally large iceberg discharges reflect decadal scale sea

surface and atmospheric cooling. They are related to thickening

of the North American ice sheet, followed by ice stream

surges, associated with the discharge of the icebergs. These

events flood the surface waters with low-salinity freshwater,

leading to a decrease in flux to the cold bottom waters, and

hence a short period global cooling.

Changes in the thermohaline circulation rigor have also

been related to other global climate changes. Droughts in the

Sahel and elsewhere are correlated with periods of ineffective

or reduced thermohaline circulation, because this reduces the

amount of water drawn into the North Atlantic, in turn cooling

surface waters and reducing the amount of evaporation.

Reduced thermohaline circulation also reduces the amount of

water that upwells in the equatorial regions, in turn decreasing

the amount of moisture transferred to the atmosphere,

reducing precipitation at high latitudes.

Atmospheric levels of greenhouse gases such as CO2 and

atmospheric temperatures show a correlation to variations in

the thermohaline circulation patterns and production of cold

bottom waters. CO2 is dissolved in warm surface water and

transported to cold surface water, which acts as a sink for the

People have built villages, towns, cities, and industrial sites near

the sea for thousands of years. The coastal setting offers beauty

and convenience but also may bring disaster with coastal storms,

tsunami, and invading armies. Coastal communities are currently

experiencing the early stages of a new incursion, that of the sea

itself, as global sea levels slowly and inexorably rise.

Sea-level rises and falls by hundreds of feet over periods of

millions of years have forced the position of the coastline to move

inland and seaward by many tens of miles over long time periods.

The causes of sea-level rise and fall are complex, including

growth and melting of glaciers with global warming, changes in

the volume of the mid-ocean ridges, thermal expansion of water,

and other complex interactions of the distribution of the continental

landmass in mountains and plains during periods of orogenic

and anorogenic activity. Most people do not think that changes

over these time frames will affect their lives, but a sea-level rise

of even a foot or two, which is possible over periods of tens of

years, can cause extensive flooding, increased severity of storms,

and landward retreat of the shoreline. Sea-level rise is rapidly

becoming one of the major global hazards that the human race is

going to have to deal with in the next century, since most of the

world’s population lives near the coast in the reach of the rising

waters. Cities may become submerged and farmlands covered by

shallow, salty seas. An enormous amount of planning is needed,

as soon as possible, to begin to deal with this growing threat. The

current rate of rise of an inch or so every 10 years seems insignificant,

but it will have truly enormous consequences. When sealevel

rises, beaches try to maintain their equilibrium profile,

moving each beach element landward. A sea-level rise of one

inch is generally equated with a landward shift of beach elements

of more than four feet. Most sandy beaches worldwide are

retreating landward at rates of 20 inches–3 feet per year, consistent

with sea-level rise of an inch every 10 years. If the glacial ice

caps on Antarctica begin to melt faster, the sea-level rise will be

much more dramatic.

What effect will rising sea levels have on the world’s cities

and low-lying areas? Many of the world’s large cities, including

New York, London, Houston, Los Angeles, Washington D.C., Cairo,

Shanghai, Brussels, and Calcutta have large areas located within a

few feet of sea level. If sea levels rise a few feet, many of the

streets in these cities will be underwater, not to mention basements,

subway lines, and other underground facilities. Imagine

Venice-like conditions in New York! If sea levels rise much more,

many of the farmlands of the midwest United States, North Africa,

Mesopotamia, northern Europe, Siberia, and eastern China will be

submerged in shallow seas. These areas are not only populated but

serve as some of the most fertile farmlands in the world. Thus,

large sea-level rise will at best displace or more likely simply eliminate

the world’s best agricultural lands, necessary for sustaining

global population levels.

What can be done to prepare for sea-level rise? Some

lessons can be learned from the Netherlands, where the Dutch

have built numerous dikes to keep the sea out of low-lying areas, at

costs of billions of dollars. If the United States had to build such

barriers around the coastlines of low-lying areas, the cost would be

unbearable and would amount to one of the largest construction

projects ever undertaken. Humans are contributing to global warming,

which in turn is probably contributing to enhanced melting of

the glaciers and ice caps. Although it is too late to stop much of the

warming and melting, it may not be too late to stop the warming

before it is catastrophic and the ice caps melt, raising sea levels by

hundreds of feet. In any case, it is time that governments, planners,

and scientists begin to make more sophisticated plans for action

during times of rising sea levels.

CO2. During times of decreased flow from cold, high-latitude

surface water to the deep ocean reservoir, CO2 can build up

in the cold polar waters, removing it from the atmosphere

and decreasing global temperatures. In contrast, when the

thermohaline circulation is vigorous, cold oxygen-rich surface

waters downwell and dissolve buried CO2 and even carbonates,

releasing this CO2 to the atmosphere and increasing

global temperatures.

The present-day ice sheet in Antarctica grew in the Middle

Miocene, related to active thermohaline circulation that

caused prolific upwelling of warm water that put more moisture

in the atmosphere, falling as snow on the cold southern

continent. The growth of the southern ice sheet increased the

global atmospheric temperature gradients, which in turn

increased the desertification of midlatitude continental

regions. The increased temperature gradient also induced

stronger oceanic circulation, including upwelling, and

removal of CO2 from the atmosphere, lowering global temperatures,

and bringing on late Neogene glaciations.

Major volcanic eruptions inject huge amounts of dust

into the troposphere and stratosphere, where it may remain

for several years, reducing incoming solar radiation and

resulting in short-term global cooling. For instance, the eruption

of Tambora volcano in Indonesia in 1815 resulted in

global cooling and the year without a summer in Europe. The

location of the eruption is important, as equatorial eruptions

may result in global cooling, whereas high-latitude eruptions

may only cool one hemisphere.

It is clear that human activities are changing the global

climate, primarily through the introduction of greenhouse

gases such as CO2 into the atmosphere, while cutting down

tropical rain forests that act as sinks for the CO2 and put

oxygen back into the atmosphere. The time scale of observation

of these human, also called anthropogenic, changes is

short but the effect is clear, with a nearly one degree change

in global temperature measured for the past few decades. The

increase in temperature will lead to more water vapor in the

atmosphere, and since water vapor is also a greenhouse gas,

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