New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 52

CO-EVOLUTION OF LIFE, ENVIRONMENT, AND CLIMATE
Earth is apparently unique in the Solar System in bearing living organisms
that profoundly modify planetary processes affecting the composition and properties
of the atmosphere, hydrosphere, and lithosphere. The geological record has provided
a compelling narrative of major changes in Earth's climate, environment, and
evolving life, played out over billions of years that has defined our planet's life-
sustaining outer shell. These interactions continue to shape the world in which we
live, and our future depends on such interactions as they unfold over the coming
centuries—and on our thoughtful and responsible stewardship of them. Yet to
understand the future, we need to know our geochemical and geobiological past.
Earth's environmental systems have experienced geochemical, climatic, and
biotic change, with conditions in the distant past remarkably different from those of
the Holocene epoch—the epoch when low and relatively stable atmospheric carbon
dioxide and largely benign climatic conditions fostered human civilizations. Earth's
deep-time record provides numerous unique analogs to the emerging climate state of
dramatically warmer temperatures and highly elevated greenhouse gas contents in the
atmosphere. But life's planetary habitat has undergone even more profound
geochemical transformations. For example, the advent of biological oxygen
production and the expansion of plants onto land are both changes that reorganized
element fluxes and concentrations in the ocean, sediments, and atmosphere on a
global scale. Only the deep-time geological and paleontological record can provide
examples of change that rival the scale of contemporary human-induced impacts on
land, biota, oceans, and climate. Thus, understanding past biosphere-geosphere
behavior is a potent approach to anticipating how linked physical, chemical, and
biological processes that characterize Earth's surface may be impacted by and
respond to human activity. Earth's biogeochemical history archived in the deep-time
geological record thus provides a major research opportunity to investigate the future
of our planet.
Understanding recent and ongoing climate change requires a full exploration
of the range of climate phenomena, rates, feedbacks, thresholds, and tipping points
captured over the long “experiment” of Earth history. Studies of the deep-time record
have revealed that Earth's climate varies between two extremes. At one extreme is a
cool, glaciated icehouse state associated with low greenhouse gas concentrations in
the atmosphere and the state in which humans evolved, while at the other extreme is a
warm greenhouse mode apparently associated with higher atmospheric greenhouse
gas levels and small-to-no ice sheets (see Box 2.6). The geological archive has been
particularly important for revealing how many physical, chemical, and biological
processes operated differently or were unique to past warmer and transitional states
than during the present cool state (NRC, 2011a).
Our ability to characterize and interpret the deep record has increased
dramatically over the past decade and continues at an accelerating pace. New tracers
(proxies) of past conditions have greatly refined our ability to extract ancient records
of Earth surface conditions, including temperature, atmospheric levels of carbon
dioxide, the chemical composition of and oxygen availability in the ocean, regional