Methane, other short-lived greenhouse gases cause long-term sea-level rise: study

In 2016, we were told that if global warming exceeds 2⁰C by 2100, about 80 per cent of global coastline could see a 6-ft rise in sea levels. At the same time, we were cautioned about the possibility of global temperature witnessing 2.9°C to 3.4°C rise even if Paris climate deal is fully implemented. Less than fortnight into 2017, a new study has now found out that short-lived greenhouse gases (GHGs), especially methane, contribute to sea-level rise over much longer time than their atmospheric lifetimes.

As the global policies are geared towards reducing climate change impacts, it is important to understand how mitigating different gases can contribute to this goal. In a joint study done by scientists at the Department of Geography, Simon Fraser University in Canada and the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology in the US, it was revealed that reducing emission of anthropogenic greenhouse gases with short lifespan can help limiting warming.

Methodology and findings

While we express concern over earth's atmospheric carbon dioxide levels crossing the 400 ppm mark, less attention has been paid on the impacts of short-lived GHGs on long-term sea-level rise. Some of these GHGs, including methane, have atmospheric lifetimes of decades or less, whereas carbon dioxide remains in the atmosphere for centuries.

In order to substantiate its claim that GHGs like methane contribute to sea-level rise through thermal expansion (TSLR), the study cited an example: at least half of the TSLR due to increases in methane is expected to remain present for more than 200 years, even if anthropogenic emissions cease to exist permanently. This is despite the fact that the atmospheric lifetime of this gas is 10 years.

Hence, though the atmospheric warming may not last more than a decade after emissions are halted, their effects in the oceans are much longer-lived. “The ocean never forgets—that’s the essential message of this paper,” said Susan Solomon of Massachusetts Institute of Technology.

Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HFCs) have been recently phased out under the Montreal Protocol. The study claims that if these gases were phased out in 2050, additional TSLR of up to about 14 cm would have been a possibility in the 21st century with the impact lasting more than 500 years.

The researchers used a climate model to understand the effect of various greenhouse gases on thermal expansion in the oceans. They applied a “business-as-usual” scenario (assuming high emissions in future) to emissions of carbon dioxide, methane, nitrous oxide and various halocarbons.

According to the researchers, when heat goes into the ocean from the atmosphere, it tends to get caught up in overturning circulation— warm water flows from the equator to the poles and sinks to the bottom of the ocean before joining a stream of cooler water flowing back toward the equator. In the process, the heat is carried all over the planet. It can take hundreds of years before the heat from the water is released back into the air. 

While it may take another decade to detect the rate of sea-level rise, the findings of the new study can help policymakers frame an action plan to reduce emissions of short-lived GHGs that could mitigate additional sea-level rise.

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