The Environment

A climax community is the final, stable community reached at the end of succession, which is self-sustaining and in equilibrium with its environment, so it shows little further directional change.

An example in a temperate region such as the UK is oak woodland.

Two greenhouse gases are carbon dioxide (CO₂) and methane (CH₄).

A human activity that increases atmospheric carbon dioxide is the burning of fossil fuels (combustion of coal, oil and gas), which releases CO₂; deforestation also raises CO₂ because fewer trees remove it by photosynthesis.

(For methane, acceptable activities include cattle farming, rice cultivation in paddy fields, or decomposition of waste in landfill sites.)

In secondary succession a soil is already present, so the slow pioneer stage in which lichens must weather bare rock and build up soil is not needed.

Because soil already exists, it can immediately retain water and nutrients, and seeds, spores and surviving root systems are often still present in or on the soil.

Larger plants can therefore establish straight away rather than having to wait for soil to form, so succession progresses through its seral stages and reaches the climax community more quickly.

Primary succession begins on bare rock, which has no soil. The first organisms to colonise are pioneer species such as lichens, which are adapted to survive the harsh conditions of low water and few nutrients.

The pioneers weather the rock both physically and chemically and, when they die, their remains are decomposed to add humus (organic matter). This begins to form a thin soil that can retain water and mineral ions, which makes the habitat suitable for the next stage.

In this thin soil, mosses and then small plants such as grasses and herbs can germinate and root. Their larger root systems break up more rock and add more organic matter, so the soil becomes deeper and more nutrient-rich.

The deeper, richer soil can now support shrubs and small trees, which add still more biomass and create shade, again changing the abiotic conditions.

Eventually a climax community develops — in a temperate climate this is typically woodland dominated by large trees such as oak. This community is stable and in equilibrium with the environment. Throughout the process, each stage changes the abiotic environment in a way that allows a more demanding community to replace it, while biodiversity generally increases.

Short-wave, high-energy radiation from the Sun passes through the atmosphere and reaches the Earth's surface, warming it. The warmed surface then re-emits energy, but as longer-wave infrared radiation.

Greenhouse gases in the atmosphere, principally carbon dioxide and methane (and also water vapour), absorb this outgoing long-wave infrared radiation. The gas molecules then re-radiate the infrared in all directions, including back towards the Earth's surface. This means that some of the energy that would otherwise have escaped to space is retained in the lower atmosphere, keeping the Earth warmer than it would be without these gases. This is the natural greenhouse effect, which keeps the planet warm enough to support life.

Human activity has increased the concentration of greenhouse gases in the atmosphere. Burning fossil fuels and deforestation raise carbon dioxide levels, while cattle farming, rice cultivation and landfill increase methane. Because there are now more greenhouse gas molecules, more of the outgoing infrared radiation is absorbed and re-radiated back to Earth, so even more energy is retained.

This extra warming is called the enhanced greenhouse effect. The result is a rise in mean global temperature, known as global warming, which is the driver of climate change.

The data show a positive correlation: as atmospheric carbon dioxide concentration increases, the mean global temperature anomaly also increases. Both rise together over the period — CO₂ climbs from 317 ppm in 1960 to 414 ppm in 2020, while the temperature anomaly rises from 0.00 °C to 0.98 °C, with the steepest temperature rise between 2000 and 2020.

The percentage increase in CO₂ concentration between 1960 and 2020 is calculated as:

percentage increase = (414 − 317) ÷ 317 × 100 = 97 ÷ 317 × 100 = 30.6% (3 significant figures).

However, these data alone do not prove that carbon dioxide causes the temperature rise. A correlation does not prove causation: the two variables changing together could be due to a confounding variable, or it is possible that warming itself releases more CO₂ (for example from the oceans), which would be reverse causation. To establish causation, a known mechanism and controlled comparison are needed.

In this case there is a plausible mechanism — CO₂ absorbs and re-radiates outgoing infrared radiation — which, together with the correlation and the large datasets, makes the causal link convincing, but the correlation in this table on its own is not sufficient proof.

A plant species can only grow successfully within a particular range of temperatures, which is determined largely by the optimum temperatures of its enzymes. Temperature is therefore one of the main abiotic factors controlling where the species can live.

As global temperature rises, the area the plant currently occupies may become too warm and fall outside its tolerance range. The species' distribution is therefore likely to shift towards cooler regions — towards the poles or to higher altitudes — where conditions remain within its tolerance.

Plants that cannot disperse their seeds fast enough into newly suitable areas, or that have no cooler region to move into, may decline in number or become locally extinct.

Warmer temperatures also increase the rate of enzyme-controlled processes such as growth and metabolism. As a result, developmental events occur earlier in the year and faster — for example the plant may germinate, leaf out and flower earlier than before.

These changes in timing can have knock-on effects: if the plant flowers before its pollinating insects are active, pollination may be reduced, which can affect reproduction and the wider food web.

Tree rings (dendrochronology), ice cores and pollen records are all examples of proxy data — they are indirect indicators of past climate rather than direct measurements of temperature, so each must be evaluated carefully.

Dendrochronology uses the width of annual growth rings: wider rings generally indicate warmer, wetter growing seasons. Its strength is that it gives an annual resolution and, by matching overlapping samples, can extend back thousands of years. Its limitation is that ring width is affected by several factors — rainfall, light availability and disease — not temperature alone, so it does not isolate temperature, and it can only be used where suitable trees grow.

Ice cores are a powerful source because trapped air bubbles directly preserve samples of the ancient atmosphere, giving past carbon dioxide and methane concentrations, while isotope ratios in the ice indicate past temperature. They can cover hundreds of thousands of years, far longer than any other source. Their limitations are that temperature itself is inferred from an isotope proxy, they are only available in polar or high-altitude glacial regions, and dating becomes less certain deeper in the core.

Pollen records preserved in peat and sediment show which plant species grew at a particular time, and because each plant needs particular conditions this indicates the past climate. They span long timescales but are again indirect, and some pollen types are preserved or dispersed better than others, which can bias the record.

Evaluating the statement: no single one of these sources, on its own, 'proves' the climate is changing, because each is indirect and has weaknesses. However, the statement is largely justified when the sources are taken together: they are independent yet broadly agree, and when combined with direct temperature records over the last 150 years they provide strong, mutually supporting evidence. The cautious conclusion is that the evidence strongly indicates climate change, rather than 'proving' it beyond all doubt — but the agreement of multiple independent proxies makes the case very convincing.

Reforestation means planting large numbers of trees on land that has been cleared or never afforested. Its main benefit is that growing trees photosynthesise and so remove carbon dioxide from the atmosphere, fixing the carbon into their biomass; the woodland therefore acts as a carbon sink. Reforestation also restores habitats and increases biodiversity. However, there are limitations: trees take many years to mature before they absorb large amounts of CO₂, so the effect is slow; the carbon is only stored while the trees live, and is released again if they are burnt, felled or left to decompose; and large-scale planting needs land that may be wanted for farming.

Carbon sequestration is the long-term capture and storage of carbon dioxide so that it is removed from the atmosphere. This can happen naturally — in forests, peat bogs and the oceans — or through technology that captures CO₂ from power-station flue gases and stores it underground in suitable rock formations. Its strength is that it directly lowers atmospheric CO₂ and, for the technological form, can store carbon for very long periods. Its limitations are that the technology is expensive and energy-intensive (which itself can produce CO₂), the storage must remain permanent because any leakage would undo the benefit, and natural sinks such as peat and forest can themselves be destroyed.

Weighing these together, both approaches genuinely remove carbon dioxide and so are worthwhile, but neither is a complete solution. Reforestation is comparatively cheap and brings extra benefits for biodiversity but is slow and reversible; sequestration can store large amounts permanently but is costly and unproven at very large scale. Crucially, both tackle the removal of CO₂ rather than its production.

In conclusion, reforestation and carbon sequestration are valuable parts of the response to climate change and should be pursued, but because they remove CO₂ relatively slowly or expensively, they cannot keep pace with continued high emissions on their own. They are most effective when combined with reducing emissions at source — for example by using renewable energy — so the best strategy uses removal and emission-reduction together.

A correlation means that two variables — here carbon dioxide concentration and global temperature — change together, in this case both increasing over time. A correlation does not, by itself, prove that one variable causes the other.

There are several reasons for caution. A confounding variable could be responsible: a third factor might be driving both changes. It is also possible that the relationship is partly the reverse of what is claimed, because a rise in temperature can itself release more CO₂, for example from the oceans, so the two reinforce each other rather than one simply causing the other. With a single correlation it is even possible, in principle, for the link to be coincidental.

However, scientists are confident that rising CO₂ is a cause of the warming because there is a well-understood physical and chemical mechanism: carbon dioxide absorbs outgoing infrared (long-wave) radiation and re-radiates some of it back towards the Earth, so increasing its concentration is expected to cause warming. In addition, the correlation is supported by very large datasets and by multiple independent sources of evidence (such as ice cores and temperature records) that all show the same pattern.

A correlation that is backed by a credible mechanism, large amounts of data and independent confirmation is far more convincing than a correlation alone. This combination is why scientists conclude that human-released carbon dioxide is contributing to global warming, even though the correlation by itself is not proof.