A6 Accelerated Climate Change

Topic

Current anthropogenic climate change constitutes an exceptional anomaly within the Holocene—the last 11,700 years of Earth's climate history. The observed rate of warming—close to 0.18°C per decade since 1970 and 0.2°C per decade during the 2010–2020 period—exceeds, in both magnitude and speed, the natural changes observed during comparable periods of the Holocene. This trend is not limited to surface temperature; ocean warming, sea-level rise, increasing atmospheric carbon dioxide concentrations, ocean acidification, rising atmospheric water vapor, and changes in climate extremes all exhibit trends that are mutually consistent and distinguishable from the climate system's natural internal variability.

The fundamental physical cause is anthropogenic radiative forcing, which is close to 2.7 W/m² above pre-industrial levels. Its primary component is carbon dioxide, contributing approximately 1.8 W/m², followed by methane at around 0.5 W/m² and nitrous oxide at nearly 0.2 W/m². Part of this warming is offset by anthropogenic aerosols, whose negative forcing is estimated at between 0.5 and 1.5 W/m². Aerosols thus mask a fraction of the warming that greenhouse gases would otherwise produce on their own.

The climate system does not respond instantaneously to forcing. Transient climate sensitivity describes the warming that occurs while forcing continues to increase and slow-responding components of the system—particularly the deep ocean—have not yet reached equilibrium. Equilibrium climate sensitivity, by contrast, represents the final warming that would be reached if forcing remained constant for a sufficiently long period. The latter is higher because it incorporates slow processes whose effects have not yet fully materialized during the transient response. The relationship between emissions and warming makes it possible to define the remaining carbon budget: the maximum additional amount of carbon dioxide that can be emitted while maintaining a certain probability of not exceeding a warming threshold, such as 1.5°C or 2°C above pre-industrial levels. This concept is based on the approximately linear relationship between cumulative CO₂ emissions and the rise in global temperature. Broadly speaking, the greater the total amount of carbon dioxide emitted, the greater the resulting warming. Consequently, each additional emission consumes a portion of the available budget and reduces the remaining margin before a specific temperature level is reached.

Anthropogenic warming also alters climate extremes. As the system's average temperature rises, the distribution of numerous meteorological variables shifts; this makes previously infrequent warm conditions more common and allows for the attainment of higher extreme values. Changes in the climate system can also modify variability and, in turn, the frequency and intensity of certain extreme events.

Thus, there is a direct link between emissions, radiative forcing, warming, and climate risks. Human activities increase greenhouse gas concentrations; these alter the planet's radiative balance, and the climate system responds with warming that unfolds over various timescales. The accumulation of emissions determines the extent of temperature rise and, consequently, how much of the carbon budget remains available. At the same time, this warming progressively alters climatic conditions and the occurrence of extremes. The remaining carbon budget thus provides a way to directly link a global temperature goal to the cumulative amount of CO₂ that can still be emitted before that threshold is exceeded.

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