Ocean acidification (OA) is one of the largest issues resulting from anthropogenic pumping of CO₂into the atmosphere. CO₂in the atmosphere dissolves into oceans, leading to a decrease in pH (called the bicarbonate buffer system):
CO₂+H₂O ↔ H₂CO₃ ↔ HCO₃¯+H⁺ ↔ CO₃¯+H⁺
Atmospheric CO₂doubling will likely lower pH of the entire ocean > 0.1 unit.
The normal variation of pH in open seawater (7.6𔃆.2),
so .1 units is very significant.
Effects on marine organisms include physiological responses (regulating acid-base imbalance) as well a dissolution of calcium carbonate support structures like shells, tests, or exoskeletons as saturation horizons rise. It is possible deep sea organisms may be particularly vulnerable. For example, the internal control of pH is critical for proper physiological functioning, so many organisms have evolved elaborate methods to regulate internal pH. However, the pH in most of the deep sea is stable over thousands of years, so deep sea organisms have not needed methods to rapidly adapt to or regulate changes in pH. Ocean acidification may be too fast for these deep sea organisms to adapt to (Seibel and Walsh, 2002).
Changes in carbonate saturation horizons are one of the biggest worries when people talk about effects of OA on organisms. The solubility of calcium carbonate (CaCO₃) increases with decreasing temperature and increasing pressure. In the north Pacific, the rate of rise for aragonite saturation horizon is around 1 m y¯¹. At atmospheric CO₂ = 780 ppm (near the end of this century), the subarctic North Pacific and Southern Ocean will be undersaturated with respect to aragonite (Fabry et al., 2008; Feely et al., 2006; Orr et al., 2005.) Major planktonic calcium carbonate producers like coccolithophores, foraminifera, and euthecosomatous pteropods (organisms responsible for nearly all the export flux of calcium carbonate to the deep sea) may be at risk as well. In lab studies, foraminifera and pteropods for example showed possible reduced calcification with decreasing pH (Feely et al., 2004; Orr et al., 2005).
Effects on marine organisms include physiological responses (regulating acid-base imbalance) as well a dissolution of calcium carbonate support structures like shells, tests, or exoskeletons as saturation horizons rise. It is possible deep sea organisms may be particularly vulnerable. For example, the internal control of pH is critical for proper physiological functioning, so many organisms have evolved elaborate methods to regulate internal pH. However, the pH in most of the deep sea is stable over thousands of years, so deep sea organisms have not needed methods to rapidly adapt to or regulate changes in pH. Ocean acidification may be too fast for these deep sea organisms to adapt to (Seibel and Walsh, 2002).
Changes in carbonate saturation horizons are one of the biggest worries when people talk about effects of OA on organisms. The solubility of calcium carbonate (CaCO₃) increases with decreasing temperature and increasing pressure. In the north Pacific, the rate of rise for aragonite saturation horizon is around 1 m y¯¹. At atmospheric CO₂ = 780 ppm (near the end of this century), the subarctic North Pacific and Southern Ocean will be undersaturated with respect to aragonite (Fabry et al., 2008; Feely et al., 2006; Orr et al., 2005.) Major planktonic calcium carbonate producers like coccolithophores, foraminifera, and euthecosomatous pteropods (organisms responsible for nearly all the export flux of calcium carbonate to the deep sea) may be at risk as well. In lab studies, foraminifera and pteropods for example showed possible reduced calcification with decreasing pH (Feely et al., 2004; Orr et al., 2005).
So
to be blunt, this is bad.
But what is going to happen? Is there another way, besides controlled laboratory experiments, to gauge where the ocean is headed? Other than just sitting and waiting around to see what happens, I mean.
We could look to the past.
The Paleocene-Eocene Thermal Maximum
About 55 million years ago, sea surface temperatures rose rapidly, about 5-10°C in only a thousand years. This rapid warming was likely due to increases in greenhouse forcing, just like today. Rather than anthropogenic however, this rapid influx of carbon to the atmosphere may have come from methane hydrates at the bottom of the ocean. For those of you who are isotopically inclined, δ¹³C records from deep sea sediment cores show a rapid initial decrease (around 20,000 years) followed by a gradual recovery (~130,000 years) back to similar δ¹³C values found before the excursion. The magnitude of the drop in δ¹³C values (-3‰) suggest the carbon source was very depleted in 13C, pointing to methane hydrates as the likely source. This carbon isotope excursion was the signature of the Paleocene-Eocene Thermal Maximum (PETM) (Zachos et al., 2005).
Part of influx of CO2 to the atmosphere dissolved into the oceans, lowering pH. This resulted in a rise in the lysocline and calcite compensation depth (CCD) which promoted the dissolution of seafloor carbonate. The CCD shoaled over 2 km within just a few thousand years, but recovery was gradual, around 60,000 years. Ultimately this CO2 would be sequestered through chemical weathering of silicate rocks (Zachos et al., 2005).
All marine communities experienced major changes, including migrations to higher latitudes, evolutionary radiations, and extinctions. There were also distinct responses between planktonic and benthic organisms. Planktonic organisms weren't particularly affected, but did experience radiation and diversification. However for benthic organisms, the PETM marked the largest extinction event in the last 90 million years. 30 to 50% of benthic species became extinct. It's important to keep in mind that gauging PETM effects on marine ecosystems relies entirely on microfossils, as no macroinvertibrate fossils have been described (Rodriguez-Tovar et al., 2011; McInerney and Wing, 2011).
Surprisingly, the driving force affecting these organisms may have been temperature rather than ocean acidification. Temperature affects bottom water oxygenation and increases metabolic rates, meaning organisms need more food to maintain base metabolism (McInerney and Wing, 2011).
Applicability?
Can we expect to see similar responses from marine organisms today? How similar was the PETM to today's global warming/ ocean acidification problem? It turns out we are likely in for much worse. for one, the rate of carbon input was much different. In the PETM, carbon was input to the atmosphere over an 8,000 year period. Contrast that with our pumping CO2 into the atmosphere over a mere 300 years. That's less than the mixing time of the ocean. The longer CO2 input rate during the PETM meant less severe acidification and carbonate dissolution in the surface ocean.
Something else to consider: there was no ice during the PETM. No glaciers, snow. Nothing. No ice means no ice/albedo feedback (warming leads to melting of ice, revealing darker surfaces which absorb more heat, leading to more warming) which means an absence of greater warming at polar latitudes. In contrast, the most drastic warming today is at the poles (McInerney and Wing, 2011; Ridgwell and Schmidt, 2010).
But what is going to happen? Is there another way, besides controlled laboratory experiments, to gauge where the ocean is headed? Other than just sitting and waiting around to see what happens, I mean.
We could look to the past.
The Paleocene-Eocene Thermal Maximum
About 55 million years ago, sea surface temperatures rose rapidly, about 5-10°C in only a thousand years. This rapid warming was likely due to increases in greenhouse forcing, just like today. Rather than anthropogenic however, this rapid influx of carbon to the atmosphere may have come from methane hydrates at the bottom of the ocean. For those of you who are isotopically inclined, δ¹³C records from deep sea sediment cores show a rapid initial decrease (around 20,000 years) followed by a gradual recovery (~130,000 years) back to similar δ¹³C values found before the excursion. The magnitude of the drop in δ¹³C values (-3‰) suggest the carbon source was very depleted in 13C, pointing to methane hydrates as the likely source. This carbon isotope excursion was the signature of the Paleocene-Eocene Thermal Maximum (PETM) (Zachos et al., 2005).
Part of influx of CO2 to the atmosphere dissolved into the oceans, lowering pH. This resulted in a rise in the lysocline and calcite compensation depth (CCD) which promoted the dissolution of seafloor carbonate. The CCD shoaled over 2 km within just a few thousand years, but recovery was gradual, around 60,000 years. Ultimately this CO2 would be sequestered through chemical weathering of silicate rocks (Zachos et al., 2005).
All marine communities experienced major changes, including migrations to higher latitudes, evolutionary radiations, and extinctions. There were also distinct responses between planktonic and benthic organisms. Planktonic organisms weren't particularly affected, but did experience radiation and diversification. However for benthic organisms, the PETM marked the largest extinction event in the last 90 million years. 30 to 50% of benthic species became extinct. It's important to keep in mind that gauging PETM effects on marine ecosystems relies entirely on microfossils, as no macroinvertibrate fossils have been described (Rodriguez-Tovar et al., 2011; McInerney and Wing, 2011).
Surprisingly, the driving force affecting these organisms may have been temperature rather than ocean acidification. Temperature affects bottom water oxygenation and increases metabolic rates, meaning organisms need more food to maintain base metabolism (McInerney and Wing, 2011).
Applicability?
Can we expect to see similar responses from marine organisms today? How similar was the PETM to today's global warming/ ocean acidification problem? It turns out we are likely in for much worse. for one, the rate of carbon input was much different. In the PETM, carbon was input to the atmosphere over an 8,000 year period. Contrast that with our pumping CO2 into the atmosphere over a mere 300 years. That's less than the mixing time of the ocean. The longer CO2 input rate during the PETM meant less severe acidification and carbonate dissolution in the surface ocean.
Something else to consider: there was no ice during the PETM. No glaciers, snow. Nothing. No ice means no ice/albedo feedback (warming leads to melting of ice, revealing darker surfaces which absorb more heat, leading to more warming) which means an absence of greater warming at polar latitudes. In contrast, the most drastic warming today is at the poles (McInerney and Wing, 2011; Ridgwell and Schmidt, 2010).
Check out this video:
We are heading for something unprecedented. The PETM at best
provides a framework for the mimimum damage today's anthropogenically induced
problems will cause deep sea marine life.
Helpful link:
