Impact of Climate Change (HL IB Biology)

• Boreal forests, or taiga, form a biome that covers much of North America, Europe, and Russia, and though they have relatively low productivity, these forests are an important carbon sink due to their size
• Boreal forests are at risk of switching from being a carbon sink to being a carbon source due to the effects of global warming on their ecosystem processes
  • This switch from sink to source is known as a tipping point
  • This further increases the positive feedback effects of global warming
• The reduction in water availability that is caused by global warming is a huge problem for boreal forests
  • Less snow falls due to increased temperatures, meaning that less water is available from snow melt water
  • This leads to drought, reducing rates of photosynthesis in the coniferous trees of boreal forests
  • Reduced photosynthesis means reduced productivity, and over long periods can kill the trees
    • Lack of water initially leads to a loss of green pigment and a process called forest browning, where the trees become brown
    • Eventually the trees will die
  • The dead trees dry out and the risk of forest fires increases
• The loss of boreal forest reduces the removal of carbon dioxide by photosynthesis, and increases the release of carbon dioxide by combustion
  • Combustion can release carbon that has been locked up for many years in the living trees, dead needles on the ground, and within the soil itself; this is known as legacy carbon combustion
  • This can tip the forests from carbon sink to source, and can be irreversible

• Many species rely on the ice that forms at the poles for their habitat
  • Sea ice forms when the ocean freezes
  • Sea ice that is attached to land is known as landfast ice
• Global warming means that there is less sea ice, and the ice that does form breaks apart and detaches from the land earlier in the year than previously, causing problems for breeding animals
  • Emperor penguins, Aptenodytes forsteri
    • These birds breed on Antarctic sea ice, laying and incubating their eggs, and raising their young
    • The early melting of sea ice is not giving them enough time to raise their young
  • Walruses, Odobenus rosmerus
    • These mammals rely on Arctic sea ice, where mothers can alternate periods of feeding their young and hunting for food in the ocean nearby
    • The early loss of ice means that nursing mothers need to care for their young further from the water's edge, leaving young without protection for longer periods when the mothers hunt for food

CC BY-SA 4, via Wikimedia Commons

Public domain, via Flickr

Emperor penguins (left) and walruses (right) rely on sea ice to breed successfully

• Weather and climate are strongly influenced by water movement in the oceans, which also play an essential role in distributing nutrients that support marine life
• Ocean currents, driven by factors like wind, temperature, and salinity gradients, redistribute heat across Earth's surface
  • Warm ocean currents carry heat from the tropics towards the poles, moderating temperatures in coastal areas
    • E.g. the Gulf Stream, a warm ocean current in the Atlantic, means that Europe has a warmer climate than Canada, despite being at a similar latitude
  • Cold ocean currents transport cold water from polar regions towards the tropics, resulting in cooler coastal temperatures and affecting marine ecosystems
• Upwelling occurs when cold, nutrient-rich water rises to the surface, primarily driven by wind that moves surface waters out of the way, allowing deeper waters to rise up to replace them
• • Upwelling brings deep, nutrient-rich waters to the surface, supporting abundant marine life and contributing to the productivity of fisheries in coastal areas

Oceanic currents map

Oceanic currents transport heat and nutrients around the world, affecting weather and climate, and influencing marine life

• Changes in oceanic currents, such as alterations in current strength or shifts in their paths, can have significant implications for regional and global climates, and for marine life
• E.g. El Niño events, part of the El Niño-Southern Oscillation (ENSO) cycle, have significant impacts on global weather patterns
  • El Niño events involve the warming of the central Pacific Ocean
  • Warm surface water prevents nutrient upwelling in the waters off Central and South America, reducing primary production and the flow of energy through marine food chains in these regions
  • El Niño can also cause shifts in atmospheric circulation, leading to droughts, floods, and other extreme weather events

• Species exist within tolerance limits, meaning that they can only survive in habitats where the environmental conditions fall within their range of tolerance
  • E.g. a marine species may only be able to survive in seawater that falls within certain temperature limits
• Climate change is causing changes to many local environmental factors; when this causes the conditions of a habitat to change beyond what a species can tolerate, the species must either migrate to a new habitat or face extinction
• This migration may involve a shift in range distribution towards the poles, or to a higher altitude, to an area where temperatures are cooler
  • A range shift towards the poles is described as a poleward shift
  • A range shift to a higher altitude is an upslope shift

Upslope range shifts in montane bird species

• Montane, i.e. mountain-dwelling, species will live at an altitude that suits their needs
  • Altitude affects temperature and oxygen availability, so will influence plant growth and rates of aerobic respiration
• Evidence gathered in the mountains of Papua New Guinea over a 50 year period shows that many bird species have migrated to higher altitudes over this time period
  • This is not the case for all species; a few have stayed in the same place or moved downslope
• E.g. data gathered from Mt Karimui show that bird species have moved upslope in this region by an average of more than 100 m

Upslope range shifts in montane birds graph

Changes in upper elevation limits of species on Mt Karimui between 1965 and 2014 show that most species have increased their upper elevation limit

Poleward range shifts in North American tree species

• The northern limit for tree survival is determined by temperature; when temperatures become too low for photosynthesis, no trees will be found 
• Various studies of North American tree species have shown range contraction, i.e. the ranges of these trees have shrunk, and northward spread for many species

• Coral reefs are built from hard calcium carbonate deposits that are secreted by organisms called coral polyps
  • Note that not all corals build reefs; reefs are built by corals described as reef-building corals
• These polyps live in a symbiotic relationship with algae, in which the algae provide carbon compounds through photosynthesis, and the coral polyp provides shelter and protection within its body
• Coral reefs are some of the most diverse ecosystems in the world; the complex structures produced by reef-building corals provide habitats for many species, supporting complex food chains and providing suitable places to breed and raise young
  • Around 25 % of the world's ocean fish species are dependent on coral reefs for survival
• Corals are highly sensitive to factors such as water temperature and pH, and global warming can have highly damaging effects on the life processes of coral polyps
• Death of coral polyps will have a knock-on effect on all other species that rely on the reef, disrupting food webs, reducing the availability of niches and therefore reducing the reef biodiversity
  • Many species will die off or migrate to other habitats
  • This leads to ecosystem collapse

Ocean acidification & corals

• The impact of increasing carbon dioxide levels on the oceans are significant for ocean biodiversity because of the effect of carbon dioxide on ocean chemistry
  • Huge amounts of carbon dioxide are dissolved by the oceans, and much of the dissolved carbon dioxide reacts with seawater to form carbonic acid (H₂CO₃)

CO₂ + H₂O → H₂CO₃ 

• • Carbonic acid then dissociates to form hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃^-)

H₂CO₃ → H⁺ + HCO₃^-

• • Hydrogen carbonate ions can then dissociate again to form more hydrogen ions and carbonate ions (CO₃^2-)

HCO₃^- → H⁺ + CO₃^2-

• Provided that this series of reactions takes place at the appropriate rate, the oceans remain slightly alkaline, and there is a steady supply of carbonate ions for organisms that need them
  • Many marine organisms need carbonate ions in order to secrete calcium carbonate for the building of the hard parts of their bodies
    • E.g. reef-building corals secrete hard exoskeletons built from calcium carbonate; these exoskeletons form the complex structures of corals which are a key part of coral reef ecosystems

• As atmospheric carbon dioxide levels increase, so too does the volume of carbon dioxide that dissolves in the oceans
• As more carbon dioxide dissolves, more carbonic acid forms and dissociates, and more hydrogen carbonate ions form and dissociate, the end result of which is increasing numbers of hydrogen ions in a seawater solution
• Increasing concentrations of hydrogen ions in solution cause that solution to become more acidic; in this case the process is known as ocean acidification
  • Note that the oceans are still alkaline, but the pH has decreased, so they are closer to neutral
• There are significant consequences to ocean acidification
  • The calcium carbonate exoskeletons of, e.g. corals, can be weakened and even dissolve
  • The reaction during which hydrogen carbonate ions dissociate to form hydrogen ions and carbonate ions reverses to buffer the increasing number of hydrogen ions, reducing the availability of carbonate ions for the building of hard exoskeletons

H⁺ + CO₃^2- → HCO₃^-

Oceanic chemistry diagram

Increased atmospheric carbon dioxide increases the number of hydrogen ions in seawater, and reduces the availability of carbonate ions

Rising ocean temperatures & corals

• High water temperatures cause the coral polyps to expel their algae symbionts; this causes the reefs to lose their bright colours and leads to coral bleaching
• Because the polyps rely on the algae for their carbon compounds, extended bleaching events can lead to the death of the polyps

Coral polyp diagram

Rising ocean temperatures cause coral polyps (above) to expel the algae within their tentacles, leading to coral bleaching and eventually death