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As ocean cooling reverses, the planet will begin warming once more. Michael Seljos

Warming slowed by cooling Pacific Ocean

The cooling of eastern Pacific Ocean waters has been counteracting the warming effect of greenhouse gases. Our research, released today in Nature, shows this natural variability in ocean cycles is responsible for the “hiatus” in global warming over the last ten years or so.

The study considers the tropical Pacific Decadal Oscillation, a climate cycle that plays out over the course of several decades. Within this large pattern fall El Niño and La Niña, well-known faster cycles that cause shifts in the distribution of warm water in the equatorial Pacific Ocean.

While El Niño and La Niña last only a few years, the Pacific Decadal Oscillation lasts several decades. The Oscillation has been in a cooling phase since 1998.

When the climate cycle that governs that ocean cooling reverses and begins warming again, the planet-wide march toward higher temperatures will resume with vigor. The study does not consider when the reversal might happen, but it brings scientists closer to understanding how to look for signs of it.

If researchers can estimate that climate cycle, they could also better estimate the end date of regional trends that are linked to ocean cooling, such as the drought in the southern United States that have caused an estimated tens of billions of dollars in agricultural damage since 2000.

As carbon dioxide levels in Earth’s atmosphere increase, global temperatures have been rising. Before 2000, global temperatures had risen at a rate of 0.13C per decade since 1950. The hiatus in warming has happened while levels of carbon dioxide, the main greenhouse gas produced by human activities, continue rising steadily. In May 2013, carbon dioxide reached 400 parts per million in the atmosphere for the first time in human history.

The disconnect led some climate watchers to speculate that increases in the concentration of carbon dioxide are not as strongly coupled to global warming as thought, even though the heat-trapping properties of carbon dioxide have been identified for more than a century.

We concluded, however, that natural variability in the form of eastern Pacific Ocean cooling is behind the hiatus. We used computer modeling to simulate regional patterns of climate anomalies. This enabled us to see global warming in greater spatial detail, revealing where it has been most intense and where there has been no warming or even cooling.

Climate models consider anthropogenic forcings such as greenhouse gases and tiny atmospheric particles known as aerosols, but they cannot reproduce a specific climate event like the current hiatus. We devised a new method for climate models to take equatorial Pacific ocean temperatures as an additional input. Then, amazingly, our model can simulate the hiatus well.

Specifically the model reproduced the seasonal variation of the hiatus, including a slight cooling trend in global temperature during northern winter season. In summer, the equatorial Pacific’s grip on the northern hemisphere loosens, and the increased greenhouse gases continue to warm temperatures, causing record heat waves and unprecedented Arctic sea ice retreat.

The last cooling phase in the tropical Pacific Decadal Oscillation – cooling waters in the eastern equatorial Pacific Ocean – lasted from roughly 1940 to the early 1970s. During that cool phase, warmer, drier weather dominated in the midwestern United States.

The current cooling phase began just after a strong El Niño year in 1998. The Texas AgriLife Extension Service at Texas A&M University estimates that the drought it traces to that same year caused nearly US$21 billion in agricultural losses through 2011. In 2011, Texas set a single-year record with losses of US$7.62 billion.

We do not know if the current cooling phase will last as long as the last one. Predicting equatorial Pacific conditions more than a year in advance is beyond the reach of current science. But we know that over the timescale of several decades, climate will continue to warm as we pump more greenhouse gases into the atmosphere.

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