Detection of anthropogenic climate change in satellite records of ocean chlorophyll and productivity

S A Henson,J P Dunne,J John,L Bopp,S C Doney,C Beaulieu,I Lima,J L Sarmiento

doi:10.5194/bg-7-621-2010

Abstract

Abstract. Global climate change is predicted to alter the ocean's biological productivity. But how will we recognise the impacts of climate change on ocean productivity? The most comprehensive information available on its global distribution comes from satellite ocean colour data. Now that over ten years of satellite-derived chlorophyll and productivity data have accumulated, can we begin to detect and attribute climate change-driven trends in productivity? Here we compare recent trends in satellite ocean colour data to longer-term time series from three biogeochemical models (GFDL, IPSL and NCAR). We find that detection of climate change-driven trends in the satellite data is confounded by the relatively short time series and large interannual and decadal variability in productivity. Thus, recent observed changes in chlorophyll, primary production and the size of the oligotrophic gyres cannot be unequivocally attributed to the impact of global climate change. Instead, our analyses suggest that a time series of ~40 years length is needed to distinguish a global warming trend from natural variability. In some regions, notably equatorial regions, detection times are predicted to be shorter (~20–30 years). Analysis of modelled chlorophyll and primary production from 2001–2100 suggests that, on average, the climate change-driven trend will not be unambiguously separable from decadal variability until ~2055. Because the magnitude of natural variability in chlorophyll and primary production is larger than, or similar to, the global warming trend, a consistent, decades-long data record must be established if the impact of climate change on ocean productivity is to be definitively detected.

Highlights

Ocean primary production (PP) makes up approximately half of the global biospheric production (Field et al, 1998), so detecting the impact of global climate change on ocean productivity and biomass is an essential task
In some instances, satellite PP algorithms are no more skilful at reproducing in situ PP measurements than biogeochemical models (Friedrichs et al, 2009). We investigate both chl and PP here because chl can change without corresponding changes in phytoplankton biomass or PP, due to the ability of cells to alter their chlorophyll to carbon ratio in response to light or nutrient stress (e.g. Laws and Bannister, 1980; Geider, 1987)
The spatial distribution of statistically significant trends are similar to the regions of large PP change between 1999 and 2004 shown in Behrenfeld et al (2006; their Fig. 3b)

Summary

Introduction

Ocean primary production (PP) makes up approximately half of the global biospheric production (Field et al, 1998), so detecting the impact of global climate change on ocean productivity and biomass is an essential task. The consequence of increasing global temperatures, in combination with altered wind patterns, is to change the mixing and stratification of the surface ocean Some species, adapted to warm temperatures and low nutrient levels (usually small picoplankton) will expand their range, whilst others that prefer turbulent, cool and nutrient-rich waters (mostly large phytoplankton, e.g. diatom species) may migrate poleward as temperatures rise. Polar and ice-edge species will have to adapt to warmer conditions and associated changes in stratification and freshwater input, or risk extinction. These shifts in species composition may alter carbon export and the availability of food to higher trophic levels.

Methods

Results

Conclusion