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How does the temperature change over the past 100 years compare to previous centuries?
Executive Summary
The past 100 years show a marked and accelerating global warming signal that is larger in magnitude and faster in rate than typical century-scale variations in many previous centuries, with instrumental analyses finding roughly 1.1–1.5°C warming relative to late‑19th-century baselines and recent decades producing the warmest years on record [1] [2] [3]. Paleoclimate reconstructions and data-assimilation studies place this century-scale rise in the context of the last millennium: while natural variability has produced warmer and cooler multi-century intervals regionally, the combination of the magnitude, global coherence, and rapid rate of the recent century’s warming distinguishes it from most earlier centuries [4] [5] [6].
1. What the instrumental records declare — recent warming is unprecedented in modern records
Instrumental temperature datasets based on surface observations show a clear, accelerating warming trend over the past century, with multiple agencies reporting that the 2010s and early 2020s contain the warmest years on record and 2024 reaching record highs relative to 1880 baselines [1] [2]. Quantitatively, the century-scale increase is commonly reported near 1.1°C–1.5°C versus late‑19th-century or mid‑20th-century baselines, and warming since 1975 accelerated to roughly 0.15–0.20°C per decade in many analyses [3]. These instrumental records are global in scope but rely on a denser network in recent decades; their strength is high temporal resolution and direct measurement, and they consistently show the top-ranked warm years clustering in the most recent decades [1] [2].
2. What paleoclimate reconstructions add — context, not contradiction
Proxy-based reconstructions of the last millennium provide longer-term perspective showing multi-century variability such as the Medieval Climate Anomaly and the Little Ice Age at regional-to-hemispheric scales, and they reveal that some past intervals were locally as warm or warmer than parts of the 20th century [5]. Studies using expanded proxy databases and data assimilation place recent warming within a broader thousand-year framework, finding the modern rise to be notable for its global coherence and speed even where some regional amplitudes fall within longer-term natural variability [4]. Proxies are essential because instrumental coverage before ~1880 is sparse; their value is temporal reach, while limitations include spatial gaps and dating/interpretation uncertainties [6].
3. Comparing magnitude and rate — why speed matters
When comparing centuries, both total warming and the rate of change matter. Instrumental analyses show the past 100 years include both a large absolute increase and a rapid acceleration since mid‑20th century, with rates since the 1980s multiple times faster than the long-term average since the 19th century [2] [3]. Paleoclimate records indicate that while some past centuries had episodes of comparable average temperatures regionally, they generally did not exhibit the global-scale, high‑latitude amplified warming and rapid multi-decade rise now observed [5] [7]. The combination of magnitude, global reach, and decadal speed sets the recent century apart in the climate record [3] [4].
4. Sources of agreement and the remaining uncertainties scientists discuss
Multiple independent teams converge on the conclusion of substantial recent warming, giving the finding high confidence; instrumental and proxy approaches each corroborate aspects of the trend, and data‑assimilation efforts bridge the two [1] [4] [8]. Remaining uncertainties include spatial gaps in proxies (especially over oceans and the Southern Hemisphere), potential biases in early instrumental records, and proxy dating and calibration issues, which affect exact estimates but not the overall conclusion of exceptional recent warming [8] [7]. Scientific discussions therefore focus on refining regional reconstructions, quantifying uncertainty margins, and improving integration of proxy and instrumental time series rather than disputing the central fact of rapid recent warming [8] [6].
5. Bottom line for comparison and what was omitted in simple summaries
In sum, the past 100 years exhibit a larger and faster global temperature increase than typical centuries in much of the last millennium, distinguished by its global coherence and acceleration in the late 20th and early 21st centuries; instrumental datasets report roughly 1.1–1.5°C increase since the late 1800s and proxies show that while some regional past warmth occurred, the modern pattern is atypical in scope and rate [1] [3] [4]. Simple comparisons often omit important context: the distinction between regional versus global signals, differences in uncertainties between instruments and proxies, and the role of external forcings (natural and anthropogenic) and aerosols in shaping mid‑20th-century variability — all issues that contemporary analyses are actively quantifying [7] [5].