| Levels of CO₂ in the atmosphere were higher in 2020 than in any year in human history. They will be higher in 2021, reaching 50% above pre-industrial levels for the first time. Concentrations will be higher still in 2022, 2030, and will likely continue to grow for the better part of the century. Climate change is a bathtub problem. Global average temperatures, sea levels and other climatic effects from floods to droughts are intimately linked to the CO₂ already in the atmospheric tub. It's not sufficient to stabilize the flow of CO₂ going into the tub when the goal is to prevent it from overflowing. That means turning off the flow of water into the tub—getting net emissions to zero and below. It is why serious net-zero emissions targets are so important. The emphasis here is on "serious." The U.K.'s 2008 Climate Change Act established a goal of decreasing the country's net emissions by 80% below 1990 levels by 2050. That alone was ambitious. In 2019, the British parliament passed a fateful amendment. It simply read: "for '80%' substitute '100%'." Of course, getting there is not as simple. Long-term goals can easily be misconstrued as cheap talk, if they aren't followed by near-term action. And 2050 isn't all that "long-term" to begin with. Almost any infrastructure project or other investment planned today will still stand well into the second half of the century. Homes built today and financed with a 30-year mortgage will still be partially owned by the bank by the time they, too, need to fit under the net-zero umbrella. The importance of near-term action makes it tempting to revert to annual CO₂ emissions as a metric for how well a country, company, or the world, is doing on meeting net-zero goals. That would be a mistake. Of course, annual net CO₂ emissions are intimately tied to the stock of CO₂ already in the atmosphere. But focusing on emissions alone opens up too many other variables to be useful. For one, there is the all-too human dimension. The intuitive bathtub analogy notwithstanding, human minds are inherently bad at judging flows versus stock. Anyone who has ever had a pot of soup boil over on the stove can attest to that. MIT professor John Sterman has written eloquently about this fundamental misconception. In a 2007 study involving more than 200 students, Sterman and author Linda Booth Sweeney showed how easy it is to confuse CO₂ emissions with concentrations. Granted, the students were not told about the bathtub analogy but instead were shown the Intergovernmental Panel on Climate Change's Summary for Policymakers. That is all the more concerning. The summary is written for non-specialists. If even MIT graduate students mix up emissions and concentrations, thinking that stabilizing the former would suffice to stabilize global climates, what hope is there for the rest of us? Then there is natural variability. This year is projected to see a particularly strong La Niña, when sea-surface temperatures in the eastern Pacific are cooler than average. The resulting wetter and cooler conditions in the tropics lead to greater-than-average plant growth, which in turn leads to more CO₂ taken out of the atmosphere. Even more consequential will be the impact of Covid-19. Global CO₂ emissions in 2020 decreased by around 7% relative to 2019. Daily emissions in mid-April, at the height of the initial wave of Covid lockdowns, decreased by around 17 million tons of CO₂, compared to the prior year. That conveniently translates to around 17%, as the world emitted a staggering 100 million tons of CO₂ on an average day in 2019. None of that is good news. If anything, Covid lockdowns showed us the limits of individual action, even if taken at a massive, collective scale. Most of us fortunate enough to do so literally didn't leave home for weeks. The totality of these measures had large, positive public health consequences, averting almost half a billion infections. Yet emissions didn't come close to "net-zero," where they need to be within three short decades. Concentrations still rose, even if by slightly less than in 2019. Emissions this year, meanwhile, are bound to bounce back. It might take another two or three years for them to reach pre-pandemic levels, but reach them they will. The proper metric to judge the Biden administration's—or any country's—climate policies, is neither this year's emissions, nor is it the net-zero decarbonization target three decades out, however important that might be as a long-term target. The real metric of climate success is the trajectory of emissions five to 10 years out. That is far enough to not be subject to the whims of annual variability. It is also close enough to be directly influenced by today's policy choices. It is where "building back better" comes in, and where it is disheartening to see the world, by and large, not doing so to the extent necessary. It is also why infrastructure investments and large-scale capital expenditures are so important. Climate success, after all, is not about doing less. It's about more investment, more jobs, and more economic activity of putting the right infrastructure in place. That investment will almost inevitably go hand-in-hand with more short-term CO₂ emissions, yet another reason why this year's emissions are a poor measure of climate success. Gernot Wagner writes the Risky Climate column for Bloomberg Green. He teaches at New York University and is a co-author of Climate Shock. Follow him on Twitter: @GernotWagner. This column does not necessarily reflect the opinion of Bloomberg LP and its owners. Like getting the Green Daily newsletter? 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