1) Global warming is caused by increased levels of greenhouse gases like carbon dioxide trapping heat in the lower atmosphere. The thin layer of atmosphere where climate occurs can be influenced by human activity. 2) Human activities like burning fossil fuels have increased atmospheric CO2 levels by about 30% since the industrial revolution, contributing to observed rising global temperatures according to computer models. 3) Potential impacts of unmitigated climate change include rising sea levels, worsening extreme weather, melting ice caps and glaciers, and damage to ecosystems. Reducing emissions from coal power plants, increasing renewable energy and energy efficiency can help address the problem.

1. Global Warming
Introduction : Global Warming is the increase of Earth's average surface temperature due to effect of
greenhouse gases, such as carbon dioxide emissions from burning fossil fuels or from deforestation, which
trap heat that would otherwise escape from Earth. This is a type of greenhouse effect.
Is global warming, caused by human activity, even remotely plausible?
Earth's climate is mostly influenced by the first 6 miles or so of the atmosphere which contains most of the
matter making up the atmosphere. This is really a very thin layer if you think about it. In the book The End
of Nature, author Bill McKibbin tells of walking three miles to from his cabin in the Adirondack's to buy
food. Afterwards, he realized that on this short journey he had traveled a distance equal to that of the layer
of the atmosphere where almost all the action of our climate is contained. In fact, if you were to view Earth
from space, the principle part of the atmosphere would only be about as thick as the skin on an onion!
Realizing this makes it more plausible to suppose that human beings can change the climate. A look at the
amount of greenhouse gases we are spewing into the atmosphere (see below), makes it even more plausible.

2. What are the Greenhouse Gases?
The most significant greenhouse gas is actually water vapor, not something produced directly by humankind
in significant amounts. However, even slight increases in atmospheric levels of carbon dioxide (CO2) can
cause a substantial increase in temperature.
Why is this? There are two reasons: First, although the concentrations of these gases are not nearly as large as
that of oxygen and nitrogen (the main constituents of the atmosphere), neither oxygen or nitrogen are
greenhouse gases. This is because neither has more than two atoms per molecule (i.e. their molecular forms
are O2 and N2, respectively), and so they lack the internal vibrational modes that molecules with more than
two atoms have. Both water and CO2, for example, have these "internal vibrational modes", and these
vibrational modes can absorb and reradiate infrared radiation, which causes the greenhouse effect.
Secondly, CO2 tends to remain in the atmosphere for a very long time (time scales in the hundreds of years).
Water vapor, on the other hand, can easily condense or evaporate, depending on local conditions. Water
vapor levels therefore tend to adjust quickly to the prevailing conditions, such that the energy flows from the
Sun and re-radiation from the Earth achieve a balance. CO2 tends to remain fairly constant and therefore
behave as a controlling factor, rather than a reacting factor. More CO2 means that the balance occurs at higher
temperatures and water vapor levels.

3. How much have we increased the
Atmosphere's CO2 Concentration?
Human beings have increased the CO2 concentration in the atmosphere by about thirty percent, which is an
extremely significant increase, even on inter-glacial timescales. It is believed that human beings are
responsible for this because the increase is almost perfectly correlated with increases in fossil fuel combustion,
and also due other evidence, such as changes in the ratios of different carbon isotopes in atmospheric CO2 that
are consistent with "anthropogenic" (human caused) emissions. The simple fact is, that under "business as
usual" conditions, we'll soon reach carbon dioxide concentrations that haven't been seen on Earth in the last
50 million years.
Combustion of Fossil Fuels, for electricity generation, transportation, and heating, and also the manufacture
of cement, all result in the total worldwide emission of about 22 billion tons of carbon dioxide to the
atmosphere each year. About a third of this comes from electricity generation, and another third from
transportation, and a third from all other sources.
This enormous input of CO2 is causing the atmospheric levels of CO2 to rise dramatically. The following
graph shows the CO2 levels over the past 160 thousand years (the upper curve, with units indicated on the
right hand side of the graph).

4. Is the Temperature Really
Changing?
Yes! As everyone has heard from the media, recent years have consistently been the warmest in
hundreds and possibly thousands of years. But that might be a temporary fluctuation, right? To
see that it probably isn't, the next graph shows the average temperature in the Northern
Hemisphere as determined from many sources, carefully combined, such as tree rings, corals,
human records, etc.

5. Is the Temperature Really
Changing?
These graphs show a very discernable warming trend, starting in about 1900. It might seem a bit surprising
that warming started as early as 1900. How is this possible? The reason is that the increase in carbon dioxide
actually began in 1800, following the deforestation of much of Northeastern American and other forested
parts of the world. The sharp upswing in emissions during the industrial revolution further added to this,
leading to a significantly increased carbon dioxide level even by 1900.
Thus, we see that Global Warming is not something far off in the future - in fact it predates almost every
living human being today.

6. How do we know if the temperature
increase is caused by anthropogenic
emissions?
Computer models strongly suggest that this is the case. The following graphs show that 1) If only natural
fluctuations are included in the models (such as the slight increase in solar output that occurred in the first
half of the 20th century), then the large warming in the 20th century is not reproduced. 2) If only
anthropogenic carbon emissions are included, then the large warming is reproduced, but some of the
variations, such as the cooling period in the 1950s, is not reproduced (this cooling trend was thought to be
caused by sulfur dioxide emissions from dirty power plants). 3) When both natural and anthropogenic
emissions of all types are included, then the temperature evolution of the 20th century is well reproduced.

7. Who are the IPCC?
In 1998, the Intergovernmental Panel on Climate Change (IPCC) was established by the World Meteorological
Organization (WMO) and the United Nations Environment Programme (UNEP), in recognition of the threat
that global warming presents to the world.
The IPCC is open to all members of the UNEP and WMO and consists of several thousand of the most
authoritative scientists in the world on climate change. The role of the IPCC is to assess the scientific,
technical and socio-economic information relevant for the understanding of the risk of human-induced
climate change. It does not carry out new research nor does it monitor climate related data. It bases its
assessment mainly on published and peer reviewed scientific technical literature.
The IPCC has completed two assessment reports, developed methodology guidelines for national greenhouse
gas inventories, special reports and technical papers. Results of the first assessment (1990--1994): confirmed
scientific basis for global warming but concluded that ``nothing to be said for certain yet''. The second
assessment (1995), concluded that `` ...the balance suggests a discernable human influence on global climate'',
and concluded that, as predicted by climate models, global temperature will likely rise by about 1-3.5 Celsius
by the year 2100. The next report, in 2000, suggested, that the climate might warm by as much as 10 degrees
Fahrenheit over the next 100 years, which would bring us back to a climate not seen since the age of the
dinosaurs.

8. Global Warming Impacts
Many of the following "harbingers" and "fingerprints" are now well under way:
Rising Seas--- inundation of fresh water marshlands (the everglades), low-lying cities, and islands with
seawater.
Changes in rainfall patterns --- droughts and fires in some areas, flooding in other areas. See the section
above on the recent droughts, for example!
Increased likelihood of extreme events--- such as flooding, hurricanes, etc.
Melting of the ice caps --- loss of habitat near the poles. Polar bears are now thought to be greatly
endangered by the shortening of their feeding season due to dwindling ice packs.
Melting glaciers - significant melting of old glaciers is already observed.
Widespread vanishing of animal populations --- following widespread habitat loss.
Spread of disease --- migration of diseases such as malaria to new, now warmer, regions.
Bleaching of Coral Reefs due to warming seas and acidification due to carbonic acid
formation --- One third of coral reefs now appear to have been severely damaged by warming seas.

9. Where do we need to reduce emissions?
In reality, we will need to work on all fronts - 10% here, 5% here, etc, and work to phase in new technologies,
such as hydrogen technology, as quickly as possible. To satisfy the Kyoto protocol, developed countries
would be required to cut back their emissions by a total of 5.2 % between 2008 and 2012 from 1990 levels.
Specifically, the US would have to reduce its presently projected 2010 annual emissions by 400 million tons of
CO2 . One should keep in mind though, that even Kyoto would only go a little ways towards solving the
problem. In reality, much more needs to be done.
The most promising sector for near term reductions is widely thought to be coal-fired electricity. Wind power,
for example, can make substantial cuts in these emissions in the near term, as can energy efficiency, and also
the increased use of high efficiency natural gas generation.
The potential impact of efficiency should not be underestimated: A 1991 report to Congress by the U.S.
National Academy of Sciences, Policy Implications of Greenhouse Warming, found that the U.S. could reduce
current emissions by 50 percent at zero cost to the economy as a result of full use of cost-effective efficiency
improvements.