Definition of Hypsometric Curve

The distribution of elevation of continents

and oceans can be portrayed on a curve showing percentage

of land at a certain elevation versus elevation, known

as the hypsometric curve, or the hypsographic curve. The

curve is a cumulative frequency profile representing the statistical

distribution of areas of the Earth’s solid surface above or

below mean sea level. The hypsometric curve is strongly

bimodal, reflecting the two-tier distribution of land in continents

close to sea level, and on ocean floor abyssal plains

1.9–2.5 miles (3–4 km) below sea level. Relatively little land

surface is found in high mountains or in deep-sea trenches.

Times when the global climate was colder, and

large masses of ice covered many continents are referred to as

ice ages. At several times in Earth’s history, large portions of

the Earth’s surface have been covered with huge ice sheets.

About 10,000 years ago, all of Canada, much of the northern

United States, and most of Europe were covered with ice

sheets, as was about 30 percent of the world’s landmass.

These ice sheets lowered sea level by about 320 feet (100 m),

exposing the continental shelves and leaving cities including

New York, Washington, and Boston 100 miles from the sea.

In the last 2.5 billion years, several periods of ice ages have

been identified, separated by periods of mild climate similar

to that of today. Ice ages seem to form through a combination

of several different factors. One of the variables is the

amount of incoming solar radiation, and this changes in

response to several astronomical effects. 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 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.

Glaciations have happened frequently in the past 55 million

years and could occur again at almost any time. In the

late 1700s and early 1800s, Europe experienced a “little ice

age” when many glaciers advanced out of the Alps and

destroyed many small villages. Ice ages have occurred at several

other times in the ancient geologic past, including in Late

Paleozoic (about 350–250 million years ago), Silurian (435

million years ago), and Late Proterozoic (about 800–600 million

years ago). During parts of the Late Proterozoic glaciation,

it is possible that the entire Earth surface temperature

was below freezing and covered by ice.

In the Late Proterozoic, the Earth experienced one of the

most profound ice ages in the history of the planet. Isotopic

records and geologic evidence suggest that the entire Earth’s

surface was frozen, though some workers dispute the evidence

and claim that there would be no way for the Earth to

recover from such a frozen state. In any case it is clear that in

the Late Proterozoic, during the formation of the supercontinent

of Gondwana, the Earth experienced one of the most

intense glaciations ever, with the lowest average global temperatures

in known Earth history.

One of the longest lasting glacial periods was the Late

Paleozoic ice age that lasted about 100 million years, indicating

a long-term underlying cause of global cooling. Of the

variables that operate on these long time scales, it appears

that the distribution and orientation of continents seems to

have caused the Late Paleozoic glaciation. The Late Paleozoic

saw the amalgamation of the planet’s landmasses into the

supercontinent of Pangea. The southern part of Pangea,

known as Gondwana, consisted of present-day Africa, South

America, Antarctica, India, and Australia. During the drift of

the continents in the Late Paleozoic, Gondwana slowly

moved across the South Pole, and huge ice caps formed on

these southern continents during their passage over the pole.

The global climate was overall much colder, with the subtropical

belts becoming very condensed and the polar and

subpolar belts expanding to low latitudes.

It seems that during all major glaciations there was a

continent situated over one of the poles. We now have

Antarctica over the South Pole, and this continent has huge

ice sheets on it. When continents rest over a polar region they

accumulate huge amounts of snow that gets converted into

several-kilometer-thick ice sheets, which reflect more solar

radiation back to space and lower global seawater temperatures

and sea levels.

Another factor that helps initiate glaciations is to have

continents distributed in a roughly north-south orientation

across equatorial regions. Equatorial waters receive more

solar heating than polar waters. Continents block and modify

the simple east to west circulation of the oceans induced by

the spinning of the planet. When continents are present on or

near the equator, they divert warm water currents to high latitudes,

bringing warm water to higher latitudes. Since warm

water evaporates much more effectively than cold water, having

warm water move to high latitudes promotes evaporation,

cloud formation, and precipitation. In cold high-latitude

regions the precipitation falls as snow, which persists and

builds up glacial ice.

The Late Paleozoic glaciation ended when the supercontinent

of Pangea began breaking apart, suggesting a further

link between tectonics and climate. It may be that the smaller

landmasses could not divert the warm water to the poles any

more, or perhaps enhanced volcanism associated with the

breakup caused additional greenhouse gases to build up in

the atmosphere, raising global temperatures.

The planet began to enter a new glacial period about

55 million years ago, following a 10-million-year-long period

of globally elevated temperatures and expansion of the

warm subtropical belts into the subarctic. This Late Paleocene

global hothouse saw the oceans and atmosphere holding

more heat than at any other time in Earth history, but

temperatures at the equator were not particularly elevated.

Instead, the heat was distributed more evenly around the

planet such that there were probably fewer violent storms

(with a small temperature gradient between low and high

latitudes), and overall more moisture in the atmosphere. It

is thought that the planet was so abnormally warm during

this time because of several factors, including a distribution

of continents that saw the equatorial region free of continents.

This allowed the oceans to heat up more efficiently,

raising global temperatures. The oceans warmed so much

that the deep ocean circulation changed, and the deep currents

that are normally cold became warm. These melted

frozen gases (known as methane hydrates) accumulated on

the seafloor, releasing huge amounts of methane to the

atmosphere. Methane is a greenhouse gas, and its increased

abundance in the atmosphere trapped solar radiation in the

atmosphere, contributing to global warming. In addition,

this time saw vast outpourings of mafic lavas in the North

Atlantic Ocean realm, and these volcanic eruptions were

probably accompanied by the release of large amounts of

CO2, which would have increased the greenhouse gases in

the atmosphere and further warmed the planet. The global

warming during the Late Paleocene was so extreme that

about 50 percent of all the single-celled organisms living in

the deep ocean became extinct.

After the Late Paleocene hothouse, the Earth entered a

long-term cooling trend which we are still currently in,

despite the present warming of the past century. This current

ice age was marked by the growth of Antarctic glaciers, starting

about 36 million years ago, until about 14 million years

ago, when the Antarctic ice sheet covered most of the continent

with several miles of ice. At this time global temperatures

had cooled so much that many of the mountains in the

Northern Hemisphere were covered with mountain and piedmont

glaciers, similar to those in southern Alaska today. The

ice age continued to intensify until 3 million years ago, when

extensive ice sheets covered the Northern Hemisphere. North

America was covered with an ice sheet that extended from

northern Canada to the Rocky Mountains, across the Dakotas,

Wisconsin, Pennsylvania, and New York, and on the continental

shelf. At the peak of the glaciation (18,000–20,000

years ago), about 27 percent of the land surface was covered

with ice. Midlatitude storm systems were displaced to the

south, and desert basins of the southwest United States,

Africa, and the Mediterranean received abundant rainfall and

hosted many lakes. Sea level was lowered by 425 feet (130 m)

to make the ice that covered the continents, so most of the

world’s continental shelves were exposed and eroded.

The causes of the Late Cenozoic glaciation are not well

known but seem related to Antarctica coming to rest over the

south pole and other plate tectonic motions that have continued

to separate the once contiguous landmasses of Gondwana,

changing global circulation patterns in the process. Two of the

important events seems to be the closing of the Mediterranean

Ocean around 23 million years ago and the formation of the

Panama isthmus at 3 million years ago. These tectonic movements

restricted the east-to-west flow of equatorial waters,

causing the warm water to move to higher latitudes where

evaporation promotes snowfall. An additional effect seems to

be related to uplift of some high mountain ranges, including

the Tibetan Plateau, which has changed the pattern of the air

circulation associated with the Indian monsoon.

The closure of the Panama isthmus is closely correlated

with the advance of Northern Hemisphere ice sheets, suggesting

a causal link. This thin strip of land has drastically

altered the global ocean circulation such that there is no

longer an effective communication between Pacific and

Atlantic Ocean waters, and it diverts warm currents to nearpolar

latitudes in the North Atlantic, enhancing snowfall and

Northern Hemisphere glaciation. Since 3 million years ago,

the ice sheets in the Northern Hemisphere have alternately

advanced and retreated, apparently in response to variations

in the Earth’s orbit around the Sun and other astronomical

effects. These variations change the amount of incoming solar

radiation on timescales of thousands to hundreds of thousands

of years (Milankovitch Cycles). Together with the other

longer-term effects of shifting continents, changing global circulation

patterns, and abundance of greenhouse gases in the

atmosphere, most variations in global climate can be

approximately explained. This knowledge may help predict

where the climate is heading in the future and may help

model and mitigate the effects of human-induced changes to

the atmospheric greenhouse gases. If we are heading into

another warm phase and the existing ice on the planet melts,

sea level will quickly rise by 210 feet (64 m), inundating

many of the world’s cities and farmlands. Alternately, if we

enter a new ice sheet stage, sea levels will be lowered, and

the planet’s climate zones will be displaced to more equatorial

regions.

See also ATMOSPHERE; GLACIER; GREENHOUSE EFFECT.

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