United States
Environmental Protection
Office of
Air and Radiation
EPA 430-F-00-011
April 2000
Global  Warming
and Our              n
    Answers to
           Frequently Asked
    A warming trend has been recorded since the late 19th century,
    with the most rapid warming occurring over the past two
    decades. If emissions of greenhouse gases continue unabated,
    scientists say we may change global temperature and our
    planet's climate at an unprecedented rate for our society.
           This brochure addresses the most frequently asked

           questions about the science of global warming and our

           changing climate. Answers are based on the

           assessments of the Intergovernmental Panel on Climate

           Change and on the most recent peer-reviewed

           scientific literature.

                                Is our  planet  warming?

How do we take
Earth's temperature?

Earth's temperature is taken through a
network of thermometers on ships, buoys
and land-based weather stations. The data
are compiled by organizations like the World
Meteorological Organization, NASA and the
U.S. National Oceanic and Atmospheric
Administration. This global temperature
record dates back to about 1860.  During
this period, measuring techniques have
changed, some weather stations relocated
and others became surrounded by cities.
Scientists have taken special care to
address these problems to ensure the
global temperature record is reliable and
consistent. To know what temperatures
were like before 1860, scientists must rely
on limited records or reconstruct Earth's
temperature history by examining tree rings,
pollen records and air locked away in
ancient ice.
The global temperature record shows an average warm-

ing of about 1 F over the past century (see graph).  This

warming has been recorded in both the northern and

southern hemispheres, and over the oceans, with some

areas substantially warmer and others actually cooler.

The ten warmest years have occurred since 1983, with

seven of them since 1990. Recent evidence shows the

20th century was the warmest in the last 1,000 years.

The 1990s were the warmest decade and 1998 was the

single warmest year of the past millennium.
                 Global Average Temperature
                                    1380   1894   1908   1922   1936   1950   1964   1978   1992 1999

                                    Source; NASA's Goddard Institute for Space Studies             "^-yefti surface a	.nln

Fundamentals of the Greenhouse Effect,
Global Warming, and Our Changing Climate

Q. What is the greenhouse effect?
A. Earth's greenhouse effect is a natural phenomenon that helps regu-
late the temperature of our planet. Simply put, the sun heats the Earth
and some of this heat, rather than escaping back to space, is trapped in
the atmosphere by clouds and greenhouse gases, such as water vapor
and carbon dioxide.  If all of these greenhouse gases were to suddenly
disappear, our planet would be 60F colder and uninhabitable.

Q. Are human activities  responsible for the warming?
A. Separating out the impact of human activity from natural climate
variation is extremely difficult.  Nonetheless, the  IPCC concluded there
is a "discernible human influence" on climate. This means the  observed
global warming is unlikely to be the result of natural variability alone
and that human activities are  at least partially responsible.

Q. What are the most important greenhouse gases?
   Where are they coming from?
A. Many greenhouse gases occur naturally, but human activities are
adding gases to the  natural mix at an unprecedented rate. Water vapor
                     The Greenhouse Effect
       Some solar radiation is
       reflected by the Earth and
       the atmosphere.
                            Solar radiation
                            passes through
                            the clear
Some of the infrared
radiation passes through
the atmosphere and some
is absorbed and re-emitted
in all directions by green-
house gas molecules. The
effect of this is to warm
the Earth's surface and the
lower atmosphere.
                                                                     What's the Intergovernmental Panel on
                                                                     Climate Change (IPCC)?

                                                                     The IPCC was formed jointly in 1988 by the United Nations
                                                                     Environment Program and World Meteorological
                                                                     Organization. The IPCC brings together the world's top
                                                                     scientists in all relevant fields, synthesizes peer-reviewed
                                                                     scientific literature on global warming studies, and pro-
                                                                     duces authoritative assessments of the current  state of
                                                                     knowledge of climate change.  The IPCC's Second
                                                                     Assessment (1996) serves as the key reference  for this
                                                                     brochure.  The IPCC's Third Assessment is scheduled for
                                                                     publication in 2001.
is the most abundant greenhouse gas; it occurs naturally and makes up
about two thirds of the natural greenhouse effect. Fuel burning and
other human activities, however, are adding large amounts of green-
house gases to the atmosphere  the most important ones being car-
bon dioxide  (C02), methane (CH4), nitrous oxide (N20), hydrofluorocarbons
(MFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Since pre-
industrial times atmospheric concentrations of C02, CH4 and N20 have
climbed by over 30%, 145% and 15%, respectively. Scientists have
confirmed this is primarily due to human activity. Burning coal, oil and
gas, and cutting down forests are largely responsible.

Q. What will happen to Earth's climate if emissions of
   these greenhouse gases continue to rise?

A. Because  human emissions of C02 and other greenhouse gases con-
tinue to climb, and because they remain in the atmosphere for decades
to centuries (depending on the gas), we're committing ourselves to a
warmer climate in the future.  The IPCC  projects an average global tem-
perature increase of 2-6F by 2100, and greater warming thereafter.
Temperatures in some parts of the globe (e.g., the polar regions) are
expected to rise even faster. Even the low end of the IPCC's projected
range  represents a rate of climate change unprecedented in the past
10,000 years.

Q. What are the potential impacts of global warming and
   a changing climate?

A. Our  health, agriculture,  water resources, forests, wildlife and coastal
areas are vulnerable to global warming and the climatic changes it will
bring.  The IPCC concluded that "climate change is likely to have wide-
ranging and mostly adverse impacts on human health, with significant
loss of  life." A few degrees of warming increases the chances of more
frequent and severe  heat waves, which can cause more heat-related
death and illness. Greater heat can also mean worsened air pollution, as
well as damaged crops and depleted water resources.  Warming is likely
to allow tropical diseases,  such as malaria, to spread northward in some
areas of the world. It will also intensify the Earth's hydrological cycle.
This means  that both evaporation and precipitation will increase. Some
areas will receive more rain, while other areas will be drier.  At the same
time, extreme events like floods and droughts are likely to become more
frequent. Warming will cause glaciers to melt and oceans to expand.
The IPCC projects that sea level will rise  one half foot to three feet over
the next century. This threatens low-lying coastal areas.  Scientists are
also concerned that warming could lead  to more intense storms.

Looking at global warming  in more detail...

Q, How serious is global warming of a few degrees?
   Couldn't warming be beneficial?

A. A warming of 1 F over the past century and a further 2-6F over the
21st century, as projected by the IPCC, may appear minor compared to
short-term weather changes from night to day and winter to summer.
But in global climate terms, a warming at this rate would be much larger
and faster than any of the climatic changes over the past 10,000 years.
Global temperatures  during the last ice age (about 20,000 years ago)
were 'only' 9F cooler than today, but that was enough to allow massive
ice sheets to reach as far south as the Great Lakes and New York City.
The warming that humans are causing will change Earth's climate in the
opposite direction, but tens or possibly a hundred times faster than nat-
ural rates of climate change.  Warming of a few degrees would lead to
more frequent droughts and heat waves, cause greater rainfall, and pos-
sibly change the strength of storms.  It is  possible that some areas
would benefit from global warming even as other areas were harmed.
Certain farming areas, for example, could enjoy a longer growing sea-
son, while others suffer from more frequent droughts.  Local effects,
however, are the most difficult to predict, making it difficult to know who
will  benefit and who will  not, and for how  long these conditions will per-
sist, as the warming  continues and the climate keeps changing.

Q. How can we talk about climate change over the next 100
   years when we can't be sure of tomorrow's weather?

A. Weather and climate are different. The methods used to forecast
changes in weather and climate differ as well. Because the weather
changes from day to day, current weather forecasts are reliable for
roughly ten days. Climate, on the other hand, can be thought of as
average weather, including weather's variability over much longer time
horizons (e.g., from year to year).  Natural changes in our planet's cli-
mate happen over the course of years, centuries and many millennia.
Long-term climate forecasts are possible because scientists understand
many of the factors that influence climate over such long periods, such
as changes in the sun's  energy and the level of greenhouse gases in
the  atmosphere.  Climate scientists do not claim to know how to predict
day-to-day fluctuations (weather) over the 21st century. Rather, they are
predicting how they think average temperature and precipitation (cli-
mate) will change due to human activities.

Q. How do we know C02 in the atmosphere is increasing?
   And that humans are responsible?

A. Careful measurements have confirmed that C02 is  increasing in the
atmosphere and that human activities are the primary  cause.  C02
measurements have been taken directly from  the atmosphere over the
past few decades. C02 trends for earlier times have been derived from
measurements of C02 trapped in air bubbles in glacial or polar ice. The
30% increase in atmospheric C02 observed since pre-industrial times
(shown in the graph  above) cannot be explained by natural causes.
C02 concentrations have varied naturally  throughout Earth's history.
However, C02 concentrations are now higher than any  seen in at least
the  past 450,000 years.
       Global Emissions and Atmospheric Concentration of COz
      1790   1815    1840   1865    1890   1915   1940   1975    1990
Q. Aren't other factors responsible for global warming?

A. Natural and human factors affect the average temperature of our
planet. Natural variability in the Earth's climate system can cause small
changes over decades to centuries.  Gradual changes in Earth's orbit
around the sun (which in turn change how sunlight hits our planet) are
thought to be the key pacemaker for the comings and goings of past
ice ages over many millennia. The sun's energy can also vary slightly
over time. Large volcanic eruptions can cool the planet for a few years
by spewing out particles that block some sunlight.  Even some of our
own pollutants, like the sulfur dioxide released from power plants and
heavy industry that contributes to acid rain, have a similar cooling
effect.  Depletion of the ozone layer  caused by our release of chloro-
fluorocarbons  has led to cooling of the upper atmosphere.
Scientists think these temporary cooling effects  have been masking
some of the long-term warming being caused by human emissions of
greenhouse gases. Over the 21st century, the ongoing  buildup of
greenhouse gases in the atmosphere is likely to be the most dominant
influence on  our planet's climate.

Q. How will global warming affect the polar ice caps?

A. Polar ice caps are some of the largest surface features on our planet
and any changes to them, however small, could  have far-reaching
effects.  Melting due to global warming is expected to reduce the size
and extent of the polar ice caps, even after taking into account the
potential for more snow and ice accumulation atop the ice sheets due to
increased precipitation. Melting of polar ice and land-based glaciers is
expected to contribute to the one half foot  to three-feet sea level rise
projected by the IPCC for the 21st century.  Shrinking ice caps may also
cause changes in ocean circulation and even storm tracks. To be sure,
not all of the  melting currently occurring is due to global warming, and
the melting of floating sea ice does not affect sea level.  Further warm-
ing will likely accelerate the shrinkage of ice caps and glaciers, however.
Of particular concern is the stability of the West Antarctic Ice Sheet. A
sudden collapse would raise sea levels  16-20 feet but the IPCC consid-
ers the likelihood of such a collapse before the year 2100 low.

I've heard these things about global warming...

Q. I've heard satellite data contradict other evidence of
   global warming.

A. Until recently, conflicting studies suggested that temperatures mea-
sured by satellites revealed a slight cooling trend, whereas the surface
temperature record showed a warming trend. This confounded the
global warming issue. Satellites began measurements in 1979 and the
surface temperature record, which reveals global warming of about 1 F,
dates back to about 1860. Satellites take temperatures through vertical
slices of the atmosphere, not at the surface.  Satellite measurements
are also known to be influenced by ozone layer depletion, which has
caused cooling of the upper atmosphere. For these and other reasons,
satellite  and surface data are not expected to be a perfect match.
Nevertheless,  the discrepancy between the two data sets was too large
to be ignored. Then scientists discovered they neglected some mea-
surement and calibration problems with the satellites, including the fact
that satellites  were falling from their orbits, which produced an artificial
cooling trend. Correcting the satellite data for these problems revealed
a small warming trend.  These corrections  though not the  last word
on the satellite vs. surface discrepancy  bring the satellite record into
better agreement with surface measurements. Any remaining discrep-
ancies do not invalidate  the fact that surface temperatures are rising.

Q. I've heard more C02 in the air could  tie beneficial tor
   plants and crops.

A. The impacts of climate change on crops and  vegetation depend on
complex interactions among  increased C02, rising temperatures, and
water and  nutrient availability. Elevated levels of C02 can essentially
fertilize plants and crops. However, plant growth is also affected by
other factors in addition  to C02  factors that will be influenced by cli-
mate change. Modest temperature increases, for example, can
enhance growth,  but if temperatures increase too much, growth actually
declines. Rising temperatures also increase the process by which
plants release C02. Higher temperatures can increase the rate of evap-
oration, drying out soils.  Insufficient water decreases plant growth.
Plants also cannot respond to more C02 unless sufficient nutrients are
available.  Furthermore,  the growth-enhancing effects of C02 may
diminish over  time. Real-world crop yields would also be subject to the
hazards of droughts and floods under a changing climate.

Q. I've heard C02 emissions  from  human activities are
   small compared to what's released tiy nature.

A. The Earth has a natural C02 cycle that  moves  massive amounts of
C02 into and out of the atmosphere. The  oceans and land vegetation
release and absorb over  200 billion metric tons of carbon into  and out of
the atmosphere each year. When the cycle is balanced, atmospheric
levels of C02 remain relatively stable.  Human activities are now adding
about 7  billion metric tons of carbon into the atmosphere every year,
which is only about 3-4% of the amount exchanged naturally.  But that's
enough to  knock the system out of balance, surpassing nature's ability to
take our C02 emissions out of the atmosphere. The oceans and land
vegetation  are absorbing about half of our emissions; the other half
remains airborne  for 100 years or longer.  This is what is causing the
rapid buildup of C02, a buildup that dwarfs natural fluctuations.
Here are some other questions you may have...
Q. How do scientists predict future climate?

A. Basic physics tells us that greenhouse gases trap Earth's heat and
cause warming. But Earth's climate is very complex, involving interac-
tions among the air, land and oceans. That's why scientists use com-
puter models to project the effects of global warming. Though the
models are far from exact, they do a reasonable job of simulating our
current climate and reproducing known changes from past climates.
Scientists are confident about the models' abilities to simulate large-
scale effects of global warming, such as global temperature increase
and average sea level rise. The models are less reliable when it comes
to simulating changes in other weather variables such as changes in
rainfall.  Also, current models are still ill-equipped to predict with any
confidence what will happen in local areas.

Q. Can you say the recent extreme weather is a
   manifestation of climate change?

A. Given our knowledge of global warming and our changing climate,
we can expect more extreme weather, including more frequent hot days
and droughts, less frequent cold days, and more precipitation (including
more snowfall in cold areas). But attributing any particular extreme
weather event to global warming remains beyond the current limits of
scientific capability.

Q. Does El Nino have anything to do with global warming
   ana climate change?

A. El Nino  is a natural phenomenon that has been occurring throughout
the centuries, though not always with the same regularity; it now occurs
about every two to seven years. El Nino is the strong warming of the
equatorial  Pacific ocean.  Its effects are felt worldwide, which demon-
strates the interconnected nature of the  Earth's climate.  Recent El Nino
events have been very strong and  have contributed to the record-set-
ting temperatures of the 1990s  evidence that El Nino events can
warm parts of the Earth.  But now scientists are examining how human-
induced global warming could affect El Nino.  Scientists are concerned
that the  accumulation of greenhouse gases in our atmosphere may
inject enough heat into the  Pacific Ocean to make El Nino events more
frequent and fierce.

Q. Does ozone layer depletion have anything to do with
   global warming and climate change?

A. The human health and environmental concerns about ozone layer
depletion are different from the risks we face from global warming.
Nevertheless, the two phenomena are related in certain ways.  Some pol-
lutants contribute to both problems and both alter the global atmosphere.
    Ozone layer depletion allows more harmful ultraviolet (UV) radiation
to reach our planet's surface. While increased UV radiation is not the
cause of global warming, it can  lead to skin cancers, cataracts and a
suppressed immune system in humans, as well as reduced yields for
crops. Ozone layer depletion is  mainly caused by chlorofluorocarbons
or CFCs. CFCs are therefore no longer produced in industrialized coun-
tries, and will eventually be eliminated worldwide.  But like C02, CFCs
are also a strong greenhouse gas.  CFCs can remain in the atmosphere
for as long as a century, meaning that their contribution to both ozone
layer depletion and climate change  will persist for  a long time.
You, your business and global warming...
Q. How might global warming affect my health and
A. Exactly how global warming will impact individual locations, let alone
individuals, is uncertain. But because global temperatures, rainfall, sea
levels and the frequency of extreme weather are expected to increase,
you could be affected in many ways.
  A lot of attention is paid to estimating the cost of reducing
  greenhouse gas emissions. But what is often overlooked is
  that global warming and climatic changes can  themselves
  impose economic  as well as health and environmental  
  costs on people and businesses.
For more  information and
to learn what you can do...
Here are some other websites
we recommend
Intergovernmental Panel on Climate Change:

U.S. Global Change Research Program:

NASA's Goddard Institute for Space Studies:

NOAA's National Climatic  Data Center:
    Your health and comfort could be affected if your region experi-
ences more frequent heat waves and worse air pollution.  These health
concerns are especially serious if you are or care for the very young,
very old, or if you have heart and respiratory problems. In the winter-
time you may feel milder temperatures. You may pay higher energy
bills for air conditioning in summer, and lower bills for heating in winter.
If you live  in the country's interior, particularly in dry areas, water short-
ages may be more frequent, leading  to more restrictions on your water
usage.  If you live along the coast, your home may be threatened by sea
level rise and  an increase in storm intensity. The cost of food may
change as farmers and the food industry adapt to new climate patterns.
And the outdoor activities that you and your family enjoy could be
affected by increased beach erosion, decreased snow fall and retreating
glaciers, and loss of forests and wildlife, where species are unable to
adapt to the changing climate.

Q. How might global warming affect my business?

A. As a business owner, your costs, competition, and planning decisions
may be affected. Your health care costs could increase if the public
health sector is burdened by increases in heat and climate-related mor-
tality and illness. Like homeowners, your business's energy costs will
reflect the need for greater cooling in  the summer and less heating in the
winter. Your property insurance premiums could go up due to more
droughts and floods and possibly more intense storms. If your business
is located along the coast, sea level rise may also affect property insur-
ance, not to mention how rising seas may directly impact your business.
If your business depends on waterways for transportation, those shipping
costs could increase in some areas due to reduced river flow and lower
lake levels, though in northern areas shipping could be eased by a longer
ice-free season. If you're in the agricultural or food industry, changing cli-
matic and growing season conditions  will require adaptations. Your com-
petitors in this sector may experience  either more or less favorable cli-
matic changes than you.  The same is true if you're in a forestry-related
business.  Some of global warming's impacts may be  most severe in
other nations less capable of adapting. This may create social  and eco-
nomic disruptions that ripple across the globe to affect your business.
For all of these reasons, long-term business planning will increasingly
have to consider the changing nature  of our planet's climate.