04 Climate Change

Climate science and warming

Climate describes long-term patterns of temperature, precipitation, winds, and other atmospheric conditions; weather is the day-to-day state of the atmosphere. Earth’s climate changes naturally, but human activities are the primary cause of the rapid warming observed since the mid-20th century.

Burning coal, oil, and natural gas, along with deforestation, agriculture, and industrial processes, adds heat-trapping greenhouse gases to the atmosphere. The most important gases identified here are carbon dioxide, methane, and nitrous oxide. They strengthen the natural greenhouse effect: sunlight warms Earth’s surface, and greenhouse gases absorb and re-emit some of the heat that the surface releases.

The IPCC assessed global surface temperature in 2011–2020 at about $1.1\, ^\circ\mathrm{C}$ above 1850–1900. Warming is not uniform: land generally warms faster than ocean, and changes differ by region and season.

A warmer climate changes the likelihood and intensity of many extremes, including heat waves and heavy rainfall. It also contributes to sea-level rise as oceans warm and land ice melts. The precise effects in a particular place depend on global warming and local conditions.

How climate risks arise

Climate risks arise through the interaction of hazard, exposure, and vulnerability. A hazard is a climate-related event or trend, such as a heat wave, drought, flood, or rising sea level. Exposure describes the people, infrastructure, or ecosystems in harm’s way. Vulnerability is their susceptibility to harm and the limits on their ability to prepare, cope, and recover.

The same heat wave can pose greater danger to outdoor workers and people without reliable cooling than to people with well-insulated homes and access to health care. The level of risk therefore depends not only on the physical hazard, but also on who and what is exposed and how able they are to respond.

Impacts and limits

Climate change affects health, water supplies, food production, ecosystems, housing, and infrastructure. Hazards can compound: drought and heat may reduce crop yields while increasing demand for water and energy. Impacts also depend on development choices, public services, and preparedness.

Adaptation can lower risk, but its options become more limited as warming increases. Some losses may not be avoidable, and adaptation has limits; reducing emissions helps prevent risks from growing beyond what adaptation can manage.

Unequal risks and responsibility

Climate risks and responsibility are unevenly distributed. Communities with fewer resources, greater exposure, or less political influence often face higher risks and have fewer means to respond. During 2010–2020, highly vulnerable regions experienced much higher mortality from floods, droughts, and storms than regions with very low vulnerability.

Many vulnerable communities have contributed relatively little to historical greenhouse-gas emissions, while people and groups with greater consumption and wealth have often contributed more. Fair climate action considers who caused emissions, who benefits from them, who bears the harms, and who has a voice in decisions.

Mitigation

Mitigation limits climate change by reducing greenhouse-gas emissions or increasing the removal of carbon dioxide from the atmosphere. Examples include replacing fossil-fuel electricity with low-emission energy, improving energy efficiency, electrifying transport and buildings, reducing methane leaks, protecting and restoring forests, and changing production and consumption patterns.

Carbon dioxide accumulates in the atmosphere, so sustained global emissions reductions are needed to limit further warming. Every increment of warming increases risks. Mitigation addresses the causes of climate change, while adaptation reduces harm from its effects; they serve complementary purposes and both are necessary.

Adaptation and maladaptation

Adaptation adjusts human or natural systems to actual or expected climate conditions to reduce harm or make use of possible benefits. Examples include heat-warning systems, shade and cooling centers, flood-resistant infrastructure, water conservation, drought-tolerant crops, and restoring wetlands that can buffer floods.

Effective adaptation is suited to local risks, includes affected communities, and accounts for future conditions. Maladaptation occurs when a response reduces one risk but increases others; for example, a flood barrier can shift floodwater toward another community. Because adaptation has limits, reducing emissions is also important to prevent risks from growing beyond what adaptation can manage.