Factors challenging our ability to detect long-term trends in ocean chlorophyll

Abstract

Global climate change is expected to affect the ocean's biological productivity. The most comprehensive information available about the global distribution of contemporary ocean primary productivity is derived from satellite data. Large spatial patchiness and interannual to multidecadal variability in chlorophyll <i>a</i> concentration challenges efforts to distinguish a global, secular trend given satellite records which are limited in duration and continuity. The longest ocean color satellite record comes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), which failed in December 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensors are beyond their originally planned operational lifetime. Successful retrieval of a quality signal from the current Visible Infrared Imager Radiometer Suite (VIIRS) instrument, or successful launch of the Ocean and Land Colour Instrument (OLCI) expected in 2014 will hopefully extend the ocean color time series and increase the potential for detecting trends in ocean productivity in the future. Alternatively, a potential discontinuity in the time series of ocean chlorophyll <i>a</i>, introduced by a change of instrument without overlap and opportunity for cross-calibration, would make trend detection even more challenging. In this paper, we demonstrate that there are a few regions with statistically significant trends over the ten years of SeaWiFS data, but at a global scale the trend is not large enough to be distinguished from noise. We quantify the degree to which red noise (autocorrelation) especially challenges trend detection in these observational time series. We further demonstrate how discontinuities in the time series at various points would affect our ability to detect trends in ocean chlorophyll <i>a</i>. We highlight the importance of maintaining continuous, climate-quality satellite data records for climate-change detection and attribution studies

Evaluation of Satellite Retrievals of Ocean Chlorophyll-a in the California Current

“…Moreover, advanced data assimilation techniques for real-time circulation and ecosystem models in the CC rely heavily on observational "truth" from satellite-derived chlorophyll. While we did not evaluate matchups for other coastal regions, it is likely, given that the standard algorithms are derived using a large percentage of data from the CC, that similar biases exist (e.g., [33,34]). …”

Variability and 20-Year Trends in Satellite-Derived Surface Chlorophyll Concentrations in Large Marine Ecosystems around South and Western Central America

“…The newly developed extended time series include almost 20 years of data at high spatial resolution and revealed significant trends in chlorophyll concentrations. Previous studies observed varying trends in global average chlorophyll concentrations, with reports of a general decline (e.g., Behrenfeld et al, 2006;Vantrepotte and Mélin, 2009;Boyce et al, 2014;Lorenzoni et al, 2017), as well as no detectable trends (Beaulieu et al, 2013) or even a global increase of 4.13% (Gregg et al, 2005). However, there is general consensus in that there is high global variability, with regions of increasing and decreasing trends.…”

Temporal changes in total and size-fractioned chlorophyll-a in surface waters of three provinces in the Atlantic Ocean (September to November) between 2003 and 2010

“…Blending in-situ and satellite observations of TCHLa (e.g. AMT and SeaWiFS) may lead to more robust estimates of trends (Aiken et al, 2009;Gregg & Casey, 2010;Gregg & Rousseaux, 2014) and minimise the impact of periods of missing data (Beaulieu et al, 2013;Racault et al, 2014a).…”