• What is the sensitivity of air and ocean (both shallow and deep) temperatures to dramatically increased CO2 levels?
• How high will atmospheric CO2 levels rise, and for how long will these high levels persist?
• How quickly do ice sheets decay and vanish, and consequently how rapidly does sea level change? Also, if the Arctic is to become permanently ice-free, how will this affect thermohaline circulation and regional and global climate patterns?
• Are there processes in the climate system that are not currently apparent or understood that will become important in a warmer world?
• How will global warming affect rainfall and snow levels, and what will be the regional consequences for flooding and drought?
• What effect will these changes have on the diversity of marine biota? What will be the impact on—and response of—terrestrial ecosystems?
• Has climate change become inevitable? How long will it take to reverse the projected changes through natural processes?
How Earth’s climate system has responded to past episodes of increasing and elevated atmospheric CO2 is a critical element of the answers to these questions.
Temperature Response to Increasing CO2
Recent syntheses suggest that climate sensitivity—the response of global mean surface temperature to a doubling of atmospheric CO2 levels—lies between 1.5 and 6.2°C (Hegerl et al., 2006; IPCC, 2007; Hansen et al., 2008). The lower end of this range (≤3°C) is based on modern data and paleoclimate records extending back no further than the Last Glacial Maximum of 20,000 years ago, and therefore these estimates factor in only the short-term climate feedbacks—such as water vapor, sea ice, and aerosols—that operate on subcentennial timescales. Climate sensitivity, however, is likely to be enhanced under higher atmospheric CO2 and significantly warmer conditions due to long-term positive feedbacks that typically are active on much longer timescales (thousands to tens of thousands of years) (Hansen et al., 2008; Zachos et al., 2008; Pagani et al., 2010). These physical and biochemical feedbacks—such as changes in ice sheets and terrestrial biomes as well as greenhouse gas release from soils and from methane hydrates in tundra and ocean sediments—however, may become increasingly more relevant on human timescales (decades) with continued global warming (Hansen and Sato, 2001; Hansen et al., 2008). Determining the deep-time record of equilibrium climate sensitivity—in particular during periods of elevated CO2 and at timescales at which long-term climate feedbacks operate—is thus a critical element in evaluating