Carbon: Too Much of a Good Thing

Carbon is the backbone of life. It is literally in our bones among everything else from plants and rocks to the air we breathe and in the oceans. With the unique ability to bond to a multitude of atoms, carbon can build large, complex carbon-based molecules necessary to sustain all living matter.

“It’s fuel and energy essential for life,” said Noah Walcutt, a Ph.D. candidate at the University of Rhode Island’s Graduate School of Oceanography (GSO), discussing the importance of carbon and how it is cycled through all living and nonliving matter during the Bay Informed discussion series in August. “Carbon is moving between all these different [sources] … over different time scales. Some of these transfers are happening daily through photosynthesis and breathing. Some are happening more slowly, like [when] rock [is] subducted from the ocean floor into the earth’s mantle and ejected back out through volcanoes.”

Despite its versatility and utility, carbon can also have devastating effects.

“Carbon dioxide is a major greenhouse gas [in the atmosphere] that acts as a blanket, preventing radiation from leaving the planet,” said Christine Gardiner, a GSO graduate student who also presented at the Bay Informed series. “It’s why we’re able to live here. We rely heavily on this effect. But when it’s too much, it becomes too hot. We need it in moderation.”

Massive amounts of carbon in the form of carbon dioxide (CO2) that spewed into the atmosphere in the form of volcanic ash are believed to be the culprit of the “Great Dying,” one of the planet’s biggest extinction events 252 million years ago. Nearly all life was snuffed out as a consequence of accelerated global temperatures and ocean acidification due to carbon. It took over 10 million years for the planet to flourish with life again, giving rise to the Jurassic Period.

Scientists know this, said Walcutt, because of different isotopic markers, or types of carbon, trapped in glaciers, sediments, and fossils.

“Where carbon exists can tell us about how our planet was in past, how it is now, and what it might be in the future,” he said, explaining that carbon radioactively decays over time. Measuring those markers tells scientists how much carbon existed either in the oceans and atmosphere and when. “Carbon that’s been buried for a very long time doesn’t have a lot of radioactivity. As a result, the ratio of radioactive carbon to nonradioactive carbon should go down, which is what we’ve seen since 1850. This is how scientists know that carbon in the Keeling Curve is from human activities burning fossil fuels.”

Dr. Charles Keeling first measured carbon dioxide in the atmosphere on a continuous basis and was the first to report global atmospheric concentrations were rising.

The Keeling Curve, named after Charles David Keeling, is one of the more well-known scientific graphs. It looks like a profile of a gently rising hill that quickly climbs a much steeper slope leading to an unknown summit – showcasing the rapid increase of carbon dioxide in the atmosphere since the 1950s. While the Earth’s history includes natural fluctuations of carbon, the rate at which carbon is being added to the atmosphere is outpacing natural sources like respiration or decomposition.

Carbon has three signature “isotopic fingerprints” based on the number of neutrons an atom carries. The most common type is carbon-12 (12C), which has six neutrons and makes up 99 percent of all carbon on Earth. While carbon-13 (13C) has an additional neutron and is less prevalent, it is a stable isotope and does not decay over time like 12C. Carbon-14 (14C) has two extra neutrons and is the least stable isotope that is continually created in the atmosphere as the sun’s ultraviolet rays react with nitrogen. This isotope radioactively decays and is what scientists use to figure out the age of carbon-based objects like fossils. Today’s naturally occurring sources of carbon dioxide, therefore, have nearly the same amount of 14C as the atmosphere, unlike ancient sources such as fossil fuels.

Fossil fuels are formed from the remains of plants and animals, said Gardiner, that melted and liquefied to form crude oil, coal, and natural gas due to the immense pressure and heat from being buried deep within the earth’s crust. When they are burned, only 12C and 13C are released, increasing their proportion in the atmosphere compared to 14C.

“[The Keeling Curve] is the smoking gun for how scientists knew that carbon was increasing over time and where is it coming from,” said Walcutt, noting that since the 1960s the amount of carbon in the atmosphere has jumped from 310 to 410 parts per million (ppm), a level which has not been seen for millions of years.

The amount of carbon dioxide released into the atmosphere that triggered the massive die-off before the rule of dinosaurs is believed to be at a comparable rate to today, say researchers from the University of Edinburgh (Newsweek). While the planet has experienced 1,000 ppm of carbon dioxide in the atmosphere, said Walcutt, it was not a condition conducive for the majority of life.

“Carbon in the atmosphere isn’t a bad thing, but the cycles that absorb carbon – the plants and the ocean – are being outpaced by the carbon that’s being put out from emissions,” explained Walcutt. “We’re throwing the equilibrium that’s existed for millions of years out of whack.”

The ocean is the largest carbon sink, housing nearly a third of all carbon released from fossil fuel burning and land-use changes, such as clear-cutting forests that also act as natural carbon sinks through photosynthesis, a process that converts sunlight and carbon dioxide into sugar as an energy source for plants.

Carbon dioxide in the atmosphere is readily dissolved to form carbonic acid (H2CO3), which is causing the oceans to rapidly acidify at a rate of one million tons per hour. This chemistry shift that would naturally take thousands of years and allow marine life to adapt is now happening in the span of 50 to 80 years (Scientific American).

Some of the carbon dissolved in the surface waters will be used for photosynthesis by phytoplankton, which also releases oxygen as a byproduct.

“There’s so much diversity you can’t see that plays a huge role in cycling carbon,” said Walcutt, describing various species of phytoplankton, or algae that play an important role in cycling carbon and creating most of the oxygen we breathe. “The ocean holds less than 1 percent of the earth’s plant biomass but produces over half of global [oxygen] production.”

This is why, he said, the ocean is referred to as the lungs of the planet. “Every other breath you take is from the ocean.”

The carbon used for photosynthesis, according to Walcutt, will make its way down to the deep ocean through the biological carbon pump in the form of “marine snow” as plant and animal remains sink into deeper water where they – and the carbon they contain –  can be sequestered for millions of years. About 1 gigaton, or 1 billion tons, of carbon are transferred from the atmosphere to the deep ocean as a result. But as the oceans continue to warm and acidify, researchers are concerned phytoplankton’s ability to consume carbon and create oxygen may be drastically reduced.

 

Black Carbon

Deposits of black carbon (soot) and dust on snow and ice accelerate melting

Carbon dioxide is not the only problematic greenhouse gas; other climate-offending carbon-based emissions include methane (CH4) and black carbon – the char and soot residue that’s left after carbon is burned.

The main sources of black carbon, according to Gardiner, are oil and gas emissions, such as from cars, coal fires, agricultural sources, and forest fires, with diesel engines accounting for 80 percent of emissions.

Black carbon, Gardiner said, is like carbon dioxide, but it’s a dark compound. “Just like wearing black clothes on a hot day, you’re going to absorb more heat than if you were wearing a white shirt.”

Even though black carbon traps more heat, she added, it has a much shorter shelf-life than carbon dioxide.

“Carbon dioxide can stay in the atmosphere for thousands of years, whereas black carbon is only there for about a week from when you burn it. It condenses to form clouds and rains back down,” she explained, emphasizing that while black carbon is a strong contributor to pollution in cities and increased global temperatures, its greatest impact is in the polar regions. “All that black carbon builds up over time on that pristine ice. The dark color causes ice to melt faster because it absorbs more heat, leading to more warming.”

Glaciers melting and retreating send a negative ripple effect. Regions that rely on ice melt for freshwater and food will have less water to sustain their supplies (Earth Institute, Columbia University), while other areas will experience significant sea level rise and flooding. All this makes black carbon a major threat to climate and health – second only to carbon dioxide. Unlike carbon dioxide, however, the effects of black carbon can more readily be reversed if sources are removed (Center for Climate and Energy Solutions).

The impacts of black carbon weren’t largely realized, Gardiner said, until the 1990s. She and other researchers are still working on how best to measure this particle to better understand its behavior and effects on health and the environment.

 

Counting Carbon

For scientists, much of the work in understanding carbon will be like that of an accountant. According to Walcutt, it’s measuring carbon inputs and outputs to better understand the reservoirs of where carbon is coming from and going to, how it’s transported, and to identify any imbalances.

“You have to count how much carbon is moving from one pool to another and see how much is leftover in the process,” he said. “It’s building budgets and keeping track of where the carbon is going.”

The total potential of global carbon reserves in the ground which includes coal, oil, oil shale, tar sands, and gas, is estimated to be around  2795 gigaton. Only a fraction needs to be released for carbon dioxide levels to reach 1,000 ppm. At the current rate of emissions, it’s been calculated that it will only take five years to exhaust the carbon budget, the amount that could be released via emissions in order to keep global temperatures from rising above 1.5 or 2 degrees Celsius – surpassing a goal set by the Paris Agreement.

By all accounts, more carbon is being added to the atmosphere than is being taken out, as is evident by rising global temperatures, acidification, droughts, floods, and storms. Balancing this carbon budget may be one of the most important tasks in ensuring a sustainable future.

 

The Bay Informed Discussion Series is sponsored by Rhode Island Sea Grant in Partnership with GSO. This series is held every third Thursday of the month at 7 p.m. at URI’s Graduate School of Oceanography’s Bay campus in Narragansett. These events are designed for the community to get involved and learn more about research at GSO.

The next event will be on September 21: Block Island Wind Farm and Coastal Impacts

For more information, visit http://web.uri.edu/chowder-marching/bay-informed/, on Facebook @bayinformed, or contact gsobayinformed@gmail.com.


–Meredith Haas | Rhode Island Sea Grant Research Communications Specialist