Download How Do We Know that Humans Are the Major Cause of Global

How Do We Know that
Humans Are the Major
Cause of Global
Warming?
http://www.ucsusa.org/global_warming/science_and_impa
cts/science/human-contribution-to-gwfaq.html#.VqY46lK3vGs
How do we know that humans are the
major cause of global warming?
The Fourth Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC)
states: it is a greater than a 90 percent certainty that
emissions of heat-trapping gases from human
activities have caused “most of the observed
increase in globally averaged temperatures since the
mid-20th century.”[1] We all know that warming—
and cooling—has happened in the past, and long
before humans were around. Many factors (called
“climate drivers”) can influence Earth’s climate—
such as changes in the sun’s intensity and volcanic
eruptions, as well as heat-trapping gases in the
atmosphere.
So how do scientists know that today’s warming is
primarily caused by humans putting too much
carbon in the atmosphere when we burn coal, oil,
and gas or cut down forests?


There are human fingerprints on carbon
overload. When humans burn coal, oil and
gas (fossil fuels) to generate electricity or
drive our cars, carbon dioxide is released
into the atmosphere, where it traps heat. A
carbon molecule that comes from fossil fuels
and deforestation is “lighter” than the
combined signal of those from other
sources. As scientists measure the “weight”
of carbon in the atmosphere over time they
see a clear increase in the lighter molecules
from fossil fuel and deforestation sources
that correspond closely to the known trend
in emissions.[2,3]
Natural changes alone can’t explain the
temperature changes we’ve seen. For a
computer model to accurately project the
future climate, scientists must first ensure

that it accurately reproduces observed
temperature changes. When the models
include only recorded natural climate
drivers—such as the sun’s intensity—the
models cannot accurately reproduce the
observed warming of the past half century.
When human-induced climate drivers are
also included in the models, then they
accurately capture recent temperature
increases in the atmosphere and in the
oceans.[4,5,6] When all the natural and
human-induced climate drivers are
compared to one another, the dramatic
accumulation of carbon from human sources
is by far the largest climate change driver
over the past half century.
Lower-level atmosphere—which contains
the carbon load—is expanding. The
boundary between the lower atmosphere
(troposphere) and the higher atmosphere
(stratosphere) has shifted upward in recent
decades. See the ozone FAQ for a figure
illustrating the layers of the
atmosphere.[6,7,8] This boundary has likely
changed because heat-trapping gases
accumulate in the lower atmosphere and that
atmospheric layer expands as it heats up
(much like warming the air in a balloon).
And because less heat is escaping into the
higher atmosphere, it is likely cooling. This
differential would not occur if the sun was
the sole climate driver, as solar changes
would warm both atmospheric layers, and
certainly would not have warmed one while
cooling the other.
Direct evidence of human contribution to
atmospheric CO2
Carbon dioxide (CO2) is the main heat-trapping gas
largely responsible for most of the average warming
over the past several decades.[2] To compare how
CO2 stacks up in influence to the many other
important heat-trapping gases contributing to
climate change see the CO2 FAQ. There is a way
that scientists can tease apart the atmospheric
concentration of CO2 to see how much of the CO2
is from natural sources and how much is from
combusted fossil fuel sources.
The atmospheric concentration of CO2 has
increased from a pre-industrial era (AD 1000 –
1750) concentration of approximately 280 parts per
million (ppm) to around 383 ppm, as measured at
Mauna Loa, Hawaii in 2007.[2,9] The carbon in the
atmospheric CO2 contains information about its
source, so that scientists can tell that fossil fuel
emissions comprise the largest source of the
increase since the pre-industrial era.
Here’s how scientists know. The same elements (i.e.
same number of protons in the nucleus) with
different mass numbers (arising from the different
numbers of neutrons in the nucleus) are called
isotopes. Each carbon molecule has six protons in
the nucleus, but there are many different isotopes
with varying numbers of neutrons in the
nucleus.[10] Carbon isotopes from different sources
are “lighter” (high negative value) or heavier (lower
negative value). For example, carbon from ocean is
the standard with a value of “0” while carbon from
fossil fuels ranges from -20 to -32.[11] While
atmospheric carbon has an average value of -5 to -9,
it is becoming “lighter” over time as carbon from
fossil fuels become more abundant in the
atmosphere (Figure 1).[9,11,12]
Figure 1. Direct Evidence of Fossil Fuel Derived
CO2 in the Atmosphere
The combination of natural drivers plus
human drivers best match reality
(1) Natural and Human Factors that Influence the
Climate (known as “climate drivers”)
Many natural and human factors influence climate.
Natural factors include the energy from the sun;
periodic volcanic eruptions of tiny particles, dust,
and salt spray—all known as aerosols— many that
can reflect sunlight; and natural carbon cycle
processes such as termite mounds in Africa that
emit methane or tiny organisms in the ocean surface
that take up carbon dioxide. Human climate drivers
include heat-trapping emissions from burning coal,
gas and oil in power plants and cars; cutting down
and burning forests; tiny pollution particles known
as aerosols; black carbon pollution more commonly
referred to as soot; and changes in land use that
change how much the Earth’s surface reflects the
sun’s energy back into space (referred to as albedo).
Some of these climate drivers result in net warming
and others lead to cooling, but all are usually
expressed as Radiative Forcing (RF) in units of
watts per square meter. When all the natural and
human-induced climate drivers are stacked up and
compared to one another, the accumulation of
human-released heat-trapping gases in the
atmosphere is so large that it has very likely
swamped other climate drivers over the past half
century, leading to observed global warming (see
Figure 2).[2,4,5]
Figure 2. Twentieth Century History of Climate
Drivers
Heat-trapping emissions (greenhouse gases) far
outweigh the effects of other drivers acting on
Earth’s climate. Source: Hansen et al. 2005, Figure
adapted by Union of Concerned Scientists [5]
(2) Cooling that partially offsets recent warming:
“Global Dimming”
only natural climate drivers do not recreate the
observed trends very well.
Figure 3. Combined Natural Plus Human
Drivers Best Match Observed Atmospheric
Temperature
The figure above also depicts the sharp cooling
influence a large volcanic eruption can have as it
spews tiny particles high into the stratosphere (the
layer of the atmosphere above the troposphere
where weather typically occurs). The massive
explosions from Krakatoa (Indonesia) in 1883 and
Mount Pinatubo (Philippines) in 1991, for example,
can be seen as the two largest downward spikes in
the blue volcanic data depicted in the figure. These
particles prevented the full energy of the sun from
reaching the surface of Earth and thus created a
cooling trend for several years.
Fossil fuel burning by humans also emits tiny
particles. Some particles reflect sunlight back to
space (aerosols), similar to the volcanic particles;
other pollution particles such as soot (black carbon)
absorb the sunlight, which leads to local warming of
the atmosphere level where the soot particles
circulate (see Aerosols FAQ). Both types of humancreated particles lead to a decrease in the amount of
sun’s energy reaching the surface of Earth. The
term “global dimming” has been used to describe
this phenomenon. There very likely would have
been even more warming in the past 60 years if it
were not for these human-made and natural tiny
particles.[5]
(3) Observed temperature changes on land and
ocean compared with natural and human climate
drivers
The IPCC has carefully documented observed
changes in air temperature, ocean temperature, ice
retreat, and sea level rise over the past century.
These observed changes are then compared in
climate computer models with natural climate
drivers and human climate drivers. The IPCC
concluded that the observed changes are unlikely to
be the result solely of natural processes. [4,6]
Figures 3 and 4 illustrate several examples from
climate models that show that the combination of
natural and human climate drivers closely match
observed historical trends. Conversely, the
corresponding figures from climate models using
Figure 4. Combined Natural Plus Human
Drivers Best Match Observed Ocean
Temperature
The sky is rising!
The boundary between the lower atmosphere
(troposphere) and the upper atmosphere
(stratosphere) has shifted upward in recent decades.
This boundary is called the tropopause. Similar to
the cases outlined above, models that have both
natural and human climate drivers match the
observed change in tropopause height better than
models with only natural climate drivers.
The likely cause of the rise in tropopause height is
from heat-trapping gases accumulating in and
heating up the troposphere and conversely blocking
heat from getting into the stratosphere, thus causing
cooling there.[6,7,8] Decreased ozone in the
stratosphere also adds to this cooling stratosphere
trend (see ozone FAQ.) This would not occur if the
sun was the sole climate driver, as solar changes
would have warmed both the stratosphere and the
troposphere.
One way to think about this is how when air of a hot
air balloon heats up it expands and the top of the
balloon rises. The same general idea can be applied
here—the troposphere volume expands and rises as
it warms, since the boundary between the
troposphere and the stratosphere is in part defined
by a change in temperature.
http://ncse.com/climate/climate-change-101/howmuch-human-responsibility-for-climate-change
How Much Does Human
Activity Affect Climate Change?

January 5th, 2012
The Earth’s climate is changing rapidly. Scientists
trying to find out what’s causing climate change
work like detectives, gathering evidence to rule out
some suspects and to ascertain just who is
responsible. It’s clear, based on over a century of
scientific investigation, that humans are responsible
for most of the climate change we’ve seen over the
last 150 years.
Humans are not the only suspects. The climate has
changed throughout the Earth’s history, well before
humans evolved. The Sun is the primary driver of
the climate. Roughly speaking, global temperatures
rise when more energy from the Sun enters the
atmosphere than returns to space through the
atmosphere. The climate cools any time more
energy returns to space than comes in from the Sun.
While humans can influence that balance, other
factors, from continental drift and changes in the
shape of the Earth’s orbit to variations in the Sun’s
activity and phenomena like El Niño, can all
influence the climate. Considering the pace of
climate change today, scientists can rule out most of
those suspects: some happen too slowly to explain
current climate change, while others move in small
cycles, not long trends, and others only influence
the climate in part of the planet. Scientists know
about these factors and can account for them when
assessing human-caused climate change.
Carbon dioxide is rising because of human
actions: Scientists can measure the increase of
atmospheric carbon dioxide over the last 150 years.
By comparing the type of carbon being added to the
atmosphere, they see that the kind of carbon
released by burning coal, gasoline, and natural gas
is diluting the naturally-occurring carbon dioxide in
the atmosphere. Data from Trudinger, et al (1999).
Image by
Steve
Newton for
NCSE, in
2012.
The potential
for human
beings to
alter climate
was first
proposed
over a
century ago, building on research in the 1850s by
John Tyndall. Light from the Sun warms the Earth’s
surface, which then gives off energy as infrared
radiation, the heat you feel from asphalt on a sunny
day. Greenhouse gases such as water vapor and
carbon dioxide (CO2), absorb this radiating energy,
heating the atmosphere and the surface. This
process results in the Earth’s temperature being
warmer than it would if it were heated only by
direct sunlight.
For over 100 years, scientists have regarded humans
as the prime suspect in current climate changes.
Around the turn of the 20th century, Svante
Arrhenius was the first to suggest that people could,
through the burning of coal, increase the amount of
greenhouse gases in the atmosphere and amplify the
natural warming effect, thereby causing the
atmosphere to warm more than it would through
strictly natural processes.
When humans burn gasoline, coal, natural gas, and
other common fuels to make electricity or drive
cars, they release a substantial amount of carbon
dioxide into the atmosphere. For every gallon (or
liter) of gasoline your car burns, 1300 times that
volume of CO2 is released (a gallon of gas weighs
about 6 pounds or 2.8 kilograms, but the released
CO2 would weigh over 19 pounds or 8.75
kilograms). Greenhouse gases are emitted from
power plants and cars, but also from landfills, from
farms and cleared forests, and through other subtle
processes. An interactive map from the
Environmental Protection Agency shows US
sources of key greenhouse gases. The World
Resources Institute cataloged (PDF) global sources
of greenhouse gases in 2005.
In the 1950s, scientists began methodically
measuring global increases in carbon dioxide. Since
then they’ve been able to confirm that the increase
has been caused primarily from the burning of fossil
fuels (and through other human activities, such as
clearing land, as well). This increase, and changes
in the type of CO2 being added to the atmosphere
provide the “smoking gun” that shows that humans
are responsible for the increased levels of carbon
dioxide in the atmosphere.
Modeling the Climate System
To clinch the case that climate change is mostly
caused by humans, scientists had to take into
account other factors: complicated atmospheric
physics, the interactions between air and land and
between air and water, changing amounts of ice and
of desert and forest, and the natural processes that
have changed the climate for 4.54 billion years. To
do all this, scientists recreate the crime scene.
Because there’s only one Earth, they do that with
computers. Climate scientists use powerful
computers to construct models based on physics of
the climate system. These models enable scientists
to make predictions and test hypotheses about what
processes affect the climate.
Modeling the
flow of water
through the
ocean: To
model climate
change,
scientists
incorporate
complex models
of the fluid dynamics of the ocean and atmosphere,
such as this image from a model of the ocean
around Africa.
In constructing such models, climate scientists start
with basic science: thermodynamic principles,
orbital dynamics, interactions between infrared
radiation and carbon dioxide and other gases, and
other such factors influencing the basic balance of
heat entering and leaving the atmosphere. They
combine these with actual measurements of data,
such as the current concentration of greenhouse
gases in the atmosphere. And they test hypotheses
about which processes affect which phenomena.
They can then see what the resultant models predict
about the climate system. When the predictions
match observations, it counts as a confirmation of
the model and its underlying assumptions. When the
predictions diverge from what is observed, climate
scientists revise their hypotheses. By such a trialand-error process, they are able to ensure that their
hypotheses become more and more accurate and
reliable.
Such models can be applied to the Earth as a whole
or to different regions of the planet. To see how
climate change affects different parts of the Earth,
climate scientists break the models down into
smaller pieces, calculate how small parts of the
Earth’s surface react to the Sun and greenhouse
gases, and then connect those small parts together,
based on measurements of how the atmosphere and
ocean interact. To ensure the accuracy of the
models at projecting future climate trends, the
models are often run backwards in time to
“retrodict” past climate changes, and then compared
with paleoclimate observations. The models through
this process have become remarkably accurate and
give the climate research community confidence
that the future projections are robust.
Scientists can do experiments with these models
that they can’t do on the planet. They can set the
atmosphere to match conditions a century ago, and
see whether the model’s predictions match what
scientists measured at the time. And they can set the
models to match conditions millions of years ago, to
better understand how past climate changed. This
allows them to verify that the models are accurate
and to fine-tune the output. They can also remove
the effects of human activities from the models, and
see how much of the climate change they observe is
still predicted by the models. These models, which
are carefully vetted by other experts, agree that
human activities, especially the burning of fossil
fuels and resulting release of carbon dioxide into the
atmosphere, are now substantially altering the
planet’s climate, as summarized by the
Intergovernmental Panel on Climate Change. In
many different sorts of climate models, from teams
competing to produce the models that most
accurately predict past climates, the consistent
result is that humans are producing most of the
climate change that we've observed in the last
century. One recent analysis, reported in Scientific
American, estimated that at least three quarters of
climate change since 1950 is due to human
activities.