News in Science

Cooler Pacific has slowed global warming, briefly

Warming pause The recent global warming 'pause' is partly due to a naturally-occurring cyclical cooling of the Pacific Ocean, say a team of scientists, who add that the pause is unlikely to last much longer.

The slowdown in the rate of rising temperatures from the 1980s and 1990s has puzzled climate researchers because heat-trapping greenhouse gas emissions from factories, power plants and cars have hit record highs.

"We know that it is important to distinguish between human-caused and natural climate variability so we can assess the impact of human-caused climate change," says Michael Mann, a climatologist from Pennsylvania State University.

Mann and colleagues looked at a combination of real-world observational data and state-of-the-art climate model simulations to understand the competing contributions to climate variability in the Northern Hemisphere.

The scientists examined temperatures in the Pacific and Atlantic Oceans back to 1850, which showed there are natural swings in winds and currents that can last decades. Their results are published today in the journal Science .

"The North Atlantic and North Pacific oceans appear to be drivers of substantial natural, internal climate variability on timescales of decades," says Mann.

The researchers conclude that the a cooler phase in the northern Pacific Ocean and the unusual slowing of warming over the past decade are tied to heat burial beneath the tropical Pacific and a tendency for sustained La Niña type conditions.

"It appears to be the Pacific that is the main driver [of the pause]" says Mann.

"The Atlantic is a minor player right now."

Pause won't last

Given the pattern of past historical variation, this trend will likely reverse and add to human-made warming in the coming decades, the researchers say.

Even though the pace of rising temperatures has slowed, last year was the warmest since records began in the 19th century, according to the UN World Meteorological Organization.

In 2013, the U.N. panel of climate scientists said the pause in warming was due to factors including natural swings such as shifts in ocean heat, sun-dimming volcanic eruptions and a decline in solar output in an 11-year cycle.

"The slowdown in warming is probably a combination of several different factors," says Mann.