Time of emergence of trends in ocean biogeochemistry

Abstract

Abstract. For the detection of climate change, not only the magnitude of a trend signal is of significance. An essential issue is the time period required by the trend to be detectable in the first place. An illustrative measure for this is time of emergence (ToE), that is, the point in time when a signal finally emerges from the background noise of natural variability. We investigate the ToE of trend signals in different biogeochemical and physical surface variables utilizing a multimodel ensemble comprising simulations of 17 Earth system models (ESMs). We find that signals in ocean biogeochemical variables emerge on much shorter timescales than the physical variable sea surface temperature (SST). The ToE patterns of pCO 2 and pH are spatially very similar to DIC (dissolved inorganic carbon), yet the trends emerge much faster -after roughly 12 yr for the majority of the global ocean area, compared to between 10 and 30 yr for DIC. ToE of 45-90 yr are even larger for SST. In general, the background noise is of higher importance in determining ToE than the strength of the trend signal. In areas with high natural variability, even strong trends both in the physical climate and carbon cycle system are masked by variability over decadal timescales. In contrast to the trend, natural variability is affected by the seasonal cycle. This has important implications for observations, since it implies that intra-annual variability could question the representativeness of irregularly sampled seasonal measurements for the entire year and, thus, the interpretation of observed trends.

The ocean carbon sink – impacts, vulnerabilities and challenges

“…Earth system modelling suggested that sea surface pCO 2 and sea surface pH trends could rise beyond the detection threshold already after 12 years from now. DIC trends would become clear after 10-30 years and trends in the sea surface temperature after 45-90 years (Keller et al, 2014). Accordingly, an earlier detection threshold for changes in mean ENSO-induced carbon cycle variability (pCO 2 , pH, biological productivity) than for ocean temperature changes during the 21st century was predicted by Keller et al (2015).…”

“…when do trends in key ocean variables emerge as robust on the background of analytical uncertainty and interannual variability? Keller et al (2014Keller et al ( , 2015 provided new insight into this issue. Earth system modelling suggested that sea surface pCO 2 and sea surface pH trends could rise beyond the detection threshold already after 12 years from now.…”

Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

“…In the ocean, Keller et al (2014) have demonstrated that the time of emergence for pCO 2 and pH is about 12 years and ∼10-30 years for DIC, with open-ocean time-series sufficiently long in duration to demonstrate long-term trends (e.g., Bates et al, 2014). Similarly, the Harrington Sound time-series of seawater CO 2 -carbonate chemistry and anomalies from 1996 to 2016 is thus of sufficient length to demonstrate trend emergence and it constitutes one of the longest, if not the longest, coastal ocean time-series in the global coastal ocean.…”

Climate and carbon cycle dynamics in a CESM simulation from 850 to 2100 CE

“…4a and b, full range). We estimate when the anthropogenically forced, global mean land and ocean uptake fluxes exceeds the envelope of preindustrial natural variability (Hawkins and Sutton, 2012;Keller et al, 2014). As a threshold criteria, it is required that the decadally smoothed uptake fluxes are larger than the upper bound of 2 standard deviations of the annual fluxes prior to 1750 CE.…”