Ensemble mean modelled global temperature before and after correction for the Interdecadal Pacific Oscillation (IPO) is applied. Graphic: Henley and King, 2017 / Geophysical Research Letters

By Chelsea Harvey
9 May 2017

(The Washington Post) – 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.

The report comes as climate agreement participants are watching the United States — where the Trump administration is debating whether to withdraw from the Paris accord — and as scientists with the Intergovernmental Panel on Climate Change are working on a special report about the 1.5-degree goal (equivalent to 2.7 degrees Fahrenheit) and the consequences of overshooting it.

That IPCC’s upcoming special report and the increasing urgency about minimizing global warming were one impetus for the study, according to co-author Benjamin Henley, a research fellow at the University of Melbourne in Australia. “We are working on a number of scientific avenues to help inform that report,” he told The Washington Post.

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. […]

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. [more]

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


ABSTRACT: Global temperature is rapidly approaching the 1.5°C Paris target. In the absence of external cooling influences, such as volcanic eruptions, temperature projections are centered on a breaching of the 1.5°C target, relative to 1850–1900, before 2029. The phase of the Interdecadal Pacific Oscillation (IPO) will regulate the rate at which mean temperature approaches the 1.5°C level. A transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026. If the Pacific Ocean remains in its negative decadal phase, the target will be reached around 5 years later, in 2031. Given the temporary slowdown in global warming between 2000 and 2014, and recent initialized decadal predictions suggestive of a turnaround in the IPO, a sustained period of rapid temperature rise might be underway. In that case, the world will reach the 1.5°C level of warming several years sooner than if the negative IPO phase persists.

SIGNIFICANCE: Global temperature is rapidly approaching the 1.5°C Paris target. In this study, we find that in the absence of external cooling influences, such as volcanic eruptions, the midpoint of the spread of temperature projections exceeds the 1.5°C target before 2029, based on temperatures relative to 1850–1900. We find that the phase of the Interdecadal Pacific Oscillation (IPO), a slow-moving natural oscillation in the climate system, will regulate the rate at which global temperature approaches the 1.5°C level. A transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026. If the Pacific Ocean remains in its negative phase, however, the projections are centered on reaching the target around 5 years later, in 2031. Given the temporary slowdown in global warming between 2000 and 2014, and recent climate model predictions suggestive of a turnaround in the IPO, a sustained period of rapid temperature rise might be underway. In that case, the world will reach the 1.5°C level of warming several years sooner than if the negative IPO phase persists.

Trajectories toward the 1.5°C Paris target: Modulation by the Interdecadal Pacific Oscillation

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