Actually this is quite interesting. The CO2 concentrations 400,000 years ago mark the Devonian period. I have included a description from wikipedia here:
Devonian - Wikipedia, the free encyclopedia
The next period is the Carbonacious period (a time when all our fossil fuels were starting to be created) ; I have included a description here:
Carboniferous - Wikipedia, the free encyclopedia
Then the Permian, which is described here:
Permian - Wikipedia, the free encyclopedia
What is interesting to note is the climate became more hospitable for humans as the CO2 levels decreased over the past 400,000 years.
Further I would point out the description prior to 400,000 years ago which is the Silurian. I have included a description here:
Silurian - Wikipedia, the free encyclopedia. the Silurian marked an increase in sea-level rise and melting glaciers. Which appears to coincide with increased CO2 levels as noted in the graph.
I think the most critical issue to point out and discuss is the latest CO2 data identified on the chart. Clearly CO2 levels have been just as high in the history of the earth---of that I don't think one can dispute (I certainly won't dispute it.) However, and this is significant, THE RATE OF CHANGE IN THE CO2 CONCENTRATION IS, IMHO, THE CRITICAL FACTOR TO BE EXAMINED.
In my mind, the question is: Is the rate of change due to significant anthropogenic influences. If the answer is yes, then it is incumbent upon us to do something, if the answer is no, then who cares--there is nothing we can do about it anyway.
In either event, the one sure way to put this issue to bed is to spend a lot of R&D on alternative energy for two very simple reasons: 1) Our purchase of oil is financing terror; and 2) Our purchase of oil is economically killing us.
Here is a case in point.
Recently, two big oil refineries were permanently shut down in the Philadelphia metro area. There are three reasons as to why they shut-down (according to the WSJ, Bloomberg, and the American Petroleum Institute.)
The three reasons are:
1) Refining capacity in the mid-west has substantially increased the supply of refined products and this supply is out-weighing demand;
2) These two particular refineries were designed to process heavy/sour crudes. For a long time heavy/sour crude was 1/3rd the cost of light sweet crude which is currently $80/barrel. Now the cost of heavy/sour crude is only slightly less than light/sweet crude;
3) The US is importing gasoline and other refined products at a cost less than what it costs to refine and distribute products in the US (mostly due to labor/transportation costs.) It costs less money to transport stocks by ship than is does by truck or rail.
Regardless of where one stands on the Climate Change battle, I think it behooves us here in the USA to find alternative sources of energy. This will positively address our economic, security, energy and environmental issues.