Changes in the climate system include an overall warming trend, more extreme weather and rising sea levels. These in turn impact nature and wildlife, as well as human settlements and societies.

The effects of human-caused climate change are broad and far-reaching, especially if significant climate action is not taken. The projected and observed negative impacts of climate change are sometimes referred to as the climate crisis.

The changes in climate are not uniform across the Earth. In particular, most land areas have warmed faster than most ocean areas, and the Arctic is warming faster than most other regions. Among the effects of climate change on oceans are an increase of ocean temperatures, a rise in sea level from ocean warming and ice sheet melting, increased ocean stratification, and changes to ocean currents including a weakening of the Atlantic      meridional overturning circulation. Carbon dioxide from the atmosphere is acidifiying the ocean.

Recent warming has strongly affected natural biological systems. It has degraded land by raising temperatures, drying soils and increasing wildfire risk. Species worldwide are migrating poleward to colder areas. On land, many species move to higher ground, whereas marine species seek colder water at greater depths. At 2 °C (3.6 °F) of warming, around 10% of species on land would become critically endangered.

Food security and access to fresh water are at risk due to rising temperatures. Climate change has profound impacts on human health, directly via heat stress and indirectly via the spread of infectious diseases. The vulnerability and exposure of humans to climate change varies by economic sector and by country. Wealthy industrialised countries, which have emitted the most CO₂, have more resources and so are the least vulnerable to global warming. Economic sectors affected include agriculture, fisheries, forestry, energy, insurance, and tourism. Some groups may be particularly at risk from climate change, such as the poor, women, children and indigenous peoples. Climate change can lead to displacement and changes in migration flows.

Changes in temperature

Global warming affects all elements of Earth’s climate system. Global surface temperatures have risen by °C (2.0 °F) and are expected to rise further in the The changes in climate are not uniform across the Earth. In particular, most land areas have warmed faster than most ocean areas, and the Arctic is warming faster than most other regions. In addition, night-time temperatures have increased faster than daytime temperatures. The impact on the environment, wildlife, society and humanity depends on how much more the Earth warms.

One of the methods scientists use to predict the effects of human-caused climate change is to investigate past natural changes in climate. To assess changes in Earth’s past climate scientists have studied tree rings, ice cores, corals, and ocean and lake sediments. These show that recent warming has surpassed anything in the last 2,000 years. By the end of the 21st century, temperatures may increase to a level not experienced since the mid-Pliocene, around 3 million years ago. At that time, mean global temperatures were about 2–4 °C (3.6–7.2 °F) warmer than pre- industrial temperatures, and the global mean sea level was up to 25 meters higher than it is today.

How much the world warms depends on human greenhouse gas emissions and how sensitive the climate is to greenhouse gases. The more carbon dioxide (CO2) emitted in the 21st century the hotter the world will be by 2100. For a doubling of greenhouse gas concentrations, the global mean temperature would rise by about 2.5–4 °C (4.5–7.2 °F). If emissions of CO2 were to be abruptly stopped and no negative emission technologies deployed, the Earth’s climate would not start moving back to its pre-industrial state. Instead, temperatures would stay elevated at the same level for several centuries. After about a thousand years, 20% to 30% of human-emitted CO₂ will remain in the atmosphere, not taken up by the ocean or the land, committing the climate to a warmer state long after emissions have stopped.

Mitigation policies currently in place will result in about 2.7 °C (2.0–3.6 °C) warming above pre- industrial levels by 2100. If all unconditional pledges and targets made by governments are achieved the temperature will rise by around 2.4 °C (4.3 °F). If additionally all the countries that adopted or are considering to adopt net-zero targets will achieve them, the temperature will rise by a median of 1.8 °C (3.2 °F). There is a substantial gap between national plans and commitments and actions so far taken by governments around the world.

Weather

The lower and middle atmosphere, where nearly all of the weather occurs, are heating due to the enhanced greenhouse effect. Increased greenhouse gases cause the higher parts of the atmosphere, the stratosphere, to cool. As temperatures increase, so does evaporation and atmospheric moisture content. As water vapour is also a greenhouse gas, this process acts as a self-reinforcing feedback.

The excess water vapour also gets caught up in storms and makes them more intense, larger, and potentially longer-lasting. This in turn causes rain and snow events to become stronger and leads to increased risk of flooding. Extra drying worsens natural dry spells and droughts, and increases risk of heat waves and wildfires. With recent climate trends clearly identified as caused by human activities, extreme event attribution estimates the impact of climate change in extreme climate events. For instance, such research can demonstrate that a specific heatwave was more intense due to climate change based on historical data for that region.

Rain and snow

Global warming increases the average precipitation (such as rain and snow) globally. Higher temperatures lead to increased evaporation and surface drying. As the air warms, its water-holding capacity also increases: Air can hold 7% more water vapour for every degree Celsius it is warmed. Changes have already been observed in the amount, intensity, frequency, and type of precipitation. Overall, climate change causing longer hot dry spells, broken by more intense heavy rainfalls. Widespread increases in heavy precipitation have occurred even in places where total rain amounts have decreased.

Climate change has increased contrasts in rainfall amounts between wet and dry seasons: wet seasons are getting wetter and dry seasons are getting drier. In the northern high latitutes, warming has also caused an increase in the amount of snow and rain. Future changes in precipitation are expected to follow existing trends, with reduced precipitation over subtropical land areas, and increased precipitation at subpolar latitudes and some equatorial regions.

Weather-related impacts

Floods

Due to an increase in heavy rainfall events, floods are expected to become more extreme under climate change. However, there are a few regions in which flooding is expected to become rarer. This depends on several factors, such as changes in rain and snowmelt, but also soil moisture. Sea level rise further increases risks of coastal flooding: if sea levels rise by a further 0.15 m (5.9 in), 20% more people will be exposed to a 1 in a 100-year coastal flood, assuming no population growth and no further adaptation. With an extra 0.75 m (2 ft 6 in), this rises to a doubling of people exposed.

Droughts

Climate change affects multiple factors associated with droughts, such as how much rain falls and how fast the rain evaporates again. Warming over land increases the severity and frequency of droughts around much of the world. In some tropical and subtropical regions of the world, there will likely be less rain due to global warming, making them more prone to drought. These regions where droughts are set to worsen are Central America, the Amazon and south-western South America, West and Southern Africa, as well as the Mediterranean and south-western Australia.

Higher temperatures lead to increased evaporation, thus drying the soil and increasing plant stress, which will have impacts on agriculture. For this reason, even regions where overall rainfall is expected to remain relatively stable, such as central and northern Europe, will experience these impacts. Without climate change mitigation, it is expected that around a third of land areas will experience moderate or more severe drought by 2100. Droughts are likely to be more intense than in the past.

Due to limitations on how much data is available about drought in the past, it is often impossible to confidently attribute a specific drought to human-induced climate change. Some areas however, such as the Mediterranean and California, already show the impacts of human activities. Their impacts are made worse because of increased water demand, population growth, urban expansion, and environmental protection efforts in many areas. Land restoration, especially by agroforestry, can help reduce the impact of droughts.

Wildfires

Globally, climate change promotes the type of weather that makes wildfires more likely. In some areas, an increase of wildfires has been attributed directly to climate change. That warmer climate conditions pose more risks of wildfire is consistent with evidence from Earth’s past: there was more fire in warmer periods, and less in colder climatic periods. Climate change increases evaporation, which can cause vegetation to dry out. When a fire starts in an area with very dry vegetation, it can spread rapidly. Higher temperatures can also make the fire season longer, the time period in which severe wildfires are most likely. In regions where snow is disappearing, the fire season may get particularly more extended.

Even though weather conditions are raising the risks of wildfires, the total area burnt by wildfires has decreased. This is mostly the result of the conversion of savanna into croplands, after which there is less forest area that can burn. Prescribed burning, an indigenous practice in the US and Australia, can reduce the area burnt too, and may form an adaptation to increased risk. The carbon released from wildfires can further increase greenhouse gas concentrations. This feedback is not yet fully integrated into climate models.

Oceans

Among the effects of climate change on oceans are an increase of ocean temperatures, more frequent marine heatwaves, ocean acidification, a rise in sea levels, sea ice decline, increased ocean stratification, reductions in oxygen levels, changes to ocean currents including a weakening of the Atlantic meridional overturning circulation. All these changes have knock-on effects which disturb marine ecosystems. The primary factor causing these changes is the Earth warming due to human-caused emissions of greenhouse gases, such as carbon dioxide and methane. This leads inevitably to ocean warming, because the ocean is taking up most of the additional heat in the climate system. The ocean absorbs some of the extra carbon dioxide in the atmosphere and this causes the pH value of the ocean to drop. It is estimated that the ocean absorbs about 25% of all human-caused CO2 emissions.

Ocean temperature stratification increases as the ocean surface warms due to rising air temperatures. The decline in mixing of the ocean layers stabilises warm water near the surface while reducing cold, deep water circulation. The reduced up and down mixing reduces the ability of the ocean to absorb heat, directing a larger fraction of future warming toward the atmosphere and land. The amount of energy available for tropical cyclones and other storms is expected to increase, while nutrients for fish in the upper ocean layers are expected to decrease, as is the ocean’s capacity to store carbon.  At the same time, contrasts in salinity are increasing: salty areas are becoming saltier and fresher areas less salty.

Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increased thermal stratification may result in a reduced supply of oxygen from surface waters to deeper waters, lowering the water’s oxygen content further. The ocean has already lost oxygen throughout its water column, and oxygen minimum zones are expanding worldwide.

Sea level rise

Between 1901 and 2018, the average global sea level rose by 15–25 cm (6–10 in), or 1–2 mm per year. This rate is increasing; sea levels are now rising at a rate of 3.7 mm (0.146 inches) per year. Human-caused climate change is predominantly the cause, as it constantly heats (and thus expands) the ocean and melts land- based ice sheets and glaciers. Between 1993 and 2018, thermal expansion of water contributed 42% to sea level rise (SLR); melting of temperate glaciers contributed 21%; Greenland contributed 15%; and Antarctica contributed 8%. Because sea level rise lags changes in Earth temperature, it will continue to accelerate between now and 2050 purely in response to already- occurring warming; whether it continues to accelerate after that depends on human greenhouse gas emissions. If global warming is limited to 1.5 °C (2.7 °F), then sea level rise does not accelerate, but it would still amount to 2–3 m (7–10 ft) over the next 2000 years, while 19–22 metres (62–72 ft) would occur if the warming peaks at 5 °C (9.0 °F).

Rising seas pose both a direct risk of flooding unprotected areas and indirect threats of higher storm surges, king tides, and tsunamis. They are also associated with second-order effects such as loss of coastal ecosystems like mangroves, losses in crop production due to freshwater salinization of irrigation water, and the disruption of sea trade due to damaged ports. Just the projected sea level rise by 2050 will expose places currently inhabited by tens of millions of people to annual flooding. This may increase to hundreds of millions in the latter decades of the century if greenhouse gas emissions are not reduced drastically. While slow increases in sea level may allow time for adaptation, such as building sea walls, the passage of time can also increase the number of people at risk, as many coastal areas have large population growth. Later in the century, millions more would be affected in cities such as Miami, Rio de Janeiro, Osaka and Shanghai under the warming of 3 °C (5.4 °F), which is close to the current trajectory.

Ice and snow

The cryosphere, the area of the Earth covered by snow or ice, is extremely sensitive to changes in global climate. There has been an extensive loss in snow on land since 1981. Some of the largest declines have been observed in the spring. During the 21st century, snow cover is projected to continue its retreat in almost all regions.

Glaciers and ice sheets decline

Since the beginning of the twentieth century, there has been a widespread retreat of glaciers. The melting of the Greenland and West Antarctic ice sheets will continue to contribute to sea level rise over long time- scales. The Greenland ice sheet loss is mainly driven by melt from the top, whereas Antarctic ice loss is driven by warm ocean water melting the outlet glaciers.

Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse. Part of the ice sheet is grounded on bedrock below sea level, making it possibly vulnerable to the self-enhancing process of marine ice sheet instability. A further hypothesis is that marine ice cliff instability would also contribute to a partial collapse, but limited evidence is available for its importance. A partial collapse of the ice sheet would lead to rapid sea level rise and a local decrease in ocean salinity. It would be irreversible on a timescale between decades and millennia.

In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to be taking place more gradually over millennia. Sustained warming between 1 °C (1.8 °F) (low confidence) and 4 °C (7.2 °F) (medium confidence) would lead to a complete loss of the ice sheet, contributing 7 m (23 ft) to sea levels globally. The ice loss could become irreversible due to a further self-enhancing feedback: the elevation-surface mass balance feedback. When ice melts on top of the ice sheet, the elevation drops. As air temperature is higher at lower altitude, this promotes further melt.

Economic impacts

Economic forecasts of the impact of global warming vary considerably, but are worse if there is only limited adaptation. Economic modelling may underrate the impact of potentially catastrophic climatic changes. When estimating losses, economists choose a discount rate which determines how much one prefers to have a good or cash now compared to at a future date. Choices of a high discount rate may also understate estimates of economic losses, as losses for future generation weight less heavily.

The total economic impacts also increase for higher temperature changes. For instance, total damages are estimated to be 90% less if global warming is limited to 1.5 °C (2.7 °F) compared to 3.66 °C (6.59 °F), a warming level chosen to represent no mitigation. One study found a 3.5% reduction in global GDP by the end of the century if warming is limited to 3 °C (5.4 °F), excluding the potential effect of tipping points. Another study noted that global economic impact is underestimated by a factor of two to eight when tipping points are excluded from consideration. In a study on a high-emission scenario, a temperature rise of 2 °C (3.6 °F) by 2050 would reduce global GDP by 2.5%–7.5%. By 2100 in this case, the temperature would rise by 4 °C (7.2 °F), which could reduce the global GDP by 30% in the worst case.

Global losses reveal rapidly rising costs due to extreme weather events since the 1970s. Socio-economic factors have contributed to the observed trend of global losses, such as population growth and increased wealth. Part of the growth is also related to regional climatic factors, e.g., changes in precipitation and flooding events. It is difficult to quantify the relative impact of socio-economic factors and climate change on the observed trend. The trend does, however, suggest increasing vulnerability of social systems to climate change.

Economic inequality

Climate change has contributed towards global economic inequality. Wealthy countries in colder regions have either felt little overall economic impact from climate change, or possibly benefited, whereas poor hotter countries very likely grew less than if global warming had not occurred.

Highly affected sectors

Economic sectors directly affected by weather are more impacted by climate change than other sectors. For instance, the agriculture, fisheries and forestry sectors are all heavily affected, but also the tourism and energy sectors. Agriculture and forestry have suffered economic losses due to droughts and extreme heat, for instance in Europe. If global warming surpasses 1.5 degrees, there may be limits to adaptation for existing tourism and for outdoor work.

In the energy sector, fossil fuel plants and nuclear power plants depend on water to cool them. Climate change can increase the likelihood of drought and fresh water shortages. In addition, higher operating temperatures reduces their efficiency and hence their output. Hydropower is affected by changes in the water cycle such as river flows. The result of diminished river flow can be a power shortage in areas that depend heavily on hydroelectric power. Brazil in particular, is vulnerable due to its reliance on hydroelectricity, as rising temperatures, lower water flow, and alterations in rainfall, could reduce total energy production by 7% annually by the end of the century. Oil and natural gas infrastructure is affected by the effects of climate change and the increased risk of disasters such as storm, cyclones, flooding and rising sea levels.

The insurance and financial services sectors also experience impacts from global warming. Insurance is an important tool to manage risks, but often unavailable to poorer households. Due to climate change, premiums are going up for certain types of insurance, such as flood insurance. Poor adaptation to climate change further widens the gap between what people can afford and the costs of insurance, as risks increase. In 2019, Munich Re noted that climate change could cause home insurance to become unaffordable for households at or below average incomes.