A view of the San Diego skyline from Centennial Park in Coronado back in May 2020. / Photo by Adriana Heldiz

San Diego County last year committed itself to reaching “net zero” emissions by 2035. Since then, it’s been studying how it will reach that lofty aim, and the early indication is that local governments as a whole are going to need a new policy trajectory entirely.

San Diego Mayor Todd Gloria is pushing the city to pursue that same goal. The update to the city’s Climate Action Plan that he proposed last year would require a series of major policy changes, all to meet a new target of achieving net zero emissions by 2035.

But we figured it might not be obvious what it is in the first place for everyone who isn’t already a climate wonk.

“Net zero,” as the name suggests, does not mean eliminating all planet-warming emissions from human activity. Rather, it means reaching a balance between the level of emissions that human society produces during a given period, and the amount of planet-warming emissions that the planet absorbs or that humans take out during that same period.

In the immediate term, that emissions-elimination would almost entirely come from natural means. Wetlands and trees pull carbon dioxide (the most prolific greenhouse gas) from the atmosphere and sequester it as part of their natural processes. By restoring and preserving those natural environments, humans can maximize the amount of carbon that the planet sequesters. A ‘net zero” policy, then, would allow humans to continue emitting an equivalent share of carbon from driving cars, operating factories, pouring concrete to repair sidewalks, heating homes with non-renewable energy, and all our other social and economic activities that contribute to climate change.

That is if the natural world could keep up with the amount of extra greenhouse gases humans are spewing into the atmosphere, which is precisely what is throwing the Earth’s climate out of whack in the first place.

So, in the middle term, governments, companies, nonprofits big and small are trying to close the gap by making promises and policies that transform fossil fuel-based resources into renewable ones. Yet, so far at least in San Diego, we know the plans on the books are not capable of reducing greenhouse gas emissions to net zero by 2035.

In the long-term, a lot of these net zero policies are banking on yet-to-be-developed technologies that mechanically remove carbon from the atmosphere, called direct air capture.

But even figuring out how many emissions humans need to cut to truly hit net zero requires both hard science and hard policymaking.

Gordon McCord, an economist specializing in sustainable development at University of California, San Diego who directed the county’s regional decarbonization study, said the focus of the analysis is to “chart pathways to reach as close to zero carbon as possible.”

“We consider ‘net zero’ only in the sense that we looked at how natural climate solutions within San Diego county could generate more negative emissions, which would be helpful to compensate for the fact that there are some hard-to-decarbonize sectors/uses,” he wrote in an email.

Gloria’s proposed Climate Action Plan update says it would reduce 80 percent of the emissions generated within the by 2035 – that leaves another 20 percent of ongoing emissions that would need to be removed from the atmosphere by natural processes or other means, if the city plans to hit the net zero commitment in the plan.

“This strategy is broad by design and includes developing more effective regional partnerships, further research into (carbon) sequestration opportunities and collaboration on pilot projects for new technology,” Alyssa Muto, the city of San Diego’s Sustainability and Mobility Department director wrote in an email.

Let’s try to break down what this would look like in simple math.

The San Diego region emitted an estimated 26 million metric tons of greenhouse gases in 2016, according to the most recent greenhouse gas emissions summary from San Diego Association of Governments. That includes electricity, natural gas, on-road transportation, solid waste, agriculture ... basically everything that makes planet-warming emissions.

The region, meanwhile, has 58 million metric tons of greenhouse gases stored in its soils, wetlands, trees, etc., according to that same regional study. As long as that land is undisturbed, that carbon will stay there. That same environment also cuts down on planet-warming gases that are in the atmosphere as well. It sucks an estimated 2 million metric tons of carbon out of the atmosphere per year and buries it in nature.

So, barring any changes to increase the regional environment’s ability to pull carbon from the atmosphere, for San Diego to get to net zero, San Diegans would theoretically have to ramp emissions down to 2 million metric tons per year to balance with nature. That means we’d need to eliminate roughly 93 percent of our existing emissions.

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