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Don Paul: Somewhere in the middle lies slightly better news on climate change

In the many articles I’ve written concerning our warming climate, I’ve often referred to a range of scenarios and possibilities for the pace and extent of warming in the global mean. The range would extend from smaller amounts of warming to the disastrous top end. The upper end might include the melting of the Greenland ice cap, which by itself would raise global sea levels by 22 feet and inundate many of the world’s  metropolitan areas. It’s been estimated 600 million people would have to evacuate coastal regions around the world in that event.

Several times in several articles I have pointed out that the worst-case scenario, while not impossible, is highly improbable. The lowest and highest projections tend to be the least likely to verify. That is not say “smack dab in the middle” is always the way to go, but a range closer to the mean tends to have a statistically better chance of working out. Below on the lower left is what we call an ensemble plume of temperature projections out to the year 2100 as forecast by some of the many climate models now in use.

A road sign warns drivers of weather conditions in downtown Washington, D.C., Oct. 28, 2012, ahead of Superstorm Sandy's landfall. (EVA HAMBACH/AFP/Getty Images)

(Source: National Academy of Sciences)


Each model or member of that plume is based on somewhat different equations, different methodologies for using laws of physics and then utilizing fairly well understood interactions between sea, land and air to make these projections. Some models, as is the case with weather models, have had better verification scores than others over the years, though this not anything like a hit-or-miss storm track in a weather model. All have captured the predictable warming trend in varying speeds and amplitudes. But unlike weather models, climate models filter out the “noise” of day-to-day weather by necessity. They must work on a different time scale. (The argument “if they can’t predict out to 10 days, why should anyone believe predictions out to a century?” is entirely a red herring and irrelevant to climate forecasts.) Superstorm Sandy may have had ties to the warming climate and oceans, but climate models are not designed to predict individual storms. They are designed to predict the probabilities of storms like Sandy in the future. In the above plume, the range in temperature increase spans from around 1.5 degrees Celsius at a minimum to what would be a calamitous 5.5 degrees Celsius at the maximum.

A new study published in the journal Nature tends to void the worst-case scenarios of 4 or more degrees Celsius as reasonably possible, as well as the best-case scenario and lowest estimates closer to just 1.5 degrees Celsius. Lead author Peter Cox, climate professor at the University of Exeter, told “Our study all but rules out very low and very high climate sensitivities.” The most recent U.N. Intergovernmental Panel on Climate Change had established a wide range from 1.5 degrees Celsius to 4.5 degrees Celsius (2.7 degrees Fahrenheit to 8.1 degrees Fahrenheit) because of scientists' inability to nail down what the earth’s average temperature would be if CO2 doubled. Cox and his co-authors use a new methodology that narrows that range to between 2.2 degrees Celsius to 3.4 degrees Celsius with the highest probability close to 2.8 degrees Celsius, or 5 degrees Fahrenheit. Early reviews from peer scientists not involved in the study find the lower probability to very high sensitivity to CO2 increases relatively hopeful because of doubts the upper end of the original range, if realized, could be managed and mitigated to avoid broad disasters for humankind. So far, they feel the new methodology is based on sound evidence.

The Paris accords were and are still a commitment to try to cap the increase at the lower end, 1.5 degrees Celsius. We already have seen serious impacts from the increase of 1 degree Celsius, with rising sea levels, degradation of ocean plant and animal life, increased storm surges, tidal flooding in such locales as Miami and a host of other negative impacts that include greater likelihood of drought and wildfires as well as floods from increasing heavy precipitation events in recent decades. Calculating what the impacts would be at the upper end of 3.4 degrees Celsius of this smaller range still could tear at the fabric of civilization.

Due to market forces, U.S. contributions to the carbon load have gone down significantly for a number of years with power generation switching from coal to natural gas as the biggest factor in that reduction. China has begun canceling its coal-based new power plants, and India is beginning to move in that direction as well. Despite that, overall carbon emissions are still likely to significantly increase globally for years to come. There may yet be tipping points not yet reached in climate change from decreased salinity in the North Atlantic due to freshwater melt-off, which could weaken or even shut down the Gulf stream, having a drastic effect on European and eastern North American climate.

This new methodology, if it continues to stand up under rigorous peer review is better news. But better news should not be confused with good news. The median in the range of 2.8 Celsius would still spell big troubles for humankind.

For those who wish to know what predictable impacts are likely at different increased temperatures, here are the projections from the National Research Council, a branch of the National Academy of Sciences:

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