Global Climate Change and the Cretaceous Greenhouse World

One of the most important earth science questions of our time is understanding how human activities may be modifying the current and future global climates. It is known that concentrations of heat-trapping gases or “greenhouse gases,” such as carbon dioxide (CO₂), are increasing in the atmosphere. As more heat is trapped, global temperatures increase. Scientists have linked human activities such as burning of fossil fuels, forest clearing, and emissions from cars to the increase in heat-trapping gases. Carbon dioxide levels have increased from a pre-industrial concentration of 280 parts per million (ppm) to 360 ppm today, and CO₂ levels continue to increase. There is wide scientific concern that the global climate system is already being affected. Noteworthy are the recent, rapid poleward retreats of sea-ice margins in the high polar latitudes, an especially sensitive part of the globe.

In January 1999, the American Geophysical Union (AGU) released a position statement on climate change and greenhouse gases that included the following passage: “The world may already be committed to some degree of human-caused climate change, and further buildup of greenhouse gas concentrations may be expected to cause further change. Some of these changes may be beneficial and others damaging for different parts of the world. AGU recommends the development and evaluation of strategies such as emissions reduction, carbon sequestration, and adaptation to the impacts of climate change."

Deep ocean basins (dark blue) and shallow inland seas (light blue) are shown in this view of the Earth 110 million years ago (Cretaceous Period). Note the opening of the central Atlantic Ocean caused by rifting between North America (upper left) and Africa (lower right). Image by Ron Blakey, Northern Arizona University.

Future global greenhouse warming may be unavoidable. Atmospheric CO₂ concentrations exceeding 1,000 ppm could become a reality only a few centuries from now¹. In order to plan for the impacts that accompany global climate change of this magnitude, an improved understanding of "Greenhouse Worlds" in the geologic past is needed. Elevated concentrations of CO₂ exceeding 1,000 ppm have not occurred during the last 1.65 million years, a period of time referred to as the “Ice Age” or Quaternary Period. The mid-Cretaceous Period (about 100 million years ago--during the "Age of Dinosaurs"), however, does represent a recent geologic analog in earth history that can be used to predict future greenhouse conditions.

The "Cretaceous Greenhouse World" refers to an episode of earth history that lasted from about 110 to 90 million years ago. During this time, submarine volcanic CO₂ emissions were released into the atmosphere at rates high enough to cause atmospheric CO₂ concentrations in excess of 1,000 ppm. This CO₂ buildup resulted from rapid sea-floor spreading related to the breakup and drifting apart of the Earth’s continents². The buildup lasted for about 10 million years, and the ensuing period of peak warming coincided with an explosive growth in the genetic diversity of flowering plants, social insects, birds, and mammals--organisms that dominate modern terrestrial ecosystems. The consequences of a similar greenhouse buildup occurring over the course of only a few hundred years, however, are likely to be highly disruptive to natural ecosystems. Plants and animals live in zones of predictable temperature and precipitation. If this climate is altered too quickly, the species may not have sufficient time to migrate and adapt.

Recent paleoclimate modeling has provided insights into the nature of global warming during the Cretaceous. These results suggest that atmospheric CO₂ concentrations during the Cretaceous were four times current CO₂ levels, and the global mean temperature during the Cretaceous was 11.2°F warmer than present³. Some important questions remain about the amount and intensity of precipitation during the Cretaceous. It has been proposed that globally averaged precipitation in the Cretaceous Greenhouse World was 28% greater than present, although scientific data to verify this are only now being developed⁴. Ongoing studies of ancient terrestrial deposits on earth are needed to help scientists understand present trends and anticipate future global climate changes.

A team of research scientists at the Iowa Department of Natural Resources -- Geological Survey Bureau and the Department of Geoscience at The University of Iowa is leading an effort to explore relationships between temperatures and the stable oxygen isotopic composition of precipitation during the Cretaceous. Elements in nature consist of atoms with different masses called isotopes. The two most abundant oxygen isotopes are ¹⁶O and ¹⁸O. As water evaporates and condenses, the relative concentration of each oxygen isotope in water undergoes change. As atmospheric moisture is transported from the equator toward the North and South Poles, there is a progressive concentration of the lighter ¹⁶O in atmospheric moisture, as the heavier ¹⁸O falls as precipitation. Rainfall that occurs at different latitudes develops an isotopic "fingerprint" by which it can be identified.

This team of scientists is studying oxygen isotopic fingerprints of buried Cretaceous soils or paleosols that are currently being mined for brick manufacture by the Sioux City Brick Company in Sergeant Bluff, Iowa. These Cretaceous deposits are located at about the same latitude now as then. The paleosols contain a soil-formed mineral, sphaerosiderite (FeCO₃), that had crystallized in water-saturated settings. The oxygen isotopic composition of these iron-carbonate minerals (see photo, below) records temperature and precipitation conditions during the Cretaceous at the latitude where they formed⁵.

This is a microscopic view through a thin slice of 100-million-year-old buried soil sampled from rocks mined at the Sioux City Brick Company in Sergeant Bluff. The spheres are nodules of the mineral sphaerosiderite, an iron carbonate that preserves records of ancient temperature and precipitation in its isotopes. (Cross-polarized light; horizontal field of view is 3.2 millimeters.) Photo by Greg Ludvigson.

Studies of sphaerosiderites in Cretaceous paleosols of North America show dramatic evidence that atmospheric moisture transport and precipitation intensity during the Cretaceous Greenhouse World were very different from that of today. Cretaceous sphaerosiderites are considerably more depleted in the heavier ¹⁸O isotope for their respective paleolatitudes than the values calculated for those same latitudes today using modern meteorological data. This difference is generally interpreted as the result of significantly greater global rainfall during the Cretaceous Period.

A major goal of this research is to gather geological information from the field that can be used to refine models simulating ancient greenhouse episodes such as the Cretaceous Greenhouse World. The knowledge gained from these models can lead to more accurate and reliable forecasting of the impacts of future global greenhouse conditions.

¹ Walker, J.C.G., and Kasting, J.F., 1992, Effects of fuel and forest conservation on future levels of atmospheric carbon dioxide: Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), v. 97, no. 3, p. 151-189.
² Caldeira, K., and Rampino, M.R., 1991, The mid-Cretaceous superplume, carbon dioxide, and global warming: Geophysical Research Letters, v. 18, no. 6, p. 987-990.
³ Barron, E.J., Fawcett, P.J., Peterson, W.H., Pollard, D., and Thompson, S.L., 1995, A "simulation" of mid-Cretaceous climate: Paleoceanography, v. 10, no. 5, p. 953-962.
⁴ Barron, E.J., Hay, W.W., and Thompson, S., 1989, The hydrologic cycle, a major variable during Earth history: Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), v. 75, no. 3, p. 157-174.
⁵ Ludvigson, G.A., González, L.A., Metzger, R.A., Witzke, B.J., Brenner, R.L., Murillo, A.P., and White, T.S., 1998, Meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology: Geology, v. 26, no. 11, p. 1039-1042.