Earth could break through a major climate threshold in the next 15 years, scientists warn

https://www.washingtonpost.com/news/energy-environment/wp/2017/05/09/earth-could-break-through-a-major-climate-threshold-in-the-next-15-years-scientists-warn/?utm_term=.121fac48ddb9

By Chelsea Harvey May 9, 2017

Global temperatures could exceed 1.5 degrees Celsius above their preindustrial levels within the next 15 years, according to a new scientific study, crossing the first threshold under the Paris climate agreement and placing the world at a potentially dangerous level of climate change.

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The study focuses on a natural planetary system known as the Interdecadal Pacific Oscillation, or IPO (it’s also sometimes referred to as the Pacific Decadal Oscillation). It’s an alternating pattern of ocean temperatures that shifts periodically between warm and cool phases, helping to drive temperature and weather patterns all over the world.

During cool, or “negative,” phases, tropical regions of the Pacific Ocean tend to be colder, and the global mean temperature is lower. The system is similar to the El Niño/La Niña cycle, the major difference being that phases of the IPO tend to last much longer — sometimes a decade or more. The phenomenon is believed to be a natural form of climate variability unrelated to human-caused climate change, although it does have the potential to influence the progression of global warming.

For most of the 2000s, the IPO has been in a negative phase, and scientists think its cooling effect has helped to slightly offset the effect of climate change, an explanation for the so-called global warming pause in the first part of the 21st century. As multiple studies have pointed out, this temporary slowdown is consistent with the overall long-term warming trend and in no way suggests that human-induced climate change is not occurring. Rather, this natural variation in the global climate helped to slightly blunt those effects.

Many scientists believe that the planet is now transitioning back into a positive, or warm, phase, which could amplify, rather than offset, human-caused climate warming. This means we could reach milestone temperature thresholds faster than we would if the IPO had remained in its negative phase.

That’s the conclusion of the new study, written by Henley and Andrew King of the University of Melbourne. Using model projections of future climate warming under a business-as-usual scenario, they suggest that the Earth could hit the 1.5-degree temperature threshold as early as 2025, while the continuation of the negative phase probably would delay this event until after 2030.

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And the 2025 date for hitting the 1.5-degree temperature threshold is looking more and more likely. Multiple studies in the past few years suggest that the transition to a positive IPO phase has begun. Henley said there’s some uncertainty about whether that has happened, but other scientists are more confident. Scientists John Fasullo and Kevin Trenberth, also of the National Center for Atmospheric Research, have published research to this effect, and both told The Post that we have been in a positive phase for several years now.

In fact, Trenberth and Fasullo suggested that the paper’s conclusions have been generally known for some time.

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Trenberth said that the 1.5 degrees — as a single, concrete number — is “pretty irrelevant.” He noted that “it is all of the other things going on when that stage is reached that really matter: the heat waves, wildfires, droughts, extreme rainfalls, etc.”

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“I guess the important thing is that policymakers should be aware of just how quickly we are approaching 1.5 degrees, and just realizing the urgency of reducing emissions,” Henley said. “It’s critical to keep pursuing the 1.5-degree goal.”