Climate variability and change
Climate variability and change refer to fluctuations in climate elements such as temperature, precipitation, and wind patterns that deviate from long-term averages, as well as persistent shifts in those averages, occurring over timescales from seasons to millennia due to internal atmospheric-ocean dynamics and external forcings including solar variations, volcanic eruptions, orbital changes, and anthropogenic greenhouse gas emissions.[1][2][3] Natural variability dominates shorter-term changes, exemplified by oscillations like the El Niño-Southern Oscillation (ENSO) that redistribute heat globally, while longer cycles such as Milankovitch orbital forcings have orchestrated ice age alternations over 100,000-year periods by altering solar insolation distribution.[4][5] Instrumental records indicate global average surface temperatures have increased by about 1.1°C since 1850, with roughly two-thirds of this rise post-1975, corroborated across datasets like HadCRUT and Berkeley Earth, though urban heat island effects and data homogenization methods introduce uncertainties in precise quantification.[6][7][8] Anthropogenic CO2 emissions, rising from pre-industrial ~280 ppm to over 420 ppm, enhance the greenhouse effect by trapping outgoing infrared radiation, as evidenced by satellite spectral measurements, yet the net climate sensitivity to doubled CO2—estimated at 1.5–4.5°C in mainstream models—remains empirically contested, with observed warming rates falling below many projections and natural factors like solar activity and ocean cycles contributing substantially to recent trends.[9][10][11] Controversies persist over attribution, as institutional narratives often emphasize human dominance while downplaying natural variability's role and discrepancies between model hindcasts and satellite-era observations, underscoring the need for skepticism toward sources with evident incentives for alarmist framing.[12][11][13]Terminology and Concepts
Definitions of Variability and Change
Climate variability encompasses fluctuations in climate statistics, such as averages, extremes, and probabilities of temperature, precipitation, and other elements, occurring over timescales from seasons to several decades, distinct from individual weather events.[2] These variations arise primarily from internal atmospheric and oceanic processes, including phenomena like the El Niño-Southern Oscillation (ENSO), which redistribute heat and moisture globally, leading to temporary deviations from long-term norms without implying a permanent alteration in the climate's baseline state.[3] For instance, ENSO events can cause widespread drought or flooding on interannual scales, but they typically revert toward the mean over time.[2] In contrast, climate change denotes a long-term, statistically detectable shift in the mean state of the climate or its variability, persisting for periods of decades or longer, identifiable through methods like trend analysis or statistical tests.[14] This definition, as articulated by the Intergovernmental Panel on Climate Change (IPCC), encompasses changes attributable to both natural external forcings—such as solar irradiance variations or volcanic eruptions—and anthropogenic factors, including alterations to atmospheric composition from human emissions.[14] Unlike variability, which oscillates around a stable reference mean, climate change involves directional trends, such as sustained increases in global mean surface temperature observed since the late 19th century, exceeding natural variability thresholds in multiple datasets.[2][15] The demarcation between variability and change hinges on duration, persistence, and statistical significance: short-term anomalies (e.g., multi-year ENSO cycles) represent variability if they do not alter the underlying climate distribution, whereas prolonged shifts, like the Holocene warming or recent anthropogenic warming, qualify as change when they exceed historical ranges or demonstrate non-reverting trends.[2] This distinction is crucial for attribution studies, as natural variability can modulate or mask underlying change signals, requiring separation via modeling and paleoclimate proxies to discern causal drivers.[14] Sources emphasizing only human-induced aspects of change, such as certain policy-oriented reports, may understate natural precedents, but empirical records confirm climate has undergone multiple natural shifts over millennia, independent of human influence.[16]Key Metrics and Indicators
Global surface temperature anomalies serve as a primary indicator of climate variability and change, with datasets from NASA and NOAA showing the 2024 annual average at approximately 1.28°C above the 20th-century baseline, marking it as the warmest year on record.[17] This warming is modulated by natural oscillations like El Niño-Southern Oscillation (ENSO), which contributed to elevated temperatures in 2023-2024, though the long-term trend since 1880 reflects a rise of about 1.1°C.[6] For 2025 through August, global temperatures ranked second highest, trailing only 2024, amid a transition to neutral ENSO conditions.[18] Atmospheric carbon dioxide (CO₂) concentrations, measured at Mauna Loa Observatory, reached a monthly average of 425.48 ppm in August 2025, continuing an upward trajectory from 315 ppm in 1958, with annual increases accelerating due to anthropogenic emissions.[19] The May 2025 peak hit 430.2 ppm, the second-largest year-over-year jump in the 67-year record, underscoring persistent radiative forcing despite natural sinks absorbing roughly half of emissions.[20] Global mean sea level has risen at an average rate of 3.4 mm per year since 1993, as tracked by satellite altimetry, totaling 8-9 inches (21-24 cm) since 1880, driven by thermal expansion and land ice melt.[21] In 2024, the rate accelerated beyond expectations, linked to extreme ocean warming, reaching a record high of 101.4 mm above 1993 levels by year-end.[22] [23] Upper ocean heat content has increased steadily since the 1970s, absorbing over 90% of Earth's excess energy, with the top 2000 meters gaining heat at rates exceeding prior decades, as evidenced by Argo float and ship-based measurements.[24] This accumulation, equivalent to roughly four Hiroshima bombs per second in recent years, reflects both radiative imbalance and ocean circulation changes.[25] Arctic sea ice minimum extent reached 4.60 million square kilometers in September 2025, tying for the 10th lowest in the satellite era (since 1979), with a long-term decline of 12.2% per decade amid reduced summer melt onset and extent.[26] Variability from weather patterns and ENSO influences annual minima, but the trend indicates diminished ice volume and thickness.[27] Other indicators include shifts in precipitation extremes and drought indices, such as the U.S. Climate Extremes Index, which tracks departures in temperature, precipitation, and drought, showing increased frequency of extremes in recent decades, though regional patterns vary due to internal variability.[28] These metrics, derived from instrumental records and proxies, quantify changes against natural baselines but require accounting for data adjustments and urban heat effects in surface observations.[29]Natural Drivers
Internal Climate Variability
Internal climate variability refers to natural fluctuations in the climate system arising from chaotic internal dynamics and interactions among components such as the atmosphere, oceans, land surface, and cryosphere, without requiring external forcings.[30] These processes generate preferred spatial patterns of variability, known as modes, that operate on timescales from intraseasonal to multidecadal and contribute substantially to climate predictability on subseasonal to decadal horizons.[30] Internal variability is generally larger in the extratropics than tropics and stronger in winter than summer, influencing regional weather patterns and masking forced trends in short-term observations.[31] The El Niño-Southern Oscillation (ENSO) represents a dominant interannual mode, with cycles typically lasting 2-7 years, driven by coupled ocean-atmosphere interactions in the tropical Pacific.[30] During El Niño phases, enhanced easterly trade winds weaken, leading to warmer sea surface temperatures (SSTs) in the central-eastern Pacific, which shift the Walker circulation and generate global teleconnections affecting precipitation and temperature; for instance, El Niño events have been linked to increased hurricane activity in the Pacific and droughts in Indonesia.[30] La Niña phases, conversely, feature cooler SSTs and strengthened trades, often resulting in opposite impacts, such as floods in Peru and drier conditions in the southern U.S.[30] ENSO variability accounts for much of the year-to-year global temperature fluctuations, with strong events capable of altering annual averages by up to 0.15°C.[30] Decadal to multidecadal modes include the Pacific Decadal Oscillation (PDO), characterized by SST anomalies in the North Pacific north of 20°N, with phases shifting roughly every 20-30 years.[32] Positive PDO phases feature cooling in the central North Pacific and warming along the coasts, impacting marine ecosystems, such as reduced salmon catches during cool phases, and modulating North American precipitation patterns.[32] The PDO enhances El Niño teleconnections to North America during positive phases, amplifying winter precipitation in the southwestern U.S. and diminishing La Niña effects.[33] The Atlantic Multidecadal Oscillation (AMO) exhibits SST variability in the North Atlantic on 60-80 year cycles, with warm phases since the mid-1990s linked to heightened Atlantic hurricane frequency and intensified multidecadal ENSO variability through alterations in the annual cycle.[34] AMO warm periods increase drought risk in the U.S. Southwest and north-central regions, while also influencing Sahel rainfall; for example, the positive AMO phase correlates with stronger Pacific ITCZ variability.[35] [36] These oscillations arise from internal ocean circulation changes, such as variations in the Atlantic Meridional Overturning Circulation, rather than direct radiative forcing.[34] Internal modes like ENSO, PDO, and AMO introduce substantial uncertainty in attributing regional climate changes, as their amplitudes can rival anthropogenic forcing on decadal scales, particularly over land areas where variability is amplified.[37] For instance, natural decadal variations may equal global warming-induced surface air temperature changes in magnitude for regional predictions.[37] While models simulate these modes, debates persist on their exact internal origins versus subtle external influences, with some analyses questioning robust multidecadal oscillations in unforced simulations.[38] Nonetheless, observational and modeling evidence confirms their role in generating low-frequency variability that overlays long-term trends.[39]
External Natural Forcings
External natural forcings encompass variations in solar output, volcanic aerosol emissions, and Earth's orbital parameters that alter the planetary energy balance independently of internal climate dynamics. These factors have driven significant climate shifts across geological timescales, with solar and volcanic influences affecting decadal to centennial variability, while orbital changes operate on millennial scales. Empirical reconstructions indicate their radiative forcings are typically small in the modern era compared to pre-industrial baselines but have modulated past climates through direct energetic imbalances.[40][41][42] Solar irradiance variations, tracked via sunspot cycles and proxy records, impose a radiative forcing of about ±0.17 W/m² over the 11-year Schwabe cycle, stemming from a 0.1% fluctuation in total solar irradiance. This equates to global temperature responses of roughly 0.1°C, as observed in satellite measurements since 1978 showing no net upward trend in irradiance amid rising temperatures. Longer-term solar minima, such as the Maunder Minimum (1645–1715), correlated with cooler European winters, but reconstructions attribute only modest global cooling of 0.1–0.3°C, underscoring limited efficacy for explaining centennial-scale changes without amplification mechanisms like cosmic ray-induced cloud cover, which remain debated.[43][44][45] Orbital forcings, formalized by Milankovitch, arise from eccentricity (100,000-year cycle modulating perihelion distance), obliquity (41,000-year axial tilt variation affecting seasonal contrast), and precession (23,000-year wobble shifting solstice timing). These redistribute insolation, with high-latitude summer peaks driving glacial terminations; for instance, obliquity maxima increase Northern Hemisphere insolation by up to 50 W/m² at 65°N, initiating deglaciations as seen in oxygen isotope records from ice cores. In the Holocene, declining obliquity has favored a gradual cooling trajectory, projecting multi-millennial declines of 0.5–1°C per 5,000 years absent countervailing influences, incompatible with 20th-century warming patterns.[46][47][48] Volcanic forcings occur via stratospheric sulfate aerosol veils from sulfur-rich eruptions, scattering incoming shortwave radiation and yielding negative forcings of 1–5 W/m² for 1–3 years. The 1991 Mount Pinatubo eruption exemplifies this, injecting 20 million tons of SO₂ and inducing 0.5°C global cooling detectable in surface temperatures. Larger historical events, like the 1815 Tambora blast, amplified aerosol lifetimes through self-lofting, but aggregate volcanic forcing over the 20th century nets near-zero due to episodic nature, contributing to short-term dips amid longer trends rather than sustained directional change.[49][50][51]Anthropogenic Factors
Greenhouse Gas Contributions
Anthropogenic greenhouse gas emissions arise predominantly from fossil fuel combustion, industrial processes, agriculture, waste management, and land-use changes such as deforestation. These activities have elevated atmospheric concentrations of long-lived greenhouse gases, with carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) comprising the majority. Fluorinated gases, though minor in volume, possess high global warming potentials. Global anthropogenic GHG emissions reached 52.9 GtCO2e in 2023, reflecting a 62% increase since 1990.[52] CO2 accounts for approximately 74-76% of total anthropogenic GHG emissions in CO2-equivalent terms, primarily from fossil fuel oxidation in energy production, transportation, and industry, supplemented by cement manufacturing and net land-use emissions. Fossil fuel-related CO2 emissions totaled about 37 Gt in 2023, with coal, oil, and natural gas as key contributors; land-use changes, including deforestation, added roughly 4-5 GtCO2 annually in recent years, though net land sinks partially offset this.[53][54][55]| Greenhouse Gas | Approximate Share of Total Anthropogenic Emissions (CO2e, %) | Primary Anthropogenic Sources |
|---|---|---|
| CO2 | 74.5 (fossil) + land-use contributions | Fossil fuel combustion, cement production, deforestation[55][54] |
| CH4 | 17.9 | Enteric fermentation in livestock, rice cultivation, fossil fuel extraction and leaks, landfills[55][56] |
| N2O | 4 | Agricultural soil management (fertilizers), manure, industrial processes[55][52] |
| Fluorinated gases | ~2-3 | Industrial refrigerants, aerosols, semiconductors[57][58] |