M6 Forced Trend

Topic

Anthropogenic climate change is the mode of climate system variability dominating the current timescale—ranging from decades to centuries—and is driven by external human-induced forcing, primarily the emission of greenhouse gases and aerosols. Unlike modes of internal climate system variability that arise from the system's own coupled internal dynamics, this mode of anthropogenic climate change is a forced response that progressively emerges above the system's internal variability as human-induced external forcing accumulates. The key parameter for this forced mode is the signal's time of emergence: the moment when the signal forced by anthropogenic climate change surpasses the noise level of the climate system's internal variability in a statistically robust manner.

Different emission pathways, grouped into Shared Socioeconomic Pathways, define various possible futures for greenhouse gas concentrations, ranging from very low to very high emission trajectories. The climate system's response to each of these pathways is calculated using Earth system models; this response can be decomposed into two components: the response forced by the emission pathway—corresponding to the change in the average across a large ensemble of simulations from the same climate model—and the residual internal variability of the climate system, corresponding to the spread among the individual simulations within that same large ensemble.

Within this pathway-forced response, the transient climate response to cumulative carbon dioxide emissions directly links the total amount of emitted carbon dioxide to the global warming ultimately reached through anthropogenic climate change, via an approximately linear relationship: the greater the total amount of emitted carbon dioxide, the greater the resulting global warming—a ratio that remains reasonably stable regardless of the specific emission pathway that produced that total amount of carbon dioxide.

The spatial patterns of this global warming exhibit three characteristic features. The first is marked Arctic amplification, occurring at a rate three to four times faster than average global warming. The second is more rapid warming over landmasses than over the ocean; this occurs because continents have a lower heat-storage capacity than the ocean and thus respond more quickly to the same human-induced external forcing. The third is a spatial pattern characteristic of greenhouse gas forcing—distinct from the patterns associated with solar or volcanic forcing—which allows the observed global warming to be specifically attributed to anthropogenic climate change rather than to these other external drivers.

Under this global warming, the hydrological cycle intensifies, resulting in increased evaporation and higher average global precipitation; however, this intensification is distributed very unevenly across regions. Already dry areas tend to become drier, while already wet areas tend to become wetter; thus, anthropogenic climate change amplifies existing regional hydrological contrasts rather than distributing the intensification of the hydrological cycle equally among all regions.

Changes in climate extremes caused by anthropogenic climate change are quantified by comparing how the probability distribution of each meteorological variable shifts and widens under global warming—much like the shifts and redistributions of precipitation described in the previous paragraph. Taken together, Arctic amplification, the contrast between land and ocean warming, the spatial pattern characteristic of greenhouse gas forcing, the redistribution of the hydrological cycle, and the shifting of climate extremes constitute the complete fingerprint that allows us to recognize anthropogenic climate change as the forced mode emerging above the climate system's internal variability—the very variability with which this description began—rather than confusing it with any of those other modes of internal variability.

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