Water stored in ice and oceans

Earth's water stores make up the hydrosphere. The atmosphere is the layer of gases surrounding Earth, while the biosphere includes living organisms and the environments they occupy. Climate change affects all three. A change in one system can therefore have consequences in another.

As temperatures rise, glaciers and ice sheets melt more. Less water stays stored as land ice where melting exceeds snowfall accumulation. The meltwater reaches the oceans and adds to their volume. Warming also causes thermal expansion: warmer seawater takes up more space. Both processes raise global mean sea level.

Sea level does not change equally everywhere. Ocean circulation redistributes water; land subsidence increases relative sea-level rise, whereas land uplift can reduce it. Higher relative sea levels increase coastal inundation and erosion. They also increase saltwater intrusion into soils and freshwater stores.

Retreating glaciers change seasonal water supply too. At first, increased melting may raise river flows. Continued ice loss, though, leaves a smaller reserve for dry seasons. Downstream communities that depend on glacier-fed rivers can face significant consequences.

Carbon stored in ice, oceans and the biosphere

A carbon store holds carbon. A carbon sink takes up more carbon than it releases over a given period. If losses exceed uptake, a store can become a source.

Trapped gases in ice include carbon dioxide, but pure ice is not a major carbon store. In frozen regions, the larger climate concern is organic carbon preserved in frozen soils, or permafrost. As these soils thaw, decomposition accelerates and releases carbon dioxide. Waterlogged conditions also allow methane to be released. These releases can intensify warming, leading to further thawing.

Oceans store carbon in dissolved forms, as well as through marine organisms and sediments. Warmer water holds less dissolved carbon dioxide. Stronger layering between surface and deeper waters can restrict the transfer of carbon to depth and weaken uptake. Absorbing atmospheric carbon dioxide also causes ocean acidification, which makes it harder for some marine organisms to form shells and skeletons.

Extreme weather: incidence and severity

Incidence describes how often an event occurs; severity describes its intensity. Climate change can alter either or both. The patterns differ between regions and types of event.

A warmer average climate makes extreme heat more likely and generally makes extreme cold less frequent. Because warmer air can contain more water vapour, rainfall can be heavier when conditions trigger precipitation. Flood risk can rise as a result, even where annual rainfall changes little.

Drought is a prolonged shortage of water relative to normal conditions. Reduced rainfall can start a drought. Higher temperatures then increase evaporation and plant water loss, drying soils further. Some regions can therefore become more drought-prone without a large decline in rainfall.

Warmer oceans can support more intense tropical cyclones with heavier associated rainfall. Cyclone numbers don't necessarily increase everywhere, since storm formation also depends on atmospheric conditions. Nor does a single event, by itself, prove that a long-term pattern is changing.

Biomes, habitats and animal migration

A biome is a large ecological region characterized by its climate and dominant vegetation. The environment in which an organism lives is its habitat. Changes in temperature and rainfall patterns alter the areas suitable for particular biomes and species.

Animal migration can change its timing, route and destination. Earlier seasonal warming may lead to earlier migration, while food availability may shift at a different rate. If breeding and arrival no longer match peak food supply, survival can fall. Ocean warming can similarly shift where fish and their predators are found.

Agriculture: yields, cultivation limits and erosion

Crop yield is the amount harvested per unit area. In cool regions, warming can lengthen growing seasons. Excessive heat, however, damages crops and speeds their development, which can shorten the time available for grain filling. Water shortages and extreme rainfall reduce yields further.

Some crops can grow more with higher carbon dioxide concentrations, but these gains depend on adequate water and nutrients. Heat stress, pests and drought may offset the gains. The crop and its location therefore shape agricultural outcomes, rather than temperature alone.

As frost becomes less restrictive, the limits of cultivation may move polewards or uphill. Newly suitable temperatures do not guarantee productive farmland, though. Soils may remain unsuitable, as may drainage and water supply. Cultivation may retreat in already hot or dry regions as water shortages intensify.

Heavier rain can detach soil and increase runoff, accelerating soil erosion. Drought can strip away protective vegetation cover, leaving dry soil exposed to wind erosion. Losing fertile topsoil then lowers productivity, linking climatic change to longer-term agricultural decline.

Health hazards

Climate change affects health directly when people face extreme conditions. It also affects health indirectly by changing water supplies, food availability and the environments in which diseases spread.

Heatwaves increase heat exhaustion and heatstroke, and put stress on the cardiovascular system. The risks are greater for older people, outdoor workers and those without access to cool housing. Floods and storms cause injuries and deaths. When they damage water supplies and sanitation, waterborne disease can increase too.

Changes in temperature and rainfall can shift the range of disease vectors such as mosquitoes and alter their seasonal activity. Climate alone does not determine disease spread: healthcare, sanitation and vector control also shape the outcome. Drought and declining crop yields can increase malnutrition. Wildfire smoke, meanwhile, can worsen respiratory illness.

Migration and displacement

Climate-related hazards can make homes unsafe or weaken livelihoods. Sudden floods and storms may force people into temporary displacement. Repeated crop failure, water shortages or coastal inundation can encourage longer-term migration.

Much of this movement stays within countries, including moves from rural areas to towns. Migration rarely has a single cause, though. Employment and family networks interact with environmental pressures, as do conflict and government decisions. Some households can afford to move; others lack the resources to leave and may remain exposed.

Places receiving migrants may gain workers while facing additional demand for housing, water and services. For people who move, access to secure work, legal rights and social support shapes the outcome. The same climatic pressure can therefore have different consequences for different households.

Ocean transport routes

A shorter distance doesn't automatically make a route cheaper or dependable. Seasonal ice and drifting ice can restrict use, along with severe weather, limited rescue facilities and the need for suitable vessels. More shipping also raises the risks of pollution and disturbance in sensitive Arctic environments.

Elsewhere, higher sea levels and more damaging coastal flooding can disrupt port terminals and storage areas, as well as road or rail connections. Climate change can open some transport opportunities while making other parts of the shipping network less reliable.

Why effects vary between places

Climate change is global. Its consequences, though, are geographically uneven: warming and rainfall changes vary by region, while local physical conditions shape how people experience them. Low-lying coasts face sea-level hazards. In dry regions, the concern is water stress; in glacier-fed basins, it is changes in seasonal runoff.

How much harm occurs also depends on exposure, meaning the people and assets in harm's way, and vulnerability, meaning their susceptibility to damage. Similar floods can have very different consequences in places with different housing quality, healthcare, income and access to services.

Effects can conflict, too. Some farmers in cool regions may benefit from a longer growing season, while heat and drought reduce yields elsewhere. New shipping opportunities may benefit trading firms but increase environmental risks for coastal communities. A gain for one place or group does not demonstrate a global net benefit.

Uncertainty about timing and scale

Future greenhouse gas emissions and natural climate variability both create uncertainty. So do differences between climate models and an incomplete understanding of some processes, especially ice-sheet responses. We can be more certain about broad trends than about exactly how they will appear locally. For example, rising global mean sea level is more predictable than the exact amount of coastal flooding at a particular settlement in a particular year.

Some effects develop gradually. Others occur once a threshold is crossed—for example, when repeated flooding makes a settlement difficult to inhabit. Oceans and ice sheets respond slowly, so consequences can continue long after the initial warming. An impact therefore need not occur at the same time as the temperature change that helped cause it.

Judging impacts on individuals and societies

To evaluate an impact well, connect the physical change to its consequences and identify who experiences them. An insured commercial farm may be able to manage the loss of a crop harvest. For a household dependent on its own production, the same loss may threaten food security.

When reading maps or projections, distinguish global averages from regional patterns. Separate short-term variability from longer-term trends, and don't treat a projected range as a single certain outcome. Uncertainty does not mean that nothing is known. Conclusions should reflect how strong the evidence is and the range of plausible outcomes.