Humans may have been interacting with oceans for thousands of years but our understanding of these vast water bodies – which cover nearly 70 per cent of Earth’s surface – is rather limited. Studies in the recent past have tried to ascertain the impacts of human induced climate change on oceans, but these studies were limited in their scope and not in a position to give the big picture.

One of the major difficulties in assessing the impact of global warming on oceans were complexities involved in distinguishing the influence of human activities from that of natural events. For instance, just what per cent of the rising ocean temperature could be attributed to human activities has been a contentious issue, even though most agreed that oceans have been getting warmer over decades.

Last week an international team, which includes a scientist from the Indian Institute of Technology (IIT), Delhi, conclusively proved that human activities have been the main reason for ocean temperatures rising in the last 50 years. This understanding is critically important for modelling the impacts of climate change correctly, says co-author Krishna Achuta Rao, climate scientist at IIT Delhi. It helps remove many uncertainties associated with futuristic projections, he says.

Ocean modelling studies have been beset with many problems. First, data from all oceans is not uniformly available. Besides, many instruments were faulty and as a result were feeding in incorrect data. These hampered reliable modelling studies.

Although other studies have indicated that human activities are responsible for ocean warming, this is the first to examine in depth how observational and modelling uncertainties hamper the conclusion that humans are primarily responsible.

"We have taken a closer look at factors that influence these results," says Peter Gleckler, climate scientist with the Lawrence Livermore National Laboratory in the US and lead author of the study, in a statement. "This study substantially strengthens the conclusion that most of the observed global ocean warming over the past 50 years is attributable to human activities," says Gleckler.

The group looked at the average temperature (or heat content) in the upper layers of the ocean. The observed global average ocean warming (from the surface to 700 metres) is approximately 0.025 degrees Celsius per decade, or slightly more than 1/10th of a degree Celsius over 50 years.

Ocean heat content data is critically important. According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), thermal expansion of seawater, which is directly linked to ocean heat content, contributes to 70-75 per cent of all ocean related projections. It is crucial in deciding the expected rise in sea level, fate of fisheries and how much carbon dioxide will be absorbed by the oceans. Since there was no clear understanding that how much of the ocean heat came from human activities, IPCC, in its last assessment report, gave an ambiguous projection of sea level rise. It said the rise in sea level due to climate change by 2099 is expected to be between 0.19 metres to 0.59 metres under different projected scenarios. Scientists hope that studies like the latest one, reported last week in the journal Nature Climate Change, will help climate scientists make more accurate projections.

“What we are trying to do is determine if the observed warming pattern can be explained by natural variability alone,” says Achuta Rao.

Although they performed a series of tests to account for the impact of various uncertainties, they found no evidence that simultaneous warming of the upper layers of all seven seas can be explained by natural climate variability alone. This shows, humans have played a dominant role, he says.

Madhavan Rajeevan Nair, an advisor in the ministry of earth sciences in New Delhi, agrees that this is a significant study. He thinks that a better understanding of heat content of the ocean is not only critical to comprehend the impact of climate change as a whole, but also to ascertain how major global wind circulations, such as monsoon winds, will behave in coming decades.

Is that why the Indian monsoon is getting later every year?