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Climate Change

Global change and the future ocean

Article: Carlos M.Duarte. Global change and the future ocean: a grand challenge for marine sciences. Frontiers in Marine Science. 2014. Volume 1, Article 63.4015–4036. doi:10.3389/fmars.2014.0006

Global Change and Its Causes

The Growing number of environmental changes, such as contamination of seafood, shortage of water and increased frequency of extreme weather (floods, drought, hurricanes, etc), has raised people’s concerns about the Earth’s ability to sustain human populations. Because terrestrial ecosystems received 90% of research on impacts from anthropogenic global change, more studies on ocean ecosystems are needed. Being aware of the scarcity of research forecasting future characteristics of ocean ecosystems, the science community established a forum to share their assessment of global change and predictions of the future state of our oceans in early 2014 in Frontiers in Marine Science.

Global change has both natural and anthropogenic drivers. While natural drivers are unpredictable and cannot be managed (for example, the impacts from large asteroids causing mass extinction in the Cretaceous-Paleogene boundary), anthropogenic drivers are predictable and can be managed. Human forces on global change consist of both growing population as well as growth in per capita resource use.  Human population is expected to exceed 9 billion by 2050, which is within the median estimates of the carrying capacity of the planet. However, this estimate is based on minimum resource requirements of individuals. More resources will be used (reducing the planet’s carrying capacity) if we continue to increase in resource per capita. Technology developments allowed people to access resources that are previously unavailable; examples include deep sea oil and conversion of atmospheric nitrogen to fertilizers.

Drivers of global changes are interconnected. The development of biofuels, for example, promotes the idea that using them can mitigate global change. Although biofuels can shrink the usage of fossil fuels, it may lead to tropical deforestation and increased water, fertilizer and pesticide demand. As a result, the mechanisms and impacts of global change are best studied in concert instead of individually.

 

Global Change and the Ocean

Figure 1. A time line of the 5 most pressing problems affecting the global ocean.

Figure 1. A time line of the 5 most pressing problems affecting the global ocean.

The earliest human pressure on ocean ecosystems was from overfishing. Humans have been gathering seafood for almost 200,000 years, yet awareness of the limitation of ocean seafood stock did not appear until recent centuries. For instance, the Steller’s Sea Cow was hunted to extinction by Dutch hunters only 28 years after their discovery in the Bering Sea. Another anthropogenic stress on ocean is from pollution which started in 3640 BP, at which time Carthaginians and Romans were mining for mercury and silver in Rio Tinto. This contributed to large heavy metal loads to the N. Atlantic Ocean.  Other forms of pollution appeared afterwards, such as persistent organic pollutants (POPs) from industrial processes and oil residues from oil leaks. Additionally, with increasing human population and their preferential settlement in coastal areas, widespread loss of habitats occurred such as clear-cutting mangroves and salt-marshes. Meanwhile, changes in land use in watersheds and river regulations have affected the delivery of nutrients to the ocean. Eutrophication happens as a result of excess nutrients. This results in poor living conditions for coastal organisms.

 

The Future Ocean

The predicted future of our oceans is still vague and qualitative. By the end of the twenty-first century, the ocean will be warmer with a reduced ice extent, higher sea level, lower pH and lower oxygen levels than at present. Also, the ocean will experience poleward expansion of the subtropical gyres. Species will migrate to new habitats where the temperature suits them better.

Figure 2. Red indicates higher pressure while blue indicates lower pressure. For an ecosystem undergoes increased pressure, the state drops from A to B following the pathway on the right. If the pressure decreases afterwards, the state of ecosystem goes back to A following the pathway on the left. At a certain pressure as indicated by either of the dashed arrows, ecosystem is at a higher state on the right pathway than the other. Therefore if the ecosystem needs to get back to the same state as before pressure changes occurred, lower pressure is required in reaching the state.

Figure 2. Red indicates higher pressure while blue indicates lower pressure. For an ecosystem undergoes increased pressure, the state drops from A to B following the pathway on the right. If the pressure decreases afterwards, the state of ecosystem goes back to A following the pathway on the left. At a certain pressure as indicated by either of the dashed arrows, ecosystem is at a higher state on the right pathway than the other. Therefore if the ecosystem needs to get back to the same state as before pressure changes occurred, lower pressure is required in reaching the state.

The response of ocean ecosystems to global change will be complex rather than linear and smooth. One important notion about the change is the threshold or so-called tipping point. This is the point by which, once passed, qualitative changes occur in an ecosystem. The recovery trajectory to the original state of the system typically does not follow the same pathway that it took when pressure increased. Take eutrophication as an example. For a coastal region where eutrophication occurs, oxygen will be used by an algae bloom, leading to hypoxia. If people try to revert the ecosystem to normal, they will meet difficulties in the system’s response to such changes. The ecosystem does not directly shift to normal once the oxygen level meets organismal requirements. A delay in returning to the original state may happen, or an oxygen level even more satisfying than that from the original state is needed to bring back the system. At an extreme, some thresholds may represent points of no return. For species that are at the edge of extinction, even when hunting pressure no longer exists, they may already have lost the ability to reestablish their community. Therefore, when the pressure goes beyond such tipping point, the system is no longer recoverable to the original state.

Acclimation and adaptation to the changing environment may help ecosystems survive global changes. For example, recent studies have already shown evidence of organisms changing their proton pumping mechanism in response to an acidifying environment (although the experiments were very restricted and did not represent a realistic system as a whole).

Modeling is a useful way for predicting future ocean conditions in spite of validation limitations. The conventional way to verify their reliability is to compare them with observations. However, we cannot wait until many years later to confirm their robustness. Even if a model’s output fits well with past events, it is questionable whether the model works as excellently for forecasting the future.

Figure 3. The predicted (thin blue lines are uncertainty bands and thick blue line is the mean; prediction was made in 2007) and observed (red lines) annual minimum sea ice extent (1979-2012).

Figure 3. The predicted (thin blue lines are uncertainty bands and thick blue line is the mean; prediction was made in 2007) and observed (red lines) annual minimum sea ice extent (1979-2012).

P.S. Global organizations and documents addressing ocean health:

Oceans Compact Initiative of the UN Secretary General

Marine Strategy Framework Directive of the EU

US Executive Order 13547 on the Stewardship of the Ocean, Our Coasts, and the Great Lakes

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