The human driver: extra greenhouse gases

Climate change is a long-term change in patterns of temperature, precipitation, wind and other features of climate. Here, the focus is deliberately tight: human-caused increases in atmospheric carbon dioxide and methane.

Anthropogenic means caused by human activity. The main anthropogenic sources of carbon dioxide are combustion of fossil fuels, combustion of biomass, and land-use change such as deforestation, which reduces photosynthetic uptake and releases stored carbon. Human activities also raise methane levels, including fossil fuel extraction and use, agriculture involving ruminants and rice paddies, landfills, and burning or decomposition of organic waste.

A greenhouse gas is an atmospheric gas that absorbs long-wave infrared radiation emitted from Earth’s surface and then re-emits radiation, reducing the rate at which energy escapes to space. Carbon dioxide and methane are greenhouse gases. Nitrogen and oxygen are not significant greenhouse gases because they do not absorb infrared radiation strongly.

Short-wave solar radiation passes through much of the atmosphere and warms Earth’s surface. Earth’s surface then emits longer-wave infrared radiation. As carbon dioxide and methane increase, this heat-retaining effect becomes stronger, so the average energy balance of Earth shifts towards warming.

Correlation is not the same as causation

A positive correlation is a relationship in which two variables tend to increase together or decrease together. A negative correlation is a relationship in which one variable tends to increase as the other decreases. Ice-core records from Antarctica show a positive correlation between atmospheric carbon dioxide concentration and global temperature over long time scales.

That correlation matters, but on its own it does not prove that carbon dioxide causes warming. Causation is a relationship in which a change in one variable directly or indirectly produces a change in another. In climate science, physics of infrared absorption, laboratory measurements, atmospheric observations, climate models, and correlations in past climate data all support the causal link. I often put it this way for students: correlation points you where to look; mechanism and repeated evidence help establish cause.

Climate change affects many biological levels: molecules and cells are affected by temperature and pH, organisms by stress and reproduction, populations by survival and range, communities by changed interactions, and ecosystems by altered energy flow and carbon storage.

Why warming can amplify itself

Positive feedback is a process in which the output of a change increases the original change. Don’t let the word “positive” mislead you: it does not mean good; it means self-amplifying.

One feedback involves the deep ocean. Carbon dioxide dissolves in seawater, but warm water holds less dissolved gas than cold water. As global temperatures rise, some ocean regions can release more carbon dioxide to the atmosphere. That extra carbon dioxide strengthens warming further.

Another feedback comes from the loss of reflective snow and ice. Albedo is the fraction of incoming solar radiation reflected by a surface. Snow and ice have high albedo; open ocean, exposed rock and dark vegetation have lower albedo. When snow and ice melt, darker surfaces absorb more solar radiation, causing more heating and therefore more melting.

Frozen soils and peat create a third feedback. Permafrost is ground that remains frozen for at least two consecutive years. It can contain previously undecomposed organic matter. When permafrost thaws, microorganisms decompose that organic matter faster, releasing carbon dioxide under aerobic conditions and methane under anaerobic, waterlogged conditions. Peatlands can behave similarly when warming and drying speed up decomposition of stored organic carbon.

Methane release matters because methane is an especially effective greenhouse gas over short time scales. Melting permafrost can therefore add methane to the atmosphere, increasing warming and causing further thawing.

Drought and forest fire form another feedback. Higher temperatures increase evaporation and can dry vegetation and soils. Drought-stressed forests burn more easily; fires release carbon dioxide from living biomass and dead organic matter, and the damaged forest then absorbs less carbon dioxide by photosynthesis. This is one reason climate change is not just a smooth, gradual line on a graph — feedbacks can steepen the curve.

Boreal forests can switch roles

A boreal forest is a high-latitude forest ecosystem dominated mainly by cold-tolerant conifers, also called taiga. Many boreal forests have traditionally acted as carbon sinks because cold conditions slow decomposition, so carbon builds up in wood, litter and soil.

Net carbon accumulation occurs when photosynthesis and biomass formation store more carbon than respiration, decomposition and fire release. Net carbon loss occurs when carbon release exceeds carbon storage. The concern is simple: a forest can move from one state to the other.

A tipping point is a threshold at which a small additional change causes a system to move into a different, self-reinforcing state. In boreal forests, warmer temperatures and reduced winter snowfall can make drought more likely. With less snow, less meltwater enters the soil in spring, while warmer air increases water loss. Drought cuts primary production because trees close stomata, photosynthesize less, grow less, and become more vulnerable to damage.

The visible result can be forest browning, a reduction in canopy greenness caused by stress, damage or death of vegetation. Browning shows reduced primary production. Dry forests also burn more frequently and more intensely.

The key phrase in the guide is legacy carbon combustion. It means combustion of carbon that had accumulated over many previous years or centuries in wood, litter, peat and soil organic matter. A severe fire doesn’t just burn this year’s growth; it can release older stored carbon. Once repeated fires, drought and reduced regrowth dominate, the forest may no longer act as a carbon sink. That links ecosystem change directly back to atmospheric carbon dioxide and global warming.

Ice is habitat, not just frozen water

Landfast ice is sea ice attached to a coastline, island or grounded ice feature, so it stays relatively fixed in place. Sea ice is frozen seawater floating on the ocean surface; it may be landfast or drifting pack ice.

For emperor penguins, Aptenodytes forsteri, stable Antarctic landfast ice can act as breeding grounds. Adults still need access to the ocean to feed, but eggs and chicks need a stable platform for long enough to complete incubation and early development. If landfast ice breaks out too early, breeding colonies may lose eggs or chicks before they can survive in the water.

In the Arctic, walruses depend on sea ice as resting habitat between feeding trips. They feed in shallow marine areas, then haul out on ice near those feeding grounds. When sea ice is lost, larger numbers may be forced onto land, which increases crowding, travel distance to feeding sites and risks to calves.

In examinations, either the common name or the scientific name is acceptable for organisms. The biology is the key part: link the physical habitat change — early breakout or loss of sea ice — to breeding, feeding, survival and population consequences.

Why the surface ocean can become nutrient-poor

Upwelling is the movement of deeper, nutrient-rich water towards the ocean surface. It matters because light is strongest near the surface, while many mineral nutrients build up deeper down as dead organisms and waste sink and decompose.

The upper ocean is usually the most biologically productive zone because photosynthetic plankton can get light. Light on its own isn’t enough, though. Producers also need nutrients such as nitrate and phosphate. When currents and winds push deep water upward, nutrient supply increases, primary production rises, and more energy enters marine food chains.

Climate change can alter ocean currents and increase stratification. Stratification is the formation of layers of water with different densities that mix only slowly. Warm surface water is less dense than cold water, and freshwater from melting ice can also reduce surface density. A warmer, fresher surface layer may sit above colder, saltier deep water like oil above vinegar in a salad dressing — not a perfect analogy, but useful.

As stratification strengthens, nutrient-rich deep water reaches the surface less often or at different times. Warmer surface water can therefore prevent nutrient upwelling, which decreases ocean primary production and reduces energy flow through marine food chains. This linking question is a classic: the distribution and productivity of marine organisms are determined by physical processes as well as biological ones.

Species track suitable climate — if they can

A species range is the geographic area where individuals of a species live and reproduce. As climate warms, the environmental conditions that suit many temperate species move towards the poles or up to higher altitudes.

A poleward range shift is a movement of a species’ distribution towards higher latitudes. In North American tree species, evidence shows northward spread at cooler range edges, along with range contraction where conditions become too warm, dry or otherwise unsuitable. Trees don't “decide” to migrate, of course. Seeds establish more successfully in newly suitable areas, and less successfully in places that have become stressful.

An upslope range shift is a movement of a species’ distribution to higher elevation. A montane species is a species adapted to mountain habitats. Tropical-zone montane bird species in New Guinea give evidence for this: as temperature zones move upward, the upper or lower limits of some bird ranges also shift upslope.

The problem is that mountains run out. Species already near a summit may have little or no higher habitat left. Range shifts therefore help explain one of the big linking questions in biology: the distribution of organisms on Earth is determined by abiotic factors such as temperature and water availability, plus biotic factors such as competition, predation and food supply.

Carbon dioxide attacks reefs in two ways

A coral reef is a marine ecosystem built by reef-forming corals that deposit calcium carbonate skeletons and provide habitat for many other organisms. Reefs show how a molecular-level change can build into ecosystem collapse.

When atmospheric carbon dioxide increases, more of it dissolves into seawater and causes ocean acidification, a decrease in ocean pH caused by increased dissolved carbon dioxide forming carbonic acid and releasing hydrogen ions. As pH falls, fewer carbonate ions are available. Corals need those carbonate ions to make calcium carbonate, so acidification reduces calcification, the biological deposition of calcium carbonate into skeletons.

Higher water temperature causes coral bleaching, the loss of colour in corals when they expel or lose many of their symbiotic photosynthetic algae. These algae provide corals with organic nutrients. A bleached coral isn’t automatically dead, but it is stressed and has less energy for growth, repair and reproduction.

Ecosystem collapse is a change in an ecosystem in which its structure and functioning are lost or greatly simplified. Corals act as foundation organisms because their skeletons create the three-dimensional reef habitat. If corals die and reef building slows, many fish and invertebrates lose shelter and feeding sites. Food webs weaken, biodiversity falls, and energy flow through the reef ecosystem is disrupted.

This section also helps link levels of organization. More carbon dioxide changes seawater chemistry at the molecular level, reducing coral calcification at the organism level. That lowers coral survival and growth at the population level, which can collapse the reef community and ecosystem.

Storing carbon biologically

Carbon sequestration means capturing carbon and storing it long term in a reservoir such as biomass, soil, peat, sediment or rock. In ecosystems, photosynthesis captures carbon, which may then be held in wood, roots, soil organic matter, peat or buried detritus.

Afforestation means planting trees in an area that has not recently been forest. Forest regeneration means forest re-establishing on land where it has been removed or degraded. Both can increase carbon storage if the trees survive, grow, and add carbon to biomass and soil.

Peat is partially decomposed organic matter that builds up in waterlogged conditions, where low oxygen slows decomposition. Peat forms naturally in waterlogged soils in temperate and boreal zones, and can also form rapidly in some tropical ecosystems. Restoring peat-forming wetlands usually involves raising water levels again, blocking drainage channels, reducing fire risk, and re-establishing wetland vegetation such as peat-forming mosses where appropriate.

This is a real scientific debate, not just politics. Plantations of fast-growing non-native trees may capture carbon quickly and produce timber, but they can reduce biodiversity, use large amounts of water, increase disease vulnerability if planted as monocultures, and fail to restore a functioning native ecosystem. Rewilding or regeneration with native species may support biodiversity and ecosystem resilience better, although carbon uptake may be slower or less predictable at first.

A sensible answer is not “trees always good” or “plantations always bad”. The best approach depends on rainfall, soil, previous land use, local biodiversity, fire risk, community needs and the time scale for carbon storage. A peatland kept wet may store carbon for centuries; a dry peatland or a fire-prone plantation can rapidly become a carbon source.

Timing is biological data

Phenology is the study of the timing of recurring biological events and how environmental variables influence that timing. Common examples are flowering, leaf budburst, bud set, bird migration and nesting.

Photoperiod is the duration of light exposure within a 24-hour day. It changes in a predictable way with season and latitude, but climate change does not shift it. For many organisms, that makes it a reliable calendar cue.

Temperature patterns don’t behave in the same way. A warm spring, late frost, hot summer or mild winter can shift biological timing from one year to the next. In deciduous trees, budburst is the opening of buds and emergence of new leaves at the start of the growing season. Bud set is the formation of protected buds when shoot growth stops before winter. Flowering, budburst and bud set may be controlled by photoperiod, temperature, or a combination of both.

Bird migration and nesting count as phenological events too. Migration is often strongly influenced by photoperiod, while food availability at the destination may depend more on temperature-driven plant growth or insect development. This creates the problem for the next section: when two interacting populations use different cues, climate change can pull their timings apart.

In data work, R² may be described as the proportion of variation in one variable that is statistically explained by another variable in a model. A high R² for temperature and budburst would suggest temperature is a strong predictor of leaf-out timing; a low R² would suggest other cues, such as photoperiod, may be more important.

When interacting species stop matching in time

Synchrony is when biological events that depend on each other happen at the same time. In ecosystems, this timing can matter for feeding, breeding, pollination, seed production and migration.

Climate change can break synchrony when one population responds to temperature while another responds to photoperiod. In a warm year, temperature-related cues may come earlier; photoperiod still follows the same astronomical pattern. One species shifts its timing, while the other shifts too little or not enough.

A clear Arctic example involves Arctic mouse-ear chickweed, Cerastium arcticum, and migrating reindeer, Rangifer tarandus. Warming conditions influence spring growth of the plant, so the peak availability of nutritious young growth can occur earlier. Reindeer migration timing may not shift in the same way. Animals can then arrive after the best forage has peaked, affecting nutrition, calf survival and population growth.

Another well-known example comes from the great tit, Parus major, and caterpillars in north European forests. Great tits rely on a high biomass of caterpillars to feed their chicks. Warmer springs can bring the caterpillar peak earlier, while bird breeding may not advance enough. Chicks then have fewer caterpillars available at the time they need them most, reducing chick growth and survival.

This is a neat example of climate change acting at the community level. The temperature has not directly killed the bird, the reindeer, the caterpillar or the plant. It has changed the timing of the interactions between them.

Warmer years can fit in an extra generation

A life cycle is the sequence of developmental stages an organism passes through from one generation to the next. In many insects, temperature controls how quickly development happens, so warmer conditions can cut the time needed to finish a generation.

Use the spruce bark beetle as the syllabus example; acceptable scientific names include Ips typographus or Dendroctonus micans. A bark beetle is an insect that develops under tree bark and can damage or kill trees by feeding, tunnelling and introducing associated microorganisms.

In cooler conditions, spruce bark beetles may need more than one year to complete development. With warmer conditions, they can complete a full life cycle within a single year. More generations can occur each year, and emergence may become synchronized, so many adults attack trees at the same time.

Climate change can also stress the host trees. Drought and heat reduce tree defences, including resin flow, which normally helps resist beetle attack. Warming therefore has a double effect: beetles develop faster, and trees become easier to attack. The result can be large outbreaks, extensive tree mortality, reduced carbon storage and increased fire risk.