The Sarmiento group studies the impacts of anthropogenic CO2 emissions and climate change on ocean chemistry and sea life.


Impact of climate change and ocean warming on fish body size

Changes in temperature, oxygen content, and other ocean biogeochemical properties also directly affect the ecophysiology of marine water-breathing organisms. Previous studies suggest that the most prominent biological responses are changes in distribution, phenology, and productivity. Both theory and empirical observations also support the hypothesis that warming and reduced oxygen will reduce body size of marine fishes. However, the extent to which such changes would exacerbate the impacts of climate and ocean changes on global marine ecosystems remains unexplored.

In collaboration with researchers at the University of British Columbia, the Sarmiento group employed a model to examine the integrated biological responses of over 600 species of marine fishes due to changes in distribution, abundance and body size. The model has an explicit representation of ecophysiology, dispersal, distribution, and population dynamics. The model results show that assemblage-averaged maximum body weight is expected to shrink by 14–24% globally from 2000 to 2050 in a warmer less oxygenated ocean under a high-emission scenario (Figure 4). About half of this shrinkage is due to change in distribution and abundance, the remainder to changes in physiology. The tropical and intermediate latitudinal areas will be heavily impacted, with an average reduction of more than 20%. The results of this study provide a new dimension to understanding the integrated impacts of climate change on marine ecosystems.


Figure 4: Change in individual-level body size of fishes in different ocean basins from 2000 (averages of 1991-2010) to 2050 (averages of 2041-2060). The thick black lines represent median values, the upper and lower boundaries of the box represents 75 and 25 percentiles and the vertical dotted lines represent upper and lower limits.


Impact of oceanic O2, CO2, and temperature changes on tuna habitats

Predicting the effects of climate change on habitat utilization in the ocean environment requires identification of underlying physiological mechanisms influenced by environmental conditions. Hemoglobin-oxygenation is hypothesized to be one of these underlying mechanisms. Oxygen extraction from seawater and delivery to tissues, a fundamental process which depends on hemoglobin, is a necessity for tuna survival, as the globe-traveling fish utilize many different regions and depths in the ocean while foraging for food.

All of the environmental factors associated with hemoglobin-oxygenation are predicted to change in the future: temperature is predicted to increase with climate change, oxygen is predicted to decrease as increases in temperature cause water column stratification to increase, and carbon dioxide is predicted to increase with ocean acidification. It is hypothesized that these changes will result in habitat compression for tuna, but the magnitude and biogeography of the compression will be different among the species of tuna. The Sarmiento group is using the P50 depth, the depth at which 50% of hemoglobin is oxygenated, as the threshold restricting tuna habitat utilization in global datasets and IPCC-class earth system model results. Preliminary results from two earth system models indicate that future climate change will have neutral to negative impacts on the habitat size of Thunnus albacares, the yellowfin tuna, by 2100.

The plan for the next year is to evaluate seven additional IPCC-class earth system models. There are many variations in how the physics, chemistry, and biology of an earth system model is constructed and also differences in the parameterizations selected for each model, all of which contribute to uncertainty in the results. For this reason, it is important to use multiple earth system models to account for the uncertainty resulting from different model formulations. The analysis will also be expanded to include all the tuna species in the genus Thunnus.