Desonie D. Climate

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Climate

The Sun’s lower UV energy and visible light waves pass through

the atmosphere unimpeded. When this radiation hits the Earth’s sur-

face, the energy is absorbed by soil, rock, concrete, water, and other

ground surfaces. The energy is then reemitted into the atmosphere

as infrared waves, which are also called heat. Greenhouse gases trap

some of this heat in the atmosphere, causing the lower atmosphere to

warm. There is a direct relationship between greenhouse gas levels

and atmospheric temperature: Higher levels of greenhouse gases

warm the atmosphere while lower levels of greenhouse gases cool

the atmosphere.

Without the greenhouse effect, Earth’s average atmospheric

emperature would be bitterly cold, about 0°F (-18°C). The planet

would be frozen and have little life. As on the Moon, temperatures

would be extremely variable: scorching when the Sun was out, and

frigid at night. But, thanks to the greenhouse effect, Earth’s aver-

age temperature is a moderate 59°F (15°C), and life is varied and

bountiful.

The dominant greenhouse gases are naturally present in the atmo-

sphere, and their levels can change due to natural processes. For

example, CO

is emitted into the atmosphere during volcanic eruptions.

known as visible light. These are the only

wavelengths the human eye can sense.

When all wavelengths of visible light are

together, the light appears white. When

they are separated into a spectrum, each

wavelength corresponds to a different

color. From the longest to the shortest

wavelengths, visible light is broken into

the colors red, orange, yellow, green,

blue, and violet. Wavelengths shorter

than violet are called ultraviolet radiation

(UV) and wavelengths longer than red are

called infrared radiation.

Due to the Sun’s high temperature,

about 7% of its radiation is made up of

shortwave UV. Because short waves carry

more energy than long waves, UV pho-

tons carry more energy than visible light

photons. Earth’s surface absorbs sunlight

in the visible and ultraviolet light wave-

lengths and then reemits the energy in

infrared wavelengths. Infrared energy is

also known as heat.

(continues)

However, some greenhouse gases, for example, chlorofluorocarbons

(CFCs),

are man-made and have only recently entered the atmosphere.

Not all greenhouse gases have the same heat-trapping ability. For

example, one CFC-12 molecule traps as much heat as 10,600 CO

molecules. Methane traps 23 times as much heat as CO

. However,

despite its lower heat-trapping ability, CO

is so much more abun-

dant than these other gases that it has a much greater impact on

global temperature: It accounts for 80% of greenhouse gas emissions

by humans.

Concentrations of particulates, which are sometimes called aero-

sols,

vary in the atmosphere. Volcanic ash, wind-blown dust, and soot

Greenhouse gases trap some of the heat that radiates off of the planet’s surface, creating

the greenhouse effect.

How Climate Works

Climate

Gas

ChemiCal

formUla

ConCentration,

(

2005, Unless noted)

ConCentration,

1750

nCrease

(%)

time horizon

(100 years)

Carbon dioxide CO

382 parts per million

(ppm; 2007)

280 ppm 36 1

Methane CH

1,774 parts per billion

(ppb)

700 ppb 153 23

Nitrous oxide N

O 319 ppb 270 ppb 18 296

Tetrafluoromethane CF

74 parts per trillion (ppt) 40 ppt 85 5,700

Hexafluorethane C

3 ppt 0 ppt * 11,900

Sulfur hexafluoride SF

5.6 ppt 0 ppt * 22,200

HFC (23, 134a, 152a) 57 ppt 0 ppt * 550, 1,300, 120

CFC-11 CFCl

251 ppt 0 ppt * 4,600

CFC-12 CF

538 ppt 0 ppt 10,600

HCFC-141b C

FCl

18 ppt 0 ppt * 700

HCFC-142b C

Cl 15 ppt 0 ppt * 2,400

Time horizon (100 years): The amount of warming the same mass of gas will contribute compared to CO

over a 100-year

time span.

Concentrations: ppm is parts per million, ppb is parts per billion, and ppt is parts per trillion. For comparison, percent (%)

can be thought of as parts per hundred.

Source: International Panel on Climate Change, Fourth Assessment Report, 2007

Greenhouse Gas Concentrations

from fires or pollutants are common aerosols. Incoming sunlight is

blocked by aerosols blown high into the atmosphere by large volcanic

ruptions. In the lower atmosphere, wind-blown dust and pollutants

reflect and scatter incoming sunlight, while other aerosols, such as

smoky soot, absorb it. Aerosols have a variable effect on climate

because of the way they react to sunlight: Those that reflect sunlight

cool the atmosphere while those that absorb sunlight warm it.

Because gravity holds gases in Earth’s atmosphere, the gases are

densest near the planet’s surface and become less dense at higher

altitude. However, the makeup of atmospheric gases is nearly the

same at all altitudes. But, despite its being similar in composition, the

atmosphere is divided into layers, primarily according to how the tem-

perature changes with altitude. The layer nearest to Earth’s surface,

rising from sea level to about 6 miles (11 kilometers), is called the tro-

posphere. Its primary heat source is the Earth’s surface, so the tropo-

sphere generally displays a decrease in temperature with altitude.

The stratosphere rises from the top of the troposphere to about

30 miles (45 km) up. Because this layer is heated by the Sun’s UV, the

stratosphere gets warmer closer to the Sun. The stratosphere contains

the ozone layer: This is the exception to the rule that the makeup

of the atmosphere is the same at all elevations. This layer, which lies

between 9 and 19 miles (15 and 30 km) up, contains a relatively high

concentration of ozone molecules. Ozone in the stratosphere is known

as “good” ozone because it serves as a protective shield for life on

arth by absorbing the lethal high-energy UV radiation.

the Water CyCle

Water moves continually between Earth’s water reservoirs: the atmo-

sphere, organisms, terrestrial water features (such as lakes and rivers),

and the oceans. The movement of water between these reservoirs is

known as the water cycle.

Much of Earth’s water is stored in the oceans, which cover 71% of

the planet’s surface. (All seawater and a small amount of lake water is

saline, or salty.) The Sun’s rays evaporate liquid water from the sea

surface into the atmosphere, where it exists as water vapor gas. When

How Climate Works

Climate

conditions are right, water vapor undergoes condensation from gas into

liquid droplets to form clouds. The droplets can come together to create

precipitation in the form of rain, sleet, hail, snow, frost, or dew.

When precipitation falls as snow, it may become frozen into a gla-

cier, which is a moving body of ice that persists over time. Glaciers

form when annual snowfall exceeds annual snowmelt. Each winter

snow falls and is compressed into firn,

a grainy, ice-like material.

If summer temperatures stay below freezing, the firn remains to be

buried by more snow the following year. The weight of many years of

accumulating firn eventually squeezes the deeper firn into ice. The

ice at the bottom of a glacier is older than the ice at the top. Glaciers

and ice sheets may store water for hundreds or even thousands

of years.

Today, glaciers are found only at high latitudes and at high alti-

tudes, where the conditions are similar to the polar areas. Over 60%

of the planet’s fresh water is trapped in glaciers. Alpine glaciers are

also called mountain glaciers because of where they are found. Con-

tinental glaciers, also called ice caps, cover large regions of rela-

tively flat ground. Only two ice caps, the Arctic in the north and the

Antarctic in the south, exist today. Together, they cover about 10% of

the planet’s surface and hold 20% of its fresh water. Much of the Arctic

ice cap lies on the Arctic Ocean and is less than 10 feet (3 meters)

thick, on average. Its thinness means that it melts relatively easily. By

contrast, the Antarctic ice cap, located on the Antarctic continent, is

10,000 feet (3,000 m) thick and is much slower to melt. Glaciers or

ice sheets can release (or calve) an ice shelf, a thick, floating platform

of ice that flows onto the ocean surface. Ice shelves are only found in

Greenland, Antarctica, and Canada.

All frozen water, including snow, glaciers, and ice shelves, is part of

the cryosphere. Permanently frozen ground, or permafrost, is also

part of the cryosphere. Permafrost is found typically at high latitudes

and some high altitude regions.

When the ice melts, the water may flow into a stream and then into

a lake or pond. Some of the water infiltrates the soil and rock to join

a groundwater reservoir beneath the ground. Groundwater moves

slowly through a rock layer or aquifer and eventually emerges into a

stream, lake, or the ocean. Water is also absorbed by living organisms.

Some of the water taken in by plants is returned to the atmosphere in

a process known as evapotranspiration.

The overall amount of water present on Earth changes very little.

What does change is its location. For example, when much of the

planet’s water was trapped in glaciers during the ice age about 10,000

years ago, the sea level was lower. But once those glaciers started to

melt, sea level began to rise.

earth’s enerGy balanCe

Solar energy arrives at the top of the atmosphere as UV or visible light.

It passes through the atmosphere unimpeded by greenhouse gases, but

about 50% of it is absorbed, scattered, or reflected by clouds. Scat-

tering

occurs when light strikes particles—atmospheric gases, water

droplets, or dust—and then flies out in all directions. Reflection

occurs when light bounces from a surface. Some surfaces reflect light

better than others: For example, a snowfield reflects much more light

than a mud pit. The measurement of the reflectivity of a surface is

called its albedo. Objects that appear black absorb all visible wave-

lengths, and those that appear white absorb none, meaning that black

objects have much lower albedo than white objects.

Of the radiation that reaches Earth’s surface, 3% is reflected

back and 47% is absorbed by water and land. After being absorbed,

some of the light energy is converted to infrared energy and reemit-

ted into the atmosphere as heat, some of which is trapped by green-

house gases. If the process stopped there, the planet would just

get hotter, but this does not happen because eventually the heat is

radiated into space.

When the amount of shortwave energy entering the Earth’s system

quals the amount of longwave radiation leaving, the planet’s heat bud-

get is in balance. When the system is not in balance, it is because the

nput of heat is greater than the output, and the planet gets warmer; or the

output is greater than the input, making the planet cooler.

How Climate Works

Climate

What shapes a reGion’s Climate?

Weather is the state of the atmosphere in a given place at a given time.

While “hot” may describe the weather for a March day in Fairbanks,

Alaska (at least relative to other March days), it does not describe the

arch climate of any part of Alaska. Climate is the long-term average

of a region’s weather. A region’s latitude and position relative to the

major wind belts are two important factors that determine that region’s

climate. The location’s climate also depends on whether or not it is

near an ocean, what types of ocean currents are nearby, where it is

relative to mountains, and the local albedo.

latitude

Although the heat budget of Earth as a whole is fairly well balanced,

that is not true for the heat budget at different locations. The low

Earth’s energy balance. The fate of incoming solar radiation in the atmosphere and at the

Earth’s surface.

latitudes near the equator take in much more solar radiation than the

high latitudes near the poles because:



The polar regions receive no sunlight at all for months at a

time in the winter, while at the equator, day length shows

little seasonal variation.

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Near the poles, even in the summer, the Sun never rises very

high in the sky, so its rays are filtered through a great wedge

of atmosphere before they reach the ground. Near the equa-

tor, the midday Sun is always overhead, so much more solar

radiation reaches the Earth directly.



The polar regions are often covered with ice and snow, and

their high albedo reflects back a high percentage of the

solar energy that comes into the atmosphere

This imbalance of entering solar radiation between the low and

high latitudes is what drives atmospheric circulation.

atmospheric Circulation

The atmosphere flows in great convection cells as it moves heat from the

warm equatorial region to the cold polar regions. Near the equator, warm

air rises. When the rising warm air reaches the top of the troposphere,

it moves toward the poles. The air cools as it flows and becomes dense

enough to sink at latitudes of about 30°N or 30°S. When this air reaches

the surface, it is sucked toward the equator by the rising air, warming as

it goes. The horizontal motion of air along the ground creates wind. When

the air returns to the equator, the convection cell is complete. Convection

cells are located at latitudes between 30°N and 30°S, 50° to 60°N, and

50° to 60°S, and at the poles. Earth’s rotation influences the direction air

moves by means of the Coriolis effect, which is the tendency of a freely

moving object to appear to move to the right in the northern hemisphere

and to the left in the southern hemisphere due to Earth’s rotation.

Atmospheric circulation cells set the framework for a region’s cli-

ate. Where the air is rising or sinking—at the equator, at 30°, at

50° to 60°, and at the poles—there is little wind. Because air cools as

How Climate Works

Climate

it rises, and cool air can hold less moisture than warm air, locations

where air rises (low pressure zones near the equator and at 50° to 60°)

have high levels of precipitation. Locations where the air sinks (high

pressure zones near 30°) experience more evaporation than precipita-

tion. Air moves horizontally from high to low pressure zones, forming

the major wind belts, which include the trade winds, between the

equator and 30°N and 30°S; the westerlies, between 30°N and 30°S

and 50° to 60°N and 50° to 60°S; and the polar winds. Convection

The six-cell model of global air circulation, showing the locations of high and low pressure

cells and the directions of the major wind belts on the Earth’s surface.

cells are the framework for atmospheric circulation, although other

factors also influence the force and direction of wind.

the effects of the ocean on Climate

Ocean currents transport heat around the Earth and influence regional

climate as they warm or cool the air above the seas. The major surface

ocean currents travel in the same direction as the major wind belts

because the wind pushes the seawater. For example, the westerly winds

drag North Pacific water from west to east, while the trade winds move

surface currents from east to west both north and south of the equator.

When these currents run into continents, the Coriolis effect causes

them to turn right in the Northern Hemisphere and left in the South-

ern Hemisphere. The currents flow along the continents until they

un into an east-west moving current going in the opposite direction.

The result is surface currents that travel in loops called gyres, which

rotate clockwise in the Northern Hemisphere and counterclockwise in

the Southern Hemisphere.

The North Atlantic gyre has an enormous influence on the climate of

northern Europe. At the southern part of this gyre, seawater is warmed

by the Sun as it moves from east to west across the equator. When it

hits the Americas, the current turns right (north) and becomes the Gulf

Stream, a swift warm water current that raises air temperatures along

the eastern United States and southeastern Canada. At the northern part

of the gyre, the Gulf Stream swings right, away from North America

and toward Europe, where it divides into two segments. One segment

moves south toward Africa, completing the gyre, while the other moves

north, along Great Britain and Norway. The northern current, called the

North Atlantic Drift, brings fairly warm Gulf Stream water into the north-

ern latitudes. This current creates air temperatures in the North Atlantic

that are 5°F to 11°F (3° to 6°C) warmer than those of other regions at

the same latitudes. As a result, although London is at 51° north latitude,

several degrees farther north than Quebec, Canada, its climate is much

more temperate: Rain instead of snow predominates in London during

winter. Besides influencing air temperature, ocean currents also affect

precipitation levels because warm water currents bring more moisture

and therefore more rain to a region than do cold currents.

How Climate Works