The oceans are warming up due to climate change. Scientists predict contradictory effects on the monsoon that is the lifeline of South Asia.
The Intergovernmental Panel on Climate Change (IPCC) has confirmed that the oceans of the world are absorbing much of the extra greenhouse gases – mainly carbon dioxide – that humans are emitting. In the process, the oceans are warming up significantly. What will be the effect of this on the summer monsoon on which millions of people in South Asia are dependent? Scientific studies predict different effects of warming over different oceans of the world, and this can become contradictory. An added complication in 2014 is the resurgence of the El Niño Southern Oscillation (ENSO) over the Pacific Ocean, a phenomenon that has weakened the monsoon current in the past. In their study called Temperature variability over the Indian Ocean and its relationship with Indian summer monsoon rainfall, D.R. Kothawale, A.A. Munot and H.P. Borgaonkar of the Indian Institute of Tropical Meteorology (IITM), Pune, found that there was a stronger monsoon whenever the winter sea surface temperature (SST) was high in the Arabian Sea and the spring SST was high in the Bay of Bengal and the equatorial southern Indian Ocean. On the other hand, there was a weaker monsoon whenever ENSO was strong, and the overall effect was a combination of these factors. Examining the long term trend in SSTs of the Arabian Sea, Bay of Bengal and equatorial southern Indian Ocean from 1901 to 2002 and for a subset period 1971–2002, the IITM scientists identified the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of ENSO. Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and equatorial southern Indian Ocean show a significant warming trend of 0.7°C, 0.6°C and 0.5°C per hundred years, respectively, and a relatively accelerated warming of 0.16°C, 0.14°C and 0.14°C per decade during the 1971–2002 period. Importantly, Kothawale, Munot and Borgaonkar found a “positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship.” In their paper written in 2006, the scientists said ENSO has a negative significant relationship with Indian monsoon rainfall. They postulated that there was a combined effect of ENSO and Arabian Sea SSTs on the Indian monsoon. If the ENSO effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and equatorial southern Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the ENSO effect, and correlation values are more pronounced than for the Arabian Sea. Critical year This study has become critical in 2014, with the ENSO becoming stronger, and with Working Group I of the IPCC confirming that ocean warming dominates the increase in energy stored in the climate system, and saying with high confidence that this accounts “for more than 90% of the energy accumulated between 1971 and 2010. It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.” This is due to greenhouse gas emissions induced by human activities. IPCC says “human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes… It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” Scientists still have trouble predicting the effect of this on critical weather systems like the monsoon, on which millions of Indian farmers depend. The IPCC says, “Changes in the global water cycle in response to the warming over the 21st century will not be uniform. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase, although there may be regional exceptions. The global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation.” On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by between 0.09 and 0.13° C per decade between 1971 and 2010. It is likely that at a depth between 700 and 2,000 m, the ocean warmed from 1957 to 2009. No significant changes in temperature have been observed between a depth of 2,000 and 3,000 m, but the IPCC says “it is likely that the ocean warmed from 3000 m to the bottom for this period (1995-2012), with the largest warming observed in the Southern Ocean. The warming of the top layer is important, because over 60% of the net energy increase in the climate system was found to be stored in the upper ocean (0–700 m) between 1971 and 2010, and about 30% below 700 m. What does this mean to the monsoon? The IPCC says, “Globally, it is likely that the area encompassed by monsoon systems will increase over the 21st century. While monsoon winds are likely to weaken, monsoon precipitation is likely to intensify due to the increase in atmospheric moisture. Monsoon onset dates are likely to become earlier or not to change much. Monsoon retreat dates will likely be delayed, resulting in lengthening of the monsoon season in many regions.” The IPCC – the world’s largest collective of scientists – also says ENSO will remain the dominant mode of interannual variability in the tropical Pacific, with global effects in the 21st century. Due to the increase in moisture availability, ENSO-related precipitation variability on regional scales will likely intensify. Natural variations of the amplitude and spatial pattern of ENSO are large and thus confidence in any specific projected change in ENSO and related regional phenomena for the 21st century remains low. According to the latest science, the strongest ocean warming is projected for the surface in tropical and Northern Hemisphere subtropical regions. At greater depth the warming will be most pronounced in the Southern Ocean. Best estimates of ocean warming in the top one hundred metres are about 0.6°C to 2.0°C; and about 0.3°C to 0.6°C at a depth of about 1,000 m by the end of the 21st century. Prehistoric clue So scientists are not sure of what will happen to the Indian summer monsoon as the oceans warm up, but there is a strong clue from the past. In their recent paper called A glimpse of the Quaternary monsoon history from India and adjoining seas, Rajeev Saraswat and Rajiv Nigam of the National Institute of Oceanography in Panaji and Thierry Correge of the University of Bordeaux have looked at the evolution of the Indian monsoon. Looking at terrestrial and marine records recovered from the Indian subcontinent and the adjoining seas, the scientists found that while the initiation of the monsoon is debated, a major intensification of the summer monsoon occurred around 8.2 million years ago. Then, a seasonal monsoon circulation with distinct summer and winter monsoon phases was established around 2.8 million years ago, the three researchers write in a paper published this year in the journal Palaeogeography Palaeoclimatology Palaeoecology. The records show that in the Quaternary period (from two million years ago to the present day) on earth, the Indian summer monsoon weakens during glacial periods as compared to interglacial periods. The glacial terminations are marked by weak monsoon activity. The world is nowhere near a glacial termination period, but it is moving in that direction due to human-induced climate change. There is reason to worry.