Global Climate Change and the Cretaceous
by Greg A. Ludvigson
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
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
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 Earths 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
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
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
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.
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,
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.
Adapted from Iowa Geology 1999, Iowa Department of Natural Resources