Changes to the Earth’s climate system caused by warming in the atmosphere and on the Earth’s surface is arguably the biggest threat to all of us in the twenty first century.
There is NO question from anybody that the planet IS warming, and that this poses serious problems (however, a very small segment of the population disagree with the broadly accepted scientific consensus that humans are the CAUSE of climate change)
What is Global Warming?
Global warming refers to the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and into the future. The global surface temperature MEASURABLY increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 years ending in 2005 
according to a report issued by Working Group I of the Intergovernmental Panel on Climate Change (IPCC).
What is a Greenhouse gas?
The IPCC concludes that most of the temperature increase since the mid-twentieth century is "very likely" the increase in man made greenhouse gas concentrations like carbon dioxide, methane and nitrous oxide. Because these molecules are physically larger than a lone Oxygen molecule, they are like the difference between a sheet on your bed and a winter duvet. They act like the walls of a greenhouse, trapping heat from the sun in the atmosphere. As the volume of greenhouse gases increases, more and more heat is trapped. Temperatures on Earth start to rise just as they do in a greenhouse. The two most problematic greenhouse gases are water vapor and carbon dioxide but greenhouse gases also include methane, ozone, chlorofluorocarbons, and nitrogen oxides. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities.
There is a lot more information available online; such as this Wikipedia
reference if you are interested.
We are responsible
Now carbon dioxide levels are at their highest levels in recorded history - purely as a result of man-made emissions such as the burning of oil and coal. The effects of climate change are evident throughout the world. They include:
- Rising Temperatures
- Glaciers receding at unprecedented speed
- Huge chunks of the Antarctic ice shelf breaking off to form giant icebergs in the Southern Ocean
- The retreat of the Arctic ice shelf
- Warmer seasons becoming longer
- Forest Fires
- Storms becoming more severe and causing ever-increasing levels of damage
If we continue to produce greenhouse gases at the current rate, sea levels will rise forcing millions of people out of their homes. And worse still, fertile agricultural lands will become deserts.
Climate change threatens to end our world as we know it. It will radically alter our environment and destroy our social and economic well-being.
The challenge for all of us is to change the way we do things.
We must reduce greenhouse gas emissions dramatically to slow down and eventually reverse the warming of the earth.
- "Summary for Policymakers" (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change