AP World History 9.3-9.4: Environment and Economic Change

Global warming

Global warming is the long-term increase in Earth's average surface temperature. Climate change is broader and includes global warming plus changes in precipitation, ice, oceans, ecosystems, and extreme weather.

Anthropogenic climate change

Climate change caused by human activities, especially fossil-fuel combustion, deforestation, and some agricultural and industrial processes that increase atmospheric greenhouse-gas concentrations.

Greenhouse gas

A gas that absorbs infrared radiation emitted by Earth's surface and atmosphere, including carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. Water vapor and clouds contribute strongly to the natural greenhouse effect but primarily vary in response to temperature.

What is the molecular basis of the greenhouse effect?

Atmospheric greenhouse gases transmit much of the incoming sunlight but absorb some infrared radiation emitted by Earth's warmed surface. The absorbed energy is re-emitted in multiple directions, slowing heat loss to space and warming the lower atmosphere.

How does the natural greenhouse effect differ from human-enhanced warming?

Naturally occurring greenhouse gases keep Earth's surface about $33\ ^\circ\mathrm{C}$ warmer than it would be without them. Human activities increase greenhouse-gas concentrations, strengthening this existing effect.

How did atmospheric carbon dioxide and methane concentrations change from 1750 to 2022?

Atmospheric $CO_2$ increased by approximately 50%, while methane increased by approximately 164%.

What fraction of 2019 human-caused greenhouse-gas emissions was attributed to each major gas?

Approximately 75% was $CO_2$, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases, measured in $CO_2$-equivalent emissions.

What are the major human sources of carbon dioxide emissions?

The largest source is burning coal, oil, and natural gas for electricity, transportation, heating, and industry. Additional emissions come from deforestation and industrial chemical processes such as cement production.

What are major sources of atmospheric methane?

Methane is released by livestock, wetlands, manure, rice cultivation, landfills, wastewater, coal mining, and oil and natural-gas extraction.

What is the main agricultural source of nitrous oxide emissions?

Microbial decomposition and transformation of nitrogen-containing fertilizers in soil produce much of the human-caused nitrous oxide released to the atmosphere.

Why is carbon dioxide generally more important for long-term warming than methane despite methane's stronger warming effect per molecule?

Methane remains in the atmosphere for about 12 years on average, whereas a significant fraction of added $CO_2$ persists much longer. Methane is more potent over short timescales, but carbon dioxide drives long-term cumulative warming.

How do greenhouse-gas emissions and sulfur dioxide emissions produce opposing climate effects?

Greenhouse gases absorb outgoing infrared radiation and warm the surface. Sulfur dioxide can form sulfate aerosols that reflect sunlight and cool the climate, partially masking greenhouse warming.

How much has Earth's average surface air temperature increased since the Industrial Revolution?

It has increased by approximately $1.5\ ^\circ\mathrm{C}$, or about $2.7\ ^\circ\mathrm{F}$, relative to the preindustrial period.

What is the approximate recent rate of increase in global surface temperature?

Multiple datasets indicate an average increase of approximately $0.2\ ^\circ\mathrm{C}$ per decade in recent decades.

Why are 20-year temperature averages used to describe long-term global warming?

Individual years are affected by internal variability such as El Niño, La Niña, and decadal ocean-atmosphere cycles. A 20-year average reduces this short-term noise and better reveals the long-term warming trend.

Why do land areas generally warm faster than oceans?

Oceans can absorb and distribute large amounts of thermal energy, and evaporation removes heat from their surfaces. Land has less heat capacity and generally responds more rapidly to changes in energy input.

What percentage of the excess heat associated with recent global warming has entered the oceans?

More than 90% of the excess energy has been stored in the oceans, with the remainder warming the atmosphere and continents or melting ice.

Why has the Arctic warmed faster than the global average?

Loss of reflective snow and sea ice exposes darker surfaces that absorb more solar radiation. Additional effects include black carbon deposited on snow and changes in oceanic heat transport.

How does ocean warming affect the atmosphere and precipitation?

Warmer oceans increase evaporation, raising atmospheric water-vapor content. This can intensify heavy precipitation even while some regions experience greater drought because evaporation and atmospheric water demand also increase.

Ocean acidification

The decrease in ocean pH caused primarily by absorption of atmospheric carbon dioxide. It results from formation of carbonic acid and increased hydrogen-ion concentration in seawater.

Why does ocean acidification threaten organisms that build calcium carbonate structures?

Increasing $[H^+]$ converts carbonate ions into bicarbonate, decreasing $[CO_3^{2-}]$. This makes precipitation of calcium carbonate less favorable and can increase dissolution of shells and coral skeletons.

Carbon sink

A natural or human-managed process that removes more carbon dioxide from the atmosphere than it releases, such as photosynthesis, carbon storage in soils, forests, or ocean uptake.

How do terrestrial ecosystems and oceans affect atmospheric carbon dioxide?

Plants and oceans absorb a substantial portion of human-caused $CO_2$ emissions each year. Carbon returns to the atmosphere through respiration, decomposition, combustion, and ocean-atmosphere exchange, so long-term removal requires storage in relatively stable reservoirs such as Earth's crust.

What is the approximate relationship between annual human-caused $CO_2$ emissions and terrestrial carbon uptake?

Land processes such as photosynthesis and soil carbon storage remove about 29% of annual global $CO_2$ emissions.

What fraction of emitted carbon dioxide has the ocean absorbed over recent decades?

The ocean has absorbed approximately 20%–30% of emitted $CO_2$ over the last two decades.

How does deforestation contribute to climate warming?

Burning or decomposing vegetation releases stored $CO_2$, and removing trees reduces future photosynthetic carbon uptake. Deforestation can also alter surface reflectivity, evaporation, clouds, and wind.

Albedo

Albedo is the fraction of incoming radiation reflected by a surface. Bright snow and ice have high albedo, while darker land and ocean surfaces absorb more solar energy.

How can melting snow and ice create a positive climate feedback?

Melting replaces bright, reflective surfaces with darker surfaces that absorb more sunlight. The additional absorbed energy causes further warming and melting.

Positive climate feedback

A process in which an initial climate change causes additional changes that amplify the original disturbance. Examples include ice-albedo feedback and increased atmospheric water vapor as temperature rises.

How do sulfate aerosols affect Earth's energy balance?

Sulfate aerosols scatter incoming sunlight and can make clouds more reflective, producing a cooling effect. Reductions in sulfur pollution therefore allow more of the greenhouse-gas warming signal to appear.

Global dimming

A period of reduced sunlight reaching Earth's surface, attributed mainly to reflective aerosols from air pollution. The cooling effect partially offset greenhouse-gas warming before pollution controls reduced aerosol concentrations.

How can black carbon contribute to warming even though many aerosols have a cooling effect?

Black carbon absorbs sunlight in the atmosphere and darkens snow and ice when deposited on them. Darkened snow and ice absorb more solar energy, accelerating melting.

Why cannot changes in solar activity explain the recent persistent global warming trend?

Satellite and historical measurements show no upward trend in incoming solar energy large enough to explain the warming. In addition, increased solar output would tend to warm the upper atmosphere, whereas greenhouse-gas warming is associated with a cooling stratosphere.

How do volcanic eruptions usually affect global temperature over several years?

Explosive eruptions can inject particles and gases that reflect or block sunlight, producing temporary cooling. Volcanic material has relatively short atmospheric persistence, so the effect generally lasts only several years.

What environmental changes are associated with recent climate change?

Heat waves and wildfires are becoming more common, while Arctic permafrost thaws, glaciers retreat, and sea ice declines. Climate change also contributes to more intense storms, droughts, and other weather extremes.

How does climate change affect biodiversity and ecosystems?

Rapid environmental changes in mountains, coral reefs, and the Arctic force many species to relocate or become extinct. Ecosystem disruption can continue even if future warming is limited.

What human consequences can result from climate change?

Climate change increases risks of flooding, extreme heat, food and water scarcity, disease, and economic loss. It can also contribute to human migration and conflict.

Climate tipping point

A threshold beyond which a climate system can undergo a large, potentially self-reinforcing change. Examples include major ice-sheet loss and long-term permafrost thaw.

Why can some climate impacts continue for centuries even if emissions decline rapidly?

The climate system has substantial thermal inertia, especially because oceans absorb and release heat slowly. Long-lived atmospheric $CO_2$, ocean acidification, melting ice, and sea-level responses can persist after emissions are reduced.

What is a carbon budget?

A carbon budget is the maximum cumulative amount of $CO_2$ that can be emitted while retaining a specified probability of staying below a temperature limit. It depends on climate sensitivity and the carbon cycle.

How can clean electricity reduce greenhouse-gas emissions?

Electricity generated from sources such as wind, solar, hydropower, and nuclear power can replace fossil fuels in transportation, building heating, and industrial processes, reducing combustion-related emissions.

What is carbon dioxide removal?

The intentional removal of $CO_2$ from the atmosphere followed by durable storage. Strategies include expanding forests and agricultural practices that increase soil-carbon storage.

Climate adaptation

Adjustments that reduce harm from climate impacts, such as flood-control infrastructure, drought-resistant crops, and planning for heat or water scarcity. Adaptation lowers risk but cannot eliminate all impacts.

Why is climate change considered an environmental-justice issue?

Lower-income communities have often contributed a smaller share of global emissions but face greater vulnerability and have fewer resources for adaptation. Climate impacts can therefore worsen existing economic and social inequalities.