Revise Climate change for IB Biology (D4.3)

Short-wave solar radiation passes largely unhindered through the atmosphere and is absorbed at Earth's surface. The warmed surface then radiates energy back outwards as long-wave infrared. Greenhouse gases absorb this outgoing infrared: an absorbed photon lifts the molecule into a higher vibrational energy state, and the excited molecule then re-emits infrared in a random direction, so a significant fraction is sent back down towards the surface. This returned radiation reduces the net rate at which the planet loses energy to space, so the surface settles at a higher temperature than it otherwise would. The four gases that matter most are carbon dioxide ($CO_2$), methane ($CH_4$), water vapour ($H_2O$) and nitrous oxide ($N_2O$). The bulk of the atmosphere — nitrogen and oxygen — does not contribute, because those symmetric diatomic molecules cannot absorb infrared in the same way. This natural greenhouse effect is essential: without it, Earth would be frozen and largely lifeless.

The enhanced greenhouse effect is the additional warming caused when human activities raise greenhouse-gas concentrations above their natural levels. Three human activities dominate. First, the combustion of fossil fuels (coal, oil and natural gas) for energy, industry and transport releases carbon that was locked underground for millions of years, adding $CO_2$ to the atmosphere. Second, deforestation removes trees that would otherwise absorb $CO_2$ through photosynthesis — it reduces a carbon sink, and burning or decay of the felled biomass releases the carbon the trees had stored. Third, agriculture releases methane (from livestock digestion and flooded rice paddies) and nitrous oxide (from nitrogen fertilisers). More greenhouse gas means more outgoing infrared is absorbed and re-emitted back to the surface, so the energy balance shifts and the surface warms until balance is restored at a higher temperature. Be precise: the plain greenhouse effect is the natural, beneficial process; the enhanced greenhouse effect is the human-caused intensification behind global warming.

Direct atmospheric measurements — most famously the continuous record begun at Mauna Loa Observatory — show $CO_2$ rising from about 280 parts per million (ppm) before the Industrial Revolution to over 420 ppm today, tracking the growth in fossil-fuel use. Antarctic ice cores extend the record back hundreds of thousands of years and show $CO_2$ and temperature rising and falling together. Instrumental temperature records over the same modern period show a clear warming trend. But a correlation, however strong, does not by itself prove that $CO_2$ causes the warming — it is always logically possible that a third factor drives both, or that the link is coincidental. What lifts the climate case from correlation to causation is the mechanism: $CO_2$ is known, from laboratory physics, to absorb and re-emit infrared, so there is a direct physical pathway from more $CO_2$ to a warmer surface. Correlation plus a verified mechanism is what makes anthropogenic climate change the accepted scientific explanation, and a strong answer states both halves.

A warming climate reshapes the living world in several linked ways. As climate bands move, so do the ranges of species: many are shifting towards the poles and to higher altitudes, tracking the conditions they can tolerate, while species that cannot move fast enough — or that are already at a mountaintop or coastline — are squeezed out. Warming also alters phenology, the timing of seasonal events such as flowering, budburst, breeding and migration; these tend to happen earlier, and problems arise when interacting species shift by different amounts, so that (for example) a chick hatches after its insect food has peaked — a phenological mismatch. Sensitive ecosystems feel this acutely. Coral reefs bleach when raised sea-surface temperatures cause corals to expel their symbiotic algae, losing colour and their main food source. Polar species such as polar bears lose the sea ice they hunt from as it melts. Meanwhile the ocean absorbs much of the added $CO_2$, forming carbonic acid and lowering pH — ocean acidification — which reduces the carbonate ions that corals, molluscs and some plankton need to build their calcium carbonate skeletons and shells. Together, range shifts, phenological mismatches, bleaching, habitat loss and acidification reduce biodiversity as vulnerable species decline or go extinct.

Mitigation means limiting the size of future climate change, and it has two complementary arms. The first is reducing emissions at source: shifting from fossil fuels to renewable energy, improving energy efficiency, and cutting the methane and nitrous oxide released by agriculture. The second is protecting and restoring the ecosystems that act as carbon sinks. Forests, peatlands, mangroves and seagrass meadows store large quantities of carbon and continue to remove atmospheric $CO_2$ through photosynthesis. Protecting an intact sink keeps that stored carbon out of the atmosphere and maintains ongoing uptake; restoring a degraded one — for example through reforestation — increases future removal of $CO_2$. Because deforestation is itself a major source of emissions, halting it is doubly valuable: it prevents a release and preserves a sink. Ecosystem conservation is therefore not a side issue but a direct climate-mitigation strategy that sits alongside cutting emissions.

D4.3 is an explain-heavy topic, and explanation marks are awarded analytically — each distinct valid biological point is worth one mark. Method marks (M) credit correct reasoning, answer marks (A) credit a correct conclusion, and error-carried-forward (ECF) means a slip early on does not cost you the marks that follow, provided your reasoning is written down. Study how each mark below is tied to a specific, named idea rather than to loose phrasing.