From analyzing air bubbles trapped in icesheets and glaciers, scientists know that global CO2 levels never exceeded 300 ppm during the 800,000 years before the Industrial Revolution. (insideclimatenews.org)
Soon we will have begun an irreversible melting of the Greenland icesheet and if that happens that will have very significant effects on the climate, well pretty much indefinately. (realclimate.org)
Proxy records suggest that the WAIS [West Antarctic Ice Sheet] might have collapsed during past interglacials (Naish et al., 2009b; Vaughan et al., 2011) and was absent during warm periods of the Pliocene when CO2 concentration was 350 — 450 ppm... and global sea level was higher than present... These reconstructions and one icesheet model simulation (Pollard and DeConto, 2009) suggest that WAIS is very sensitive to the subsurface ocean temperature. (hubpages.com)