The warming of the planet is the central characteristic of climate change, defined as the long-term increase in the global average surface temperature. This temperature rise indicates a fundamental shift in the planet’s energy balance, where the system retains more heat than it releases. Understanding this heating is foundational to comprehending the consequences that cascade across the environment and human society.
Quantifying Global Heating
Determining the extent of global heating requires establishing a reliable baseline for comparison. Scientists use the period between 1850 and 1900 to define the pre-industrial temperature level, predating significant human-caused greenhouse gas emissions. Instrumental records from land and ocean measurements calculate the global average temperature. This average is calculated over a 30-year span to filter out short-term natural fluctuations and reveal the long-term trend of warming.
The data confirm that the Earth’s average surface temperature has risen by approximately $1.1^{\circ} \text{C}$ to $1.2^{\circ} \text{C}$ since the pre-industrial era. The rate of this temperature increase has accelerated significantly, with the warming trend over the past four decades being the fastest since at least 1850. To reconstruct temperatures before the 19th century, researchers analyze proxy data like ice cores and tree rings. These records extend the climate record back thousands of years and confirm the current warming is unprecedented.
The Primary Drivers of Warming
The primary cause of rising global temperature is the enhanced greenhouse effect, driven by the increase in heat-trapping gases due to human activity. These gases absorb infrared radiation emitted by the Earth’s surface. By trapping this energy, the gases prevent it from escaping into space, thereby heating the lower atmosphere.
Carbon dioxide ($\text{CO}_2$) is the most significant anthropogenic greenhouse gas, responsible for about three-quarters of the observed warming. The majority of $\text{CO}_2$ emissions originate from the combustion of fossil fuels—coal, oil, and natural gas—for electricity, transportation, and industry. Methane ($\text{CH}_4$) is another potent heat-trapping gas, sourced primarily from livestock farming, fossil fuel production, and the decay of waste in landfills.
Nitrous oxide ($\text{N}_2\text{O}$) contributes to the enhanced greenhouse effect, mainly released through agricultural activities like the use of nitrogen-based fertilizers. While water vapor is the most abundant natural greenhouse gas, its concentration increases as the air warms. The long atmospheric residence time of gases like $\text{CO}_2$ means that the warming influence from past emissions will persist for centuries, even if current emissions are reduced.
Direct Physical Changes
The increased heat retained by the Earth system manifests in measurable physical alterations. Over 90 percent of the excess heat is absorbed by the oceans, increasing ocean heat content. This warming causes water molecules to spread out, resulting in thermal expansion of seawater volume.
Thermal expansion is one major contributor to global sea level rise, alongside the melting of land-based ice masses. The cryosphere, including glaciers and the ice sheets of Greenland and Antarctica, is losing mass at an accelerating rate. The freshwater runoff from this melting land ice flows into the ocean, adding volume that raises the global sea level.
The oceans are also undergoing ocean acidification, a direct consequence of absorbing excess atmospheric $\text{CO}_2$. When carbon dioxide dissolves in seawater, it forms carbonic acid, which lowers the water’s $\text{pH}$ level. This change reduces the availability of carbonate ions, making it difficult for marine organisms like corals and shellfish to build their calcium carbonate shells. The warming planet contributes to the thawing of permafrost, which risks releasing trapped carbon and methane, creating a feedback loop that could accelerate future warming.
Ecological and Societal Impacts
The physical changes from global heating translate into widespread consequences for living systems and human civilization, often characterized by changes in the intensity and frequency of extreme weather events. Warmer air holds more moisture, intensifying the water cycle and leading to more extreme precipitation events and associated flooding. Conversely, increased heat drives longer and more frequent periods of drought and intense heatwaves, particularly over land areas.
These climate shifts profoundly affect food security and water resources worldwide. Agricultural yields are reduced by prolonged drought and heat stress, while changes in seasonal timing disrupt established planting and harvesting cycles. Water scarcity is exacerbated in arid and semi-arid regions as precipitation patterns change and glacial meltwater supplies diminish.
Ecological systems are strained as habitats shift faster than species can adapt, increasing the risk of extinction for many populations. Marine ecosystems, especially coral reefs, face widespread bleaching and degradation due to rising sea temperatures and ocean acidification.
For human populations, the consequences include direct health risks from heat stress and respiratory illnesses from more intense wildfires. The changing climate expands the geographical range of disease-carrying vectors, such as mosquitoes and ticks, increasing the potential spread of infectious diseases. Coastal communities face increasing threats to infrastructure and displacement due to chronic flooding and higher storm surges brought by rising sea levels.