Hi Phil

When Mars first formed two processes blew away much of its original atmosphere - asteroid impacts and the enhanced Solar Wind of the early Sun. After 3.9 billion years ago the impactor rates went down and are too rare to matter these days. The Solar Wind settled down too, but only after Mars lost its magnetic field and a lot of atmosphere. The rest was locked up in the soil, in clathrates and, perhaps, in carbonates - though Martian acids probably prevented most carbonate formation, as revealed by the ME Rovers.

While it's true that Mars lost ~ 90-99% of its original atmosphere to the solar wind (NOT cosmic rays) that was when the wind was much, much stronger than at present. Yet even today nitrogen would escape fairly readily (still over millions of years) due to the solar wind. Carbon dioxide however is stable against the solar wind and Mars is losing none. Add some oxygen and things might change slightly, but with minimal nitrogen it's still pretty stable.

What will breakdown continually is the flurocarbons used to warm Mars - some last only a decade or two. Others can last much, much longer and apparently C3F8 is one such. To make Mars air breathable to current humans will require a furocarbon greenhouse, since the initial greenhouse warming will be from mostly carbon dioxide.

Above 5% CO2 many humans find it toxic and from 1-5% many suffer discomfort. For comparison Earth's current CO2 is just 0.0378% contributing ~ 0.17 optical depth to the greenhouse - the other 2.3 is from water vapour. Because of Mars' low gravity the actual optical depth of the same partial pressure of gas increases by sqrt(1/0.378) ~ 1.63. Not much for current CO2 levels, but important for higher levels. An optical depth of ~ 2.8 is achieved at 5% CO2, but for near Earth-like warmth over ~ 9 is needed in total. Water would add ~ 2, so the flurocarbons will be needed in quantity.

If humans can be engineered with carbonate forming and removing processes in their blood then higher levels will be tolerable - some fish and crocodilians endure very high CO2 and low O2 levels. Personally I don't see why carbon dioxide can't be a substantial fraction, but that's a matter for biochemists and physiologists to tackle the other side of 2100 AD. At 0.3 bar CO2, the optical depth is ~ 6.9. Water almost adds the rest, provided Mars is darkened to a Bond albedo of 0.1 (currently it's 0.25.) The less flurocarbon, the less it will be broken down. Another 0.1 - 0.2 bar can be oxygen, perhaps allowing modified human life.

Adam