from inundation of densely populated coasts to ocean acidification to the poleward spread of tropical diseases, underscore the need to determine how much of the warming is caused by human activities and what can be done about it. Earth science has an important role in answering both questions.
The immediate grand challenge in climate science is predicting how climate will change over the coming decades. However, the broader challenge is to account for both the long-term consistency of Earth’s climate and its multiple and varied excursions in the context of a constantly evolving global geological and biological framework. Only when we are able to capture past climate changes in models will we have confidence in our predictions of future climate. Reliable models have not been available because the conditions that characterized ancient climates—such as ground surface temperature, sea surface temperature, and mean annual precipitation—vanished thousands or millions of years ago, along with the climate they shaped. Lacking real-time data for ancient events, geologists are assembling toolkits of “proxy” data. The temperature and precipitation of continental regions, for example, can often be inferred from evidence left in the sediments of lake beds or in ancient preserved soils. Earth’s large-scale surface temperature structure, as well as information on ancient ocean currents, is also reflected in fossil and geochemical records of deep-sea sediments and in records of sea-level change. Similarly, atmospheric temperatures for at least the past 100,000 years or so are recorded in glacial ice and retrievable through deep drill cores in the ice. However, the further we journey into Earth’s past, the more different Earth was from our modern planet. To understand Earth’s climate in geologically ancient times, we need to know an enormous amount about the geology and geography of the ancient Earth; this is where geological science and climate science become inseparable.
The climate system is regulated by how much energy Earth receives from the Sun and how much is radiated back into space (Figure 3.1). How much energy is absorbed depends on the reflectivity (or albedo) of Earth’s atmosphere and surface. The albedo depends on how much of Earth’s surface is covered by water, land, or ice; how the continents are arranged; the extent of land vegetation; and the amount of reflective material (clouds and particles) in the atmosphere. It is generally believed that the key determinant of Earth’s ability to capture energy from the Sun is the amount of greenhouse gases, predominantly carbon dioxide, present in Earth’s atmosphere. Increasing the CO2 content of the atmosphere stimulates warming, which is then amplified by increasing amounts of water vapor that can evaporate from the oceans at higher temperature. Hence the cornerstone of any broader understanding of Earth’s climate is the question of what controls the amount of CO2 in the atmosphere.
The various processes that contribute to the CO2 content of the atmosphere are referred to collectively as the carbon cycle. The carbon cycle is a key regulator of climate change. The overarching issue is the fraction of Earth’s carbon that is present in the atmosphere in the form of CO2 or other greenhouse gases like CH4. For the modern Earth, most of the carbon is stored in rock, and most of that is stored deep within the mantle and core. Estimates suggest there is 500,000 times more carbon stored in Earth’s mantle than in the atmosphere (McDonough and Sun, 1995; Salters and Stracke, 2004), and there is likely to be more carbon in Earth’s core than in the mantle. Most of the carbon not stored in the mantle and core is found in sedimentary rocks as the mineral calcite or as organic material like kerogen and petroleum. Most of the rest is either dissolved in the oceans, stored in soils, or present as living plant and animal tissue. Only a very tiny fraction (roughly one-millionth) is present in the atmosphere and acting to help warm Earth’s surface. The Venusian atmosphere, which contains about 200,000 times more CO2 than Earth’s preindustrial atmosphere, is clear evidence that the distribution of carbon between a planet’s interior and atmosphere can be very different from that of Earth.
Even though the amount of carbon in Earth’s atmosphere is small, changes in the amount have a major effect on the surface temperature. Although the carbon in Earth’s core is not likely to be transferred to the atmosphere, there are ways that at least some fraction of the enormous amounts of carbon in Earth’s mantle, crust, and oceans could be. Similarly, there are ways to transfer the carbon in the atmosphere to the oceans and to sediments and then to subduct them into the mantle.