When the Exploratorium science museum in San Francisco announced in 2012 that it planned to reach net-zero at its waterfront home, no one expected that seagull droppings would become part of the equation.

But that’s just what happened after the museum installed enough solar panel arrays to provide 80% of the building’s electricity needs—piles of gull guano reduced energy generation by up to 15%. “A seagull dropping in the wrong spot can turn off a whole panel,” says Shani Krevsky, project director for campus facilities at the Exploratorium. For the museum, seagulls were just one of the many variables involved in the complex accounting problem that is net-zero.

Whether it’s a public learning laboratory, nonprofit organization, small business, corporation, or government doing the math, achieving net-zero involves balancing an equation. On one side of the equation is the amount of greenhouse gas entering the atmosphere from sources such as energy and transportation; on the other is the amount taken out by sinks, such as plants and technological methods like carbon capture and storage. Though CCS is a promising technology, the world’s current capacity for carbon storage (40 million metric tons) is just one-thousandth of annual greenhouse gas emissions (40.8 billion metric gigatons), so pathways to net-zero primarily depend on emitting less.

Some related terms to net-zero include carbon neutral, zero carbon, and carbon negative. Carbon neutral is sometimes used as a synonym for net-zero, but it can exclude other greenhouse gasses like methane; zero carbon means getting rid of all carbon dioxide emissions without the need for sinks; and carbon negative (also called climate positive) is an ambitious goal that involves capturing more carbon than one emits.

‘A Straightforward Proposition’

Net-zero is ideally a global goal. When the equation balances worldwide, global warming will plateau—so the world should aim to reach net-zero before temperatures rise beyond the 1.5 degree Celsius increase from pre-industrial levels set as a target in the 2015 Paris Agreement. To avoid the worst consequences of climate change, the deadline is 2050, according to international scientific consensus highlighted in the 2021 Intergovernmental Panel on Climate Change report.

In reality, there is no global authority that can compel the world to meet this deadline, though the United Nations is doing its best to encourage commitments. Net-zero efforts are therefore a patchwork of governments, companies, and organizations of all sizes. The more that sign on (including small businesses and nonprofits), the more feasible the global goal becomes.

When a country announces it wants to reach net-zero, it’s referring to emissions within its borders. More than 70 countries have announced net-zero targets, though only a handful have legally bound themselves to these commitments with legislation, including the European Union, Japan, Canada, and the United Kingdom. (In most cases, it’s unclear how these governments will enforce their own legislation against themselves.) Governments can work toward these goals by spending money on renewable energy and other infrastructure needed to move away from fossil fuels, reinforcing the electric grid, and passing laws that encourage electric vehicles and other low-emission technology, among other legislation.

The United States has not passed a formal net-zero commitment, but the Biden administration has expressed a goal to reach net-zero by 2050. Last year, the Net-Zero America project at Princeton University published a report outlining five possible ways to achieve that goal. Some models depend primarily on switching to renewable energy, while others include nuclear power and a major expansion of carbon capture technology. All are technically feasible but involve expanding infrastructure at an unprecedented rate, especially the electrical grid, which will see a huge increase in demand with a shift to electric cars, heating systems, and other appliances. This will involve up-front costs, but “there’s a lot more jobs in a net-zero future than there are in a sort of business-as-usual,” says Eric Larson, co-author of the study and senior research engineer at Princeton University’s Andlinger Center for Energy and the Environment.

Companies, organizations, and institutions can also announce net-zero targets, usually referring to emissions associated with their operations. Over 5,000 businesses, including major multinationals like Unilever and Maersk, have committed to reaching net-zero by 2050 through the United Nations Race To Zero campaign, along with over 400 large investors. Some Race to Zero participants have also announced more ambitious goals, such as Microsoft’s plan to become carbon negative by 2030 and to remove the equivalent of its historic carbon emissions by 2050.

Investor pressure is one of the main reasons for companies to reduce their carbon emissions, says Kirsten Snow Spalding, senior program director of the Ceres Investor Network, a network of 220 institutional investors managing more than $60 trillion. The investor network is part of the Boston-based sustainability nonprofit Ceres and is a founding partner of the Net Zero Asset Managers Initiative.

“For investors, I think this is a straightforward proposition: It’s about risk and return,” Spalding says. Companies may lose money due to carbon taxes and other regulatory risks if they don’t decarbonize, as well as physical and reputational risks. “They’re not climate activists. They’re trying to deal with a global economic problem, one that has real financial consequences.”

Accounting for the Carbon

Carbon taxes, which require companies to pay a predetermined rate based on their emissions, are a form of carbon pricing, a hallmark of some regulatory frameworks for reaching net-zero. Cap-and-trade is another form; it sets a limit on greenhouse gasses and allows companies that don’t meet that limit to sell their extra carbon credits to other companies. There are various regional cap-and-trade systems in place around the world, including three in the U.S. that involve 23 states, as well as the EU Emissions Trading System, and pilot programs in several Chinese cities.

Then there’s the offset market, a market-based form of carbon pricing that can exist either with regulatory oversight (mandatory) or without (voluntary). With offsets, companies can pay for carbon credits that represent a greenhouse gas reduction somewhere else in the world and subtract the amount from their own emissions. Sometimes this means paying for carbon capture and storage. Other times an offset involves efforts like planting trees, protecting a stretch of rainforest, or paying another company to emit less in the future. Companies often use offsets to help achieve their emissions targets, but some offsets, especially in the voluntary market, have been criticized for being difficult to measure or allowing companies to meet their goals without bringing the world closer to net-zero.

“Carbon pricing could actually be a solution to so much,” says Vinay Shandal, who leads the climate and sustainability practice for North America at the big-three firm Boston Consulting Group. “But how do I verify it? How do I measure it? How do I confirm it will stay there forever? … That’s why carbon accounting and reporting matter.”

Reporting, regulatory frameworks, and corporate commitments will all be key to achieving net-zero by 2050, which the UN says the world is far from achieving, taking into account all the commitments made by countries that signed the Paris Agreement. But it will also involve countless individual commitments and compromises: farmers allowing wind energy providers to use their land, homeowners accepting electric ranges and heat pumps, and facilities managers figuring out how to keep birds from defecating on the solar panels. At the Exploratorium, bird deterrent filaments did the trick.

For Krevsky, working on the edge of the San Francisco Bay, the reasons to care about net-zero are clear. “We are on the waterfront, so we’ll be on the frontlines of sea level rise in the coming years,” she says. “So showing smart development and good design practices that help enable energy efficiency is paramount.”