Carbon dioxide is a greenhouse gas.

This means Carbon Dioxide traps heat from the sun in the atmosphere. Most of the time this is beneficial to us, because without greenhouse gases to trap heat, the planet would be a lot colder.

95% of Carbon Dioxide emissions are from natural sources

Natural decay of organic material releases 8x more carbon dioxide than human beings

This greenhouse gas is released to the atmosphere by a variety of natural sources, and over 95% of total CO2 emissions would occur even if humans were not present on Earth.
For example, the natural decay of organic material in forests and grasslands, such as dead trees, results in the release of about 220 gigatonnes of carbon dioxide every year. This is over 8 times the amount emitted by humans.

Natural sources of atmospheric carbon dioxide further include volcanic outgassing and the respiration processes of living organisms. (When we breathe we exhale carbon dioxide, and so do most other living beings except plants which absorb carbon dioxide and release oxygen.)

So why are we so worried about the tiny percentage of CO2 emitted by humans?

Although natural sources represent most CO2 emissions, they do not contribute to the recent observed increase in concentrations because natural sources are balanced by natural sinks that remove this greenhouse gas from the atmosphere.
The increase in carbon dioxide concentration arises because the increase from human activity is not completely balanced by a corresponding sink.

Why is an increase in carbon dioxide worrisome?

The analysis of ice cores clearly shows a relationship between atmospheric carbon di0xide levels and global temperatures: The higher the concentration of this gas, the warmer the planet. The graphic below shows this relationship over the past 400,000 years. Temperature variations are in blue and carbon dioxide concentration in the atmosphere in red.

Temperature and atmospheric carbon dioxide correlation

Not only did they discover this relationship between carbon dioxide levels and temperature, they also discovered that up until now, even during the warmest periods between ice ages, carbon dioxide levels never exceeded three hundred parts per million.

Temperature and atmospheric carbon dioxide correlation

Carbon dioxide levels are now higher than at any point in human history.

The oldest human fossil found (named Lucy and found in Ethiopia) has been dated back to between 3 and 3.6 million years ago. However, Lucy was not a Homo Sapiens.
The earliest Homo Sapien found to date is 200,000 years old.
It’s still possible a Homo Sapien could be found that dates back 400,000 years or longer, but at the moment that’s all we know: Humans as we know them, have walked the Earth for no more than 200,000 years.
Considering that Dinosaurs were on the Earth for 250 million years, it’s truely remarkable what humans have achieved in only 200,000 years!
Unfortunately not all human achievements are good.

Rise of carbon dioxide since the Industrial Revolution

Today carbon dioxide levels are more than four hundred parts per million, far above carbon dioxide levels ever registered since human beings evolved.
This enormous rise has taken place in a very short time. Shorter ice cores from Antarctica and Greenland have shown that between 1000 AD and the mid-1800s Carbon Dioxide levels remained fairly constant at about two hundred ninety parts per million.
Since then, in pace with the industrial revolution, CO2 levels have climbed to levels not reached at any time in the past half million years.

CO2 levels have risen thirty percent over the last hundred and fifty years alone.
Scientists link this finding to a simultaneous increase in CO2 emissions caused by human activities. Therefore present day scientists agree that Global Warming is, with virtual certainty, due to human actions.

How will today’s CO2 emissions influence our future?

When we emit carbon dioxide it stays in the atmosphere for a very long time; somewhere between decades and centuries. This is because of the way carbon dioxide breaks down by 50 year halve-times.
This means that even if we stop all carbon dioxide emissions right now, only half of human emitted carbon dioxide will remain after 50 years. Unfortunately that doesn’t mean that 50 years later all CO2 is gone. Instead, it’s again only half of the remaining carbon dioxide which is gone. 50 years later again half will be gone and so on.

In numbers it would look like this:
If we had 100 parts of CO2 to start with, we would have 50 parts after 50 years.
After 100 years there would be 25 parts and after 150 years 12.5 parts.
After 200 years there would still be 6.25 parts and so on.
It would take centuries for all carbon dioxide to disappear.

Therefore carbon dioxide will continue to trap heat in the atmosphere long after we stop emitting any greenhouse gas. The consequences of what we do now, will be felt for centuries to come. That’s why it’s so important that we stop emitting carbon dioxide as soon as we can.

The cooling effect of Aerosols.

Cars and power plants which are the primary sources of carbon dioxide emissions also produce aerosols such as smoke and fumes, which actually have a cooling effect on global temperatures. Some scientists think that the magnitude of this cooling approximately cancels out the warming effect of carbon dioxide.

Cool, you may think. We can just keep on driving our cars, without a worry about global warming.

Unfortunately the problem is a bit more complex.

Aerosols are short lived.

Scientists are finding it hard to determine the exact amount of cooling caused by aerosols.
Even if aerosols have as great a cooling effect as some scientists believe, it doesn’t mean we can continue to send our carbon dioxide pollution into the air without consequences.

Aerosols are short-lived, lasting only a few days, several weeks at the most, while carbon dioxide continues to heat the atmosphere for decades to centuries. So if we just keep on driving our cars and running our power plants, continuing to emit carbon dioxide it may have little influence in the short-term because of the aerosol protection, but in the long-term carbon dioxide emissions will catch up with us, and by then it will be too late.

Another thing to keep in mind is that some aerosols even have the opposite effect and contribute to Global Warming.

Black Carbon Aerosols.

It turns out that black carbon, an aerosol emitted by diesel engines, the open burning of forests, and the residential burning of coal, wood, dung, and agricultural residues actually has a warming effect.
Scientists recently discovered that black carbon aerosols likely account for forty-five percent or more of the warming which has occurred in the Arctic during the last three decades.

Sources:
http://co2.cms.udel.edu/Increasing_Atmospheric_CO2.htm
http://en.wikipedia.org/wiki/Atmospheric_CO2