Every oceanographer I know who is studying the controversial idea of coaxing the ocean to absorb extra carbon dioxide from the atmosphere remembers the moment they decided to start this contentious work. For me, it came during the Pacific Northwest heat wave of June 2021, which sent temperatures soaring above 49 degrees Celsius (120 degrees Fahrenheit) and set boreal forests ablaze. I had spent years studying ocean circulation and Earth’s carbon cycle, but not marine carbon dioxide removal (mCDR)—techniques for reducing CO2 in the oceans so they, in turn, can draw more CO2 from the air. Nevertheless, just before the heat wave began, I offered to help organize a virtual panel discussion on mCDR for an ocean research conference.
Most of the questions that arose during the session were about fears that such research could create a moral hazard, allowing people to claim that drawing down CO2 lessens the urgency of reducing fossil-fuel emissions. During the panel, a First Nations scholar from the University of British Columbia, Candis Callison, talked about how to involve local shoreline communities where ocean field trials might be conducted. Callison was a brilliant voice, helping the scientists understand more about public discourse on climate change.
Just days later wildfires flared up and damaged several First Nations reserves—including the home, Callison informed me, where her relatives lived. This tragic event vividly reminded me of the dangers we already faced after one degree C of global warming. The tragedies could become much worse, given that even optimistic scenarios indicated the world would warm by at least another degree. I decided mCDR research was important. If it ultimately showed that the methods were futile or hazardous, the research could prevent prolonged investment in a false hope. If the work revealed safe ways to stimulate the ocean to take up more CO2, then those could be new tools to help stabilize the climate.
Starting in the 1950s, scientists began analyzing air bubbles trapped in ice cores drilled from the Greenland and Antarctic ice sheets to understand climate history. By the 1980s, they realized that the world’s oceans could inhale or exhale enough CO2 to substantially contribute to Earth’s long-term cycles of ice-sheet expansion and retreat across continents. The leading hypothesis at that time for the seesawing of carbon concentrations in the ocean over thousands of years was that the surface water contained iron, which blew in from arid landscapes during cold periods, and its levels regulated phytoplankton growth across the seas. More iron would cause more growth, which would pull more CO2 from the air. Oceanographer John Martin of Moss Landing Marine Laboratories in California proposed that artificially fertilizing the ocean with iron could influence climate.
Martin’s iron hypothesis prompted more than a dozen artificial iron-enrichment experiments between 1992 and 2009. Researchers released iron on the ocean’s surface and tracked for days or weeks how the area’s water chemistry and organisms changed. Results confirmed that iron enrichment could lead to a phytoplankton bloom when other conditions were favorable. Whether or not oceanographers considered these experiments “geoengineering,” the studies yielded extraordinary insight into the interacting biological and chemical processes that could alter climate on long timescales.
