It's called a "global warming hiatus" and it's been a puzzle to scientists. If earth is indeed warming up, then why have the temperatures of earth's surface remained relatively steady for the last 15 years despite increasing emissions of carbon dioxide? Some climate change skeptics pointed to this warming hiatus as evidence that global warming is not real.

Scientists have debated the cause of the hiatus. Could it be the result of a prolonged period of reduced solar activity? Or a cooling of the tropical Pacific Ocean due to a global weather pattern known as La Niña?

Researchers now say they have identified another possible cause: According to a study in this week's issue of the journal Nature Geoscience, led by climatologist Francisco Estrada of the National Autonomous University of Mexico and Vrije Universiteit in Amsterdam, the global warming pause is an unintented consequence of an international ban in the late 1980s against chlorofluorocarbons, or CFCs, chemicals that were destroying the Earth's ozone layer.

Cause And Effect

In their study, Estrada and his team used a statistical method to match sharp declines in global temperature with reductions in greenhouse gases during different periods of the 20th century.

The team then looked for specific human actions that could have affected the concentration of greenhouse gases—and thus global warming.

One underlying cause that the team identified was the Montreal Protocol, an international treaty signed in 1989 that phased out CFCs, which not only harm the ozone layer but are also potent greenhouse gases.

While curbing greenhouse gases was not a focus of the Montreal Protocol, the preamble of the treaty did state that its signers are "conscious of the potential climatic effects of emissions" of CFCs.

The signers "recognized [then] that they may also have an impact on climate," Estrada said in an email interview.

Felix Pretis, a statistician at the University of Oxford in the UK, cautioned that the Montreal Protocol alone can't account for the full magnitude of reduced warming seen since 1998.

"While it has contributed to the warming hiatus, it does not entirely explain it," Pretis said.

The full explanation of the warming hiatus is unlikely to be a single issue, scientists say, and most likely has involved other factors such as natural climate variability and a pause in the increase of atmospheric methane, another potent greenhouse gas. Scientists don't fully understand the methane matter but think it may be related to changes in agricultural practices.

Not The First Time

The current warming hiatus is not without precedent. The scientists also identified slowdowns in global warming during the two world wars and the Great Depression in the 1930s, when greenhouse gas emissions were in decline due to global economic downturns.

Their analysis further revealed a pronounced rise in both temperature and greenhouse gas concentrations in the 1960s, which marks the onset of sustained global warming.

The new study is the latest evidence the rate of global warming is tightly coupled with human activity, noted Oxford's Felix Pretis.

And Estrada is hopeful that the new findings will help spur steps to mitigate the effects of climate change, leading to "actions ... to reduce greenhouse gases more broadly, in particular [carbon dioxide]."