New breakthrough study attempted to answer the key question: When might scientists first realize that burning fossil fuels is significantly changing the global climate? Using a broad range of thought experiment with modern climate models, the study came to the surprising conclusion that noticeable human influence on atmospheric temperature was probably present and detectable more than 130 years ago, that is, at the end of the 19th century.

Scientific foundations and simulation approach As early as the mid-to-late 19th century, Eunice Foote and John Tyndall had discovered the heat-trapping properties of CO2, and Svante Arrhenius had estimated the impact of fossil fuel combustion on CO2 increases. During this period, daily surface temperatures began to be measured systematically, and work with weather balloons led to the discovery of the stratosphere. In addition, ice core analysis showed an increase in atmospheric CO2 at a rate of about 2.5 ppmv per decade between 1860 and 1899. The study answers the question of detecting human influence using "thought experiment" with state-of-the-art climate modelsThe scientists assumed that the ability to monitor global atmospheric temperature changes with the precision of today's satellite radiometers existed as early as 1860. They then applied the standard method "fingerprints"to distinguish between human and natural influences on climate and determine the time of initial detection. The experiment examined various start dates of monitoring from 1860 to 1986 and analyzed the stratosphere and troposphere.

Key findings: Detection in the stratosphere as early as the late 19th century The most significant and earliest detectable human impact was in the stratosphere. Significant cooling of the middle to upper stratosphere, caused mainly by anthropogenic increases in carbon dioxide and ozone depletion, would be identifiable already around 1885, before the arrival of gasoline-powered cars. This means that a human fingerprint on atmospheric temperature would be detectable with high certainty after just 25 years of monitoring, if it had started in 1860. Even with limited measurements for the mid-latitudes of the Northern Hemisphere, human-induced stratospheric cooling would have been detectable by 1894, just 34 years after the supposed start of monitoring. This early detection in the stratosphere is due to the large anthropogenic cooling and its significantly different pattern from natural noise.

Differences in the troposphere and the influence of natural factors On the contrary, in the troposphere, recognition of human influence was more difficult and later. Globally averaged anthropogenic signals are smaller there and their patterns are more similar to natural noise. Reliable identification of “fingerprints” in the troposphere would not have been possible with the start of monitoring in 1860 and 40 years of measurements. Consistent detection of anthropogenic fingerprints in tropospheric temperature is only achieved for monitoring start dates from 1960 or later.

The detection time is also affected by natural influences, such as volcanic eruptions (e.g. Krakatoa, Pinatubo) and fluctuations in solar radiation. These phenomena can temporarily dampen or enhance the anthropogenic signal, which is reflected in fluctuations in the signal-to-noise ratio and can delay detection. Nevertheless, especially after 1940, anthropogenic signals are strong enough to overcome the combined noise of internal variability and volcanic or solar radiation. Also important is the higher signal-to-noise ratio (S/N) in the stratosphere, which allows for earlier and more robust detection. The reliability of the findings is high, as the CMIP6 models do not underestimate tropospheric variability, and even in the stratosphere the detectable signal is significantly larger than the assumed noise.

Implications for the present This study strongly suggests that significant human interference with the Earth's climate is not a new phenomenon, but has existed for more than 130 years. While it is unclear whether early knowledge in the 19th century would have prompted change, we know with high certainty today that sustainable pathways are essential to avoid dangerous anthropogenic interference with the climate. Projected future climate changes are significantly greater than those we have seen in the recent past. Humanity is on the threshold of dangerous anthropogenic interference, and our immediate decisions will determine whether we cross that threshold. Spring

The study was published in the journal pnas.org

Glossary of key terms

• Anthropogenic: Caused by or resulting from human activity, especially in terms of environmental impacts.
• Arrhenius, Svante: A Swedish chemist who modeled climate in the late 19th century and estimated that doubling CO2 could increase surface temperature.
• Atmospheric Temperature: Air temperature at different heights in the Earth's atmosphere.
• CMIP6 (Coupled Model Intercomparison Project Phase 6): The sixth phase of an international climate model comparison project, providing simulations for climate research.
• Detection: The process of determining whether an observed signal (e.g., climate change) is statistically different from natural variability.
• EOF (Empirical Orthogonal Function): A statistical technique used to decompose a data set into a set of orthogonal spatial patterns (fingerprints) and their corresponding time series.
• Emissivity: A measure of a material's ability to radiate energy in the form of thermal radiation. For the stratosphere, increased emissivity due to CO2 means more heat is radiated into space, leading to cooling.
• Foote, Eunice: American scientist who demonstrated the heat-absorbing properties of CO2 in 1856.
• Fossil Fuels: Natural fuels, such as coal, oil, and natural gas, are formed from geological processes acting on the remains of dead organisms. Burning them releases CO2.
• "Fingerprint" (Fingerprint): A specific spatiotemporal pattern of climate change that is unique to particular factors (e.g., human influences vs. natural forces). Used to detect and attribute climate change.
• Thought Experiment: A hypothetical scenario designed to explore the implications of a hypothesis or theory.
• Hunga Tonga (Hunga Tonga-Hunga Ha'apai): An underwater volcano whose eruption in 2022 injected a large amount of water vapor into the stratosphere, affecting atmospheric temperature.
• Krakatoa (Krakatoa): A volcano whose major eruption in 1883 had a significant but temporary effect on global temperatures, causing stratospheric warming and tropospheric cooling.
• MSU/AMSU (Microwave Sounding Unit/Advanced Microwave Sounding Unit): Satellite instruments used to measure atmospheric temperatures in the troposphere and lower stratosphere (TLT, TMT, TLS layers).
• Pinatubo (Mount Pinatubo): A volcano whose major eruption in 1991 caused short-term global cooling of the troposphere and warming of the stratosphere.
• Signal-to-Noise Ratio (S/N): The ratio of the detectable signal strength to the background noise level. A higher S/N ratio indicates clearer signal detection.
• Solar Cycle: An approximately 11-year cycle in the variation of solar radiation that affects stratospheric temperatures.
• Stratosphere: The layer of the Earth's atmosphere above the troposphere, extending from approximately 10 km to 50 km, where the temperature increases with height.
• SSU (Stratospheric Sounding Unit): A satellite instrument used to measure atmospheric temperatures in the stratosphere.
• Ocean Thermal Inertia: The ocean's ability to absorb and release heat slowly, leading to longer reaction times to temperature changes in the troposphere following volcanic eruptions.
• Troposphere: The lowest layer of the Earth's atmosphere, extending from the surface to approximately 10-15 km, where most weather occurs.
• Tyndall, John: Irish physicist who demonstrated in 1859 that certain gases, such as water vapor and CO2, absorb heat and are responsible for the greenhouse effect.
• Internal Variability: Natural fluctuations in the climate system that are not caused by external influences (e.g. solar activity, volcanic eruptions or anthropogenic emissions).