Boldface mine:

We are only certain of the atmospheric CO2 concentrations over the past 420000 years, the time for which we have an archive of ancient air trapped in bubbles of Antarctic ice (IPCC 2001). During this period the atmospheric concentrations have always been lower than today. With less certainty, we can reconstruct CO2 concentrations much further back in time.

To do this we can use inferred reconstructions from ‘proxy’ data as diverse as boron isotopes in carbonates (Pearson and Palmer 2000), rocks derived from ancient soils (Cerling 1991), the stomata on fossil leaves, and models of the processes we believe control atmospheric CO2 (Berner 1994; Bergman et al 2004).

Such studies agree that atmospheric CO2 has probably been relatively low for the past few million years, but that in the more distant past it was much higher than it is today. In the warm world of the Cretaceous for example (100 million years ago), concentrations were between three and ten times higher than present. Four hundred million years ago, before the land was colonised by forests, they were probably 20 times higher.

400 million years ago was the Devonian Period, sometimes called the "Age of Fish." Nature, and the oceans, are obviously more robust than we give them credit for. This means that life and the ecosystem will survive changes. To be fair, this does not mean the changes will be problem-free, or even without danger, for a single species, like humanity. Nature likes life. She isn't fussy about the form.

It is still not clear to me exactly how temperature and CO2 interact, and what parts are cause, and what parts are effect. The two numbers obviously are linked, but they were tracking each other before humans had even discovered fire.