the long-lasting effects of this anthropogenic perturbation on the climate system, has permanent change—from a human point of view—become inevitable? How many thousands of years will it take for natural processes to reverse the projected changes?
The importance of these questions to science and to society prompted the National Science Foundation, the U.S. Geological Survey, and Chevron Corporation to commission the National Research Council (NRC) to report on the present state of scientific understanding of Earth’s geological record of past climates, and to identify focused research initiatives to better understand the insights offered by Earth’s deep-time record into the response of Earth systems to potential future climate change. Throughout this report, the “deep time” geological record refers to that part of Earth’s history that is older than historical or ice core records, and therefore must be reconstructed from rocks. Although the past 2 million years of the Pleistocene are included in “deep time,” most of the research described or called for in this report focuses on the long record of Earth’s history prior to the Pleistocene.
The study of climate history and processes during the current glacial (icehouse) state shows the sensitivity of Earth’s climate system to various external and internal factors and the response of key components of the Earth system to such change. The resolution and high precision of these datasets capture environmental change on human timescales, thereby providing a critical baseline against which future climate change can be assessed. However, this more recent paleoclimate record captures only part of the known range of climate phenomena; the waxing and waning of ice sheets in a bipolar glaciated world under atmospheric CO2 levels at least 25 percent lower than current levels.
In contrast to this reasonably well documented record of recent climate dynamics and at least partial understanding of the short-term feedbacks that have operated in icehouse states of the recent past, the climate dynamics of past periods of global warming and major climate transitions are considerably less well understood. Paleoclimate records offer potential for a much improved understanding of the long-term equilibrium sensitivity of climate to increasing atmospheric CO2 and of the impact of global warming on atmospheric and ocean circulation, ice sheet stability and sea level response, ocean acidification, regional hydroclimates, and the health of marine and terrestrial ecosystems. Deep-time paleoclimate records uniquely offer the temporal continuity required to understand how both short-term (decades to centuries) and long-term (millennia to tens of millennia) climate system feedbacks have played out over the longer periods of time in Earth’s history. This understanding will provide valuable insights on how Earth’s climate would respond to, and recover from, the levels of greenhouse gas forcing that are projected to occur if no efforts are made to reduce emissions.