Tag: global warming hiatus

As seas exchange heat, the Indian Ocean is becoming a marine hothouse

The Wire
May 21, 2015

Since about 1998, the rate at which the Earth’s surface temperature has been becoming hotter due to anthropogenic global warming has slowed. It slipped from about 0.12 kelvin per decade since the late 1800s to about 0.05 kelvin per decade. For a time, climate deniers jumped on this statistic to refute that the burning of fossil fuels was warming the planet. However, scientists found out that such variations were due to the internal variability of the world’s climate, and that such hiatuses would occur again.

In fact, during the hiatus period the subsurface Pacific Ocean was found to have absorbed a significant amount of heat. But recent measurements of the sea’s depths have actually signalled that the temperature there is dropping, not increasing. If the Pacific Ocean had absorbed the heat from Earth’s atmosphere yet its subsurface waters were cooling, where is the heat?

As it turns out, the Pacific has been leaking it into the Indian Ocean for the last decade, via currents running along the Indonesian archipelago. A team of researchers from France and the US found that the upper 700 m of the Indian Ocean accounted for more than 70% of the global heat gain in 2003-2012.

“The model simulation and hydrographic data both showed that Indian Ocean heat content did not increase much prior to 2000,” said Sang-Ki Lee, an oceanographer at the National Oceanic and Atmospheric Administration, and lead author of the study. “We believe that the massive heat transport from the Pacific Ocean to the Indian Ocean occurred during the past decade is a fairly unusual phenomenon.” They published their findings in Nature Geoscience on May 19.

To support the measurements they made, Lee and his team simulated the warm water’s flow from the Pacific into the Indian Ocean in a computer program. “It is not clear how good the model is in simulating the features of the Indian Ocean, specially at subsurface levels,” P.N. Vinayachandran, associate professor at the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, Bengaluru, told The Wire. “The reliability of results would depend on this factor.”

Nonetheless, the hydrographic data mustered by Lee and co. and their model’s ability fit in seamlessly with the onset of the hiatus in 1998 lends a platform from which to explore the consequences of their find. According to their model, the warm water was transported into the Indian Ocean through the Indonesian Throughflow. It is a series of deep straits in the archipelago through which, due to a pressure gradient between the two oceans, 15 million cubic metres of warm water flowed per year.

The resultant cooling of the Pacific is likely to influence the El Nino southern oscillation, an anomalous heating and cooling of the Pacific’s waters off the tropical South American coast. Normally, they are about eight degrees Celsius cooler than the waters along eastern Indonesia. However, during years in which the trade winds blowing from South America to Australia and Indonesia fall off, the water along the coast of Peru becomes warmer. The result is floods in Peru and droughts in Australia and Indonesia.

The study’s authors state that the heat being pumped into the Indian from the Pacific Ocean was driven by a converse event – of the waters having becoming warmer off the coast of eastern Indonesia. Warm waters are typically nutrient-deficient and don’t support fisheries. They are also detrimental to marine ecosystems in general, greatly endangering creatures that can’t relocate to cooler waters quickly (such as coral reefs) and in turn other creatures dependent on them (like the orange-spotted filefish).

Moreover, the study reconciles the Pacific and Indian Oceans’ warming trends while also emphasizing how little we know about heat absorption by the world’s oceanic basins despite their significant effects on climate. It also raises important questions about where and how the stored heat will be released and with what socioeconomic consequences.

“It is possible that the upper ocean warmth in the Indian Ocean may be carried to the North Atlantic Ocean to increase the frequency and amplitude of Atlantic hurricanes and to accelerate the melting of Arctic sea-ice,” Lee said, but that would be on a much longer timescale.

On shorter timelines, during the Indian summer monsoon, deeper, colder water rises to the surface along the Somalian, Arabian and western Indian coasts. Lee explained: “Due to the increased upper ocean warmth the upwelling may bring warmer water to the surface releasing more water vapour to the atmosphere and thus increasing the moisture transport to India and the Southeast Asia.” This would bring more rain – and possibly floods.

Another possibility, which Lee says their team will next investigate, is the Indian Ocean transporting its heat into the Atlantic Ocean via the Agulhas current that bends around the coast of South Africa. This presents more devastating consequences because the Atlantic has already been on a warming trend since the 1950s.

Finally, the oceans aren’t bottomless either. Once a threshold is reached, they will release the heat back into the atmosphere. A study in Nature Climate Change from February this year predicted the event would be like a burst, starting from around 2020, a sustained release that would be associated with “warming across South America, Australia, Africa and Southeast Asia.” The event will also accelerate the melting of ice in the Antarctic and lead to rising sea levels.

Recent studies – including Sang-Ki Lee’s – present an important challenge. The global impact of climate change hasn’t yet visited humans the way it visited the golden toad (by wiping it out) and there is still talk by governments to reverse its impact. But with the warming hiatus predicted to end by 2020 and the Indian Ocean shown to be a prominent player in global climate variations, the world could receive a brutal preview of what life might be like at the end of this century in the next 10 years.

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.