Impacts of Climate Change

This page is a set of case studies, and they are worth learning as case studies rather than as a list of facts. Examiners ask you to explain an impact, not to recite one, so for each example below make sure you can say what changed, what it did to the organisms, and why.

Boreal forests: from sink to source

Boreal forest, also called taiga, is the belt of coniferous forest covering huge areas of North America, Europe and Russia. Its productivity is relatively low – it is cold, and the growing season is short – but the biome is so enormous that it works as an important carbon sink anyway. Size beats rate here.

These forests are now at risk of switching from carbon sink to carbon source. That switch is a tipping point, and once past it the forest is adding carbon dioxide to the atmosphere rather than removing it, which feeds straight back into the positive feedback cycles from the previous page.

The mechanism runs through water, which is not where most students expect it to run.

Polar habitats and sea ice

Plenty of species depend on ice that forms at the poles as an actual habitat, not just as scenery. Sea ice forms when the surface of the ocean freezes; sea ice that stays attached to land is called landfast ice.

Global warming means there is less sea ice overall, and the ice that does form breaks up and detaches from the land earlier in the year than it used to. For animals that breed on it, timing is everything.

Changes in ocean currents

Water movement in the oceans has a strong influence on weather and climate, and it also plays an essential role in distributing the nutrients that support marine life. Ocean currents are driven by wind, by temperature and by salinity gradients, and what they do is redistribute heat around the planet.

• Warm currents carry heat from the tropics towards the poles, moderating coastal temperatures. The Gulf Stream in the Atlantic is why Europe has a warmer climate than Canada, despite sitting at a similar latitude.
• Cold currents carry cold water from polar regions towards the tropics, cooling coasts and shaping the marine ecosystems there.

The second job of the ocean is feeding it. Upwelling happens when cold, nutrient-rich water rises to the surface. It is driven mainly by wind pushing surface water out of the way, so that deeper water rises to replace it. That deep water brings nutrients up into the sunlit surface layer, which supports abundant marine life and makes coastal fisheries productive.

Changes in current strength or in the paths currents take therefore have significant implications for regional and global climate and for marine life. The clearest example is El Niño, part of the El Niño-Southern Oscillation (ENSO) cycle:

• El Niño events involve a warming of the central Pacific Ocean.
• Warm surface water prevents nutrient upwelling off Central and South America, which reduces primary production and the energy flowing through marine food chains.
• El Niño also shifts atmospheric circulation, causing droughts, floods and other extreme weather a long way from the Pacific.

Range shifts

Every species exists within tolerance limits. It can only survive where the environmental conditions fall inside the range it can tolerate – a marine species, for example, may only manage in seawater within certain temperature limits.

Climate change alters local environmental factors. When conditions move beyond what a species can tolerate, that species has exactly two options: migrate to a new habitat, or face extinction. There is no third option.

Migration here means a shift in range distribution towards somewhere cooler, and cooler means one of two directions.

The two examples to learn

Montane birds in Papua New Guinea. Montane simply means mountain-dwelling. These species live at the altitude that suits them, because altitude affects both temperature and oxygen availability, and so influences plant growth and rates of aerobic respiration. Evidence gathered over a 50 year period shows that many bird species there have moved to higher altitudes. On Mt Karimui, bird species have shifted upslope by an average of more than 100 m. Be careful with the wording, though: this is not true of every species. A few stayed put, and a few moved downslope.

North American trees. The northern limit for tree survival is set by temperature – where it is too cold for photosynthesis, no trees grow. Studies of North American tree species have found range contraction, meaning their overall ranges have shrunk, together with northward spread for many species.

Coral reefs: two separate threats

Coral reefs are built from hard calcium carbonate deposits secreted by animals called coral polyps. Not all corals build reefs – the ones that do are called reef-building corals.

The polyps live in a symbiotic relationship with algae. The algae photosynthesise and supply the polyp with carbon compounds; the polyp provides the algae with shelter and protection inside its own body. It is a genuinely mutual arrangement, and it is also the reef’s weak point.

Reefs are among the most diverse ecosystems on Earth. The complex structures that reef-building corals produce create habitats for enormous numbers of species, supporting complex food chains and giving animals places to breed and raise young – around 25 % of the world’s ocean fish species depend on coral reefs. So when polyps die, the effect is not limited to the polyps: food webs are disrupted, niches disappear, reef biodiversity falls, species die off or move away, and the result can be ecosystem collapse.

Threat one: rising temperature and bleaching

Corals are highly sensitive to water temperature and pH. High water temperatures cause the polyps to expel their algal symbionts. The reef loses its bright colours, which is why this is called coral bleaching – the white you see is the calcium carbonate skeleton showing through.

Bleaching is not instant death. But because the polyps rely on the algae for their carbon compounds, an extended bleaching event starves them, and prolonged bleaching leads to the death of the polyps.

Threat two: ocean acidification

This one is chemistry, and it is worth walking through slowly because the exam sometimes gives you the equations and asks you to explain them.

The oceans dissolve huge amounts of carbon dioxide. Much of the dissolved carbon dioxide reacts with seawater to form carbonic acid, which then dissociates, and the product of that dissociation dissociates again.

Provided this series of reactions runs at the usual rate, the oceans stay slightly alkaline and there is a steady supply of carbonate ions for the organisms that need them. Many marine organisms need carbonate to secrete calcium carbonate for the hard parts of their bodies – and reef-building corals secrete hard exoskeletons made of exactly that.

Now add more carbon dioxide to the atmosphere. More dissolves in the oceans, more carbonic acid forms and dissociates, more hydrogen carbonate forms and dissociates, and the end result is more hydrogen ions in solution. Rising hydrogen ion concentration makes any solution more acidic, and in seawater that process is called ocean acidification.

The oceans are still alkaline – the pH has fallen, so they are closer to neutral, but they have not become an acid. The damage comes from two consequences:

• Calcium carbonate exoskeletons can be weakened and even dissolve.
• The last reaction in the chain reverses in order to buffer all those extra hydrogen ions. Hydrogen ions and carbonate ions combine back into hydrogen carbonate, which mops up the acidity but reduces the availability of carbonate ions for building hard exoskeletons.

Worked examples