I've hosted and thumbnailed the plots as it makes them more appealing. Sources at the end.

[1]
This chart shows carbon dioxide concentrations (or proxies of) reconstructed to 500 million years ago. You can see that generally co2 concentration is all over the place, but certainly it used to be much higher than it is now (perhaps 8000 ppm). In fact, we didn't have any ice caps until 34 million years ago due to these high co2 concentrations[4]. I would consider the ice caps a pretty important feature of our planet (understatement).
For scale, life has been around for some 3.5 billion years (about 7 time further than this chart shows) and the dinosaur extinction event happened about 65 million years ago (way on the left hand side). So, mammals have dominated only for this tiny region on the left, where co2 concentrations were certainly < 1000 ppm.

[2]
This shows co2 and temperatures back to 400,000 years. They are correlated, though whether one lags the other is moot. The point is they are related in some incredibly complicated way to each other and everything else. co2 concentration can be seen to vary between ~180 and ~290 ppm, temperature anomaly ~-8 to ~3 degrees C.
For scale, homo sapiens appeared around 200,000 years ago, or half way along this chart.

[3]
Here we have much more recent co2 concentrations, the last 1100 years, using ice core data and measurements from Mauna Loa (Hawaii). Having "zoomed in" this far the co2 concentration looks pretty flat, until about 1850 where it takes a huge jump from ~290 ppm (the maximum value seen in the last 400,000 years) to the current 391 (a 35% increase)[5].
So what have we learnt? CO2 concentration gives us a good idea of the temperature of the Earth. Generally more = hotter. This concentration has varied greatly throughout the Earth's history, but has been fairly low, and relatively stable, since the dinosaurs became extinct.
In the last 400,000 years, twice as long as humans have existed, the co2 concentration has kept within extremely tight limits (only 180-290 ppm).
Now, since 1850, we have seen co2 concentrations jump from 290 to 390.
This is why we are concerned.
Making this post has been extremely educational for me, and I hope it will be for you.
[1] http://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere
[2] http://maps.grida.no/go/graphic/tem...in-the-atmosphere-over-the-past-400-000-years
[3] http://iter.rma.ac.be/en/img/CO2-concenNEW_EN.jpg (not sure about the reliability of this one, but probably legit)
[4] http://www.treehugger.com/files/200...link-between-c02-and-antarctic-ice-sheets.php
[5] http://co2now.org/

[1]
This chart shows carbon dioxide concentrations (or proxies of) reconstructed to 500 million years ago. You can see that generally co2 concentration is all over the place, but certainly it used to be much higher than it is now (perhaps 8000 ppm). In fact, we didn't have any ice caps until 34 million years ago due to these high co2 concentrations[4]. I would consider the ice caps a pretty important feature of our planet (understatement).
For scale, life has been around for some 3.5 billion years (about 7 time further than this chart shows) and the dinosaur extinction event happened about 65 million years ago (way on the left hand side). So, mammals have dominated only for this tiny region on the left, where co2 concentrations were certainly < 1000 ppm.

[2]
This shows co2 and temperatures back to 400,000 years. They are correlated, though whether one lags the other is moot. The point is they are related in some incredibly complicated way to each other and everything else. co2 concentration can be seen to vary between ~180 and ~290 ppm, temperature anomaly ~-8 to ~3 degrees C.
For scale, homo sapiens appeared around 200,000 years ago, or half way along this chart.

[3]
Here we have much more recent co2 concentrations, the last 1100 years, using ice core data and measurements from Mauna Loa (Hawaii). Having "zoomed in" this far the co2 concentration looks pretty flat, until about 1850 where it takes a huge jump from ~290 ppm (the maximum value seen in the last 400,000 years) to the current 391 (a 35% increase)[5].
So what have we learnt? CO2 concentration gives us a good idea of the temperature of the Earth. Generally more = hotter. This concentration has varied greatly throughout the Earth's history, but has been fairly low, and relatively stable, since the dinosaurs became extinct.
In the last 400,000 years, twice as long as humans have existed, the co2 concentration has kept within extremely tight limits (only 180-290 ppm).
Now, since 1850, we have seen co2 concentrations jump from 290 to 390.
This is why we are concerned.
Making this post has been extremely educational for me, and I hope it will be for you.
[1] http://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere
[2] http://maps.grida.no/go/graphic/tem...in-the-atmosphere-over-the-past-400-000-years
[3] http://iter.rma.ac.be/en/img/CO2-concenNEW_EN.jpg (not sure about the reliability of this one, but probably legit)
[4] http://www.treehugger.com/files/200...link-between-c02-and-antarctic-ice-sheets.php
[5] http://co2now.org/