Impacts of Climate Change

Warming does not affect everything evenly. Some ecosystems shift a few metres up a mountain. Some flip from storing carbon to releasing it. And some, like coral reefs, get hit twice over by the same molecule — once through heat and once through chemistry. This page works through five impacts, and the chemistry one is where most marks are won and lost.

Boreal forests: from carbon sink to carbon source

Boreal forests, or taiga, form a biome covering much of North America, Europe and Russia. Their productivity is relatively low, but they are so vast that they are an important carbon sink.

The worry is that global warming could make them switch from being a carbon sink to being a carbon source. That switch is a tipping point, and it would further increase the positive feedback effects of global warming.

The route to that tipping point runs through water:

• Higher temperatures mean less snow falls, so there is less melt water available.
• That leads to drought, which reduces rates of photosynthesis in the coniferous trees.
• Reduced photosynthesis means reduced productivity. Lack of water first causes a loss of green pigment — a process called forest browning — and over long periods the trees die.
• Dead trees dry out, so the risk of forest fires increases.

The consequence is a double shift. Loss of boreal forest reduces removal of carbon dioxide by photosynthesis and increases release of carbon dioxide by combustion. Fires can burn carbon that has been locked up for many years in living trees, in the dead needles on the ground and within the soil itself — this is legacy carbon combustion. It can tip a forest from sink to source, and can be irreversible.

Polar habitat change

Many species depend on ice that forms at the poles for their habitat. Sea ice forms when the ocean freezes; sea ice that is attached to land is landfast ice. Global warming means there is less sea ice, and the ice that does form breaks apart and detaches from the land earlier in the year than it used to.

• Emperor penguins breed on Antarctic sea ice, laying and incubating eggs and raising their young there. Early melting does not leave them enough time to raise their chicks.
• Walruses rely on Arctic sea ice, where mothers can alternate between feeding their young and hunting in the ocean nearby. Early ice loss means nursing mothers must care for young further from the water’s edge, leaving the young unprotected for longer while the mothers hunt.

Changes in ocean currents

Weather and climate are strongly influenced by water movement in the oceans, which also plays an essential role in distributing nutrients that support marine life. Ocean currents, driven by wind, temperature and salinity gradients, redistribute heat across Earth’s surface.

• Warm currents carry heat from the tropics towards the poles, moderating coastal temperatures. The Gulf Stream is why Europe has a warmer climate than Canada despite being at a similar latitude.
• Cold currents transport cold water from polar regions towards the tropics, cooling coastal temperatures and affecting marine ecosystems.
• Upwelling occurs when cold, nutrient-rich water rises to the surface, driven mainly by wind moving surface water out of the way so deeper water rises to replace it. It brings nutrients to the surface, supporting abundant marine life and productive coastal fisheries.

Changes in current strength or in the paths currents take have significant implications for regional and global climates and for marine life. El Niño events, part of the El Niño–Southern Oscillation (ENSO) cycle, involve warming of the central Pacific. Warm surface water prevents nutrient upwelling off Central and South America, which reduces primary production and therefore the flow of energy through marine food chains in those regions. El Niño can also shift atmospheric circulation, producing droughts, floods and other extreme weather.

Range shifts

Species exist within tolerance limits — they survive only where environmental conditions fall within the range they can tolerate. When climate change pushes local conditions beyond what a species can tolerate, that species must migrate to a new habitat or face extinction.

Migration usually means a shift in range distribution towards cooler conditions, which can be reached in two ways:

• A shift towards the poles is a poleward shift.
• A shift to higher altitude is an upslope shift.

Upslope shifts in montane birds

Montane species — mountain-dwelling ones — live at an altitude that suits their needs. Altitude affects both temperature and oxygen availability, so it influences 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. This is not true of all species — a few have stayed put or moved downslope. Data from Mt Karimui show that bird species there have moved upslope by an average of more than 100 m.

Poleward shifts in North American trees

The northern limit for tree survival is set by temperature: where it becomes too cold for photosynthesis, no trees are found. Studies of North American tree species have shown range contraction — the ranges have shrunk — along with northward spread for many species.

Threats to coral reefs

Coral reefs are built from hard calcium carbonate deposits secreted by organisms called coral polyps. Not all corals build reefs; those that do are described as reef-building corals.

The polyps live in a symbiotic relationship with algae: the algae supply carbon compounds made by photosynthesis, and the polyp provides shelter and protection within its body. Reefs are among the most diverse ecosystems on Earth, and around 25 % of the world’s ocean fish species depend on them.

Threat 1: rising temperature and bleaching

Corals are highly sensitive to water temperature and pH. High water temperatures cause the polyps to expel their algae symbionts, so the reef loses its bright colours — coral bleaching. Because the polyps rely on the algae for their carbon compounds, extended bleaching events lead to the death of the polyps.

The death of coral polyps has a knock-on effect on every species that relies on the reef. Food webs are disrupted, the availability of niches falls, and reef biodiversity drops. Many species die off or migrate to other habitats, and the result can be ecosystem collapse.

Threat 2: ocean acidification

This is where students lose marks, because it is a chain of reactions rather than a single step. Huge amounts of carbon dioxide dissolve in the oceans, and much of the dissolved gas reacts with seawater to form carbonic acid.

Provided this series of reactions proceeds at the right 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 ions to secrete calcium carbonate for the hard parts of their bodies — reef-building corals secrete hard exoskeletons from it, and those exoskeletons form the structures that the whole reef ecosystem is built on.

As atmospheric carbon dioxide rises, more dissolves in the ocean, so more carbonic acid forms and dissociates, and the end result is increasing numbers of hydrogen ions in seawater. A rising hydrogen ion concentration makes the solution more acidic — this is ocean acidification. Note that the oceans remain alkaline; their pH has simply decreased, moving them closer to neutral.

There are two significant consequences. Calcium carbonate exoskeletons can be weakened and even dissolve. And the reaction that produces carbonate ions reverses to buffer the extra hydrogen ions, which reduces the availability of carbonate ions for building hard exoskeletons in the first place.

Worked examples