Past Climate Changes: Informing Decisions about the Future
Earth’s climate is changing rapidly with unknown consequences for ecosystems and society. Atmospheric greenhouse gas concentrations, including carbon dioxide (CO2), have risen to levels not recorded for at least 3 million years. Oceans are becoming more acidic due to the uptake of carbon dioxide from the atmosphere. Atmospheric and ocean temperatures are increasing, Arctic sea-ice cover is decreasing, and glaciers and parts of the Greenland and Antarctic Ice Sheets are melting, contributing to rising sea level. Changes in precipitation are resulting in extreme droughts and floods in many regions, affecting crops, fisheries and other commercial activities. Because instrumental records of rainfall and temperature cover only a century or so of Earth’s 4.5 billion year history, it is necessary to study past climate changes using the geological record to provide a baseline to evaluate recent changes and improve the understanding of likely impacts of future climate change.
Interdisciplinary research in paleoclimatology, the study of past climate change, involves scientific studies relevant to today’s climate challenges. There is now strong evidence that Earth’s climate changes occur over timescales ranging from decades to millions of years. The causes of these climatic changes include cycles in Earth’s orbit (its tilt, precession and orbital shape), changes in greenhouse gas concentrations, solar output, volcanic activity, the position of the continents, ocean circulation, and interactions among the atmosphere, ocean, land surface, glaciers and ice sheets. Paleoclimate research documents natural climate variability and past responses of the Earth system to climate extremes and abrupt changes, providing key insights into potential rates of change, tipping points, and impacts on ecosystems.
Climate variability occurs over time scales ranging from “deep time” (tens of millions of years) to the current interglacial period that covers the last ~11,000 years. For example, Earth’s climate is influenced by quasi-periodic glacial-interglacial (ice age) cycles caused by changes in the Earth’s orbital geometry relative to the sun, changing the seasonal and geographic distribution of the sun’s energy. These orbital climate changes (Milankovitch cycles) are characterized by fluctuations in global temperature, sea level, and the global carbon cycle. Warm climatic extremes, known as hyperthermal events, also punctuate Earth history and provide evidence of the Earth system response to elevated atmospheric CO2 concentrations and other factors. One such event, the Paleocene Eocene Thermal Maximum (PETM) 55 million years ago, experienced a global temperature spike of 6°C in 20,000 years, probably caused at least in part by elevated CO2 concentrations. Paleoclimate records of natural CO2 fluctuations and ecosystem response provide decision-makers direct information about the fate of human fossil fuel carbon emissions and the Earth’s climate sensitivity to future elevated CO2 concentrations.
Paleoclimate research also investigates abrupt climate change. One example of abrupt climate transition occurred during the last deglaciation from 18,000 to 10,000 years ago when ice-sheet collapse caused rapid sea-level rise, changes in climate and altered ocean circulation. Another type of abrupt change involves severe drought, such as droughts documented by tree-ring, sediment, coral and speleothem proxy records of rainfall in North America over the past millennium.
In sum, paleoclimate reconstructions are informing decisions about mitigation response to regional rainfall extremes, management and engineering in low-lying coastal regions, energy development, water management and agriculture, international security, commercial transportation in polar regions, and managing threatened species and ecosystems. Such information provides the foundation to understand climate impacts across society and to improve climate predictions of future climate change.