The Skeptical Science study finding a 97 percent expert consensus that humans are causing global warming led readers to ask several related climate questions. For example, how does the consensus jibe with claims that global surface warming has slowed over the past 10 to 15 years?
Accelerated ocean and global warming
The answer to this question lies in recent research into ocean warming. Only about 2 percent of global warming is used in heating the Earth’s atmosphere, while about 90 percent heats the oceans. Several recent studies, including one that I led last year along with the Skeptical Science team and oceanographer John Church, have shown that the warming of the oceans and thus the planet as a whole has accelerated over the past 10 to 15 years.
Most recently, a study published in the journal Geophysical Research Letters examined ocean temperature records made by the HMS Challenger in the 1870s. Their analysis confirmed that there has been an immense amount of energy absorbed by the oceans as a result of human-caused global warming.
Their estimated accumulation of ocean heat is equivalent to around 10 billion Hiroshima atomic bomb detonations over the past 140 years, or an average of about 2 detonations per second since the 1870s. Half of that energy has accumulated in the oceans since 1970, at an average rate of about 4 atomic bomb detonations per second due to accelerated global warming. Peter Sinclair of Climate Crocks interviewed climate scientist Kevin Trenberth about the significance of the recent acceleration in ocean and global warming.
As Dr. Trenberth noted in that interview, global warming has also acted to accelerate global ice loss. For example, this video created by Andy Lee Robinson shows that Arctic sea ice has lost an astonishing three quarters of its volume over just the past three decades.
Recent research has also shown that while the ocean has more efficiently absorbed heat over the past decade, acting to temporarily slow the warming we experience at the surface, this trend will probably reverse in the near future. We simply can’t rely on the oceans to indefinitely slow global surface warming.
How much warming are humans causing?
In response to the 97 percent consensus survey, some readers also asked exactly how much of the observed global warming is due to human activities. In our survey of the climate science researchers, of the 237 papers that took a position on the amount of human-caused warming, over 96 percent responded that humans have caused at least half of the warming over the past 50 years. This is actually a conservative estimate, since the research estimating the various contributions to global warming almost universally puts the human contribution at close to 100 percent over the past 50 years.
Another new study published in the Journal of Geophysical Research investigated this question by testing various different models for the Earth’s natural internal climate variability, to see if it could possibly account for a significant amount of recent global warming. Combining their approach with several other recent studies into the causes of global warming, they found it could not. Human activity was found to be responsible for between about 70 and 100 percent of the observed global surface warming over the past 30 to 60 years.
A paper falling in the 2–3 percent of peer-reviewed research disputing this conclusion was also recently published in a rather obscure physics journal, the International Journal of Modern Physics B. Led by the University of Waterloo’s Qing-Bin Lu, the paper suggested that chlorofluorocarbons (CFCs – which are responsible for the hole in the ozone layer, and are also greenhouse gases) could explain most of the observed global warming, rather than carbon dioxide.