3.1 2.a - Observed warming and the global energy balance

Evidence of modern warming

Global warming means a sustained rise in the average temperature of the climate system, not that every place becomes warmer every day. Instrumental evidence combines land weather stations, sea-surface measurements, ocean instruments, balloons and satellites. Different instruments observe different places or layers, so their coverage, calibration and methods must be checked.

A temperature anomaly is the difference from an average over a stated reference period. Anomalies help combine places with different normal temperatures: a cold mountain and a warm coast can both be one degree above their own baseline. Changing the baseline changes the displayed anomaly but does not remove the underlying warming trend. Sparse historical coverage, especially over oceans and polar regions, creates uncertainty that scientists estimate rather than conceal.

The IPCC's 2023 Synthesis Report assessed global surface temperature in 2011–2020 as about $1.1\,^{\circ}\mathrm{C}$ above 1850–1900. This is a dated decade-average estimate, not a statement about every year after 2020. Atmospheric observations also show warming in the troposphere, the lower atmosphere, while the stratosphere above has cooled. “The atmosphere has warmed” therefore needs a height and period, not an assumption that every layer responds identically. IPCC AR6 Synthesis Report, 2023.

Ocean warming provides a further line of evidence. Water can absorb a large amount of energy with a relatively small temperature rise, so ocean heat content is a more complete indicator than sea-surface temperature alone. Much of the excess energy enters the oceans and is redistributed by mixing and currents. A year with slower surface warming can therefore still involve continued planetary heat accumulation. NASA summarises the independent observational evidence.

Weather variability, including El Niño and La Niña, can raise or lower individual annual values. A long series and several independent indicators distinguish this variability from a sustained trend. Agreement across surface records, the lower atmosphere and ocean heat storage strengthens the conclusion because they have different measurement limitations.

Worked resource critique — three presentation limitations

Illustrative resource: a line chart is titled “Global temperature anomaly”. Its vertical axis gives degrees Celsius but no reference period. It plots only 1990, 2000, 2010 and 2020, joining them with straight lines. The source note says “land stations”, with no ocean observations. Identify three limitations of the presentation technique. First, the missing reference period prevents the viewer interpreting what zero anomaly represents. Second, straight lines between ten-year observations conceal annual variability and may imply an unobserved smooth change. Third, land-only observations do not represent the complete global surface implied by the title. These are three distinct, visible limitations tied to the resource's baseline, temporal detail and spatial coverage. There is no basis for criticising absent units: the stated vertical axis already supplies them.

Ice, snow, water vapour and sea level

Warming changes water storage in the atmosphere, oceans and cryosphere, the frozen parts of Earth. These responses provide evidence beyond thermometer readings, but each has additional controls and a different response time.

Valley glaciers shrink when losses from melting and ice discharge exceed gains from snowfall. Glacier fronts can lag behind climatic change, and one glacier may temporarily advance because of unusual snowfall or ice dynamics. Widespread changes in mass are therefore more informative than a photograph of a single terminus. Greenland and Antarctic ice sheets also lose mass through surface melting, changes in snowfall and ice discharge into the sea. Their size means important responses can continue long after the original temperature change.

Sea level rises partly because warming seawater expands and partly because land ice transfers water into the oceans. Floating sea ice already displaces seawater, so its melting has little direct effect on global sea level. Local relative sea level also depends on land uplift or subsidence and ocean circulation. A rising tide gauge at one sinking coast cannot, on its own, measure the global trend. The IPCC's 2021 ocean and cryosphere assessment distinguishes these contributions.

Less snow cover and declining sea ice are further indicators, especially when changes are compared over consistent seasons. Snow can still fall in a warming climate: winter conditions may remain below freezing, and a warmer atmosphere can carry more moisture. The important question is the pattern in extent, duration and mass, not whether snow has disappeared everywhere.

Atmospheric water vapour generally increases as warming raises the amount of moisture air can hold at saturation. Availability of water and relative humidity also matter, so this is not a promise of more rainfall in every place. Because water vapour is a greenhouse gas, this response strengthens the initial warming. It is mainly a feedback to temperature at the global scale, whereas sustained additions of carbon dioxide can provide the initiating forcing.