Global warming is progressing slower than we thought, at least compared to the most severe emissions scenarios outlined by the Intergovernmental Panel on Climate Change (IPCC), according to a new study.
"Based on our analysis, a middle-of-the-road warming scenario is more likely, at least for now," researcher Patrick T. Brown, a doctoral student in climatology at Duke University, said in a statement. "But this could change."
The new findings, based on 1,000 years of temperature records, 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.
Scientists refer to this natural variability as "climate wiggles," which can slow down or speed up the rate of global warming as time passes. They can even accentuate or offset the effects of increases in greenhouse gas concentrations - which saw a record high in 2013.
If scientists don't properly explain and account for these climate wiggles, they may skew the reliability of climate models and lead to over-interpretation of short-term temperature trends.
So to better and more accurately understand the future of global warming, a team at Duke University relied on empirical data, rather than the more commonly used climate models, to estimate decade-to-decade variability.
"At any given time, we could start warming at a faster rate if greenhouse gas concentrations in the atmosphere increase without any offsetting changes in aerosol concentrations or natural variability," explained Wenhong Li, who was involved in the study. (Scroll to read on...)
The researchers examined whether climate models, such as those used by the IPCC, accurately account for natural variability that can occur in the rate of global warming. They tested their accuracy with 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."
During this global warming "pause," or hiatus, the average surface temperature on Earth barely increased, and caused some people to question the reality of global warming. But this and other research has cited natural variability as reasons for the slowdown, including factors like natural cooling fluctuations, solar irradiance and heat uptake by the oceans.
And now these latest findings add another intriguing insight into this phenomenon.
"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."
In fact, under the IPCC's middle-of-the-road scenario, there was a 70 percent chance that at least one hiatus lasting 11 years or longer would occur between 1993 and 2050.
"That matches up well with what we're seeing," Brown said.
However, it should be noted that these results do not suggest that global warming is letting up, and that this rate of warming will remain steady in coming years. Warming could very much increase in the years to come.
"Our analysis clearly shows that we shouldn't expect the observed rates of warming to be constant," Li warned. "They can and do change."
The results were published in the journal Scientific Reports.
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