A drop in carbon dioxide appears to be the driving force
that led to the Antarctic ice sheet’s formation, according to a recent study
led by scientists at Yale and Purdue universities of molecules from ancient
algae found in deep-sea core samples.
The key role of the greenhouse gas in one of the biggest
climate events in Earth’s history supports carbon dioxide’s importance in past
climate change and implicates it as a significant force in present and future
The team pinpointed a threshold for low levels of carbon
dioxide below which an ice sheet forms in the South Pole, but how much the greenhouse
gas must increase before the ice sheet melts—which is the relevant question for
the future—remains a mystery.
Matthew Huber, a professor of earth and atmospheric
sciences at Purdue, says roughly a 40% decrease in carbon dioxide occurred
prior to and during the rapid formation of a mile-thick ice sheet over the
Antarctic approximately 34 million years ago.
A paper detailing the results was published in Science.
“The evidence falls in line with what we would expect
if carbon dioxide is the main dial that governs global climate; if we crank it
up or down there are dramatic changes,” Huber says. “We went from a
warm world without ice to a cooler world with an ice sheet overnight, in
geologic terms, because of fluctuations in carbon dioxide levels.”
For 100 million years prior to the cooling, which occurred
at the end of the Eocene epoch, Earth was warm and wet. Mammals and even
reptiles and amphibians inhabited the North and South poles, which then had
subtropical climates. Then, over a span of about 100,000 years, temperatures
fell dramatically, many species of animals became extinct, ice covered Antarctica and sea levels fell as the Oligocene epoch
Mark Pagani, the Yale geochemist who led the study, says
polar ice sheets and sea ice exert a strong control on modern climate,
influencing the global circulation of warm and cold air masses, precipitation
patterns and wind strengths, and regulating global and regional temperature
“The onset of Antarctic ice is the mother of all climate
‘tipping points,'” he says. “Recognizing the primary role carbon
dioxide change played in altering global climate is a fundamentally important
There has been much scientific discussion about this sudden
cooling, but until now there has not been much evidence and solid data to tell
what happened, Huber says.
The team found the tipping point in atmospheric carbon
dioxide levels for cooling that initiates ice sheet formation is about 600 ppm.
Prior to the levels dropping this low, it was too warm for the ice sheet to
form. At the Earth’s current level of around 390 ppm, the environment is such
that an ice sheet remains, but carbon dioxide levels and temperatures are
increasing. The world will likely reach levels between 550 and 1,000 ppm by
2100. Melting an ice sheet is a different process than its initiation, and it
is not known what level would cause the ice sheet to melt away completely,
“The system is not linear and there may be a
different threshold for melting the ice sheet, but if we continue on our
current path of warming we will eventually reach that tipping point,” he
says. “Of course after we cross that threshold it will still take many
thousands of years to melt an ice sheet.”
What drove the rise and fall in carbon dioxide levels
during the Eocene and Oligocene is not known.
The team studied geochemical remnants of ancient algae
from seabed cores collected by drilling in deep-ocean sediments and crusts as
part of the National Science Foundation’s Integrated Ocean Drilling program.
The biochemical molecules present in algae vary depending on the temperature,
nutrients and amount of dissolved carbon dioxide present in the ocean water.
These molecules are well preserved even after many millions of years and can be
used to reconstruct the key environmental variables at the time, including
carbon dioxide levels in the atmosphere, Pagani says.
Samples from two sites in the tropical Atlantic Ocean were
the main focus of this study because this area was stable at that point in
Earth’s history and had little upwelling, which brings carbon dioxide from the
ocean floor to the surface and could skew measurements of atmospheric carbon
dioxide, Huber says.
In re-evaluating previous estimates of atmospheric carbon
dioxide levels using deep-sea core samples, the team found that continuous data
from a stable area of the ocean is necessary for accurate results. Data
generated from a mix of sites throughout the world’s oceans caused inaccuracies
due to variations in the nutrients present in different locations. This
explained conflicting results from earlier papers based on the deep-sea samples
that suggested carbon dioxide increased during the formation of the ice sheet,
Constraints on temperature and nutrient concentrations
were achieved through modeling of past circulation, temperature, and nutrient
distributions performed by Huber and Willem Sijp at the University of New South
Wales in Australia. The collaboration built on Huber’s previous work using the National Center for Atmospheric Research
Community Climate System Model 3, one of the same models used to predict future
climates, and used the UVic Earth System Climate Model developed at the
University of Victoria, British Columbia.
“The models got it just about right and provided
results that matched the information obtained from the core samples,” he
says. “This was an important validation of the models. If they are able to
produce results that match the past, then we can have more confidence in their
ability to predict future scenarios.”
In 2004 the
team used evidence from deep-sea core samples to challenge the longstanding
theory that the ice sheet developed because of a shift from warm to cool ocean
currents millions of years ago. The team found that a cold current, not the
warm one that had been theorized, was flowing past the Antarctic coast for
millions of years before the ice sheet developed.
plans to investigate the impact of an ice sheet on climate.
“It seems that the polar ice sheet shaped our modern
climate, but we don’t have much hard data on the specifics of how,” he
says. “It is important to know by how much it cools the planet and how
much warmer the planet would get without an ice sheet.”