3.1 4.a, 4.b - Climate models, feedbacks and unequal impacts

The carbon cycle in climate models

A climate model represents physical relationships governing energy, water, air, oceans and ice. It divides the system into numerical regions and calculates how conditions change through time. An Earth-system model also represents interactions such as vegetation and carbon cycling. It is a simplified representation tested against observations and physical understanding, not a crystal ball.

The carbon cycle matters because emissions do not translate one-for-one into a permanent atmospheric increase. Carbon moves between stores through transfers. Photosynthesis removes carbon dioxide from the atmosphere; respiration and decomposition return it. Oceans exchange carbon dioxide with the air and transport carbon into deeper water. Fossil-fuel combustion and land clearance disturb these exchanges by adding carbon or changing uptake.

A sink has a net inward carbon transfer over a stated period; a source has a net outward transfer. A forest can absorb carbon through photosynthesis and release it through respiration at the same time. Its net balance depends on the difference. Counting gross uptake as if it were permanent net storage exaggerates the benefit. Warming, drought, fire, nutrient supply and land management can alter that balance.

Models therefore calculate both human emissions and the response of land and ocean sinks. If a warming ocean takes up less additional carbon dioxide, a larger fraction of emissions can remain in the atmosphere. This strengthens greenhouse forcing compared with a world with stronger uptake. The carbon budget and the energy budget are connected: changing carbon storage changes radiation, and warming changes carbon transfers. IPCC AR6 Working Group I Chapter 5 assesses these carbon-cycle interactions, 2021.

Worked example — an increasing atmospheric store

Illustrative dataset: annual emissions are 10 carbon units and net land and ocean uptake totals 6 units.

Worked example — lower emissions with continuing accumulation

Illustrative dataset: emissions fall to 8 carbon units while uptake remains 6.

The atmospheric store grows more slowly but still grows, because inputs exceed removal. Slower growth of a stock differs from a falling stock; the annual emissions trend alone cannot establish the direction of atmospheric change.

Positive and negative climate feedbacks

A feedback is a response that acts back on the original change. Positive feedback amplifies that change; negative feedback opposes it. “Positive” does not mean beneficial and “negative” does not mean harmful. The sign describes the direction of the system response.

Ice-albedo feedback is positive. Warming melts reflective snow or sea ice; darker land or water absorbs more solar energy; further warming encourages more melting. The chain needs the return link to temperature. Saying only that ice melts describes an impact, not a complete feedback.

Water-vapour feedback also amplifies warming. A warmer atmosphere can hold more water vapour, and, where moisture is available, increased vapour strengthens infrared absorption. The resulting warming permits further moisture increase. This does not mean water vapour independently drives the same long-term atmospheric accumulation as sustained fossil carbon dioxide emissions: water cycles rapidly through evaporation and precipitation.

A fundamental negative feedback is the increase in radiation emitted to space as Earth warms. A warmer body emits more long-wave energy, tending to reduce the initial imbalance. Positive feedbacks can make the warming needed to restore balance larger without causing indefinite runaway warming. Carbon uptake by vegetation can also oppose an initial carbon-dioxide rise, but nutrient limits, drought, disturbance and eventual saturation restrict that response. Cloud changes contain competing effects through sunlight reflection and infrared absorption, so their net contribution must be assessed rather than assigned a sign from one feature.

Models incorporate these processes, but some are smaller than a model grid or incompletely understood. Scientists compare models, observations and past climates to constrain their strength. A threshold is a level beyond which the system response changes substantially, sometimes producing persistent change. It should be attached to a specific process and timescale; every positive feedback is not automatically a tipping point. IPCC AR6 Working Group I Chapter 7, 2021.

Worked synoptic connection — climate and inherited coasts

Examine how climate change can influence coastal landscapes. Climate feedbacks affect the strength of warming, while coastal forms respond through water storage, relative sea level and sediment processes. Warming that reduces reflective snow and ice increases solar absorption, reinforcing the initial temperature change. Loss of land-based ice then transfers water into the oceans; warming seawater also expands. Floating sea-ice loss is significant through albedo but has little direct sea-level effect, so the two mechanisms must be distinguished.

Rising global sea level changes the setting for marine erosion and deposition where it produces a local relative rise. The inherited shape remains important. Geirangerfjord in Norway occupies a glacially eroded trough: inundation creates the marine setting, but the earlier ice erosion supplies its steep-sided form. Present waves, currents and slope processes modify that inheritance rather than recreate it entirely. At a shingle barrier such as Chesil in Dorset, higher water can promote overwash, transferring sediment landwards. Defra and the Environment Agency's February 2008 barrier study identified Chesil's reduced connection with an original offshore gravel source. Available material and room to migrate consequently influence the response to a rising sea.

The relationship also varies with land movement. Post-glacial uplift can reduce or reverse a local relative rise even while the global ocean rises; fixed development can prevent a barrier migrating into new space. Climate change therefore supplies changing energy and water-storage conditions, while geology, inherited landforms, sediment supply and human decisions determine the local outcome. The explicit connection is from feedback and warming through sea-level processes to different coastal adjustments, rather than two separate accounts of climate and coasts. UNESCO's fjord account and the 2008 barrier study provide the named landscape evidence.

Scenarios, temperatures and sea levels

A scenario is a coherent set of assumptions about future drivers such as energy, population, technology, land use and policy. A projection asks what climate response follows if those assumptions occur. It is conditional; a forecast instead attempts to estimate what will actually happen. Scenario names are not probabilities.

The IPCC's Shared Socioeconomic Pathway scenarios combine a socioeconomic pathway with an approximate radiative-forcing level in 2100. For example, SSP2-4.5 is an intermediate emissions pathway; 4.5 refers to an approximate forcing in watts per square metre, not a temperature rise. Very low and very high scenarios allow researchers to compare consequences of substantially different emissions futures.