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Friday, April 24, 2015

Duke Study: Global Warming Is Not “Worst Case Scenario”

What the "pause" means for climate change

A new study from climate researchers at Duke University suggests that the worst-case scenarios for global warming — the scenarios that see the most severe and rapid increases in surface temperatures over the next century — are unlikely to be accurate.

Instead, observed data indicates that warming is on track to be more moderate and gradual.

“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.

The research, published today in the peer-reviewed journal Scientific Reports, uses empirical data, rather than the more commonly used climate models, to estimate decade-to-decade variability.

One of these “wiggles” or short-term trends is the much-reported “pause,” slowdown, or hiatus in the warming trend from 2002-2013 (last year did show some warming). Most climate temperature models failed to predict this, leading to a lot of controversy about where “missing heat” went (the deep oceans was one plausible theory, since contradicted by a study from NASA) and whether “global warming has stopped.”

To see if current climate models could account for such variations, the researchers created their own model based on empirical temperature records from 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.”

In other words, the climate’s natural variability, not related to changes in “forcing” — i.e., changes in solar radiation, greenhouse gases, etc. — can account for the last decade’s unusually flat trend, as well as the previous three decades’ unusually steep rise in temperatures. The empirical record shows that natural sources of variability can suppress or exacerbate temperature changes in the short-term and explain much of the deviations in temperature trends in the 20th and 21st centuries.

The upshot of this appears to be twofold. First, global warming has not stopped, at least not permanently. We are likely to see the resumption of the upward trend. Most of the warming trend last century occurred in this step-wise pattern, increasing from 1900-1950, flat-lining until 1975, and then increasing until about 2000 before flat-lining again. What this paper suggests is that such variation is consistent with current understanding of how climate changes.

Second, decade-long pauses in warming are more likely under “middle-of-the-road” scenarios, not the worst-case runaway warming projections.

 “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.”

Under the IPCC’s middle-of-the-road scenario, there was a 70 percent likelihood that at least one hiatus lasting 11 years or longer would occur between 1993 and 2050, Brown said.  “That matches up well with what we’re seeing.”

Figure 3(e) from the paper shows the probability of flat or “negative” trends under three of the IPCC’s “Representative Concentration Pathways” (essentially estimates of the increase in energy trapped by the atmosphere by 2100).

RCP 4.5 (the green line) is a more moderate warming scenario, RCP 6.0 (blue line) is the middle-of-the-road scenario, and RCP 8.5 (black line) is the most extreme warming projection in the IPCC report.

Based on the Duke researchers’ empirical model, there is a 50% probability of at least one 11-year pause between 1993 and 2050 under the models that lead to RCP 4.5, a 70% probability under RCP 6.0, but only about a 30% probability under the worst-case RCP 8.5 scenarios. 

The hiatus is more consistent with mid-range projections of the pace of warming, suggesting that we likely live in a world where climate change is not occurring particularly fast. The moderate and middle of the road scenarios are estimated to produce between 1.1 – 3.1°C of warming this century, which is not insignificant but not necessarily catastrophic, either.

More on global warming in the Freeman this month:

  • Daniel Bier is the executive editor of The Skeptical Libertarian.