11.1 Global temperature trends and patterns

Global temperatures have risen significantly since pre-industrial times, with the most recent years consistently ranking as the warmest on record. This warming trend is driven mainly by human activities and shows distinct patterns across Earth's surface, with land warming faster than oceans and polar regions warming faster than the tropics.

Both natural variability and human factors influence temperature. Short-term fluctuations can come from events like El Niño or volcanic eruptions, but the long-term warming trend is primarily due to increased greenhouse gas emissions.

Global Temperature Trends and Patterns

Global temperature trends

Global average surface temperature has increased by approximately 1.1°C (2.0°F) since pre-industrial times (roughly the 1850–1900 baseline). That warming hasn't been steady; it accelerated sharply in recent decades.

• The rate of warming has been roughly 0.2°C (0.36°F) per decade since 1981, about double the rate over the full record.
• The warmest years on record have all occurred in the 21st century, with 2016 and 2020 among the hottest (and more recent years continuing the trend).
• Each of the last four decades has been warmer than the one before it.

Spatial patterns of warming

Warming isn't spread evenly across the planet. Two major patterns stand out: the land-ocean contrast and polar amplification.

Land vs. ocean:

• Land surfaces have warmed faster than ocean surfaces. Water has a much higher heat capacity than land, meaning oceans can absorb enormous amounts of energy with a smaller temperature increase. Oceans also redistribute heat through currents and deep mixing, which slows surface warming.
• As a result, continental interiors tend to show larger temperature increases than nearby ocean regions.

Polar amplification:

• The Arctic has warmed roughly two to three times faster than the global average. Antarctic warming is real but less dramatic, partly because the Southern Ocean absorbs a lot of heat.
• Several factors drive polar amplification:
  • Ice-albedo feedback: As ice and snow melt, the darker land or ocean surface underneath absorbs more solar radiation, which causes further warming and more melting.
  • Changes in atmospheric and oceanic circulation that transport extra heat toward the poles.
  • Thinner sea ice exposing more open water, which releases stored heat into the Arctic atmosphere during winter.

Causes of temperature change

Natural variability can push global temperatures up or down on short timescales (year to year, or even decade to decade):

• El Niño events release heat from the tropical Pacific into the atmosphere, temporarily boosting global temperatures. La Niña events do the opposite, producing a mild cooling effect.
• Large volcanic eruptions (like Mt. Pinatubo in 1991) inject sulfate aerosols into the stratosphere. These aerosols reflect incoming solar radiation and can cool the planet by ~0.2–0.5°C for a year or two.
• Solar output varies slightly over the ~11-year solar cycle, but this effect is small compared to greenhouse gas forcing.

Anthropogenic (human-caused) factors are the dominant driver of the long-term warming trend:

• Greenhouse gases like $CO_2$, $CH_4$, and $N_2O$ trap outgoing heat in the atmosphere. Atmospheric $CO_2$ concentrations have risen over 50% above pre-industrial levels, from about 280 ppm to over 420 ppm.
• The main sources are fossil fuel combustion, deforestation, agriculture, and industrial processes.
• Other human influences include land-use changes (which alter surface reflectivity) and aerosol emissions (which can have both warming and cooling effects depending on the type).

Analysis of temperature data

Scientists track global temperature using several independent datasets maintained by NASA (GISTEMP), NOAA, and the UK Met Office (HadCRUT). The fact that these separate analyses agree closely strengthens confidence in the observed warming.

• Data come from land-based weather stations, ocean buoys, ships, and satellites. Each source is quality-controlled and adjusted for known biases like urban heat island effects or changes in measurement instruments over time.
• Results are typically reported as temperature anomalies, which are deviations from a long-term baseline average (often 1951–1980 or 1961–1990). Using anomalies rather than absolute temperatures makes it easier to compare locations with very different climates and to spot the overall trend.
• Maps of temperature anomalies clearly show the land-ocean contrast and polar amplification discussed above. Time series plots reveal both the long-term upward trend and shorter-term variability from events like El Niño and volcanic eruptions.