🌊Oceanography Unit 11 – Oceans and Climate Change

Ocean Basics

• Oceans cover approximately 71% of the Earth's surface and contain 97% of the planet's water
• The average depth of the ocean is about 3,688 meters (12,100 feet)
  • The deepest point is the Mariana Trench in the Pacific Ocean, which reaches a depth of 11,034 meters (36,201 feet)
• Oceans play a crucial role in regulating the Earth's climate by absorbing and redistributing heat, carbon dioxide, and water vapor
• Major ocean currents, such as the Gulf Stream and the Kuroshio Current, transport warm water from the equator to the poles, influencing regional climates
• Oceans are home to a vast array of marine life, including phytoplankton, which produce approximately 50% of the world's oxygen through photosynthesis
• The ocean is divided into five main basins: the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans
• Oceans are a major source of food, energy, and minerals for humans, with industries such as fishing, oil and gas extraction, and deep-sea mining

Climate Change 101

• Climate change refers to long-term shifts in global or regional climate patterns, primarily attributed to the increased levels of atmospheric carbon dioxide produced by the use of fossil fuels
• The Earth's average surface temperature has risen about 1.18°C (2.12°F) since the late 19th century, with most of the warming occurring in the past 40 years
• The primary greenhouse gases responsible for climate change are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)
  • These gases trap heat in the atmosphere, leading to the "greenhouse effect"
• Human activities, such as burning fossil fuels, deforestation, and agriculture, have significantly increased the concentration of greenhouse gases in the atmosphere
• Climate change impacts include rising sea levels, more frequent and intense heatwaves, droughts, and extreme weather events, as well as changes in precipitation patterns and ocean acidification
• The Paris Agreement, adopted in 2015, aims to limit global temperature rise to well below 2°C above pre-industrial levels and pursue efforts to limit the increase to 1.5°C
• Mitigating climate change requires reducing greenhouse gas emissions through the adoption of renewable energy sources, energy efficiency, and sustainable land use practices

Ocean-Climate Interactions

• Oceans and the atmosphere are closely linked, with the oceans absorbing about 30% of the carbon dioxide released by human activities
• The oceans act as a buffer against climate change by absorbing excess heat from the atmosphere
  • Since 1955, the oceans have absorbed more than 90% of the excess heat trapped by greenhouse gases
• Changes in ocean temperatures and currents can significantly impact global climate patterns, such as the El Niño-Southern Oscillation (ENSO)
• Melting of land-based ice sheets and glaciers, caused by rising temperatures, contributes to sea level rise and alters ocean circulation patterns
• Warming oceans lead to thermal expansion, which accounts for about one-third of the observed sea level rise since 1993
• Changes in ocean temperature and salinity can affect the formation of deep water masses, which play a crucial role in global ocean circulation and climate regulation
• Feedback loops between the oceans and the atmosphere can amplify or dampen the effects of climate change, making it essential to understand these interactions for accurate climate projections

Impacts on Marine Ecosystems

• Climate change and ocean acidification are causing significant shifts in marine ecosystems, affecting the distribution, abundance, and survival of marine species
• Rising ocean temperatures lead to coral bleaching events, where corals expel their symbiotic algae, often resulting in widespread coral mortality
  • The Great Barrier Reef has experienced several mass bleaching events in recent years, with the 2016 and 2017 events affecting over 75% of the reef
• Warmer waters can cause species to migrate to higher latitudes or deeper waters in search of suitable habitats, altering community structures and food webs
• Changes in ocean circulation and upwelling patterns can affect the availability of nutrients and oxygen in marine ecosystems, impacting primary productivity and species distribution
• Ocean acidification, caused by the absorption of excess atmospheric CO2, reduces the availability of carbonate ions necessary for calcifying organisms to build their shells and skeletons
  • This affects species such as corals, mollusks, and some plankton, which form the foundation of many marine food webs
• Climate change can exacerbate the impacts of other stressors on marine ecosystems, such as overfishing, pollution, and habitat destruction, leading to cumulative effects on biodiversity and ecosystem functioning
• The loss of marine biodiversity and changes in ecosystem structure can have significant implications for the provision of ecosystem services, such as food security, coastal protection, and tourism

Ocean Acidification

• Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, primarily caused by the absorption of excess atmospheric carbon dioxide (CO2)
• Since the beginning of the industrial era, the average pH of the ocean surface has decreased by approximately 0.1 units, representing a 30% increase in acidity
• When CO2 dissolves in seawater, it forms carbonic acid (H2CO3), which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-)
  • The increased concentration of hydrogen ions lowers the pH of the water, making it more acidic
• Ocean acidification reduces the availability of carbonate ions (CO32-), which are essential for calcifying organisms to build their shells and skeletons
• The saturation state of calcium carbonate minerals, such as aragonite and calcite, decreases with increasing acidity, making it more difficult for organisms to calcify and maintain their structures
• Ocean acidification can have significant impacts on marine ecosystems, particularly on coral reefs, which are highly sensitive to changes in pH and carbonate saturation
  • Reduced calcification rates and increased dissolution of coral skeletons can compromise the growth and resilience of coral reefs
• Other organisms affected by ocean acidification include mollusks, echinoderms, and some plankton species, which play crucial roles in marine food webs and biogeochemical cycles
• The impacts of ocean acidification can have cascading effects on marine ecosystems, altering species interactions, community structure, and ecosystem functioning

Sea Level Rise

• Sea level rise is one of the most significant consequences of climate change, caused by the thermal expansion of ocean water and the melting of land-based ice sheets and glaciers
• Global mean sea level has risen about 21-24 centimeters (8-9 inches) since 1880, with about a third of that occurring in the last 25 years
• Thermal expansion accounts for about one-third of the observed sea level rise since 1993, as warmer water occupies more volume than colder water
• The melting of land-based ice, particularly from the Greenland and Antarctic ice sheets, is a major contributor to sea level rise
  • The Greenland ice sheet has lost an average of 279 billion tons of ice per year between 1993 and 2019, while the Antarctic ice sheet has lost about 148 billion tons per year
• Sea level rise can have significant impacts on coastal communities and ecosystems, increasing the risk of flooding, erosion, and saltwater intrusion into freshwater aquifers
• Low-lying islands and coastal regions are particularly vulnerable to sea level rise, with some communities facing the prospect of displacement or relocation
• Sea level rise can also affect coastal infrastructure, such as ports, roads, and power plants, as well as agricultural land and freshwater resources
• The rate and magnitude of future sea level rise depend on the trajectory of greenhouse gas emissions and the response of the Earth's climate system to warming temperatures

Mitigation and Adaptation Strategies

• Addressing the challenges posed by climate change and its impacts on the oceans requires a combination of mitigation and adaptation strategies
• Mitigation strategies focus on reducing greenhouse gas emissions to limit the magnitude of future climate change
  • This includes transitioning to renewable energy sources, improving energy efficiency, and adopting sustainable land use practices, such as reforestation and reduced deforestation
• Adaptation strategies aim to reduce the vulnerability of human and natural systems to the impacts of climate change that are already occurring or are expected to occur in the future
• Coastal adaptation measures include the construction of sea walls, levees, and other hard infrastructure to protect against flooding and erosion
  • Nature-based solutions, such as the restoration of coastal wetlands and mangroves, can also provide protection while offering co-benefits for biodiversity and ecosystem services
• Marine protected areas (MPAs) can help to build the resilience of marine ecosystems to the impacts of climate change by reducing other stressors, such as overfishing and pollution
• Assisted migration and genetic diversity management may be necessary to help species adapt to changing environmental conditions and avoid local extinctions
• Improving monitoring and early warning systems can help to detect and respond to the impacts of climate change on the oceans, such as coral bleaching events and harmful algal blooms
• Engaging local communities and stakeholders in the development and implementation of adaptation strategies is crucial for ensuring their effectiveness and social acceptability

Future Projections and Research

• Climate models project that the Earth's average surface temperature will continue to rise throughout the 21st century, with the magnitude of the increase depending on future greenhouse gas emissions
• Under a high-emissions scenario (RCP8.5), global mean sea level is projected to rise by 0.61-1.10 meters (2.0-3.6 feet) by 2100, relative to 1986-2005
  • A low-emissions scenario (RCP2.6) projects a sea level rise of 0.38-0.73 meters (1.2-2.4 feet) over the same period
• The frequency, intensity, and duration of marine heatwaves are expected to increase under future climate change, with significant implications for marine ecosystems and the communities that depend on them
• Ocean acidification is projected to continue, with the global average surface pH potentially dropping by an additional 0.3-0.4 units by 2100 under a high-emissions scenario
• The impacts of climate change on the oceans are expected to have significant socio-economic consequences, affecting industries such as fisheries, aquaculture, and coastal tourism
• Ongoing research aims to improve our understanding of the complex interactions between the oceans and the climate system, as well as the impacts of climate change on marine ecosystems and human societies
  • This includes advances in ocean observation systems, climate modeling, and interdisciplinary studies that integrate natural and social sciences
• Developing effective mitigation and adaptation strategies requires a strong foundation in scientific research, as well as collaboration among scientists, policymakers, and stakeholders at local, regional, and global scales
• Addressing the challenges posed by climate change and its impacts on the oceans is crucial for achieving the United Nations' Sustainable Development Goals, particularly SDG 13 (Climate Action) and SDG 14 (Life Below Water)