4.9.2.3,4.9.2.4 - Climate change and carbon footprints

Climate consequences and risk

An increase in average global temperature can change climate patterns and create environmental risks extending far beyond a warmer afternoon.

An average is taken across many places and times. A rise in the global average does not mean every place warms equally or that cold weather disappears. Climate change includes patterns of temperature, rainfall and extremes, with different regional outcomes. Four potential effects are:

The IPCC's 2021 assessment reported a global mean sea-level rise of 0.20 m between 1901 and 2018, with an assessed range of $0.15$ to $0.25\,\mathrm{m}$. This is a dated global estimate: it does not mean that every coast rose by exactly $20\,\mathrm{cm}$. Local land movement and ocean conditions affect relative sea level and exposure to flooding.

Melting floating sea ice has a different direct sea-level effect from melting land ice, because it already displaces water. Its loss can still affect climate and habitats. Similarly, habitat changes do not guarantee every species becomes extinct: the outcome depends on its tolerance, rate of change and available habitat.

Scale concerns how wide an effect is, how large it is and how long it lasts. Greenhouse gases can affect the global system, but the consequences and vulnerability are uneven. Risk combines the likelihood of harm with how serious that harm would be. A possible flood threatens more damage where many people or irreplaceable habitats occupy low ground. Uncertainty about the exact date or size does not make a serious potential consequence irrelevant.

Use supplied regional evidence to discuss implications rather than claiming a single outcome everywhere. For an illustrative coastal wetland, rising sea level may drown habitat, but whether it can shift inland depends on the local land and barriers. The same global temperature change can therefore have different local consequences.

Carbon footprint accounting

A carbon footprint includes greenhouse gases emitted across the whole life cycle of a product, service or event, rather than only during its use.

A product's life cycle can include extracting raw materials, manufacture, transport, use, maintenance and disposal or recycling. An event can also involve making equipment, travel, power, food and waste. Looking only at electricity used by a finished product can miss substantial earlier emissions.

Different greenhouse gases have different warming influences. A footprint may be expressed as carbon dioxide equivalent, written $\ce{CO2}$-equivalent, so emissions are combined using a stated comparison method and time period. Do not add raw masses of different gases as if their effects were identical. In the examples here, the supplied figures are already expressed in the same equivalent units; no gas conversion factor needs to be invented.

Comparing a product and its redesign

Illustrative product: materials and manufacture contribute $40\,\mathrm{kg}$, transport $70\,\mathrm{kg}$, use $170\,\mathrm{kg}$, and disposal $20\,\mathrm{kg}$, all in $\ce{CO2}$-equivalent. A redesign provides the same service with contributions of $60$, $50$, $90$ and $20\,\mathrm{kg}$.

Comparing transport to an event

Illustrative event: attendees' travel contributes $54\,\mathrm{kg}$, venue electricity $28\,\mathrm{kg}$ and food/waste $18\,\mathrm{kg}$, all in $\ce{CO2}$-equivalent. Changing the travel plan reduces its contribution to $34\,\mathrm{kg}$, leaving the other stages unchanged.

Travel falls by $20\,\mathrm{kg}$, so the total must fall by the same amount. Dividing by the travel contribution alone would give the percentage reduction in travel emissions, rather than in the whole event footprint.

The redesign emits more during manufacture but less overall. That conclusion depends on equal service and the stated lifetime: a product replaced sooner or used less may give a different comparison. Check that both estimates have the same boundaries and cover the same useful outcome. Missing data, energy-source assumptions and disposal routes can introduce uncertainty.