Technology to drain heat-trapping CO2 from the atmosphere may slow global warming, but will not reverse climate damage to the ocean on any meaningful timescale, according to research published Monday.
At the same time, a second study reported, even the most aggressive timetable for reducing greenhouse-gas emissions will need a big boost from largely untested carbon removal schemes to cap warming to two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.
Above that threshold, say scientists, the risk of climate calamity rises sharply. Earth is currently on a 4 Celsius (7.2 Fahrenheit) trajectory.
Both studies, coming months before 195 nations meet in Paris in a bid to forge a climate pact, conclude that deep, swift cuts in carbon dioxide (CO2) emissions are crucial.
Planetary-scale technical fixes - sometimes called geo-engineering - have often been invoked as a fallback solution in the fight against climate change.
But with CO2 emissions still rising, along with the global thermostat, many scientists are starting to take a hard look at which ones might be feasible.
Research has shown that extracting massive quantities of CO2 from the atmosphere, through intensive reforestation programs or carbon-scrubbing technology, would in theory help cool the planet.
But up to now, little was known about the long-term potential for these measures for restoring oceans, rendered overly acidic after two centuries of absorbing CO2.
Increased acidification has already ravaged coral, and several kinds of micro-organisms essential to the ocean food chain, with impacts going all the way up to humans.
Scientists led by Sabine Mathesius of the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, used computer models to test different carbon-reduction scenarios, looking in each case at the impact on acidity, water temperatures and oxygen levels.
If humanity waited a century before sucking massive amounts of CO2 out of the atmosphere, they concluded, it would still take centuries, maybe even a thousand years, before the ocean would catch up.
In the meantime, they researchers say, corals will have disappeared, many marine species will have gone extinct and the ocean would be rife with dead spots.
"We show that in a business-as-usual scenario, even massive deployment of CO2 removal schemes cannot reverse the substantial impacts on the marine environment - at least not within many centuries," Mathesius said.
Even in a scenario in which large-scale carbon removal begins in 2050 - assuming such technology is available - the ocean does not fare well.
"Immediate and ambitious action to reduce CO2 emissions is the most reliable strategy for avoiding dangerous climate change, ocean acidification, and large-scale threats to marine ecosystems," the researchers concluded.
Scientists commenting on the study said it should sound an alarm.
"The threat of ocean acidification alone justifies dramatic and rapid reduction of CO2 emissions," said Nick Riley, a research associate at the British Geological Survey (BGS).
The second study, led by Thomas Gasser of the Institut Pierre-Simon Laplace, near Paris, uses state-of-the-art models to measure the trade-off between reducing emissions and carbon-removing technologies.
They show that even if nations strike a deal in Paris adhering to the most aggressive CO2-slashing pathway outlined by UN scientists, it may not be enough to keep Earth on a 2 C trajectory.
"Our results suggest that negative emissions" - the use of carbon removing technology - "are needed even in the case of very high mitigation rates."
To have a chance of meeting the 2 C target, 0.5 to 3.0 gigatonnes of carbon - up to a third of total annual CO2 emissions today from industry - would need to be extracted every year starting more or less immediately, they calculate.
The study exposes "an elephant in the room," Riley said. "The target to keep warming within the 2 C rise is looking increasingly unattainable."