Global Carbon and Other Biogeochemical Cycles and Feedbacks

Four Decades of Trends and Drivers of Global Surface Ocean Acidification

“…The driver decomposition Equations 3 and 4 confirms the expectation that the majority of the decreasing trend in Ω ar and increasing trend in [H + ] F is driven by the anthropogenic increase in atmospheric pCO 2 causing an increase in surface ocean C ant (dashed lines in Figures 2a and 2c). This conclusion is in line with IPCC's assessments (Bindoff et al, 2019;Canadell et al, 2021) as well as with prior work (Lauvset et al, 2015;Lauvset & Gruber, 2014). However, hidden behind the dominant role of C ant are substantial and relevant contributions from the other drivers.…”

“…Our global trend analyses represent a major step forward relative to prior data-based assessments of the global trends in OA, which were based on a limited number of time series stations (Canadell et al, 2021), based on spatial aggregation of data over large-scale biomes (Lauvset et al, 2015) or using linear regression models only (Iida et al, 2021). Not only were we able to substantially reduce the uncertainties of the trends, but we also pointed out the substantial regional differences in the trends of the most important OA parameters with pH experiencing, on average, the highest rates of changes in the higher latitudes, while the largest changes in Ω ar are found in the tropics.…”

“…Building on this work and its own synthesis of the data from various time series sites across the world's oceans, the Intergovernmental Panel on Climate Change (IPCC) concluded in its special report on the Ocean and Cryosphere (Bindoff et al, 2019), that "pH in open ocean surface water has changed by a virtually certain range of −0.017 to −0.027 pH units per decade since the late 1980s." IPCC's Working Group 1 report in AR6 confirmed, in essence, this very large range for the global mean rate, and also discussed the spatial variability around this mean trend (Canadell et al, 2021). An alternative approach was taken more recently by Jiang et al (2019) who combined a climatological seawater CO 2 product with model results to obtain trends in the OA parameters.…”

“…We interpret these differences to be primarily the result of the relatively small number of time series sites that were used by the IPCC for their assessment. We are not aware of any study that investigated the spatial pattern of the trend in Ω ar in a systematic manner, although IPCC AR6 (Canadell et al, 2021) mentioned the regional differences.…”

“…A systematic difference we can confirm here. In its global assessment, IPCC AR6 (Canadell et al, 2021) discussed the regional differences as well, pointing out, for example, that the central and eastern upwelling zones of the Pacific exhibited a faster pH decline of −0.022 to −0.026 decade −1 (see also Sutton et al (2014)) compared to the western tropical Pacific, where the trends are only in the range of −0.010 to −0.013 decade −1 (see also Ishii et al (2020)). This strong east-west gradient in the tropical Pacific is fully confirmed by our results (Figure 2d).…”