Climate Change

Oceans cool the planet by releasing short-lived halogens that contribute 8-10 per cent of cooling: Study

Cooling by halogens could increase to 18-31 per cent by 2100 which has not been accounted yet by climate models

By Rohini Krishnamurthy
Published: Thursday 29 June 2023
Photo: iStock_

Oceans do more than just absorbing carbon dioxide and moderating the climate. They also cool the planet by releasing short-lived halogens such as chlorine, bromine and iodine, a new study has found.

Currently, these halogens contribute 8-10 per cent of cooling. This could increase to 18-31 per cent by 2100, the study published in Nature projected.

Climate models that make future projections do not account for this. “We are showing these short-lived halogens cause a substantial impact and this cannot be ignored,” Anoop S Mahajan from the Indian Institute of Tropical Meteorology, told Down To Earth.

Short-lived halogens, which have a lifetime of less than six months in the atmosphere, are naturally produced by the oceans. 

However, human activities have amplified their release into the atmosphere. Mahajan explained that human activities cause pollutants such as ozone to deposit on the ocean, which then convert the soluble short-lived halogens into insoluble ones, forcing them out of the sea water and into the atmosphere. 

The team wanted to study the role that ocean plays in controlling the climate. Halogens cause a depletion of ozone in the troposphere. Ozone is a greenhouse gas that traps outgoing radiation, leading to warming.

The team used models to quantify how short-lived halogen impact the global radiative balance, which includes solar radiation emitted by the sun and terrestrial radiation released by the Earth’s surface and atmosphere. They studied to the global energy balance across pre-industrial, present-day and future climates.

They calculated that short-lived halogens from oceans reduces warming by depleting ozone. Its cooling effect was -0.24 ± 0.02 Watts per square metre (W m−2).

However, their effect on methane is opposite. Short-lived halogens increase methane’s lifetime in the atmosphere by destroying hydroxyl radicals (OH), explains Mahajan. OH is a sink as it is known to break down this greenhouse gas.

These short-lived halogens increased global methane burden by 14 per cent and 9 per cent for pre-industrial and present-day conditions, respectively. This leads to a warming effect of 0.09 ± 0.01 W m−2 of warming. 

Similarly, these halogens increase the levels of water vapour, a greenhouse gas, in the atmosphere, causing a warming effect of 0.011 ± 0.001 W m−2.

These short-lived halogens reduce the formation of cooling aerosols, which are minute particles suspended in the atmosphere that reflect sunlight. It causes a small warming of 0.03 ± 0.01 W m−2.

Though these halogens drive an increase in warming by influencing methane, water vapour and aerosols, they compensate this by destroying ozone, which exerts a cooling effect, Mahajan explained. Overall, the net cooling effect was found to be −0.13 ± 0.03 W m−2. 

Halogen emissions from the ocean are not the same across the world. “Over continents, the emissions are small while it is bigger in polar regions and some places with higher ozone levels,” Mahajan said.

The cooling by chlorine, bromine and iodine since the preindustrial era has risen by −0.05 ± 0.03 watts per square metre, which represents a 61 per cent spike. “This is driven by the anthropogenic amplification of natural halogen emissions,” the researchers wrote in their study.

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