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How are
people changing
the climate?
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Unit 1:
1. Man-made climate change? - more
In the Basics-section you will find links to the Read more-section. Follow those
links if you want to explore an issue in more depth (rather than reading this Read
more-section from beginning to end).
In this unit you can read about:
-
Observed changes in concentrations of greenhouse gases in the atmosphere
Observed changes in the Earth's climate
The inertia of the climate system
Feedback effects
Abrupt changes.
This unit also has worksheets.
Part 1: Changes in concentrations
1.Man-made climate change?
Observed changes in concentrations of greenhouse gases in the
atmosphere.
 Carbon dioxide (CO2) has increased from 280
ppm (280 litres of CO2 in a million litres of
air) in the period 1000 to 1750 to about 370
ppm today. This is about a 31% increase and
today’s levels are the highest atmospheric
CO2 concentrations in at least 420,000 years,
probably 20 million years.
1. Increase in greenhouse gas
concentrations (in ppm) from
1750 to 2000. Graphics: Elmar
Uherek.

Methane (CH4) has increased from 0.7 ppm in
1750 to 1.7 ppm in 2000. This is an increase
of about 151%.

Nitrous oxide (N2O) has increased from 0.27 ppm in 1750 to 0.32 ppm in
2000. This is about a 17% increase.

Tropospheric ozone (O3) has increased by about 35% from 1750 to 2000.
This figure, does however, vary from region to region. Ozone in the lower
atmosphere is a greenhouse gas, it is formed and broken down by
chemical reactions in the atmosphere, and humans emit other
substances which influence these chemical reactions.
(Source: IPCC 2001 and http://cdiac.esd.ornl.gov/pns/current_ghg.html).
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Part 2: Changes in climate
1. Man-made climate change?
Observed changes in the Earth’s climate
Observed changes in the Earth’s climate
The 1990's appear to be the warmest decade since 1860 and 1998 was
the the warmest year. Over the last 1000 years, the 1900's probably saw
the greatest temperature increase in the Northern Hemisphere. Less is
known about the Southern Hemisphere.
Weather
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Average global temperatures on both the land and the ocean surface have
increased by 0.6°C. Temperatures have increased more over land areas
than over the oceans.
The differences between day and night temperatures have decreased
because night temperatures have increased more than day temperatures.
The number of hot days has increased.
The number of cold days with frost has decreased.
The average rainfall in the Northern Hemisphere increased by 5–10%
throughout the 1900's. Some areas, such as the Mediterranean region and
the northern and western regions of Africa, have had less rain, while other
areas have become wetter.
The number of extreme rainfall events at mid- and high-latitudes (high
latitudes are regions close to the North or South poles) have increased.
Areas in Asia and Africa have had more frequent and more intense dry
periods during the summer.
Other physical conditions
 Sea level has increased, on
average, by 1 - 2 mm per year
during the 1900's.
 The period when ice covers lakes
and rivers became about two weeks
shorter per year during the 1900's.
 Glaciers outside the polar regions
have receded.
1. Traces of the past: By drilling deep into mountain
glaciers and the ice caps on Antarctica and
Greenland, scientists can analyze old ice to find out
about the climate in the past. The ice core on the
picture contains dust from a volcanic eruption,
carried by the wind from far away. Photo: Marzena
Kaczmarska/NPI
 Permafrost (ground or bedrock
that is always frozen) in polar and
mountain regions has thawed.
El Niño is a weather phenomenon in the Pacific Ocean that appears regularly,
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about every five years around Christmas (which is why it is called “El Niño”, as
this means “Baby Jesus”). El Niño has always affected the climate in large areas
of the world and led to drought and catastrophic flooding. Scientists believe that
global warming may make El Niño more intense and occur more often. This trend
has been visible over the last 20 to 30 years.
Part 3: Inertia of climate
The inertia of the climate system
The inertia of the climate system
One important characteristic of the climate system is it’s inertia – that is,
it takes a long time from when something happens (cause) to when we
see the total consequence (effect). For example, it takes a very long
time from:

when people emit CO2 or another gas until we can measure new stable
concentrations of the gas in the atmosphere.

when the concentration of greenhouse gases increases until we can see
the effect on the temperature.

when the temperature changes until we can see the biological effects in
various species of plants and animals such as their extinction, mutations,
or relocation to new habitats.

when the temperature in the atmosphere increases to when the sea level
rises.
An illustration of this inertia is the reduction in the number of glaciers in certain
areas. There are about a third less glaciers than 135 years ago. This reduction
isn't, however, simply the result of global warming but rather because the Earth
is still returning to its normal state after the last Ice Age!
Because the climate system is so slow, the climate will continue to warm up even
after greenhouse gas emissions are stabilized or reduced. Sea level will also
continue to rise for many hundreds of years after CO 2 emissions are stabilized.
The slowness also adds to the uncertainties in our knowledge about climate
change. Both the degree to which people add to the greenhouse effect and the
possible consequences of human actions are difficult to study because it takes so
long from when a gas is emitted to when we can measure the changes. When
scientists try to predict the future, it takes a long time from when they make the
calculations to when they can check to see if they were right.
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1.
SLOW RESPONSE: This figure shows what might happen if man-made emissions of CO2 stop
growing at some point during the next hundred years, and then start falling. After CO 2
emissions are reduced and concentrations in the atmosphere stabilize, surface air
temperatures continue to rise slowly for a century or more. Expansion of the ocean due
to warmer water continues long after CO2 emissions have been reduced and melting of ice
sheets continues to contribute to sea-level rise for many centuries. Source: IPCC
The slowness also means that the consequences of our actions are less noticeable
now than they will be in the future. Emissions from one generation are most likely
to affect future generations. Likewise, anything we do to slow down climate
change now will only show results in the future.
Part 4: Feedback effects
When the Earth warms up, a large number of changes take place in the
atmosphere, the oceans and on the land surface. Some of these changes
can, in turn, affect the temperature. These are called feedback effects.
Some of these feedback effects increase global warming, while others
reduce it.
Feedback from water vapor
Water vapor is one of the most important feedback effects. A slight warming of
the Earth due to more sunlight or an increased greenhouse effect, will lead to an
increase in the amount of water vapor in the atmosphere. As water vapor is also
a greenhouse gas, the extra water vapor will increase the greenhouse effect even
more, leading to even greater warming. Thus water vapor has an amplifying
effect on global warming.
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Feedback from snow and ice cover
The feedback effects from ice and snow-covered
surfaces are similar. When the climate is cold,
there is a lot of ice and snow on Earth. These
shiny surfaces reflect sunlight away from the
ground and make it even colder. A warmer
climate means less ice and snow. This leads to
less reflection of solar radiation to outer space
and increased warming.
1. Ice cover in the Arctic ocean
around Greenland. Photo: NASA.
Feedback from clouds
2. Clouds. Photo: The NOAA Photo Library
Another important feedback mechanism is
changes in the cloud cover. When it gets
warmer on Earth, the amount of water
vapor in the atmosphere increases and
more clouds may be formed. This can either
increase or decrease warming, depending
on what type of clouds they are. All clouds
both cool the Earth by reflecting sunlight
back into space and warm it up by
absorbing heat from the surface in the same
way that greenhouse gases do. Thin cirrus
clouds (which appear high up in the
atmosphere when the weather is
fine) generally have a warming effect. Low
cumulus and stratus clouds, on the other
hand, have a cooling effect. However, we
still don't know much about how climate
change will affect the formation of different
cloud types.
Part 5: Abrupt changes
The geological history of the Earth shows that there have been many abrupt
changes in climate in the past. During the last Ice Age, sudden climate changes
occurred about every thousand years.
Scientists have drilled deep into the ice on Greenland and have analyzed layers of
ice that have been there for tens of thousands of years. The analyses show
that the average temperature on Greenland changed several times by 8 to16 oC
over a short period of time – as little as a decade or two! The climate has been
far more stable since the Ice Age ended and there have only been moderate
climate variations since. These include the Little Ice Age in Europe between 1400
and 1850 when temperatures where much lower than average.
Sources of abrupt changes
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A gradual warming of the Earth – for example, due to stronger solar radiation or
an increased greenhouse effect – can lead to abrupt changes in the climate
system when a threshold is reached. For example, the abrupt climate changes
during the last Ice Age probably occurred when the large inflows of freshwater
from melting glaciers stopped flowing into the oceans changing ocean circulation
patterns. Scientists think it unlikely that we will see dramatic changes in ocean
currents in the future but they cannot rule out the possibility that the strength of
the ocean currents will change quickly and lead to rapid climate changes in
Europe.
Another possible source of abrupt climate change is the enormous amounts of the
greenhouse gas methane (CH4) trapped in the frozen ground in the Arctic. If
global warming causes the permafrost to thaw and the methane to be released,
this can lead to very rapid warming.
1. Abrupt changes: Reconstructed summer temperature in the Scandes Mountains, Sweden over the
last 10,000 years. The curve shows a quite abrupt cooling episode that took place approximately
8,200 years ago. This event is also seen in temperature reconstructions from other locations in
Europe and in ice cores from Greenland. It was probably caused by shifts in the ocean currents,
caused by huge amounts of freshwater which were released to the oceans when the melting ice caps
still left over North America after the Ice Age suddenly released huge amounts of water that had been
trapped in lakes behind the ice. The temperature reconstruction was possible because scientists know
what temperatures pine trees need to grow in the Scandes mountains. From plant remains, the
scientist determined the maximum height above sea level where pine trees could grow at various
times. A high limit for pine trees means a relatively warm climate. Source: Dahl and Nesje, The
Holocene 6(4) 381-398
Modelling abrupt changes
Climate models are best able to estimate gradual changes in climate resulting
from higher concentrations of greenhouse gases and are often unable to predict
abrupt changes. Calculating the likelihood and consequences of abrupt climate
changes is very uncertain – partly because we do not know exactly where the
“thresholds” lie, or what causes the abrupt changes. As a result we know little
about when, where and how abrupt climate changes resulting from a warmer
atmosphere will occur.
Consequences of abrupt changes
Sudden and unexpected climate changes often have serious consequences.
Abrupt changes do not allow us the time or opportunity to prepare. For animal
and plant life, abrupt changes are particularly serious, especially for species that,
for example, have long lifetimes, are not very mobile or are specially adapted to
one habitat. Sudden climate changes can give such species little time to find new
homes and they might, therefore, face extinction.
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