Climate Sensitivity

Man-made Global Warming Likely to Be Moderate, Says Study

Alternatively: model-predicted catastrophic warming is unlikely



In his Earth Day proclamation last week, "The Warning to Humanity," California Gov. Jerry Brown (D) declared:

Increasing levels of gases in the atmosphere from human activities, including carbon dioxide released from fossil fuel burning and from deforestation, may alter climate on a global scale. Predictions of global warming are still uncertain–with projected effects ranging from tolerable to very severe–but the potential risks are very great.

Perhaps the governor and the rest of us can take a bit of comfort from a new study published the day before his proclamation by researchers at Duke University. They found that future temperature increases due to man-made warming are tending in the tolerable direction. That study compared climate model outputs with model outputs primed with empirical temperature data from over the past 1,000 years. As the Duke University press release, "Global Warming More Moderate Than Worst-Case Models," notes:

A new study based on 1,000 years of temperature records suggests global warming is not progressing as fast as it would under the most severe emissions scenarios outlined by the Intergovernmental Panel on Climate Change (IPCC).  

"Based on our analysis, a middle-of-the-road warming scenario is more likely, at least for now," said Patrick T. Brown, a doctoral student in climatology at Duke University's Nicholas School of the Environment. "But this could change."

The Duke-led study shows that natural variability in surface temperatures—caused by interactions between the ocean and atmosphere, and other natural factors—can account for observed changes in the recent rates of warming from decade to decade.   

The researchers say these "climate wiggles" can slow or speed the rate of warming from decade to decade, and accentuate or offset the effects of increases in greenhouse gas concentrations. If not properly explained and accounted for, they may skew the reliability of climate models and lead to over-interpretation of short-term temperature trends. …

To test how accurate climate models are at accounting for variations in the rate of warming, Brown and Li, along with colleagues from San Jose State University and the USDA, created a new statistical model based on reconstructed empirical records of surface temperatures over the last 1,000 years.

"By comparing our model against theirs, we found that climate models largely get the 'big picture' right but seem to underestimate the magnitude of natural decade-to-decade climate wiggles," Brown said. "Our model shows these wiggles can be big enough that they could have accounted for a reasonable portion of the accelerated warming we experienced from 1975 to 2000, as well as the reduced rate in warming that occurred from 2002 to 2013."  

Further comparative analysis of the models revealed another intriguing insight.

"Statistically, it's pretty unlikely that an 11-year hiatus in warming, like the one we saw at the start of this century, would occur if the underlying human-caused warming was progressing at a rate as fast as the most severe IPCC projections," Brown said. "Hiatus periods of 11 years or longer are more likely to occur under a middle-of-the-road scenario."

Yes indeed, the science is settled.