Author: Katrina Outland
Institution: Hawaii Pacific University
Date: July 2005
Melting ice caps, massive flooding, and Armageddon-scale storms reminiscent of The Day After Tomorrow - it seems that everyone has been uttering the new doomsday phrase, "global warming." Environmentalists worry about it, big businesses dismiss it, and many media representations wildly distort it. Misconceptions run rampant and amidst all the controversy, it is almost impossible for the rest of us to decide whom to believe. What exactly is global warming, what causes it, and can we mitigate its effects?
Unfortunately, the complete answers to these questions are complex and depend on many factors. However, the accepted definition among scientists, including NASA and the Intergovernmental Panel on Climate Change (IPCC), is that global warming is the recent increase in the earth's surface temperature, most likely caused by an increase in greenhouse gases. These greenhouse gases include water vapor, carbon dioxide (CO2), methane, and other natural- or human-produced gases that trap solar radiation in the atmosphere. Of these, CO2 is the most significant human contribution to global warming, according to the Australian Greenhouse Office and the US Department of Energy.
Since the main greenhouse gas is largely emitted by humans, is there a way to mitigate global warming? There is no simple answer. The phenomenon is influenced by various global processes, such as deforestation and industrialization. But there is an aspect of global warming that is not often discussed in the media: the role of the oceans. While most scientists agree that our best chance of reducing global warming is by cutting back on fossil fuel burning, some scientists are looking towards the oceans for additional means.
Nature's Thermostat, the Oceans
The oceans are vital in maintaining a climate suitable for life. One of the most important ways they accomplish this is through the carbon cycle, which consists of two main processes: the direct air-surface exchange and the biological pump. Both processes can take some of the greenhouse gas CO2 out of the air. According to Chris Winn, dean of marine science at Hawaii Pacific University (HPU), the air-surface exchange is a natural equilibration process, allowing CO2 to flow back and forth between the ocean and atmosphere.
"[CO2] used to be in relative equilibrium," says Winn. "But as we add it [into the air], the pressure in the atmosphere rises and forces CO2 into the oceans. Now there's more going in than coming out."
The ocean absorbs excess CO2 , at first this sounds ideal. Unfortunately, there are two problems in this assumption. The first, as Winn explained, is that CO2 does not dissolve as easily in warmer water, and in some regions there might actually be an overall flux of CO2 back into the atmosphere. The second problem is speed. According to Michael McElroy, professor of environmental science at Butler University, the air-surface exchange can take hundreds or even thousands of years.
"There is no hope that this process will take place fast enough to help control the buildup of CO2 [in the atmosphere]," explained McElroy in a 2002 interview.
The second half of the ocean's carbon cycle is the biological pump. Phytoplankton , tiny, plantlike creatures that drift near the ocean surface , absorb CO2 , die, and sink, carrying their carbon with them. This "rain of detritus," first described by Alexander Agassiz in the late 1800s, allows carbon from the surface to redeposit in the depths, either as CO2 or in some organic form.
However, as with the air-surface exchange, the biological pump acts very slowly. Because both parts of the ocean's carbon cycle take so long, how can they help slow down global warming today?
The Need for Speed: Human Interference in the Carbon Cycle
As humans, we don't like to wait for anything. Even if the oceans could absorb all the excess CO2 in the atmosphere, most of us do not want to sit around for a few thousand years to find out. Instead, scientists have been searching for ways to boost the ocean's natural carbon cycle.
One proposal is to bypass the slow air-surface exchange by carbon sequestration,the direct transport and storage of CO2 in the deep ocean. This "injection" could be done by a stationary outfall pipe on the sea floor, much like a sewage pipe, pumping CO2 from onshore or an offshore ship, according to Eric Vetter, professor of ecology at HPU.
Carbon sequestration was one of the "hot" issues discussed at the last conference of Greenhouse Gas Control Technologies (GHGT) held in Vancouver, Canada in September 2004. Speakers at the conference brought up the most recent efforts of sequestering CO2, including the international Carbon Sequestration Leadership Forum (for more details, click here).
Another proposal involves speeding up the biological pump by human-induced phytoplankton blooms. The hypothesis is if more phytoplankton is present, then it can absorb more CO2. One of the more popular methods of inducing the blooms is to provide the phytoplankton with iron, a rare nutrient that usually limits their growth. Scientists have conducted several experiments of iron enrichments throughout the Pacific and Southern oceans. In a study published in 1996, Rutgers University professor Michael Behrenfeld found that adding iron to low-nutrient areas resulted in a massive phytoplankton bloom and a reduction in surface CO2 concentrations. Nicholas Meskhidze of the Georgia Institute of Technology even found evidence that sulfur dioxide (SO2)in air pollution can help make iron more soluble, also boosting phytoplankton growth.
What's the Catch?
At first glance, induced phytoplankton blooms and carbon sequestration seem like ideal ways to reduce global warming. However, no system is perfect and the costs may outweigh the benefits.
Trying to influence the biological pump to reduce global warming may have unexpected consequences , much like the idea to use the warfare chemical DDT as a pesticide did. Ecological systems are highly complex, and artificially boosting the growth of phytoplankton might have unforeseen effects. For example, scientists Andrew Bakun and Scarla Weeks published a 2005 study in which they found that massive unnatural phytoplankton blooms in several areas around the world caused "dead zones." After the phytoplankton died and sank, they released methane and hydrogen sulfide as they decayed, killing sea life and perhaps even adding to global warming.
Professor of biological oceanography James McCarthy explains that phytoplankton growth depends on many factors, including nutrient supply, sunlight, and circulation. No one knows how changing global temperatures may affect the currents and consequent growth of plankton. Even iron enrichment depends on too many factors to be easily predicted, according to studies done by researcher Philip Boyd from the University of Otago in New Zealand. Water temperature alone can change the results from one area to another. In short, we can not yet predict if human interference in the biological pump will be effective enough to mitigate global warming.
This ecological manipulation is a complicated device, so what about carbon sequestration? The concept certainly sounds simpler: taking excess CO2 from the air and putting it someplace where it cannot contribute to global warming.
In actuality, it is not so simple. Some scientists, including Eric Vetter, have done experiments to see how a CO2 outfall pipe might affect the community of animals living on the sea floor. Vetter and his colleagues found that high levels of CO2 act as a toxin, temporarily incapacitating animals in the area. In addition, scavengers refused to enter the areas, so Vetter predicts that organic matter will not decay in these areas. On top of these possible ecological problems, carbon sequestration is expensive.
"We could potentially mitigate the effects [of global warming]," claims Vetter, "but this is no silver bullet; this is not going to solve our problems."
The oceans are nature's long-term thermostat, but can we as humans use its carbon cycling to mitigate the effects of global warming? Perhaps. but it will probably not be our main focus. The oceans are undoubtedly an important component, but any speedy cures are unlikely.
[link = http://www.greenhouse.gov.au/agriculture/framework/append2.html]Australian Greenhouse Office[link].
Bakun, A, and Weeks, SJ. (2004). Greenhouse gas buildup, sardines, submarine eruptions,
and the possibility of abrupt degradation of intense marine upwelling ecosystems. EcologyLetters. 7(11).
Behrenfeld, MJ et al. (1996). Confirmation of iron limitation of phytoplankton photosynthesis
photsynthesis in the equatorial Pacific Ocean. Nature. 383:508-511.
Boyd, PW. (2002). The role of iron in the biogeochemistry of the Southern Ocean and
equatorial Pacific: a comparison of in situ iron enrichments. Deep-Sea Resaerch II.
Greenhouse Gas Control Technologies (GHGT):
7th Convention (2004).
8th Convention (2006).
Meskhidze, N and W Chameides. (2005). Dust and Pollution: A Recipe for Enhanced Ocean
Fertilization. Journal of Geophysical Research , Atmospheres.
Shaw, J. (2002). The Great Global Experiment. Harvard Magazine.