11. Global Climate Change

The Climate System and Greenhouse Effect

Earth’s climate system consists of the atmosphere, hydrosphere, cryosphere, geosphere, and biosphere. These components exchange energy and matter, so a change in one can affect the others.

Global climate change refers to long-term changes in average climate conditions and climate variability. It includes changes in temperature, precipitation, sea level, ocean chemistry, and the frequency or intensity of some extreme events.

The energy balance

Earth receives mostly shortwave radiation from the Sun. Clouds, atmospheric particles, snow, ice, and bright land surfaces reflect some of that energy. The surface and atmosphere absorb the remainder and later emit energy as longwave, or infrared, radiation.

The greenhouse effect occurs when atmospheric gases absorb and re-emit some outgoing infrared radiation. This natural process keeps Earth’s average surface temperature much warmer than it would otherwise be. Human activities have strengthened it by increasing atmospheric concentrations of Greenhouse gases (GHGs).

Important greenhouse gases include water vapor $\mathrm{H_2O}$, carbon dioxide $\mathrm{CO_2}$, methane $\mathrm{CH_4}$, nitrous oxide $\mathrm{N_2O}$, ozone $\mathrm{O_3}$, and fluorinated gases. Carbon dioxide is the most important long-lived human-emitted greenhouse gas. Water vapor is abundant and usually acts mainly as a feedback because warmer air can hold more water vapor.

Takeaway

The natural greenhouse effect makes Earth habitable, but increased human emissions create an enhanced greenhouse effect and a persistent warming influence.

Evidence and Causes of Current Warming

A radiative forcing is a change in the balance between incoming and outgoing energy. Increased carbon dioxide produces positive forcing and tends to warm the climate; some reflective aerosols produce negative forcing and tend to cool it. Aerosol cooling is temporary because aerosols are short-lived, whereas many greenhouse gases accumulate or persist for much longer.

Multiple independent observations support the conclusion that current warming is occurring and is primarily caused by human activities:

• Instrumental records show substantial warming since the late nineteenth century.

• Atmospheric measurements and air bubbles in ice cores show a rapid modern increase in greenhouse-gas concentrations compared with natural variations over hundreds of thousands of years.

• The isotopic composition of atmospheric carbon is becoming relatively poorer in carbon-13 and carbon-14, consistent with carbon from ancient plant material in fossil fuels.

• Oceans are warming and absorbing much of the excess heat; they are also becoming more acidic as they absorb carbon dioxide.

• Glaciers, ice sheets, snow cover, and Arctic sea ice have declined in many regions.

• Global sea level is rising because seawater expands as it warms and land ice melts.

• Many species are shifting their ranges or changing migration, flowering, breeding, and feeding times.

• Human-caused warming increases the likelihood or intensity of many heat extremes and contributes to heavier precipitation in many regions, while increasing drought risk in some areas.

Natural factors such as volcanic eruptions, solar variability, orbital changes, and internal ocean–atmosphere variability influence climate, but they do not explain the sustained pattern of recent warming. For example, volcanic eruptions generally cause short-term cooling, and observed solar changes are too small to account for the long-term trend.

Takeaway

Confidence comes from the agreement among physical principles, atmospheric measurements, paleoclimate evidence, ecological observations, and climate-model results rather than from one observation alone.

Climate Feedbacks and Amplification

A climate feedback responds to an initial climate change and can amplify or reduce it. Feedbacks are therefore different from forcings: a forcing initiates a change, while a feedback modifies the response.

Amplifying feedbacks

• Water-vapor feedback: Warming increases evaporation. Because water vapor is a greenhouse gas, additional moisture can cause further warming.

• Ice–albedo feedback: Snow and ice reflect much incoming sunlight. When they melt, darker land or ocean surfaces are exposed and absorb more solar energy.

• Permafrost–carbon feedback: Thawing permafrost allows decomposition to release carbon dioxide and methane.

• Vegetation and wildfire feedbacks: Heat, drought, pests, and fire can reduce carbon storage in forests and soils, releasing carbon and weakening a natural sink.

Stabilizing or counteracting processes

Some processes partially counter warming. Increased plant growth can remove additional carbon dioxide where water and nutrients are sufficient, and a warmer Earth emits more infrared radiation to space. These processes do not cancel the long-term effect of human greenhouse-gas emissions.

Clouds are especially complex because they can reflect sunlight while also trapping infrared radiation. Their overall effect depends on cloud type, altitude, location, and changes in coverage.

Takeaway

Feedbacks can substantially increase or partially reduce the initial response, so the climate response to greenhouse-gas emissions is not determined by the direct effect of carbon dioxide alone.

Human Causes and Ecosystem Impacts

Anthropogenic climate change results mainly from burning coal, petroleum, and natural gas; deforestation and other land-use changes; agriculture; industrial processes; and waste disposal and wastewater treatment. Fossil-fuel combustion is the dominant source of the modern increase in atmospheric carbon dioxide. Oceans and terrestrial ecosystems absorb a substantial portion of human carbon dioxide emissions, while the remainder accumulates in the atmosphere.

Climate change affects ecosystems through temperature, precipitation, seasonal timing, wildfire, ocean chemistry, sea-level rise, and extreme events. Important effects include:

• Species shifting toward higher latitudes or elevations as suitable temperatures change.

• Phenological mismatches, such as a bird arriving after the peak abundance of insects needed by its young.

• Coral bleaching when warm water causes corals to expel their symbiotic algae.

• Freshwater stress because warmer water holds less dissolved oxygen and can favor harmful algal blooms.

• Increased wildfire risk, tree mortality, and insect outbreaks under heat and drought.

• Greater biodiversity loss among species with small ranges, specialized habitats, slow reproduction, or limited ability to migrate.

• Inundation, erosion, and saltwater intrusion in coastal wetlands, mangroves, estuaries, beaches, and low-lying islands.

Climate change can interact with habitat destruction, invasive species, pollution, overharvesting, and disease. These combined pressures may create compound or cascading impacts greater than those caused by any single stressor.

Takeaway

Climate change alters both physical conditions and biological relationships, making ecosystem effects interconnected rather than isolated.

Water, Agriculture, and Food Security

Climate change alters evaporation, precipitation, snowpack, soil moisture, groundwater recharge, runoff, and streamflow. A warmer atmosphere can hold more water vapor, increasing the potential for intense precipitation. At the same time, warmer conditions can increase evaporation and worsen drought when precipitation is limited.

Potential water-resource consequences include reduced snowpack, earlier spring snowmelt, decreased water availability in some dry regions, more intense flooding in some areas, saltwater intrusion into coastal aquifers, poorer water quality after floods or fires, increased competition among cities, farms, industries, and ecosystems, and reduced hydropower reliability where streamflow changes.

Agriculture depends on suitable temperature ranges, precipitation, soil moisture, pollinators, nutrients, and reliable water supplies. Climate change can reduce crop yields through heat stress, drought, flooding, and shifting growing seasons. It can also increase irrigation demand, expand some pests and diseases, increase livestock heat stress, shift fisheries, and raise food-safety risks after floods.

Some crops may benefit temporarily from higher carbon dioxide concentrations or longer frost-free seasons, but those benefits depend on adequate water and nutrients and may be offset by heat extremes, pests, drought, and extreme precipitation. Adaptation options include drought-tolerant varieties, adjusted planting dates, efficient irrigation, soil-moisture conservation, agroforestry, crop diversification, improved weather information, and modified livestock management.

Takeaway

Water and food impacts vary by region: a place may receive more annual precipitation yet still experience water stress if rainfall arrives in short, damaging storms rather than being stored as snow or groundwater.

Health, Society, and Climate Risk

Climate change affects human health through direct hazards and indirect pathways. Heat waves can cause heat stress, heat exhaustion, and heatstroke and can worsen cardiovascular, respiratory, kidney, and mental-health conditions. Changes in temperature and precipitation can alter the range or season of some mosquito-, tick-, food-, and water-borne diseases. Wildfire smoke and ground-level ozone can increase respiratory and cardiovascular illness.

Droughts, floods, crop losses, and contamination can contribute to food and water insecurity. Storms, floods, fires, and sea-level rise can cause injuries, deaths, housing loss, displacement, and interruptions in medical care. Disaster exposure, livelihood loss, displacement, and persistent climate anxiety can contribute to stress, depression, and post-traumatic stress.

Climate change can damage infrastructure, reduce labor productivity, disrupt transportation and supply chains, increase energy demand for cooling, and raise costs for disaster response and public health. Impacts can cross political boundaries: a drought may reduce agricultural production, raise food prices, interrupt exports, and affect consumers far from the drought location.

Climate risk depends on hazard, exposure, and vulnerability:

A dangerous heat wave is a hazard. People living in the affected area are exposed. Older adults without air conditioning, outdoor workers, and people with limited access to health care may be especially vulnerable.

Takeaway

The same climate hazard can produce very different outcomes depending on who or what is exposed and how much capacity exists to prepare, respond, and recover.

Climate Justice and Responses

Climate justice applies fairness, human rights, and accountability to climate change. Contributions to greenhouse-gas emissions are unequal among countries, communities, industries, and individuals, while exposure, vulnerability, and adaptive capacity also differ.

Many low-income countries, Indigenous peoples, small-island states, children, and marginalized communities have contributed relatively little to cumulative emissions but face severe risks. Climate policies can also create burdens if costs, land rights, jobs, or pollution are distributed unfairly.

A just response considers:

• Procedural justice: who participates in decisions and whose knowledge and rights are recognized.

• Distributional justice: who receives benefits and who bears costs.

• Meaningful consultation with Indigenous communities.

• Protection of land and water rights.

• Affordable clean energy and support for workers during energy transitions.

• Accessible disaster preparedness.

• Adaptation finance directed toward highly vulnerable communities.

Mitigation and adaptation

Mitigation reduces the causes of climate change by lowering greenhouse-gas emissions or increasing carbon removal. Strategies include replacing fossil fuels with low-carbon energy, improving energy efficiency, electrifying suitable transportation and industrial processes, protecting and restoring ecosystems, reducing methane emissions, improving public transportation, reducing food waste, and shifting toward lower-emission production systems.

Adaptation reduces harm from climate impacts. Examples include heat-alert systems, cooling centers, floodplain management, wildfire planning, drought-resistant crops, water conservation, ecosystem restoration, stronger infrastructure, coastal setbacks, and climate-resilient health systems.

Mitigation can produce co-benefits such as cleaner air, while ecosystem restoration can store carbon, reduce flooding, and provide habitat. Adaptation has limits, especially when warming becomes greater or faster, so effective responses combine rapid emissions reductions with equitable adaptation.

Takeaway

Responding to climate change requires both mitigation to limit future warming and adaptation to manage impacts that cannot be fully avoided, with attention to fairness and community rights.