Global warming refers to the long-term heating of Earth’s climate system observed since the pre-industrial period, generally considered to be between 1850 and 1900. This warming is primarily driven by human activities that increase heat-trapping greenhouse gas levels in the atmosphere. While Earth’s climate has naturally shifted throughout its vast geological history, the current warming trend is notable for its unprecedented rate.
Earth’s Historical Climate Shifts
Earth’s climate has never been static, undergoing dramatic changes over millions of years, including alternating ice ages and warmer interglacial periods. These natural, long-term shifts are largely influenced by Milankovitch cycles, which describe variations in Earth’s movement around the Sun. These cycles involve changes in orbital eccentricity, axial tilt (obliquity), and axial precession, collectively affecting the amount and distribution of solar energy reaching the planet.
Orbital eccentricity refers to changes in the shape of Earth’s orbit around the Sun, shifting from nearly circular to more elliptical over roughly 100,000-year cycles. Axial tilt, or obliquity, describes the variation in the angle of Earth’s spin axis relative to its orbit over approximately 41,000 years. Axial precession, or wobble, is the gradual shift in the orientation of Earth’s axis. These subtle astronomical shifts redistribute solar radiation across Earth’s surface and seasons, influencing the growth and retreat of ice sheets and other climatic fluctuations over thousands to hundreds of thousands of years. Natural variations in atmospheric carbon dioxide levels also occurred, but unlike the present, their origin was natural.
The Human Factor in Warming
The Industrial Revolution marked a turning point for global greenhouse gas emissions. The widespread adoption of fossil fuels like coal, oil, and natural gas to power factories and transportation led to a significant increase in atmospheric carbon dioxide. This human-driven rise in greenhouse gases has created an “enhanced” greenhouse effect, trapping more heat and causing Earth’s surface temperature to rise.
Since the 18th century, atmospheric carbon dioxide concentrations have risen by over 40%, while methane concentrations have more than doubled. Nitrous oxide levels have also increased by approximately 20% from pre-industrial times. These increases are directly attributable to human activities. The current rate and magnitude of greenhouse gas increase far exceed natural historical fluctuations.
Self-Reinforcing Climate Processes
Initial warming can trigger self-reinforcing climate processes, known as positive feedback loops, which further amplify the warming effect. One example is the ice-albedo feedback. Ice and snow are highly reflective surfaces. As global temperatures rise, ice and snow melt, exposing darker surfaces like ocean water or land, which absorb significantly more solar energy.
This reduction in reflectivity, or albedo, leads to increased heat absorption, accelerating further melting and warming, particularly in polar regions where ice loss is most pronounced. Another feedback loop involves the thawing of permafrost, ground that has remained frozen. Permafrost contains vast amounts of trapped organic matter.
As permafrost thaws, microbes decompose this organic material, releasing heat-trapping greenhouse gases, primarily carbon dioxide and methane, into the atmosphere. Methane is a potent greenhouse gas, intensifying global warming.
A third amplifying process is the water vapor feedback. Warmer air can hold more moisture, so as Earth’s temperature rises, more water evaporates from oceans and land surfaces, increasing atmospheric water vapor. Since water vapor is itself a powerful greenhouse gas, this additional moisture traps even more heat, leading to further warming and reinforcing the cycle. This feedback roughly doubles the warming that would occur from increased carbon dioxide alone.
Signs of a Changing Climate
Observable and measurable evidence confirms that global warming is actively progressing. Global average temperatures, both over land and oceans, have risen significantly. Earth’s average surface temperature in 2024 was about 1.47 degrees Celsius (2.65 degrees Fahrenheit) warmer than the late 19th-century pre-industrial average. The rate of warming has accelerated, with the past decade, from 2015 to 2024, experiencing the ten warmest years on record since 1880.
Melting glaciers and ice sheets contribute substantially to rising sea levels. The Greenland and Antarctic ice sheets, which store about two-thirds of Earth’s freshwater, are losing mass at an accelerated rate. Between 1992 and 2020, these ice sheets collectively lost 7,560 billion metric tons of ice, contributing to global mean sea level. Global average sea level has risen 21-24 centimeters (8-9 inches) since 1880, with the rate more than doubling.
Changes in precipitation patterns are also evident, with some regions experiencing increased heavy rainfall events and others facing more frequent and intense droughts. The frequency and intensity of extreme weather events, such as heatwaves, heavy rainfall, and wildfires, have increased globally. Ocean acidification is occurring as the ocean absorbs human-induced carbon dioxide emissions, reducing seawater pH and making it more challenging for marine organisms to build their shells and skeletons.