A massive new study of ancient atmospheric carbon dioxide levels and corresponding temperatures paints a daunting picture of where Earth's climate might have gone. The study covers the past 66 million years of the geological record, linking current concentrations to deep time. Among other things, the research shows that the last time atmospheric carbon dioxide reached sustained human-driven levels today was 14 million years ago - much longer than some existing assessments suggest.
The report asserts that long-term climate is highly sensitive to greenhouse gases, with cascading effects that could last for thousands of years.
The study was completed by more than 80 researchers from 16 countries over a period of seven years. The research was recently published in the journal Science.
"We've long known that adding carbon dioxide to the atmosphere increases temperatures," said geochemist Bärbel Hönisch of Columbia University's Lamont-Doherty Earth Observatory. "This study gives us a clearer understanding of how sensitive climate is over long timescales."
Redefining climate sensitivity
Leading estimates suggest that, over scales of decades to centuries, each doubling of atmospheric carbon dioxide increases average global temperatures by 1.5 to 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit). However, at least one widely read recent study suggests that the current consensus underestimates the Earth's sensitivity, suggesting that each doubling would result in 3.6 to 6 degrees Celsius of warming. Regardless, looking at current trends, all estimates put Earth warming close to or exceeding the 2°C it is likely to reach this century, which many scientists agree we must avoid if possible.
Historical trends in carbon dioxide concentrations
At the end of the 17th century, the amount of carbon dioxide in the air was about 280 parts per million (ppm). Today, the concentration of carbon dioxide in our air reaches 420 parts per million, an increase of about 50%; by the end of the century, our air may reach 600 parts per million or more. So we're already somewhere along an uncertain warming curve, with temperatures rising by about 1.2 degrees Celsius (2.2 degrees Fahrenheit) since the late 19th century.
Regardless of how temperatures end up behaving, most estimates of future warming come from studies of the relationship between past temperatures and carbon dioxide levels. To do this, the scientists analyzed a variety of materials, including air bubbles in ice cores, the chemical composition of ancient soil and ocean sediments, and the anatomy of fossil plant leaves.
Members of the consortium are not collecting new data; instead, they work together to classify published studies to assess their reliability based on evolving knowledge. Based on the new findings, they eliminated some studies they considered outdated or incomplete and recalibrated others based on the latest analysis techniques. They then calculated a new 66-million-year curve of carbon dioxide versus temperature, based on all the evidence to date, and reached consensus on a so-called "Earth system sensitivity," according to which every doubling of carbon dioxide would raise Earth's temperature by 5 to 8 degrees Celsius.
Understanding Earth System Sensitivity
It should be noted that Earth system sensitivity describes climate changes over hundreds of thousands of years, not decades or centuries that are closely relevant to humans. In the long term, the increase in temperature may come from intertwined Earth processes that go beyond the direct greenhouse effect of carbon dioxide in the air, the authors say. These processes include the melting of polar ice caps, which will reduce the Earth's ability to reflect solar energy; changes in land plant cover; and changes in clouds and atmospheric aerosols, which may raise or lower temperatures.
"If you want us to tell you what the temperature will be in 2100, this doesn't tell you. But it does have implications for current climate policy," said co-author Dana Royer, a paleoclimatologist at Wesleyan University. "It reinforces what we already know. It also tells us that there are slow cascading effects that will last for thousands of years."
The study will be helpful to climate modelers trying to predict what will happen in the coming decades, Honig said, because they will be able to incorporate new powerful observations into their studies and distinguish between processes operating on short and long time scales. She noted that all data for the project is available in an open database and will be updated on a rolling basis.
Improve the relationship between carbon dioxide and temperature
The new study, which covers the so-called Cenozoic Era, does not fundamentally change the well-established relationship between carbon dioxide and temperature, but it enhances understanding of some periods and refines measurements of others.
The most distant period, from about 66 million years ago to 56 million years ago, has been a mystery because the Earth was largely ice-free at the time, but some studies suggest carbon dioxide concentrations were relatively low. This casts some doubt on the relationship between carbon dioxide and temperature. However, when the team eliminated what they considered the least reliable estimates, they determined that carbon dioxide concentrations were actually quite high, around 600 to 700 parts per million, comparable to concentrations likely to be reached by the end of the century.
The researchers confirmed long-held belief that the hottest period was about 50 million years ago, when carbon dioxide levels reached 1,600 parts per million and temperatures were 12 degrees Celsius higher than today. But by about 34 million years ago, carbon dioxide levels dropped enough that today's Antarctic ice sheets began to form. After some ups and downs, there was a further long-term decline in carbon dioxide, during which the ancestors of many modern plants and animals began to evolve. The authors of the paper said this shows that changes in carbon dioxide affect not only the climate but also ecosystems.
Impact on modern climate
The new assessment shows that about 16 million years ago, carbon dioxide concentrations were last consistently higher than today's levels, at about 480 parts per million; by 14 million years ago, carbon dioxide concentrations had dropped to today's human-caused levels of 420 parts per million. After that, carbon dioxide concentrations continued to fall, reaching about 270 or 280 ppm about 2.5 million years ago, triggering a series of ice ages. Carbon dioxide levels were at or below this level when modern humans emerged about 400,000 years ago, and continued until we began massive destruction of the atmosphere about 250 years ago.
Study co-author Gabriel Bowen, a professor at the University of Utah, said: "No matter how many degrees the temperature changes, it is clear that we have brought the planet into conditions our species has never seen before. This should make us pause and question what is the right path forward."
The consortium has now grown into a larger project that aims to map the evolution of carbon dioxide and climate throughout the Cenozoic Era, from 540 million years ago to the present.
Compiled from /ScitechDaily