Hopes that future efforts to extract excess carbon dioxide from the atmosphere could spare the planet the worst impacts of climate change have been dimmed by a new study finding the acidification of oceans could take centuries to reverse.
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The world's oceans have already become about 30 per cent more acidic since pre-industrial times as seas absorb about one quarter or more of the excess carbon dioxide, triggering a chemical reaction. Combined with heat stress caused by warming waters, the rising acidity levels are already affecting complex ecosystems from plankton to shellfish and corals.
Researchers at Germany's Potsdam Institute for Climate Impact Research examined the prospects for massive geo-engineering efforts to remove CO2 from the atmosphere and how long it would take centuries for the oceans to become less acidic.
Assuming carbon dioxide removal could be ramped up to 90 gigatonnes a year – or about twice current annual emissions – oceans would not be brought back to pre-1750 levels until at least 2700, the researchers said in a paper published this week in Nature Climate Change journal.
"Geoengineering measures are currently being debated as a kind of last resort to avoid dangerous climate change – either in the case that policymakers find no agreement to cut CO2 emissions, or to delay the transformation of our energy systems," Sabine Mathesius, a Potsdam Institute researcher, said in a statement.
"[I]n a business-as-usual scenario of unabated emissions, even if the CO2 in the atmosphere would later on be reduced to the pre-industrial concentration, the acidity in the oceans could still be more than four times higher than the pre-industrial level," Ms Mathesius said. "It would take many centuries to get back into balance with the atmosphere."
While more acidic conditions make it harder for creatures to form shells, warming waters were also likely to mix less, reducing oxygen and nutrient transfers, "factors that would tend to prolong the deep ocean 'memory' of anthropogenic changes", the paper said.
Pete Strutton, an associate professor at the Institute for Marine and Antarctic Studies at the University of Tasmania, said the paper showed "what we should already realise".
"What's needed is aggressive efforts to reduce emissions now, rather than aggressive programs in 50 years' time to remove it," Professor Strutton said.
Ocean circulation can already be very slow – with some of the oldest water taking 1000 years to return to the surface. "Stratification impacts are relatively difficult to quantify right now but we're definitely seeing the warming right now,'' he said.
Hans Joachim Schellnhuber, a co-author of the paper and also the senior scientific adviser to Pope Francis for his recent climate encyclical, said "the chemical echo of this century's CO2 pollution will reverberate for thousands of years".
"If we do not implement emissions reductions measures in line with the 2 degrees Celsius target in time, we will not be able to preserve ocean life as we know it," Professor Schellnhuber said.
The decarbonisation efforts considered included mass planting of biomass which would suck CO2 from the atmosphere. That biomass would then be burnt and the emissions captured and stored.
