Background
The authors note that changes in seawater chemistry -- such as the pH decline that may be caused by rising atmospheric CO2 concentrations -- have the potential to negatively impact fertilization kinetics in free-spawning marine invertebrates -- but that ocean warming could do the opposite and "may enhance fertilization due to positive effects on sperm swimming speeds and heightened sperm-egg collisions," such that the net effect of both phenomena acting in unison could well be negligible.

What was done
To explore the degree of likelihood of this scenario occurring in the real world, Byrne et al. investigated the effects of projected near-future oceanic warming and acidification for conditions that have been predicted for southeast Australia within the timeframe of 2070-2100: an increase in sea surface temperature of 2 to 4°C and a decline in pH of 0.2 to 0.4 unit. This they did in a fertilization study of the sea urchin Heliocidaris erythrogramma via multi-factorial experiments that incorporated a titration of sperm density (10-10³ sperm per ml) across a range of sperm-to-egg ratios (10:1-1500:1).

What was learned
Quoting the five Australian researchers, "across all treatments there was a highly significant effect of sperm density, but no significant effect of temperature or interaction between factors." In fact, they state that "low pH did not reduce the percentage of fertilization even at the lowest sperm densities used, and increased temperature did not enhance fertilization at any sperm density." In addition, they remark that "a number of ecotoxicology and climate change studies, where pH was manipulated with CO2 gas, show that sea urchin fertilization is robust to a broad pH range with impairment only at extreme levels well below projections for ocean acidification by 2100 (pH 7.1-7.4, 2,000-10,000 ppm CO2)," citing the work of Bay et al. (1993), Carr et al. (2006), and Kurihara and Shirayama (2004).

What it means
Interestingly, neither seawater warming nor seawater acidification (caused by contact with CO2-enriched air) had either a positive or a negative effect of sea urchin fertilization, suggesting, as the five scientists concluded, that "sea urchin fertilization is robust to climate change stressors."

References
Bay, S., Burgess, R. and Nacci, D. 1993. Status and applications of echinoid (Phylum Echinodermata) toxicity test methods. In: Landis, W.G., Hughes, J.S. and Lewis, M.A. (Eds.) Environmental Toxicology and Risk Assessment. American Society for Testing and Materials, Philadelphia, Pennsylvania, USA, pp. 281-302.

Carr, R.S., Biedenbach, J.M. and Nipper, M. 2006. Influence of potentially confounding factors on sea urchn porewaer toxicity tests. Archives of Environmental Contamination and Toxicology 51: 573-579.

Kurihara, H. and Shirayama, Y. 2004. Effects of increased atmospheric CO2 on sea urchin early development. Marine Ecology Progress Series 274: 161-169.