The ocean warming that has occurred over the past 50 years is primarily caused by humans, new research shows.
The new research, from the Lawrence Livermore National Laboratory, shows that the observed ocean warming over the last 50 years is only consistent with climate models when the models include the influence of increasing greenhouse gas emissions in the 20th century.
This research isn’t the first to identify the human influence on ocean warming, but it’s the first to do an in-depth examination of how the different observational and modeling variables affect the conclusion that the warming is human-caused.
“We have taken a closer look at factors that influence these results,” said Peter Gleckler, an LLNL climate scientist and lead author of the new study that appears in the June 10 edition of the journal Nature Climate Change. “The bottom line is that this study substantially strengthens the conclusion that most of the observed global ocean warming over the past 50 years is attributable to human activities.”
The researchers looked at the average temperature (heat content) of the upper layers of the ocean. “The observed global average ocean warming (from the surface to 700 meters) is approximately 0.025 degrees Celsius per decade, or slightly more than 1/10th of a degree Celsius over 50 years.” The sub-surface warming is somewhat less than that because of the slow transfer of surface warming to lower depths. “Nevertheless, because of the ocean’s enormous heat capacity, the oceans likely account for more than 90 percent of the heat accumulated over the past 50 years as Earth has warmed.”
For this study, the international research team examined the causes of ocean warming using improved observational estimates. The researchers also used results from a large multi-model archive of control simulations (these don’t include the effects of humans, only natural variability), which they compared to simulations that included the effects of fossil fuel emissions during the 20th century.
“By using a ‘multi-model ensemble,’ we were better able to characterize decadal-scale natural climate variability, which is a critical aspect of the detection and attribution of a human-caused climate change signal. What we are trying to do is determine if the observed warming pattern can be explained by natural variability alone,” Gleckler said. “Although we performed a series of tests to account for the impact of various uncertainties, we found no evidence that simultaneous warming of the upper layers of all seven seas can be explained by natural climate variability alone. Humans have played a dominant role.”
Image Credits: Timo Bremmer/LLNL, CSIRO/Steve Rintoul