Climate change and oceans
Climate change significantly impacts the world's oceans, with human-induced global warming leading to a variety of environmental changes. Scientists project that by 2100, global temperatures may rise by 2 to 4 degrees Celsius, which could result in substantial sea level increases—potentially up to 25 meters over several centuries. As carbon dioxide (CO2) levels have reached unprecedented highs, the oceans have absorbed much of this excess CO2, leading to dangerous levels of ocean acidification and threatening marine life, particularly species with calcium shells.
Warmer ocean temperatures disrupt the food web, adversely affecting phytoplankton and zooplankton populations, which are critical to marine ecosystems. Coral reefs, known for their biodiversity, face severe threats from increased temperatures, leading to coral bleaching and heightened susceptibility to disease. Additionally, climate change contributes to the rise of harmful algal blooms that can jeopardize human health and local economies. The overall decline in ocean health poses serious risks to global fisheries, which are vital food sources for billions of people. The intersection of climate change and ocean health highlights the urgency of addressing environmental issues to mitigate future impacts on both marine and human communities.
Subject Terms
Climate change and oceans
Rising temperatures affect sea levels and the salinity and acidity of the oceans, which in turn have impacts on aquatic life and the lives of people and animals that live near shorelines. Rising sea levels, the results of melting ice and thermal expansion of seawater, are likely to constitute the most notable challenge related to global warming for many millions of people around the world.
The scientific community accepts climate change, and specifically human-induced global warming, as an observed fact. Many researchers have projected that the Earth’s temperature will likely rise between 2 and 4 degrees Celsius (3.6 and 7.2 degrees Fahrenheit) by 2100, bringing it to a level near that of the middle Pliocene, three million years ago, when the seas were 15 to 35 meters (50 to 115 feet) higher than in the 2000s. The carbon dioxide (CO2) level at that time reached 425 parts per million (ppm); by 2010 the CO2 level was at 390 ppm, increasing at a rate of about 2 to 3 ppm per year. In 2013 the CO2 level surpassed 400 ppm, and in 2017 it surpassed 410 ppm for the first time in millions of years. May 2023 saw the CO2 level reach 424 ppm, a new modern record.
Various studies have linked climate change to significant effects on the world's oceans. In an article published in the March 2004 issue of Scientific American, James E. Hansen, director of the National Aeronautics and Space Administration’s Goddard Institute for Space Studies, warned that temperature increases connected to continued growth in CO2 emissions and other greenhouse gases could provoke large rises in sea level, with potentially catastrophic effects on coastal populations. According to Hansen, a temperature increase of 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) could raise sea levels by about 25 meters (82 feet) within a few centuries. Subsequent reports have projected an even larger and faster sea level rise, principally due to melting ice from Greenland and the West Antarctic ice sheet. By the 2010s, some areas had already seen noticeable shifts in sea level. According to the US Environmental Protection Agency (EPA), from 1993 to 2016 the global average sea level rose by 0.11 to 0.14 inches each year, about two times faster than long-term historical trends.
According to the National Oceanic and Atmospheric Administration (NOAA), 2022 was the sixth warmest year in terms of average global land and ocean surface temperature since global records began in 1880. Measurements for the year saw an increase of 1.55°F (0.86°C) over the twentieth century average of 57.0°F (13.9°C). Additionally, NOAA reported that the ten warmest years on record have all occurred since 2010, with the years 2014–2022 constituting the nine warmest years ever recorded. By 2023, according to NOAA, global mean sea level had reached 101.2 millimeters (4 inches) above 1993 levels, when modern satellite record-keeping began.
While rising sea levels have perhaps the most immediate impact on human populations, other effects of climate change on oceans have wide-ranging consequences. Warmer water temperatures and increased ocean acidification can significantly harm aquatic life and overall biodiversity. This in turn threatens human health due to the importance of fishing and aquaculture as food sources. Changes in ocean temperature can also disrupt normal weather patterns, creating more frequent and more severe extreme weather events.
Rising Acidity in the Oceans
In 2003 scientists Ken Caldeira and Michael E. Wickett noted in the journal Nature that CO2 levels were rising in the oceans more rapidly than at any time since the age of the dinosaurs. As the oceans absorb more CO2 and become more acidic, their capacity to hold more CO2 in the future is strongly reduced, further escalating the greenhouse gas problem.
Concerns about rising ocean acidification were reinforced in January 2009, when 155 scientists from twenty-six nations, organized by several international groups under the aegis of the United Nations, issued the Monaco Declaration, warning of severe damage to the oceans by rising acidity. In June 2009, seventy science academies around the world called for the inclusion of ocean acidity on the agenda of international climate change studies, noting that the oceans already had become more acidic than at any time during the past 800,000 years.
Carbon dioxide is being injected into the oceans much more quickly than nature can neutralize it. Seawater is usually alkaline, about 8.2 pH. The pH scale is logarithmic, so a 0.1 decrease in pH—the change from the beginning of the Industrial Revolution to the early twenty-first century—indicates a 30 percent increase in the concentration of hydrogen ions.
Scientists have investigated what continued ocean acidification might do to animals with calcium shells. One study investigated 328 colonies of massive Porites corals on the Great Barrier Reef off Australia; these corals in the past have grown to more than 6 meters (20 feet) tall over decades to centuries. Results from sixty-nine sections of the reef found that calcification had declined 14.2 percent between 1990 and 2005, impeding the corals’ growth by 13.3 percent. Scientists continued to monitor this situation into the 2020s, noting that carbon dioxide levels continued to increase at that time.
The Ocean Food Web
Warming sea surface temperatures may interfere with phytoplankton production, with impacts rippling through the food web. Cooler, upwelling ocean water breaks through warm surface waters less frequently, reducing the nutrients available for plants and animals living in the oceans. Such decreases in productivity have been detected and studied near the California coast, where scientists have documented a measurable decrease in the abundance of zooplankton, the second level in the food web. By the 1990s, the abundance of zooplankton was 70 percent lower than it had been during the 1950s. A 2014 study suggested that climate change would decrease phytoplankton and zooplankton biomass by approximately 6 and 11 percent, respectively. However, scientists noted that much of plankton ecology was still not understood, making accurate predictions about climate change's effects on plankton populations difficult. In 2015 the results of the first global plankton survey were reported, giving researchers a baseline to work with and confirming that plankton are sensitive to ocean temperature and other issues related to climate change.
Similarly, researchers note that warming ocean water temperatures causes coral bleaching, in which corals turn white after expelling the beneficial algae that live within them. Coral bleaching stresses the coral colonies and makes them more susceptible to disease, predation, and starvation. This threatens coral reef ecosystems as a whole, which are among the most biodiverse habitats on the planet.
Scientists have documented the effects of climate change on larger marine animals as well. Various studies have noted decreased reproduction and increased mortality in seabirds and marine mammal populations in warming water. For example, a 1999 report published by the World Wide Fund for Nature and the Marine Conservation Biology Institute noted that the population of the sooty shearwater, a seabird, off the California coast declined 90 percent during the late 1980s and early 1990s, and the population of the Cassin’s auklet declined 50 percent. Other studies have found marked increases in disease incidence among species such as turtles, echinoderms, mollusks, and various marine mammals. Marine plants can also suffer from ocean warming, with declines in species such as eelgrass posing drastic changes to habitats and entire ecosystems.
By the 2020s many scientists had also noticed the potential for global warming to influence the frequency and size of algal blooms, which are rapid growths of algae or cyanobacteria in water. Algal blooms, which can be toxic to humans and other life, often occur in shallow coastal water but are also observed in ponds, lakes, and other inland bodies of water. In addition to potentially poisoning humans, algal blooms can create unpleasant or uninhabitable water conditions and severely disrupt local ecosystems as well as economic activity. Researchers have identified multiple ways in which climate change can create conditions in which algal blooms become more common; warmer ocean temperatures and higher CO2 levels are major factors, but increased freshwater salinity, unpredictable rainfall patterns, and increasingly common severe weather may also play a role.
The threats posed by climate change to ocean ecology and biodiversity have the potential to drastically impact human populations. Seafood is a vital nutrition source for billions of people worldwide, and the warming ocean puts the vitality of global fisheries at risk. In addition to directly harming many marine species relied upon by humans for food, climate change also compounds problems caused by overfishing and ocean pollution.
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