Humans are releasing planet-warming carbon dioxide at about 10 times faster than the most rapid event of anytime in at least the past 66 million years. This leaves us without a historical analogue to guide predictions for how climate change will affect the world in coming years, a new study has found.
The study, published Monday in the journal Nature Geoscience, comes about a week after news broke that the level of carbon dioxide in the Earth’s atmosphere spiked by the largest amount on record in 2015, and on the heels of the hottest year and mildest first two months of 2016 on record.
February, for example, had the highest temperature departure from average of any month on record since at least 1880, NASA and the National Oceanic and Atmospheric Administration found.
To reach their conclusions, the researchers examined ancient sediments off the coast of Millville, New Jersey, looking at the chemical composition of sediment layers that were laid down millions of years ago, containing clues about the planet’s climate history. Specifically, they researched trends in carbon and oxygen isotopes.
The carbon isotopes represent new carbon coming into the climate system, and the oxygen isotopes represent the climate's response to this pulse of excess carbon.
The study focuses on what the isotope ratios reveal about what occurred during a time period in Earth’s geological history known as the Paleocene-Eocene Thermal Boundary, or PETM. The PETM took place about 56 million years ago.
The isotopes changed in virtual lockstep with each other, indicating that the carbon release during the PETM was more likely to have taken place over a long period of time, due to what is known about the lag effects in the climate system and how the climate responds to sudden, massive injections of carbon dioxide.
There has been a long-running debate in the scientific community about just what caused the massive climate change and sizable species extinctions at the PETM boundary, and how quickly carbon was released into the atmosphere, thereby warming the air and seas while also acidifying the oceans.
While mass extinctions of marine creatures occurred during the PETM, there was not widespread species loss on land, according to study co-author Richard Zeebe of the University of Hawaii at Manoa.
During the PETM, the total amount of carbon released was about equal to all the fossil fuels thought to be currently left on Earth. This period likely saw the largest carbon release in at least the past 66 million years, according to the study.
“I think to me it’s completely clear we have entered a completely new era in terms of what humans can do on this planet”
The study also used computer model simulations of Earth’s climate and carbon cycle in order to estimate rates of change from the record without needing precise knowledge of the precise ages of each sediment layer in the record.
The global temperature increase during the PETM is thought to be between 4 and 8 degrees Celsius, or 7.2 to 14.4 degrees Fahrenheit, according to Scott Wing, a paleobiologist and curator at the Smithsonian Museum of Natural History in Washington, who did not contribute to the new study.
“As far as we know the PETM, the Paleocene-Eocene boundary is the event in the last 66 million years with the largest carbon release and most likely also the fastest, and that’s why we have studied this event,” Zeebe told Mashable.
Zeebe and his colleagues found that the maximum emissions rate during the beginning of the PETM was likely on the order of about 1.1 billion tons per year. Overall, the PETM onset is thought to have lasted more than 4,000 years.
This compares to today’s emissions rates of closer to 10 billion tons per year during a period of just a few centuries.
In other words, the study concludes that the current global emissions rate is about 10 times as fast as what occurred during the PETM.
Specifically, the study shows that the temperature increase associated with the first jump in carbon emissions when the PETM began was synchronous rather than instantaneous. This means they evolved together during the course of several thousand years, which suggests a relatively slow rate of carbon emissions spread out across a long period of time.
A more rapid increase in emissions would have produced more of a lag between the carbon release and temperature increase, since the climate system does not respond instantaneously to a sharp rise in greenhouse gases, given that it takes time to cycle carbon through the planet’s oceans and land-based carbon sinks, like forests.
We are now operating with no analogue
One of the biggest implications of the study’s findings is that the impacts of modern-day climate change on the seas and land will be more severe than what was seen during the PETM.
The impacts of modern-day climate change on the seas and land will be more severe.
This is especially the case in the oceans, where carbon is being absorbed at rapid rates, making the waters more acidic and interfering with the ability of species that rely on calcium carbonate to make their shells, such as scallops and oysters, to function normally.
Zeebe said the ocean acidification lessons of the study are particularly significant.
“... Unfortunately because we’re doing it so fast, our conclusion is that the rate of acidification will be much more severe in the future, and also that the effects on the calcifiers are likely to be more severe than what we see during the PETM,” he told Mashable.
A high rate of warming can overwhelm the ability of many marine and land species to adapt to climate change.
Zeebe said ecosystems on land will also see larger changes than what occurred during the PETM because the pace of climate change is now more rapid than it has been even during that time.
Wing said the study is important for providing new insight into an important question: “We want to know how fast the PETM happened because the rate of carbon addition is critical to determining its effects on everything."
“If the rate of PETM addition was 10 billion tons per year, then it is very much like current human addition, and we might expect the future to look like the PETM in many ways," he said.
"If the rate of PETM carbon addition was a tenth of our rate, then the future will likely be much more extreme than the PETM in many ways,” Wing said.
More evidence of the 'anthropocene'
Zeebe has conducted numerous studies on the planet’s climate history using chemical signatures contained in various marine organisms, known as foraminifera.
This has given him a unique perspective on the Earth’s geological history, and how to place modern-day climate change within a long-term perspective.
Zeebe is one of many researchers who favor naming the period starting with the industrial revolution as a new geological epoch, known as the Anthropocene.
“I think to me it’s completely clear we have entered a completely new era in terms of what humans can do on this planet,” he said.
“... If you look at the past and if you study the geologic record, every time when there was massive carbon release there were major changes on the planet and there were significant, large changes in climate."
Experts who did not participate in the new study said it helps to shed insight on a critical question concerning the PETM and modern climate change, although there are some questions about how the study's authors arrived at their conclusions.
Paul Pearson, a researcher at Cardiff University in Wales, who was not involved in this research, said the study shows that the carbon input into the Earth’s atmosphere during the PETM, as well as the climate’s response to those emissions, were likely gradual over thousands of years.
“This means that the rate of carbon input was probably much lower than the modern anthropogenic [man-made] rate,” he told Mashable. “This makes anthropogenic change a 'no-analogue state' in their jargon - in other words we are now changing the Earth much faster than ever before.”
Peter Stassen, a postdoctoral fellow at the University of Leuven in Belgium who wasn't involved with the new research, said the study does imply that we’re entering a time period for which we have no historical guide.
“We are probably entering a new era of climate alteration (Anthropocene) that is unknown from the deep-time geological history in respect to the rate of variations in atmospheric CO2 levels,” he wrote in an email.
"We’re still underestimating the long-term consequences of our fossil fuel burning"
The study does come with important caveats.
For example, precisely dating the age of sediment during the PETM is difficult, although Zeebe and his colleagues tried to get around this by using statistical approaches to estimate the emissions rate rather than pinpointing the exact years at which emissions and climate responses occurred.
Pearson, of Cardiff University, said the study adds valuable insight to climate science.
“I think the conclusion is valid, although I am concerned about one aspect: whether the isotope shifts, especially oxygen, are a primary record of environmental change or if there could be some other explanation,” he said.
“Other cores in the New Jersey area show quite [a] different magnitude of shift… which is currently difficult to understand.”
Wing said the study is important in that it addresses a nagging question about the PETM, but he is not yet sure that it’s methods and conclusions are completely robust.
“What I have to figure out is how sensitive Zeebe’s analysis is to the assumptions that they have to make about rate of ocean warming, and how good the data are, since the dissolution of carbonate is a problem in most marine records,” he told Mashable. “It will take some time to evaluate these factors.”
According to Zeebe, it would be an error to assume that we currently know by how much the world will warm during the next few centuries based on emissions rates and historical climate records.
If anything, he says, past climate data, such as that revealed in the new study, shows that warming may be on the upper end of projections.
“We should be careful, in terms of the long-term consequences of our fossil fuel burning,” Zeebe warns, “because it is possible, and this is what these records tell us, that we’re still underestimating the long-term consequences of our fossil fuel burning.”