How have human-caused greenhouse gas emissions influenced the trend in the hottest years?
Executive summary
Human-caused emissions of greenhouse gases—chiefly carbon dioxide, methane and nitrous oxide—have amplified Earth’s natural greenhouse effect and are the primary driver of the persistent upward shift in global hottest years over the past century [1] [2]. Multiple independent national and international assessments conclude that anthropogenic greenhouse gases have already warmed the planet by about 1.0–1.3°C since preindustrial times and that most of the warmest years on record are linked to these emissions [3] [4].
1. The physical link: how added greenhouse gases push peak annual temperatures upward
Greenhouse gases trap infrared radiation and slow Earth’s heat loss to space; adding more of them increases the amount of energy retained in the climate system, producing higher average and peak temperatures—an effect documented since foundational 19th‑century work and reinforced by modern observations and models [2] [5]. Climate models that omit rising greenhouse‑gas concentrations cannot reproduce the observed century‑scale warming, whereas models that include human emissions do reproduce both the magnitude and pattern of temperature increases, including record hot years [2] [5].
2. The observational record: a growing cluster of hottest years
Global temperature datasets from multiple institutions show consistent warming: the last few decades contain the majority of the warmest years on record, with global surface temperatures rising roughly 1.0–1.3°C above 1850–1900 averages in recent decades and even higher anomalies in individual recent years and the 2011–2020 decade [1] [3] [4]. Agencies such as NOAA and international assessments attribute virtually all of that long‑term trend to human activities—principally CO2 emissions from fossil fuel burning and land‑use changes—rather than natural variability alone [3] [6].
3. Attribution studies: why scientists say emissions made the hottest years far more likely
Formal detection-and-attribution analyses quantify how much human influences increased the probability and intensity of extreme heat and record‑setting years; these studies find that well‑mixed greenhouse gases contributed about 1.0–2.0°C of the observed warming since 1850–1900, while aerosols and natural drivers partly offset or slightly modified that signal [3]. Organizations synthesizing the evidence (IPCC, NOAA, NASA, Royal Society) conclude that without anthropogenic emissions it is highly unlikely the recent succession of record hot years would have occurred in the same way [7] [3] [5].
4. Which gases and sources matter most for the trend in hottest years
Carbon dioxide is the dominant long‑lived contributor: since preindustrial times its concentration has risen markedly and it accounts for the largest share of the long‑term heating influence from human emissions, while methane and nitrous oxide add additional warming with different timescales and potency [8] [9] [10]. Fossil fuel combustion, cement production and land‑use change drive most CO2 increases, while agriculture, waste and the energy sector are large methane and N2O sources—collectively determining the trajectory of future hottest years depending on emissions pathways [11] [9] [12].
5. Future pathways: emissions determine whether hottest years keep climbing
Climate model projections show that the magnitude and frequency of record hot years depend directly on future greenhouse‑gas emissions: higher emissions scenarios push global and regional temperatures further upward and make new hottest years more extreme, whereas aggressive mitigation slows the rate of warming and reduces the likelihood of ever‑higher record years [10] [3]. The IPCC and agencies stress that while some warming is already locked in, the pace and ceiling of future hottest years remain controllable through emission choices [4] [3].
6. Caveats, competing influences and limits of the record
While there is broad scientific agreement that human greenhouse gases are the dominant cause of the long‑term warming and the clustering of warmest years, other factors also affect year‑to‑year variability—volcanic eruptions, solar variability, aerosols and internal climate oscillations like El Niño—that can amplify or suppress warming in individual years and regions [3] [2]. The reporting sources used here synthesize multiple datasets and models; if a specific new observational claim or extreme‑year attribution outside those assessments is in question, that would require targeted, up‑to‑date attribution analysis not covered explicitly in the provided sources [3] [5].
7. Bottom line: human emissions turned up the thermostat on the hottest years
The convergence of physical theory, long‑term observations, and attribution studies across NASA, NOAA, IPCC and other scientific bodies is clear: human‑caused greenhouse‑gas emissions have amplified the planet’s greenhouse effect, raised global mean temperatures by roughly 1°C or more since preindustrial times, and are the primary reason recent decades contain most of the hottest years on record; how much hotter future record years become depends directly on future emissions choices [5] [4] [10].