12.6 Climate Change and Ecosystems

Earth absorbs shortwave solar radiation and emits energy as longer-wave infrared radiation. Greenhouse gases including water vapor, carbon dioxide, methane and nitrous oxide absorb particular infrared wavelengths and emit radiation in all directions. This reduces the rate at which energy escapes to space from lower atmospheric layers, warming the surface until outgoing energy again balances incoming energy. The natural greenhouse effect makes Earth habitable; increasing greenhouse-gas concentrations strengthens it.

Burning fossil fuels transfers carbon from geological stores to the atmosphere, while cement production and deforestation add further carbon dioxide. Agriculture, waste, fossil-fuel extraction and altered wetlands emit methane and nitrous oxide. Greenhouse gases differ in absorption, atmospheric lifetime and concentration, so comparing them requires a stated time horizon. Water vapor mainly acts as a feedback because warmer air can hold more of it; carbon dioxide is a long-lived forcing that initiates warming.

Global warming is the rise in mean surface temperature; climate change includes associated shifts in rainfall, extremes, winds, ocean circulation, ice and seasons. A local cold event does not disprove a global long-term trend. Attribution compares observed spatial and temporal patterns with model expectations under natural and anthropogenic drivers, alongside physical measurements of ocean heat, radiation, ice and greenhouse-gas isotopes.

Ice and snow have high albedo and reflect a large fraction of incoming radiation. Warming melts them, exposing darker ocean or land that absorbs more radiation and causes further warming. Thawing permafrost allows microbial decomposition of formerly frozen organic matter, releasing carbon dioxide under aerobic conditions and methane in anaerobic conditions. Warmer oceans can hold less dissolved carbon dioxide, weakening one carbon sink. Each is a positive feedback because the response amplifies the initial temperature change.

Forests can approach tipping risk when heat, drought, fire and fragmentation increase mortality faster than regeneration. Tree loss reduces transpiration and can lower regional rainfall, which further stresses remaining forest. The exact threshold is uncertain and differs among regions, but uncertainty does not imply absence of risk. Preventing deforestation and restoring connected, diverse forest reduces several pressures at once.

Species track suitable climate by shifting poleward, uphill, deeper or into new local microhabitats, but movement is limited by dispersal, barriers, soil, partners and available space. Polar and alpine species can run out of habitat at the edge of a continent or mountain summit. Sea-ice loss removes platforms used for breeding, resting and hunting and changes the timing and location of under-ice production that supports polar food webs.

Ocean circulation transports heat, oxygen and nutrients. Where winds and currents drive surface water away, deeper nutrient-rich water rises in upwelling and supports phytoplankton production. Surface warming strengthens density stratification and can inhibit vertical mixing, reducing nutrient resupply to sunlit water. Changes in winds or large-scale circulation can shift upwelling regionally; the ecological effect depends on both nutrient delivery and whether organisms can respond to new timing.

Warming also lowers oxygen solubility and increases metabolic oxygen demand, expanding stress in poorly mixed waters. Carbon dioxide dissolving in seawater forms carbonic acid and shifts carbonate equilibria, lowering carbonate-ion availability. This ocean acidification is chemically distinct from warming even though both arise largely from carbon dioxide emissions. Calcifying organisms may then require more energy to build or maintain shells and skeletons.

Reef-building corals depend on mutualistic photosynthetic dinoflagellates. The symbionts supply much of the coral's usable carbon while receiving inorganic nutrients, carbon dioxide and shelter. Heat and intense light disrupt photosynthetic systems and generate damaging reactive molecules. Corals expel symbionts or lose their pigments, revealing pale skeleton through transparent tissue: bleaching.

A bleached coral is stressed but not necessarily dead. Survival depends on event duration and intensity, food supply, disease and the return of compatible symbionts. Repeated bleaching depletes reserves, reduces growth and reproduction and can shift reefs toward algal dominance after coral mortality. Acidification separately reduces calcification and reef repair. Local control of pollution, sediment and overfishing improves resilience, but cannot substitute for reducing global greenhouse-gas forcing.

Mitigation reduces the magnitude of climate change by cutting emissions or increasing durable carbon storage. Adaptation reduces harm under changes that occur. Protecting peatlands avoids oxidation of dense carbon stores; rewetting drained peat can restore carbon accumulation but may temporarily alter methane release. Restoring mangroves protects coasts, supports nurseries and stores carbon. Planting trees is valuable in suitable ecosystems, but planting on natural grassland or drained peat can damage biodiversity and carbon balance.

Ecological restoration should match the original system, restore hydrology and disturbance regimes, use genetically and functionally diverse native species and reconnect climate refuges. Assisted migration may help species cross human-made barriers but risks unforeseen interactions. Because future conditions are uncertain, monitoring and adaptive management are essential: interventions are treated as testable actions that can be revised as evidence accumulates.