That is how much CO2 emissions would have to be reduced just to stabilize atmospheric carbon dioxide at current levels. The reason is that CO2 stays in the atmosphere for a very long time after we put it there. Quoting from the AR4 physical basis page 824;
"While more than half of the CO2 emitted is currently removed from the atmosphere within a century, some fraction (about 20%) of emitted CO2 remains in the atmosphere for many millennia. Because of slow removal processes, atmospheric CO2 will continue to increase in the long term even if its emission is substantially reduced from present levels."
The eighty percent figure does not appear directly, but can be interpolated from the graphs at the bottom of page 824.
What this means is that reducing CO2 output by less than eighty percent simply moves the peak atmospheric level forward in time assuming that humans wind up burning all the fossil fuels they can get to. (There are various international mechanisms that might keep this total down, but I am pessimistic about their implementation in relevant time frames as I will discuss in other posts.)
Based on my informal surveying I find that this figure is not understood by most people, yet it is critical to our understanding of the issue. Governments in developed countries are already straining in discussing figures like fifty percent cuts in thirty or fifty years. Developing countries are currently not even discussing restricting their growth rates, let alone reducing from current levels. Simple math shows that there is no possible way for this to result in an eighty percent reduction in current output anywhere in the foreseeable future. Instead it would seem just stabilizing output at current levels or even somewhat higher levels on a global basis will be challenging.
Once people understand the eighty percent figure most realize that the currently proposed approaches to solving the issue are completely unrealistic. We need to accept this and implement policies that could actually achieve the objective of lowering atmospheric CO2 levels which will certainly get higher before we can start reducing them.
At this point I believe the only possible way to reduce atmospheric CO2 is to come up with solutions that pull it back out of the atmosphere. All alternatives that achieve this need to be explored. Of course breakthroughs in low cost energy production will help reduce the size of this task.