12.1 Causes and evidence of global climate change

Greenhouse Gases and Emissions

Anthropogenic Greenhouse Gas Sources

Greenhouse gases trap heat in the atmosphere by absorbing and re-emitting infrared radiation. Without them, Earth's average temperature would be about -18°C instead of the habitable ~15°C we have now. The problem isn't that greenhouse gases exist; it's that human activities have dramatically increased their concentrations.

Carbon dioxide ($CO_2$) is the biggest driver of recent climate change. Burning fossil fuels (coal, oil, natural gas) for energy, transportation, and industry has pushed atmospheric $CO_2$ from about 280 ppm before the Industrial Revolution to over 420 ppm today. That's roughly a 50% increase.

Deforestation makes the problem worse in two ways: it releases the carbon stored in trees back into the atmosphere, and it removes trees that would otherwise absorb $CO_2$ through photosynthesis.

Methane ($CH_4$) is released from agricultural activities (livestock digestion, rice paddies), landfills, and natural gas production. It has a shorter atmospheric lifetime than $CO_2$ (about 12 years vs. centuries), but over a 20-year period, each molecule of methane traps roughly 80 times more heat than a molecule of $CO_2$.

Radiative forcing measures the change in Earth's energy balance caused by a given factor, expressed in watts per square meter ($W/m^2$). Positive radiative forcing means the factor adds energy to the climate system, driving warming. The combined positive radiative forcing from anthropogenic greenhouse gases is the dominant cause of observed global warming.

Other Greenhouse Gases and Their Sources

• Nitrous oxide ($N_2O$) comes from agricultural soil management (especially synthetic fertilizers), industrial processes, and fossil fuel combustion. It's about 270 times more effective at trapping heat than $CO_2$ over a 100-year period.
• Tropospheric (ground-level) ozone ($O_3$) acts as a greenhouse gas and forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Vehicle exhaust and industrial emissions are major sources of these precursor chemicals.
• Water vapor ($H_2O$) is actually the most abundant greenhouse gas, but humans don't directly control its atmospheric concentration. Instead, as the Earth warms from other greenhouse gases, more water evaporates, which traps more heat, creating a positive feedback loop.
• Chlorofluorocarbons (CFCs) are extremely potent greenhouse gases that also deplete the stratospheric ozone layer. The Montreal Protocol (1987) phased out most CFCs, but some of their replacements, called hydrofluorocarbons (HFCs), are also strong greenhouse gases and are now being phased down under the Kigali Amendment.

Evidence of Climate Change

Temperature and Sea Level Observations

Global warming refers to the long-term rise in Earth's average surface temperature driven by the enhanced greenhouse effect. Earth's average surface temperature has increased by approximately 1.1°C since the pre-industrial era (roughly 1850–1900), with most of that warming occurring since the 1970s.

Temperature anomalies show how much warmer or cooler a given year or region is compared to a long-term baseline average. Every year from 2015 through the early 2020s has ranked among the warmest on record globally, and the trend continues upward.

Sea level rise results from two main processes:

1. Thermal expansion — ocean water physically expands as it absorbs heat
2. Melting of land-based ice — glaciers and ice sheets (Greenland, Antarctica) add water to the ocean as they lose mass

Global mean sea level has risen about 21–24 cm since 1880, and the rate of rise is accelerating. Current estimates put the rate at roughly 3.6 mm per year.

Paleoclimate Proxies

Paleoclimate proxies are indirect records that allow scientists to reconstruct climate conditions from before the era of direct measurements.

• Ice cores are drilled from ice sheets in Greenland and Antarctica. Tiny air bubbles trapped in the ice preserve samples of the ancient atmosphere, allowing direct measurement of past $CO_2$ and methane levels going back hundreds of thousands of years. The ratio of oxygen isotopes ($^{18}O / ^{16}O$) in the ice itself serves as a proxy for past temperatures, since lighter water molecules evaporate more readily in warmer conditions.
• Tree rings form annually, with wider rings during warm, wet growing seasons and narrower rings during cold or dry years. By analyzing ring width and density, scientists can reconstruct regional climate conditions. Very old trees like bristlecone pines provide records stretching back thousands of years, and overlapping samples from different trees can extend the record even further.

Other proxies include ocean sediment cores, coral growth bands, and pollen records, all of which help fill in the picture of Earth's climate history.

Climate History and Feedbacks

Earth's Climate History

Paleoclimatology is the study of how Earth's climate has changed over geologic time. Earth has experienced dramatic natural climate swings driven by factors like:

• Changes in solar output
• Major volcanic eruptions (which inject reflective aerosols into the stratosphere)
• Milankovitch cycles — slow, predictable shifts in Earth's orbit and axial tilt that alter how much solar energy different parts of the planet receive over tens of thousands of years

Ice ages are a well-known example of natural climate change, with colder global temperatures and massive continental ice sheets advancing and retreating in cycles.

The critical distinction is this: the current rate and magnitude of warming is unprecedented in at least the last 2,000 years and cannot be explained by natural factors alone. The scientific consensus, supported by multiple independent lines of evidence, attributes the observed warming primarily to human greenhouse gas emissions.

Climate Feedbacks

Feedback loops in the climate system either amplify or dampen an initial temperature change. Understanding them is essential for predicting how much warming a given increase in greenhouse gases will ultimately produce.

Positive feedbacks (amplify warming):

• Ice-albedo feedback — As Arctic sea ice and snow cover melt, they expose darker ocean water and land surfaces. These darker surfaces absorb more sunlight instead of reflecting it, which causes further warming and more melting. Albedo is a measure of reflectivity: ice has high albedo, open ocean has low albedo.
• Water vapor feedback — Warmer air holds more water vapor (following the Clausius-Clapeyron relation). Since water vapor is a greenhouse gas, this extra moisture traps additional heat, which causes more evaporation, and so on.

Negative feedbacks (dampen warming):

• Planck feedback — As Earth's surface temperature rises, it emits more infrared radiation to space (following the Stefan-Boltzmann law: energy radiated increases with the fourth power of temperature). This acts as a natural thermostat that partially offsets warming.

The net effect of all feedbacks combined determines climate sensitivity, which is how much Earth's temperature will ultimately rise for a given increase in greenhouse gas concentrations. Current best estimates place equilibrium climate sensitivity at roughly 2.5–4°C of warming for a doubling of atmospheric $CO_2$.