The effects of climate change span the physical environment, ecosystems and human societies. It also includes the economic and social changes which stem from living in a warmer world. Human-caused climate change is one of the threats to sustainability.
Climate change has already impacted ecosystems and humans. In combination with climate variability, it makes food insecurity worse in many places and puts pressure on fresh water supply. This, in combination with extreme weather events, leads to negative effects on human health. Climate change has also contributed to desertification and land degradation in many regions of the world. This has implications for livelihoods as many people are dependent on land for food, feed, fibre, timber and energy. Rising temperatures, changing precipitation patterns and the increase in extreme events threaten development because of negative effects on economic growth in developing countries. Climate change already contributes to migration in different parts of the world.
The future impact of climate change depends on the extent to which nations implement prevention efforts, reduce greenhouse gas emissions, and adapt to unavoidable climate change effects. Much of the policy debate concerning climate change mitigation has been framed by projections for the twenty-first century. The focus on a limited time window obscures some of the problems associated with climate change. Policy decisions made in the next few decades will have profound impacts on the global climate, ecosystems and human societies, not just for this century, but for the next millennia, as near-term climate change policies significantly affect long-term climate change impacts.
Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels. Without mitigation, increased energy demand and the extensive use of fossil fuels may lead to global warming of around 4 °C. With higher magnitudes of global warming, societies and ecosystems will likely encounter limits to how much they can adapt.
Observed and future warming
Global warming refers to the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change, and has been demonstrated by the instrumental
temperature record which shows global warming of around 1 °C since the pre-industrial period, although the bulk of this (0.9 °C) has occurred since 1970. A wide variety of temperature proxies together prove that the 20th century was the hottest recorded in the last 2,000 years. Compared to climate variability in the past, current warming is also more globally coherent, affecting 98% of the planet. The impact on the environment, ecosystems, the animal kingdom, society and humanity depends on how much more the Earth warms.
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." This has been brought about primarily through the burning of fossil fuels which has led to a significant increase in the concentration of GHGs in the atmosphere.
Physical impacts
A broad range of evidence shows that the climate system has warmed. Evidence of global warming is shown in the graphs (below right) from the US National Oceanic and Atmospheric Administration (NOAA).
Some of the graphs show a positive trend, e.g., increasing temperature over land and the ocean, and sea level rise. Other graphs show a negative trend, such as decreased snow cover in the Northern Hemisphere, and declining Arctic sea ice, both of which are indicative of global warming. Evidence of warming is also apparent in living (biological) systems such as changes in distribution of flora and fauna towards the poles.
Human-induced warming could lead to large-scale, abrupt and/or irreversible changes in physical systems. An example of this is the melting of ice sheets, which contributes to sea level rise and will continue for thousands of years. The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change.
Wildlife and nature
Recent warming has strongly affected natural biological systems. Species worldwide are moving poleward to
colder areas. On land, species move to higher elevations, whereas marine species find colder water at greater depths. Of the drivers with the biggest global impact on nature, climate change ranks third over the five decades before 2020, with only change in land use and sea use, and direct exploitation of organisms having a greater impact.
The impacts of climate change in nature and nature's contributions to humans are projected to become more pronounced in the next few decades. Examples of climatic disruptions include fire, drought, pest infestation, invasion of species, storms, and coral bleaching events. The stresses caused by climate change, added to other stresses on ecological systems (e.g. land conversion, land degradation, harvesting, and pollution), threaten substantial damage to or complete loss of some unique ecosystems, and extinction of some critically endangered species. Key interactions between species within ecosystems are often disrupted because species from one location do not move to colder habitats at the same rate, giving rise to rapid changes in the functioning of the ecosystem.
Regional effects
rising temperature, resulting in local effects, such as melting ice. In other cases, a change may be related to a change in a particular ocean current or weather system. In such cases, the regional effect may be disproportionate and will not necessarily follow the global trend.
There are three major ways in which global warming will make changes to regional climate: melting or forming ice, changing the hydrological cycle (of evaporation and precipitation) and changing currents in the oceans and air flows in the atmosphere. The coast can also be considered a region, and will suffer severe impacts from sea level rise.
The Arctic, Africa, small islands, Asian megadeltas and the Middle East are regions that are likely to be especially affected by climate change. Low-latitude, less-developed regions are at most risk of experiencing negative impacts due to climate change. Developed countries are also vulnerable to climate change. For example, developed countries will be negatively affected by increases in the severity and frequency of some extreme weather events, such as heat waves.
Projections of climate changes at the regional scale do not hold as high a level of scientific confidence as projections made at the global scale. It is, however, expected that future warming will follow a similar geographical pattern to that seen already, with the greatest warming over land and high northern latitudes, and least over the Southern Ocean and parts of the North Atlantic Ocean. Land areas warm faster than ocean, and this feature is even stronger for extreme temperatures. For hot extremes, regions with the most warming include Central and Southern Europe and Western and Central Asia.
On humans
The vulnerability and exposure of humans to climate change varies from one economic sector to another and will have different impacts in different countries. Wealthy industrialised countries, which have emitted the most CO2, have more resources and so are the least vulnerable to global warming. Economic sectors that are likely to be affected include agriculture, human health, fisheries, forestry, energy, insurance, financial services, tourism, and recreation. The quality and quantity of freshwater will likely be affected almost everywhere. Some people may be particularly at risk from climate change, such as the poor, young children and the elderly. According to the World Health Organization, between 2030 and 2050, "climate change is expected to cause about 250,000 additional deaths per year." As global temperatures increase, so does the number of heat stress, heatstroke, and cardiovascular and kidney disease deaths and illnesses. When air pollution worsens, so does respiratory health, particularly for the 300 million people worldwide living with asthma; there is more airborne pollen and mold to torment hay fever and allergy sufferers.
Abrupt or irreversible changes
Self-reinforcing feedbacks amplify and accelerate climate change.
The climate system exhibits threshold behaviour or tipping points when these feedbacks lead parts of the Earth system into a new state, such as the runaway loss of ice sheets or the destruction of too many forests. Tipping points are studied using data from Earth's distant past and by physical modelling. There is already moderate risk of global tipping points at 1 °C above pre-industrial temperatures, and that risk becomes high at 2.5 °C.
Tipping points are "perhaps the most ‘dangerous’ aspect of future climate changes", leading to irreversible impacts on society. Many tipping points are interlinked, so that triggering one may lead to a cascade of effects. A 2018 study states that 45% of environmental problems, including those caused by climate change are interconnected and make the risk of a domino effect bigger.
Irreversible change
Warming commitment to CO
2 concentrations.
If emissions of CO
2 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 CO
2 will remain in the atmosphere, not taken up by the ocean or the land, committing the climate to warming long after emissions have stopped. Pathways that keep global warming under 1.5 °C often rely on large-scale removal of CO
2, which feasibility is uncertain and has clear risks.
Irreversible impacts
There are a number of examples of climate change impacts that may be irreversible, at least over the timescale of many human generations. These include the large-scale singularities such as the melting of the Greenland and West Antarctic ice sheets, and changes to the AMOC. In biological systems, the extinction of species would be an irreversible impact. In social systems, unique cultures may be lost due to climate change. For example, humans living on atoll islands face risks due to sea level rise, sea surface warming, and increased frequency and intensity of extreme weather events.
Global catastrophic risk
A global catastrophic risk is a hypothetical future event which could damage human well-being on a global scale, even endangering or destroying modern civilization. An event that could cause human extinction or permanently and drastically curtail humanity's potential is known as an existential risk.
intelligence and destructive biotechnology or nanotechnology. Insufficient or malign global governance creates risks in the social and political domain, such as a global war, including
nuclear holocaust
, bioterrorism using genetically modified organisms, cyberterrorism destroying critical infrastructure like the electrical grid; or the failure to manage a natural pandemic.Problems and risks in the domain of earth system governance include global warming, environmental degradation, including extinction of species, famine as a result of non-equitable resource distribution, human overpopulation, crop failures and non-sustainable agriculture.
Examples of non-anthropogenic risks are an asteroid impact event, a supervolcanic eruption, a lethal gamma-ray burst, a geomagnetic storm destroying electronic equipment, natural long-term climate change, hostile extraterrestrial life, or the predictable Sun transforming into a red giant star engulfing the Earth.
