Desonie D. Climate

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Climate

away from Earth. This phenomenon is called global dimming, which

brings about a decrease in temperature.

The filtering of incoming solar radiation by polluted clouds has

been seen everywhere around the planet. Between the 1950s and

the early 1990s, sunlight was reduced 9% in Antarctica, 10% in the

United States, 16% in parts of the British Isles, and nearly 30% in

Russia. The most extreme reduction was found in Israel, which expe-

ienced a 22% decrease in sunlight in a 20-year period that ended in

the early 1980s.

Researchers suggest that global dimming has already had dire

effects. The horrendous Ethiopian drought that culminated in 1984

and brought about one million deaths and upset 50 million lives, can

be explained by global dimming. In Africa, under normal conditions,

he summer Sun heats the air north of the equator, generating a low-

pressure cell that sucks the summer monsoon rains into the Sahel,

he semiarid region south of the Sahara desert. (Monsoons bring warm,

moist air from the ocean onto the land, often accompanied by intense

rains.) The summer monsoon is crucial because it is the only rain that

countries in that part of the world, such as Ethiopia, receive. But for

twenty years, in the 1970s and 1980s, the monsoon rains did not come

into the Sahel.

Global dimming could have been behind this drought. In those

decades, large amounts of pollutants from Western Europe and North

America wafted over northern Africa, dimming the incoming solar

radiation. A large enough dimming could have kept temperatures

in the area from increasing enough for the air to rise and suck the

monsoon rains northward. With no monsoon rains, the Sahel became

rought-stricken.

The Ethiopian drought eventually abated. This can also be ex-

plained by global dimming. In the past two decades or more, Western

Europe and North America have decreased their pollutant emissions

by burning cleaner fuels and installing pollution control devices on

smokestacks and exhaust systems. Because of this, fewer pollutants

now drift over northern Africa, so the air there heats enough to rise

and bring the monsoon rains into the Sahel. Additional pollution

When the Jets Were Grounded

An amazing scientific experiment one

that could not have been performed under

any other circumstance came out of

a national tragedy. David Travis of the

University of Wisconsin, Whitewater, had

been studying the effects of jet plane con-

trails on climate for 15 years. As shown in

the photograph, he could see from satel-

lite photos that contrails sometimes cover

as much as 50% to 75% of the sky. He

assumed that they had an effect on tem-

perature, but he could not tell what kind

and how much. Travis found the opportu-

nity of his research career when, after the

terrorist attacks on September 11, 2001,

nearly the entire commercial air fleet of the

United States was grounded. He knew that

by studying the atmosphere in a state that

it had never been in during his lifetime

contrail free he could learn the most

about the effects of contrails on climate.

Travis collected data from more than

5,000 weather stations all over the contig-

uous 48 states. What he discovered was

startling: Like clouds, contrails moderate

daily temperature swings. Contrails reduce

the amount of solar radiation that hits the

ground, making days cooler, and trap heat

being reemitted from Earth, making nights

warmer. During the three- day period in

which the planes did not fly, there was an

amazing 1.8°F (1°C) increase in tempera-

ture across the United States the great-

est single temperature effect ever seen.

Travis notes that this large effect was

due to just one source other sources of

pollutants were not restricted. Therefore,

he reasoned, the effect of global dimming

from all sources must be huge.

A contrail forms when hot air from a jet engine

mixes with cool air in the surrounding environment.

Contrails can linger in the sky and over time spread

out into thin cloud like formations. (NASA / MODIS

Rapid Response Team / Goddard Space Flight Center)

Human Causes of Climate Change

Climate

control measures could decrease or end global dimming regionally and

globally. But the flip side of a decrease in this pollution could well be

an increase in global warming.

WrapUp

Human activities are releasing greenhouse gases that have been

sequestered in fossil fuels and biomass for millions of years back into

he atmosphere. Man-made greenhouse gases such as CFCs that have

never before existed on Earth are also being added. Changes in how

eople use the land—as when they change forests to farmlands or

cities—also alter the composition of the atmosphere. Human activities

cause climate to cool because atmospheric pollutants block sunlight.

Still, scientists have found that global dimming has been counteracting

some portion of global warming. As air pollution is reduced, the effect

may likely be a further increase in global warming.

John Christy, an atmospheric scientist at the University of Ala-

bama, Huntsville, and a former climate skeptic, stated in a 2004

interview with National Public Radio: “It is scientifically inconceiv-

able that after changing forests into cities, turning millions of acres

into farmland, putting massive quantities of soot and dust into the

atmosphere and sending quantities of greenhouse gases into the air,

that the natural course of climate change hasn’t been increased in the

past century.”

How Scientists Learn

About Past, Present,

and Future Climate

arth formed about 4.55 billion years ago, but humans— the

genus and species Homo sapiens— evolved only about 200,000

years ago. Indeed, nearly all of Earth’s past climate occurred

before there were people around to witness it. Only during the past

millennium have people been chronicling important climatic events,

and only in the past century have they kept accurate and consistent

weather records. This chapter discusses some of what is known about

past climate. Much of this knowledge is the result of the inventive use

of the available evidence.

measUrements

Meteorology is the study of Earth’s atmosphere with the goal of predicting

the weather. Since the late 1800s, meteorologists have been measuring

weather characteristics such as temperature, precipitation, and wind

speed and direction from land- based stations and, more recently, from

weather balloons. In 1960, meteorologists began to use satellites to gather

Climate

weather data. Because satellites see a large and clear picture from high

above Earth’s surface, they are extremely important for chronicling global

climate change. Satellites have gathered decades’ worth of information on

pollution, fires, ocean temperature, ocean current patterns, ice boundar-

es, volcanic ash clouds, and many other climate-related features. For

example, images gathered each year for two decades detail the loss of ice

cover during Arctic summers over that time period.

Climate proxies

Paleoclimatologists (paleoclimatology is the study of past climate)

have developed many innovative techniques for getting information

about the history of Earth’s climate. These scientists use biological or

physical clues, known as climate proxies, to unravel past climate

patterns over the entire planet or over specific regions. These clues

are found in ice cores, tree rings, and sediments, for example,

and can be used to reconstruct past climate with surprising depth

and accuracy. Some climate proxies, for example, preserve evidence

of past temperatures. Using these proxies, paleoclimatologists have

reconstructed Earth’s climate history in differing amounts of detail

that reach back millions of years.

Unique tools are used for different time scales. Ice cores yield

climate data that cover hundreds of thousands of years. Sediments in

the ocean floors go back millions of years. Sedimentary rocks from

the surface of the earth are rocks that are made of sediments or that

precipitate from water. These rocks can give general information about

climate that goes back billions of years. Although tree ring cores taken

from living trees are only useful for as many years as the tree has lived,

logs that are preserved by becoming petrified in rock or sediments

yield information about time periods that are much further back.

ice Cores

The most powerful window into past climate is the ice contained in

glaciers and ice caps. To collect an ice core, scientists drill a hollow

pipe into an ice sheet or glacier. The cores taken from the Greenland

and Antarctic ice caps supply data that span long periods of time: Two

Greenland cores go back 100,000 years, while one Antarctic core

goes back 420,000 years, through four glacial cycles. The European

Project for Ice Coring in Antarctica (EPICA), about two miles (3,190

m) long, has cut through eight glacial cycles covering 740,000 years

and is still being drilled. Mountain ice cores are thinner and have

much shorter records, but they can be collected from regions scattered

around the Earth.

Polar ice caps may contain as many as 100,000 layers of ice. Each

layer yields age and weather data from the time when it was deposited.

High up in an ice core, close to the surface, each layer represents one

year: The age of the sample can be determined simply by counting

backwards from the top of the core. Deeper in the core, where the lay-

ers have been compacted by overlying ice, only multiple year blocks

can be distinguished. To determine the age of a deeper layer, events

with known ages that have left evidence of their occurrence in the

layer can be identified. Useful recent events are nuclear bomb tests

that have left deposits of radioactive isotopes in the ice. Volcanic

ash is valuable for ice of any age because the ash can be correlated

to specific eruptions far back in time. Age can also be determined by

the chemistry of marine sediments.

Gases and particles trapped in snowfall can be analyzed by

examining ice layers. These substances represent atmospheric con-

ditions at the time the snow fell. Scientists can analyze CO

in the

gases to determine the concentration of that greenhouse gas at the

time and to ascertain its source, whether from volcanic eruptions or

urning fossil fuels, for example. The presence of Beryllium-10 in

the ice core is evidence of the strength of solar radiation. Ash indi-

cates a volcanic eruption, dust an expansion of deserts, and pollen

the types of plants that were on the planet at the time. The amount

of pollen found in the ice layer may be an indicator of the amount of

precipitation that fell.

Paleoclimatologists can discern air temperature at the time the

snow fell by measuring the ratios of different isotopes of oxygen and

hydrogen. These isotope ratios also reveal global sea level.

How Scientists learn about Past, Present, and Future Climate

Climate

By using isotopes and many other chemical and physical indicators

trapped in the ice, scientists can reconstruct atmospheric temperature,

ocean volume, precipitation, the composition of the lower atmosphere,

volcanic eruptions, solar variability, the productivity of plankton at

the sea surface, the extent of deserts, and the presence of forest fires.

Most importantly for paleoclimatologists, they can construct a record of

climate change through the period of time represented by the core.

sediments

Sediments collect in layers on seafloors and lake bottoms. Like ice,

sediments can be drilled as cores with the oldest layer located at the

bottom. Sediments can be found dating back millions of years. They

Isotopes and Their Scientific Uses

An atom is the smallest unit of a chemi-

cal element (a substance that cannot

be chemically reduced to simpler sub-

stances) having the properties of that ele-

ment. At an atom’s center is its nucleus,

which contains protons that have small,

positive electrical charges and neutrons

that have no charge. An atom’s atomic

weight is the sum of its protons and neu-

trons. A particular element, for example,

potassium, will always have the same

number of protons in its nucleus, but

it may have a different number of neu-

trons. Potassium always has 19 protons

but it can have 20, 21, or 22 neutrons.

Therefore, the atomic weight of a potas-

sium nucleus can be 39, 40, or 41, which

creates the different isotopes of potas-

sium: potassium-39, potassium-40, or

potassium-41.

Oxygen isotopes are especially impor-

tant in deciphering Earth’s past climate.

Nearly all oxygen is either

O (8 protons

and 8 neutrons) or

O (8 protons and 10

neutrons). Either isotope may become part

of a water molecule. H

O containing

is “light” and so is slightly more likely to

evaporate than “heavy” H

O containing

When water evaporates, the water vapor is

enriched in

O while the liquid water left

behind is enriched in

O. Therefore, the

O of the liquid is relatively high, and

the

O of the vapor is relatively low.

Similarly, a heavy water molecule (one con-

taining

O) is slightly more likely to con-

dense to form a raindrop or snowflake than

a light water molecule. For that reason, the

O of the raindrop is higher than that

of the remaining vapor. Hydrogen isotopes

work the same way, with lighter

H (one

proton) more likely to be in water vapor and

heavier

H, (one proton and one neutron)

more likely to be in liquid water.

Air cools as it rises or moves toward the

poles and releases some of its moisture.

Because

O is more likely to condense

into a raindrop, the first precipitation to

fall has a relatively high

O ratio.

With time, the

O is depleted from the

air so the

O ratio of the raindrops

decreases. Because air moves toward the

poles from the equator,

O decreases

with increasing latitude.

As a result of these processes, higher

O indicates warmer air tempera-

tures; lower

O indicates cooler tem-

peratures. Therefore,

O is a proxy

for temperature.

H can also be used to

reconstruct the temperature at the time of

precipitation. These isotopes can be used

as a proxy for temperature in ice cores and

marine sediments.

As temperature increases and ice sheets

melt, fresh water enriched in light oxygen

returns to the sea. Low

O ratios indi-

cate less ice cover and, therefore, higher

temperatures.

O is also a proxy for

local rainfall: Because the

O precipitates

first, low

O means that a large amount

of rain has already fallen.

can be composed of sand, rock fragments, clay, dust, ash, preserved

vegetation, animal fossils, and pollen.

Perhaps the most useful sediments are the remains of plankton

that once floated at the ocean surface before dying and sinking to the

bottom. These tiny shells harbor many kinds of information on the

conditions of the atmosphere and surrounding seawater. One type of

lankton—small, coiled foraminifera (forams)—are extremely sensi-

tive to ocean temperature, and therefore each of the species in this

group is present only within a specific, narrow temperature range. The

presence of a particular species in a core yields the sea surface tem-

perature (SST) at the time the organisms lived. One species of foram,

Neogloboquadrina pachyderma, changes its coiling direction from left

Isotopes and Their Scientific Uses

An atom is the smallest unit of a chemi-

cal element (a substance that cannot

be chemically reduced to simpler sub-

stances) having the properties of that ele-

ment. At an atom’s center is its nucleus,

which contains protons that have small,

positive electrical charges and neutrons

that have no charge. An atom’s atomic

weight is the sum of its protons and neu-

trons. A particular element, for example,

potassium, will always have the same

number of protons in its nucleus, but

it may have a different number of neu-

trons. Potassium always has 19 protons

but it can have 20, 21, or 22 neutrons.

Therefore, the atomic weight of a potas-

sium nucleus can be 39, 40, or 41, which

creates the different isotopes of potas-

sium: potassium-39, potassium-40, or

potassium-41.

Oxygen isotopes are especially impor-

tant in deciphering Earth’s past climate.

Nearly all oxygen is either

O (8 protons

and 8 neutrons) or

O (8 protons and 10

neutrons). Either isotope may become part

of a water molecule. H

O containing

is “light” and so is slightly more likely to

evaporate than “heavy” H

O containing

When water evaporates, the water vapor is

enriched in

O while the liquid water left

behind is enriched in

O. Therefore, the

O of the liquid is relatively high, and

the

O of the vapor is relatively low.

Similarly, a heavy water molecule (one con-

taining

O) is slightly more likely to con-

dense to form a raindrop or snowflake than

a light water molecule. For that reason, the

O of the raindrop is higher than that

of the remaining vapor. Hydrogen isotopes

work the same way, with lighter

H (one

proton) more likely to be in water vapor and

heavier

H, (one proton and one neutron)

more likely to be in liquid water.

Air cools as it rises or moves toward the

poles and releases some of its moisture.

Because

O is more likely to condense

into a raindrop, the first precipitation to

fall has a relatively high

O ratio.

With time, the

O is depleted from the

air so the

O ratio of the raindrops

decreases. Because air moves toward the

poles from the equator,

O decreases

with increasing latitude.

As a result of these processes, higher

O indicates warmer air tempera-

tures; lower

O indicates cooler tem-

peratures. Therefore,

O is a proxy

for temperature.

H can also be used to

reconstruct the temperature at the time of

precipitation. These isotopes can be used

as a proxy for temperature in ice cores and

marine sediments.

As temperature increases and ice sheets

melt, fresh water enriched in light oxygen

returns to the sea. Low

O ratios indi-

cate less ice cover and, therefore, higher

temperatures.

O is also a proxy for

local rainfall: Because the

O precipitates

first, low

O means that a large amount

of rain has already fallen.

How Scientists learn about Past, Present, and Future Climate

Climate

to right when the surface water temperature warms above 46°F (8°C).

With temperature information from all around the seafloor, scientists

can reconstruct the patterns of ocean currents during any time period

from the past 150 million years or more.

The isotope ratios of marine fossils also contain useful informa-

tion.

O in shells, teeth, bone, and other hard tissues yield the

temperature at the time the organism lived. The shells of some forams

contain an average of 2% more

O during a glacial period than a

warm period.

O can also be used to calculate the amount of the

Earth that was covered by ice. Because heavy water is more likely to

precipitate near the equator, snow that falls in the high latitudes is

light. During an ice age, this light snow is trapped in ice sheets, and

therefore seawater is enriched in

O. The

O value of seawater

is therefore directly related to the amount of

O-enriched ice that

covers the Earth. Oxygen isotopes also give information on sea level:

A rise or fall in

O of just 0.01% indicates a 33 foot (10 m)

change in sea level.

sedimentary rock

Sedimentary rock does not give as detailed a picture of past climate

as ice cores and marine or lake sediments. Sedimentary rocks’ value

is that they are widespread and are available from much further back

in Earth history, even going back hundreds of millions of years. Sedi-

mentary rocks contain the only remaining paleoclimate information for

much of Earth history.

Many sedimentary rocks can only be deposited in a restricted range

of climates. Modern coral reefs, for example, only grow in the tropics;

therefore, fossil reefs indicate that the region was tropical at the time

the coral reef grew. Rock that was formed from glacial deposits indi-

cates a cold climate, while coal was formed in a warm and wet environ-

ment. Limestone forms in warm shallow seas. The ratio of

O in

sedimentary rocks indicates how much rain was falling when the rocks

were deposited and where the moisture may have come from. Scientists

can use these data to reconstruct atmospheric circulation patterns.

tree rings

Each year a tree grows a new layer of wood under the bark. This cre-

ates a tree ring, which varies in size depending on the temperature and

precipitation conditions at the time of growth. Narrow rings are from

cool, dry years and wider rings represent warm, wet years. Tree rings

are useful only in locations where there is an annual seasonal cycle of

temperature and precipitation, such as in the temperate zones.

Trees do not live more than a few centuries, but evidence of past

climate can be reconstructed from logs found in ice, permafrost, or

glacial sediments. Tree ring data from petrified trees can yield infor-

mation from much further back in time. The age of these trees can be

determined using radiocarbon dating, which measures the abundance

of carbon isotopes that undergo radioactive decay at a known rate.

Climate models

Scientists input the information gathered from modern measurements,

paleoclimate data, and current ideas on how land, atmosphere, oceans,

and ice interact into a supercomputer to construct climate models. A

climate model can be created for a local area or for the entire Earth.

Climate is very complex, and climate models are difficult to con-

struct. Many aspects of climate are not well understood. Simple cli-

mate models look at a single atmospheric characteristic and its effect

on a single condition, such as the effect of rising carbon dioxide levels

on surface air temperatures. Simple models can be combined to gener-

ate more complex models. For example, the effect of rising temperature

on separate layers of the atmosphere can be combined into a model

of the changes in temperature and circulation expected for the entire

atmosphere. The more factors that are put into the model, the more

complex it is, and the less certainty scientists may have regarding the

accuracy of the outcome.

To check the validity of a new model, scientists try to replicate

events that have already occurred. For example, they might construct

a model to predict the effect of increased air temperature on sea

How Scientists learn about Past, Present, and Future Climate