If humans continue spewing carbon pollution into the atmosphere, the oceans are going to get a lot simpler — meaning fewer organisms and biodiversity — according to a new summary of the latest marine research.
A study released this week in the journal Proceedings of the National Academy of Sciences (PNAS) was the first in the world to comprehensively analyze how human greenhouse gas emissions will cause a dramatic collapse in the globe’s largest ecosystem.
As carbon dioxide levels increase in the atmosphere, ocean temperatures rise — in fact, last year saw the hottest average sea surface temperatures on record. And fully 90 percent of human-caused warming ends up in the oceans.
Another effect of increasing CO2 levels is ocean acidification, where carbon dioxide saturates the ocean and forms carbonic acid. This means the water is more acidic and makes life harder for many types of marine life — such as those found off the coast of Maine. Over the last 200 years, the oceans have absorbed 150 billion metric tons of human carbon pollution, making them 30 percent more acidic. Right now they take in about a third of human emissions.
Rising temperatures combined with increasingly acidic oceans mean changes for all marine life, but most research into the nature of that change focused on specific species, life stages, or ecosystems. Marine ecologists Ivan Nagelkerken and Sean Connell at the University of Adelaide looked at a huge number of quantitative studies like this and made broader conclusions about what the oceans will look like if temperatures and pH levels continue to rise.
“The analysis suggests that the impact will affect a range of species, ecosystems, and latitudes,” Nagelkerken told ThinkProgress. “The rates of warming and acidification will accelerate even faster compared to current change, and other stressors may show similar patterns.”
Nagelkerken and Connell conducted a meta-analysis of 632 experiments across ecosystems and latitudes to find the winners and losers in future hot, acidic water.
The world will see more plankton in temperate waters as water warm. Plankton off North American coasts have developed into toxic algal blooms in particularly warm waters. Production of tropical plankton, however, will decrease.
Microbes will thrive in acidifying waters. “Microorganisms have fast generation times and occur in a wide range (of extreme) environments, which might explain why they perform well under climate change stress,” Nagelkerken said.
Herbivores’ metabolic rates and consumption levels increase as the temperature does, but they do not grow or thrive with this increased food intake. In fact, as the oceans acidify, herbivores will become weaker and smaller.
Carnivores also must consume more prey as their metabolic costs increase, though with fewer options as herbivores decrease in abundance and size, or become contaminated by toxic algae. On the west coast of the United States, sea lions have been suffering from fish shoals moving north as waters warm, as well as a giant toxic algal bloom that contaminated shellfish they would usually try to eat.
“Species diversity and abundances of tropical as well as temperate species decline with acidification,” the study reads, “with shifts favoring novel community compositions dominated by noncalcifiers and microorganisms.” This means a reduction in the total number of species, and a severe reduction in reef-building species like corals and shellfish. Currently, a massive coral bleaching event is sweeping across oceans from Hawaii to India to the Caribbean. A recent study found that fish numbers also have an enormous impact on the health of coral reefs.
“There will be a species collapse from the top of the food chain down,” Nagelkerken said in a release.
One way to confirm what is likely to happen to species as waters get more acidic is to study organisms at natural warm CO2 vents on the ocean floor. The research suggests that species will not acclimate to these more extreme conditions.
“This conceptualization of change across whole communities and their trophic linkages forecast a reduction in diversity and abundances of various key species that underpin current functioning of marine ecosystems,” the study reads.
Species do worst when dealing with warmer waters, ocean acidification, overfishing, nutrient runoff, and other pollution. Nagelkerken said that because even under the most optimistic scenarios the world will keep emitting carbon dioxide in the short term, warming and acidification will not stop. So reducing the other stressors, like pollution and overfishing, can buy time for some species to adapt or acclimate.
Nagelkerken said that additional research would be needed to be able to make conclusions about how ocean life would respond along a gradient of temperatures and CO2 levels. This would allow researchers to predict threshold levels, or tipping points, beyond which ocean diversity could get particularly grim.
“The future simplification of our oceans has profound consequences for our current way of life, particularly for coastal populations and those that rely on oceans for food and trade,” the study said.