A new analysis of climate change data confirms the world's oceans have warmed over the past 50 years, but not because of natural events alone. While previous studies have linked ocean warming to human activities, the new research uses a larger number of climate models and an improved set of observational data to reach the same conclusion.
Peter Gleckler is a climate scientist at Lawrence Livermore National Laboratory in California and the lead author of the study. Based on the new analysis, he says, the only way to explain the ocean warming is by factoring in the effects of increased greenhouse gas emissions over the past half century.
“If we don’t do it, then it is highly unlikely that we can explain this rather significant warming just by natural variability alone. And that’s really what this work is about, is to try and separate out what we call a signal, this warming signal, from natural variability.”
The scientists ran tests designed to highlight natural swings in the Earth’s climate system.
Gleckler says the computer simulations for these natural variations covered thousands of years of climate history.
“That’s much longer than what we have at the observational record. So we can look at the variability and how it changes over many decades and hundreds of years and how it differs from one model to the next. But when we factor all that in together, pretty much no matter how we slice it, we still get this result, the ‘signal’ is so robust."
What they found was that natural variations accounted for only about 10 percent of the oceans’ warming. The rest was due to rising concentrations of heat-trapping greenhouse gases, such as carbon dioxide, in the earth’s atmosphere. And most of that extra heat, they found, has been absorbed by the oceans.
The average temperature in the upper layers of the ocean, from the surface down to 700 meters, has warmed one-tenth of one degree Celsius over the past 50 years. Gleckler says while that may not seem like much, the increase is quite significant.
“It could have impacts on the circulation of the ocean, possibly on the circulation of the atmosphere as well.”
Gleckler says the simultaneous warming of the upper layers of all seven seas cannot be explained by natural climate variability alone. The inescapable conclusion, he says, is that human activities - specifically greenhouse gas emissions from our cars, our buildings and our power plants - have played a major role.
The findings are reported in the journal Nature Climate Change.