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Without it,
scientists estimate
that the average temperature on Earth
would be colder by approximately 30 degrees Celsius,
far too cold to sustain our current ecosystem.
Second,
we need to clarify the way humans contribute to the greenhouse effect.
While the greenhouse effect
is an essential environmental prerequisite
for life on Earth,
there really can be too much of a good thing.
The problems begin when human activities distort and accelerate the natural process
by creating more greenhouse gases in the atmosphere
than are necessary to warm the planet to an ideal temperature,
for example,
burning natural gas,
coal and oil,
including gasoline for automobile engines,
raises the level of carbon dioxide in the atmosphere.
Some farming practices and land-use changes
increase the levels of methane and nitrous oxide.
Many factories produce long-lasting industrial gases
that do not occur naturally,
yet contribute significantly to the enhanced greenhouse effect
and global warming that is currently under way.