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Previous story Climate change is a major factor behind increased migration at U.S. southern border, experts say Next story

STORY BY HANNAH MIAO

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impact of Hurricanes is one of many reasons migrants are making the dangerous journey to the U.S Twin hurricanes intensify drivers of migration, including violence, food insecurity and poverty.
Published on April 19, 2021 4:49 AM

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Hurricanes Eta and Iota struck Central America last November, bringing torrential rain, flash floods, landslides and crop damage across Honduras, El Salvador, Guatemala and Nicaragua.

An estimated 7.3 million people in the region were affected by the twin hurricanes as of December, according to the United Nations.

The impact of the hurricanes is one of many reasons migrants from Central America are making the dangerous journey to the U.S. southern border to seek refuge — and just one example of climate-exacerbated drivers of displacement and migration.

"Climate change is reinforcing underlying vulnerabilities and grievances that may have existed for decades, but which are now leading to people having no other choice but to move," Andrew Harper, special advisor on climate action for the UNHCR, the United Nations Refugee Agency, said in an interview.

President Joe Biden and his administration have faced pressure from across the political spectrum to stem the flow of migration at the U.S. southern border.

U.S. Customs and Border Patrol reported encountering more than 172,000 people attempting to cross the southern border in March, a 71% increase compared with the previous month and a 34% increase from the same time frame in 2019. The vast majority of people arrive at the border are being expelled due to public health ordinance Title 42, although seeking asylum in the U.S. is a legal right.

CBP cited "violence, natural disasters, food insecurity, and poverty" in Mexico, Honduras, Guatemala and El Salvador for the rising numbers of encounters at the border.

New Jersey high school wrestling coach is CEO of $100 million firm that owns one deli Biden says he is ready to "take further actions' if Russia escalates against U.S., opens door to cooperation Bipartisan House group pushes to scrap SALT cap as infrastructure debate heats up "Climate change is never the sole driving factor behind migration decisions," said Kayly Ober, senior advocate and program manager for the Climate Displacement Program at Refugees International. "We see a confluence of events."

Ober said in addition to the sudden-onset disasters like Hurricanes Eta and Iota, longer-term climate challenges like drought contribute to instability, particularly in what's known as the Dry Corridor — a region running along the Pacific coast of Guatemala, El Salvador, Honduras and Nicaragua.

Krish O'Mara Vignarajah, president and CEO of Lutheran Immigration and Refugee Services, told CNBC at least a third of the migrants LIRS works with cite climate-related reasons as a primary factor for their displacement.

"You may see migrants who are initially internally displaced due to crop failures. But then because of that initial displacement, they become more vulnerable to gang violence and persecution, which then leads to international migration because the situation becomes worse," Vignarajah said.

Sarah Blodgett Bermeo, a professor of public policy and political science at Duke University, recently co-authored a study investigating root causes of migration from Honduras.

Using available data from 2012 to 2019, the study found that negative rainfall was associated with greater numbers of Honduran families apprehended at the U.S. southern border. Higher levels of violence, as measured by homicide rates, increased the magnitude of the association even further.

The Atlantic's Caitlin Dickerson: Congress needs to make "meaningful' changes to address border surge "As climate change continues to have impacts around the world, we're going to see more and more of these mixed migration flows, where people are coming for a variety of reasons from the same country," Bermeo said...

BACKGROUND

Climate change includes both global warming driven by human emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale. The largest driver of warming is the emission of greenhouse gases, of which more than 90% are carbon dioxide and methane. Fossil fuel burning for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and manufacturing. The human cause of climate change is not disputed by any scientific body of national or international standing. Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapour , and changes to land and ocean carbon sinks.

Temperature rise on land is about twice the global average increase, leading to desert expansion and more common heat waves and wildfires. Temperature rise is also amplified in the Arctic, where it has contributed to melting permafrost, glacial retreat and sea ice loss. Warmer temperatures are increasing rates of evaporation, causing more intense storms and weather extremes. Impacts on ecosystems include the relocation or extinction of many species as their environment changes, most immediately in coral reefs, mountains, and the Arctic. Climate change threatens people with food insecurity, water scarcity, flooding, infectious diseases, extreme heat, economic losses, and displacement. These impacts have led the World Health Organization to call climate change the greatest threat to global health in the 21st century. Even if efforts to minimize future warming are successful, some effects will continue for centuries, including rising sea levels, rising ocean temperatures, and ocean acidification.

Many of these impacts are already felt at the current level of warming, which is about 1.2 °C . The Intergovernmental Panel on Climate Change has issued a series of reports that project significant increases in these impacts as warming continues to 1.5 °C and beyond. Additional warming also increases the risk of triggering critical thresholds called tipping points. Responding to climate change involves mitigation and adaptation. Mitigation – limiting climate change – consists of reducing greenhouse gas emissions and removing them from the atmosphere; methods include the development and deployment of low-carbon energy sources such as wind and solar, a phase-out of coal, enhanced energy efficiency, reforestation, and forest preservation. Adaptation consists of adjusting to actual or expected climate, such as through improved coastline protection, better disaster management, assisted colonization, and the development of more resistant crops. Adaptation alone cannot avert the risk of 'severe, widespread and irreversible' impacts.

Under the 2015 Paris Agreement, nations collectively agreed to keep warming 'well under 2.0 °C ' through mitigation efforts. However, with pledges made under the Agreement, global warming would still reach about 2.8 °C by the end of the century. Limiting warming to 1.5 °C would require halving emissions by 2030 and achieving near-zero emissions by 2050.

Terminology Before the 1980s, when it was unclear whether warming by greenhouse gases would dominate aerosol-induced cooling, scientists often used the term inadvertent climate modification to refer to humankind's impact on the climate. In the 1980s, the terms global warming and climate change were introduced, the former referring only to increased surface warming, while the latter describes the full effect of greenhouse gases on the climate. Global warming became the most popular term after NASA climate scientist James Hansen used it in his 1988 testimony in the U.S. Senate. In the 2000s, the term climate change increased in popularity. Global warming usually refers to human-induced warming of the Earth system, whereas climate change can refer to natural as well as anthropogenic change. The two terms are often used interchangeably.

Various scientists, politicians and media figures have adopted the terms climate crisis or climate emergency to talk about climate change, while using global heating instead of global warming. The policy editor-in-chief of The Guardian explained that they included this language in their editorial guidelines 'to ensure that we are being scientifically precise, while also communicating clearly with readers on this very important issue'. Oxford Dictionary chose climate emergency as its word of the year in 2019 and defines the term as 'a situation in which urgent action is required to reduce or halt climate change and avoid potentially irreversible environmental damage resulting from it'.

Observed temperature rise Main articles: Temperature record of the last 2,000 years and Instrumental temperature record

Global surface temperature reconstruction over the last 2000 years using proxy data from tree rings, corals, and ice cores in blue. Directly observational data is in red.

NASA data shows that land surface temperatures have increased faster than ocean temperatures. Multiple independently produced instrumental datasets show that the climate system is warming, with the 2009–2018 decade being 0.93 ± 0.07 °C warmer than the pre-industrial baseline . Currently, surface temperatures are rising by about 0.2 °C per decade, with 2020 reaching a temperature of 1.2 °C above pre-industrial. Since 1950, the number of cold days and nights has decreased, and the number of warm days and nights has increased.

There was little net warming between the 18th century and the mid-19th century. Climate proxies, sources of climate information from natural archives such as trees and ice cores, show that natural variations offset the early effects of the Industrial Revolution. Thermometer records began to provide global coverage around 1850. Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, did not occur at the same time across different regions, but temperatures may have reached as high as those of the late-20th century in a limited set of regions. There have been prehistorical episodes of global warming, such as the Paleocene–Eocene Thermal Maximum. However, the modern observed rise in temperature and CO 2 concentrations has been so rapid that even abrupt geophysical events that took place in Earth's history do not approach current rates.

Evidence of warming from air temperature measurements are reinforced with a wide range of other observations. There has been an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity. Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are flowering earlier in spring. Another key indicator is the cooling of the upper atmosphere, which demonstrates that greenhouse gases are trapping heat near the Earth's surface and preventing it from radiating into space.

While locations of warming vary, the patterns are independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, global average land temperatures have increased almost twice as fast as global average surface temperatures. This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation. Over 90% of the additional energy in the climate system over the last 50 years has been stored in the ocean, with the remainder warming the atmosphere, melting ice, and warming the continents.

The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice, because of how the land masses are arranged around the Arctic Ocean. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat. Localized black carbon deposits on snow and ice also contribute to Arctic warming. Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world. Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, further changing the climate.

Drivers of recent temperature rise Main article: Attribution of recent climate change

Contributors to climate change in 2011, as reported in the fifth IPCC assessment report The climate system experiences various cycles on its own which can last for years , decades or even centuries. Other changes are caused by an imbalance of energy that is 'external' to the climate system, but not always external to the Earth. Examples of external forcings include changes in the composition of the atmosphere , solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun.

To determine the human contribution to climate change, known internal climate variability and natural external forcings need to be ruled out. A key approach is to determine unique 'fingerprints' for all potential causes, then compare these fingerprints with observed patterns of climate change. For example, solar forcing can be ruled out as a major cause because its fingerprint is warming in the entire atmosphere, and only the lower atmosphere has warmed, as expected from greenhouse gases . Attribution of recent climate change shows that the primary driver is elevated greenhouse gases, but that aerosols also have a strong effect.

Greenhouse gases Main articles: Greenhouse gas, Greenhouse gas emissions, Greenhouse effect, and Carbon dioxide in Earth's atmosphere

CO 2 concentrations over the last 800,000 years as measured from ice cores and directly The Earth absorbs sunlight, then radiates it as heat. Greenhouse gases in the atmosphere absorb and reemit infrared radiation, slowing the rate at which it can pass through the atmosphere and escape into space. Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence. While water vapour and clouds are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore considered feedbacks. On the other hand, concentrations of gases such as CO 2 , tropospheric ozone, CFCs and nitrous oxide are not temperature-dependent, and are therefor considered external forcings.

Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels , has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2018, the concentrations of CO 2 and methane had increased by about 45% and 160%, respectively, since 1750. These CO 2 levels are much higher than they have been at any time during the last 800,000 years, the period for which reliable data have been collected from air trapped in ice cores. Less direct geological evidence indicates that CO 2 values have not been this high for millions of years.

The Global Carbon Project shows how additions to CO 2 since 1880 have been caused by different sources ramping up one after another. Global anthropogenic greenhouse gas emissions in 2018, excluding those from land use change, were equivalent to 52 billion tonnes of CO 2. Of these emissions, 72% was actual CO 2, 19% was methane, 6% was nitrous oxide, and 3% was fluorinated gases. CO 2 emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity. Additional CO 2 emissions come from deforestation and industrial processes, which include the CO 2 released by the chemical reactions for making cement, steel, aluminum, and fertilizer. Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, coal mining, as well as oil and gas extraction. Nitrous oxide emissions largely come from the microbial decomposition of inorganic and organic fertilizer. From a production standpoint, the primary sources of global greenhouse gas emissions are estimated as: electricity and heat , agriculture and forestry , industry and manufacturing , transport , and buildings .

Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for CO 2. Natural processes, such as carbon fixation in the soil and photosynthesis, more than offset the greenhouse gas contributions from deforestation. The land-surface sink is estimated to remove about 29% of annual global CO 2 emissions. The ocean also serves as a significant carbon sink via a two-step process. First, CO 2 dissolves in the surface water. Afterwards, the ocean's overturning circulation distributes it deep into the ocean's interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted CO 2.

Aerosols and clouds Air pollution, in the form of aerosols, not only puts a large burden on human health, but also affects the climate on a large scale. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed, a phenomenon popularly known as global dimming, typically attributed to aerosols from biofuel and fossil fuel burning. Aerosol removal by precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for a few years. Globally, aerosols have been declining since 1990, meaning that they no longer mask greenhouse gas warming as much.

In addition to their direct effects , aerosols have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. This effect also causes droplets to be more uniform in size, which reduces the growth of raindrops and makes clouds more reflective to incoming sunlight. Indirect effects of aerosols are the largest uncertainty in radiative forcing.

While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.

Changes of the land surface

Global tree cover loss, annually Humans change the Earth's surface mainly to create more agricultural land. Today, agriculture takes up 34% of Earth's land area, while 26% is forests, and 30% is uninhabitable . The amount of forested land continues to decrease, largely due to conversion to cropland in the tropics. This deforestation is the most significant aspect of land surface change affecting global warming. The main causes of deforestation are: permanent land-use change from forest to agricultural land producing products such as beef and palm oil , logging to produce forestry/forest products , short term shifting cultivation , and wildfires .

In addition to affecting greenhouse gas concentrations, land-use changes affect global warming through a variety of other chemical and physical mechanisms. Changing the type of vegetation in a region affects the local temperature, by changing how much of the sunlight gets reflected back into space , and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also contribute to changing temperatures by affecting the release of aerosols and other chemical compounds that influence clouds, and by changing wind patterns. In tropic and temperate areas the net effect is to produce significant warming, while at latitudes closer to the poles a gain of albedo leads to an overall cooling effect. Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.

Solar and volcanic activity Further information: Solar activity and climate Physical climate models are unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity. As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system. Solar irradiance has been measured directly by satellites, and indirect measurements are available from the early 1600s. There has been no upward trend in the amount of the Sun's energy reaching the Earth. Further evidence for greenhouse gases being the cause of recent climate change come from measurements showing the warming of the lower atmosphere , coupled with the cooling of the upper atmosphere . If solar variations were responsible for the observed warming, warming of both the troposphere and the stratosphere would be expected, but that has not been the case.

Explosive volcanic eruptions represent the largest natural forcing over the industrial era. When the eruption is sufficiently strong sunlight can be partially blocked for a couple of years, with a temperature signal lasting about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature trends. Present-day volcanic CO2 emissions are equivalent to less than 1% of current anthropogenic CO2 emissions.

Climate change feedback Main articles: Climate change feedback and Climate sensitivity

Sea ice reflects 50% to 70% of incoming solar radiation while the dark ocean surface only reflects 6%, so melting sea ice is a self-reinforcing feedback. The response of the climate system to an initial forcing is modified by feedbacks: increased by self-reinforcing feedbacks and reduced by balancing feedbacks. The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and probably the net effect of clouds. The primary balancing feedback to global temperature change is radiative cooling to space as infrared radiation in response to rising surface temperature. In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO 2 on plant growth. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.

As air gets warmer, it can hold more moisture. After initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. As water vapour is a potent greenhouse gas, this further heats the atmosphere. If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become more high and thin, they act as an insulator, reflecting heat from below back downwards and warming the planet. Overall, the net cloud feedback over the industrial era has probably exacerbated temperature rise. The reduction of snow cover and sea ice in the Arctic reduces the albedo of the Earth's surface. More of the Sun's energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes. Arctic amplification is also melting permafrost, which releases methane and CO 2 into the atmosphere.

Around half of human-caused CO 2 emissions have been absorbed by land plants and by the oceans. On land, elevated CO 2 and an extended growing season have stimulated plant growth. Climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow in the future. Soils contain large quantities of carbon and may release some when they heat up. As more CO 2 and heat are absorbed by the ocean, it acidifies, its circulation changes and phytoplankton takes up less carbon, decreasing the rate at which the ocean absorbs atmospheric carbon. Climate change can increase methane emissions from wetlands, marine and freshwater systems, and permafrost.

Future warming and the carbon budget Further information: Carbon budget, Climate model, and Carbon cycle

Average climate model projections for 2081–2100 relative to 1986–2005, under low and high emission scenarios Future warming depends on the strengths of climate feedbacks and on emissions of greenhouse gases. The former are often estimated using various climate models, developed by multiple scientific institutions. A climate model is a representation of the physical, chemical, and biological processes that affect the climate system. Models include changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing. Computer models attempt to reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere. Models project different future temperature rises for given emissions of greenhouse gases; they also do not fully agree on the strength of different feedbacks on climate sensitivity and magnitude of inertia of the climate system.

The physical realism of models is tested by examining their ability to simulate contemporary or past climates. Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations. The 2017 United States-published National Climate Assessment notes that 'climate models may still be underestimating or missing relevant feedback processes'.

Various Representative Concentration Pathways can be used as input for climate models: 'a stringent mitigation scenario , two intermediate scenarios and one scenario with very high emissions '. RCPs only look at concentrations of greenhouse gases, and so do not include the response of the carbon cycle. Climate model projections summarized in the IPCC Fifth Assessment Report indicate that, during the 21st century, the global surface temperature is likely to rise a further 0.3 to 1.7 °C in a moderate scenario, or as much as 2.6 to 4.8 °C in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects.

Four possible future concentration pathways, including CO 2 and other gases' CO 2-equivalents A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affect – and interact with – the physical climate. With this information, these models can produce scenarios of how greenhouse gas emissions may vary in the future. This output is then used as input for physical climate models to generate climate change projections. In some scenarios emissions continue to rise over the century, while others have reduced emissions. Fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century. Emissions scenarios can be combined with modelling of the carbon cycle to predict how atmospheric concentrations of greenhouse gases might change in the future. According to these combined models, by 2100 the atmospheric concentration of CO2 could be as low as 380 or as high as 1400 ppm, depending on the socioeconomic scenario and the mitigation scenario.

The remaining carbon emissions budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases. According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of CO 2, depending on exactly how the global temperature is defined. This amount corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget; for instance, it may be 100 gigatonnes of CO 2 smaller due to methane release from permafrost and wetlands.

Impacts Main article: Effects of climate change Physical environment Main article: Physical impacts of climate change

Historical sea level reconstruction and projections up to 2100 published in 2017 by the U.S. Global Change Research Program The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations. Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency. Extremely wet or dry events within the monsoon period have increased in India and East Asia. The maximum rainfall and wind speed from hurricanes and typhoons is likely increasing.

Global sea level is rising as a consequence of glacial melt, melt of the ice sheets in Greenland and Antarctica, and thermal expansion. Between 1993 and 2017, the rise increased over time, averaging 3.1 ± 0.3 mm per year. Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm. Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet and the possibility of a 2-meter sea level rise by 2100 under high emissions.

Climate change has led to decades of shrinking and thinning of the Arctic sea ice, making it vulnerable to atmospheric anomalies. While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2.0 °C . Higher atmospheric CO 2 concentrations have led to changes in ocean chemistry. An increase in dissolved CO 2 is causing oceans to acidify. In addition, oxygen levels are decreasing as oxygen is less soluble in warmer water, with hypoxic dead zones expanding as a result of algal blooms stimulated by higher temperatures, higher CO 2 levels, ocean deoxygenation, and eutrophication.

Tipping points and long-term impacts The greater the amount of global warming, the greater the risk of passing through "tipping points', thresholds beyond which certain impacts can no longer be avoided even if temperatures are reduced. An example is the collapse of West Antarctic and Greenland ice sheets, where a temperature rise of 1.5 to 2.0 °C may commit the ice sheets to melt, although the time scale of melt is uncertain and depends on future warming. Some large-scale changes could occur over a short time period, such as a collapse of the Atlantic Meridional Overturning Circulation, which would trigger major climate changes in the North Atlantic, Europe, and North America.

The long-term effects of climate change include further ice melt, ocean warming, sea level rise, and ocean acidification. On the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic CO 2 emissions. This is due to CO 2's long atmospheric lifetime. Oceanic CO 2 uptake is slow enough that ocean acidification will continue for hundreds to thousands of years. These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. Sea level rise will continue over many centuries, with an estimated rise of 2.3 metres per degree Celsius after 2000 years.

Nature and wildlife Main article: Climate change and ecosystems Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes. Higher atmospheric CO 2 levels and an extended growing season have resulted in global greening, whereas heatwaves and drought have reduced ecosystem productivity in some regions. The future balance of these opposing effects is unclear. Climate change has contributed to the expansion of drier climate zones, such as the expansion of deserts in the subtropics. The size and speed of global warming is making abrupt changes in ecosystems more likely. Overall, it is expected that climate change will result in the extinction of many species.

The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles faster than species on land. Just as on land, heat waves in the ocean occur more frequently due to climate change, with harmful effects found on a wide range of organisms such as corals, kelp, and seabirds. Ocean acidification is impacting organisms who produce shells and skeletons, such as mussels and barnacles, and coral reefs; coral reefs have seen extensive bleaching after heat waves. Harmful algae bloom enhanced by climate change and eutrophication cause anoxia, disruption of food webs and massive large-scale mortality of marine life. Coastal ecosystems are under particular stress, with almost half of wetlands having disappeared as a consequence of climate change and other human impacts.