There is no particular "meaning" to the 400ppm level other than as human beings we tend to like nice round numbers - hence the pseudo "science" of numerology. 400 ppm does not represent a “tipping point” or some other sort of point of no return, it is merely a symbolic milestone.
It is entirely true to say that life as we know it would not exist without the CO
2 and the greenhouse effect. However, it is facile(*) to suggest that more carbon in the atmosphere is unequivocally a good thing. Changing weather and climate patterns are more likely to result in poor harvests rather than the enhanced ones some people anticipate due to higher CO
2 levels because we are growing things in the “wrong” place. These impacts may be compounded by impacts on populations of pollinator species and land use issues caused by the demands of a burgeoning global population. I would accept that higher CO
2 is good for life generally, but it is hard to see how it is good for Homo sapiens specifically.
Whilst in the controlled conditions of a lab or commercial greenhouse, increased CO
2 can significantly boost growth in some plants, the evidence in the real world does not support it, where, more often than not, other factors – such as soil fertility and water availability are the limiting factors to growth/ production. There are also species that respond to increasing temperatures by reducing growth (often to limit moisture loss).
It is also expected, that increased energy in the atmosphere is likely to result in increasing numbers of extreme weather events – these can devastate production on a local/ regional scale. Good examples of this include the shortage of the right quality wheat for weetabix due to the poor summer in the UK last year
http://www.bbc.co.uk/news/uk-england-northamptonshire-22248961 or the devastation of the Italian basil crop in 2006
http://news.bbc.co.uk/1/hi/world/europe/5267796.stm whilst, of course, any one weather event is impossible to (scientifically) link to increasing CO
2, it is an increasingly reasonable connection to make.
We are entering almost entirely uncharted territory climate wise. This is because the paleo-historic temperature increases, whether resulting from Milankovitch cycles, or other mechanisms, appears to drive increasing atmospheric CO
2 levels. In the current situation, the reverse is true - the consensus is that increasing CO
2 will drive temperatures upwards. The only occasion when there is clear evidence that this happened before is the Paleocene–Eocene Thermal Maximum. The PETM resulted in a mass extinction (and subsequent re-radiation) of foraminifera species (a kind of plankton) but also in the diversification of mammal species. Unfortunately, the cause of the PETM is unclear, although there is a lot of academic interest in it.
* as is much of the assumption heavy analysis / group think posted on Anthony Watt’s site.
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I feel compeled to put my pedants hat on and say that it is weathering not erosion that made the rocks so soft.
