The global warming hiatus could last another five years. Its aftermath is the real problem.

Whether you’ve been fending off climate-change skeptics on Twitter or have been looking for reasons to become a climate-change skeptic yourself, you must’ve heard about the hiatus. It’s the name given to a relatively drastic drop in the rate at which the world’s surface temperatures have increased, starting since the late 1990s, as compared to the rate since the early 1900s. Even if different measurements have revealed different drops in the rate, there’s no doubt among those who believe in anthropogenic global-warming that it’s happening.

According to one account: between 1998 and 2012, the global surface temperature rose by 0.05 kelvin per decade as opposed to 0.12 kelvin in the decades preceding it, going back to the start of the previous century. To be sure, the Earth has not stopped getting warmer, but the rate at which it was doing so got turned down a notch for reasons that weren’t immediately understood. And even as climate-scientists have been taking their readings, debate has surged about what the hiatus portends for the future of climate-change.

Now, a new study in Nature Climate Change has taken a shot at settling just this debate. According to it: The chances that a global-warming hiatus will happen for 10 consecutive years is about 10%, but that it will happen for 20 consecutive years is less than 1%. Finally, it says, if a warming hiatus has lasted for 15 years, then the chances it will last for five more years could be as high as 25%. So that means the current letoff in warming is somewhat likely to go on till 2020.

The study was published on February 23, titled pithily, Quantifying the likelihood of a continued hiatus in global warming. It focuses on the effects of internal variability, which – according to the IPCC – is the variability due to internal processes in the climate system (such as the El Niño Southern Oscillation) and excluding external influences (such as volcanic eruptions and sulphate aerosol emissions).

At the least, the statistically deduced projections empower climate scientists by giving them a vantage point from which to address the slowdown in warming rates since the start of this century. But more significantly, the numbers and methods give observers – such as those in the government and policy-makers – a perspective with which to address a seeming anomaly that has come at a crucial time for tackling anthropogenic global warming.

Global mean land-ocean temperature index from January 1970 through January 2014. The colored line is the monthly mean and the black line is the five-year running mean. The global warming hiatus referenced in literature commonly starts circa 2000.
Image: DHeyward/CC-BY-SA 3.0

Its timing (as if it could be timed) was crucial because it coincided with the same decade in which most of the faster-growing economies on the planet were circling each other in the negotiation bullring, wanting to be perceived as being committed to protecting the environment while reluctant about backing down on growth-rate reforms. The slowdown was a not-insurmountable-yet-still-there stumbling block to effectively mobilizing public debate on the issue. Needless to say, it also made for fodder for the deniers.

Wryly enough, the Nature Climate Change study shows that it is not an anomaly that’s about to let anybody off the hook but a phenomenon actually consistent with what we know about internal climate variability, and that such an event though rare could last two full decades without defying our knowledge. In fact, throw in coincident external variability and we have the additional possibility of longer and stronger hiatus periods in reality.

Anyway, there is yet more cause for alarm with this assertion because it suggests that some natural entity – in this case the sub-surface Pacific Ocean – is absorbing heat and causing the hiatus. Once a threshold is reached, that accumulated heat will be released in a sustained burst of about five years. The study’s authors term this the period of ‘accelerated warming’, when the oceans release 0.2 W/m2 of energy in “a pattern … that approximates a mirror image of surface temperature trends during hiatus periods”.

The analysis was based on data obtained from the Coupled Carbon Cycle Climate Model Intercomparison Project (Phase 5), which assesses changes in the climate due to changes in the carbon cycle in the presence of external variables. And simulations using it helped the researchers highlight a worrying discrepancy from previous predictions for the Arctic region:

Hiatus decades associated with internal variability in models generally exhibit cooling over the Arctic whereas recent observations indicate a strong warming. Our results indicate that, following the termination of the current global warming hiatus, internal climate variability may act to intensify rates of Arctic warming leading to increased climate stress on a region that is already particularly vulnerable to climate change.

The Arctic isn’t the only region that’s in trouble. The authors also predict that the period of accelerated warming will be “associated with warming across South America, Australia, Africa and Southeast Asia”. This doesn’t bode well: developing nations have been found to be especially susceptible to the adverse effects of anthropogenic warming because of their dependence on agriculture and for being under-prepared for catastrophic weather events.

Even if climate talks are beginning to focus on goals for the post-2020 period, this predicted asymmetry of impact won’t be at the top of negotiators’ minds at the 21st annual Conference of the Parties to the UNFCCC in Paris on November 30. However, should it transpire, the slowdown-speedup tendencies of climate variability could further muddle negotiations already fraught with shifting alliances and general bullheadedness.