California’s SB 253 just became the most aggressive corporate emissions disclosure law in the United States. ~4,160 entities across both SB 253 and SB 261 now face mandatory greenhouse gas reporting, including Scope 3 emissions from their entire value chain (Harvard Law School Forum on Corporate Governance, Oct 2025). That’s not a soft suggestion. It’s a legal obligation with penalties up to $500,000 per year.

Whether you’re a sustainability director scrambling to build an emissions inventory or a CFO trying to understand the financial exposure, this guide breaks down everything you need to know. We’ll cover who’s affected, what’s required, when it’s due, and how companies are actually preparing for it. No filler. Just the details that matter.

What Is California SB 253?

SB 253, formally the Climate Corporate Data Accountability Act, is a California state law signed by Governor Newsom on October 7, 2023. It requires large companies doing business in California to publicly disclose their greenhouse gas emissions across all three scopes. CARB unanimously approved final implementing regulations on February 26, 2026 (National Law Review, 2026), making it the broadest mandatory emissions disclosure program in the U.S.

Here’s what makes this law different from nearly every other climate reporting requirement globally: there’s no materiality filter. Companies can’t decide that certain emission categories aren’t “material” and skip them. If it falls under GHG Protocol methodology, you report it. Period.

The law also mandates third-party assurance, which phases in over several years. That means an independent auditor will verify your numbers. It’s not a voluntary disclosure where you pick favorable metrics and publish a glossy PDF.

Who Does SB 253 Apply To?

~4,160 entities appear on CARB’s preliminary list across both SB 253 and SB 261, with over 2,600 under SB 253 specifically, according to CARB’s preliminary entity list (Harvard Law School Forum, Oct 2025). The law targets any U.S. or foreign entity with total annual revenues exceeding $1 billion that “does business in California,” as defined by the state’s Tax and Revenue Code. Both public and private companies are covered.

That last detail catches a lot of people off guard. Most climate disclosure rules, like the now-defunct SEC rule, only applied to publicly listed companies. SB 253 doesn’t care whether you’re on the NYSE or privately held. Revenue is the trigger.

What “Doing Business in California” Means

California defines this broadly. If your company is organized in California, has sales exceeding the current threshold (around $735,000), or has property or payroll in the state, you likely qualify. Given California’s role as the world’s fifth-largest economy, most billion-dollar companies have some California nexus.

CARB published a preliminary list of affected entities and opened a 45-day comment period. If your company isn’t on the list but meets the criteria, you’re still obligated to report. Don’t assume the absence of a letter means you’re off the hook.

Related: For a detailed comparison, see What Are SB 253 and SB 261? What Businesses Need to Know.

What Does SB 253 Require?

SB 253 mandates annual disclosure of all greenhouse gas emissions, including Scope 3 supply chain emissions, which represent 70-90% of a typical company’s carbon footprint (CDP, 2024). Companies must follow GHG Protocol Corporate Standard methodology, and there is no materiality-based exemption allowing firms to exclude specific categories.

Here’s the breakdown of what you actually need to report.

Scope 1: Direct Emissions

These come from sources your company owns or controls. Think company vehicles, on-site fuel combustion, industrial processes. Most large companies already track Scope 1 with reasonable accuracy.

Scope 2: Indirect Energy Emissions

Purchased electricity, steam, heating, and cooling. Again, most companies with existing sustainability programs have this covered, though location-based versus market-based accounting can trip people up.

Scope 3: Value Chain Emissions

This is where things get hard. Scope 3 covers 15 categories of upstream and downstream emissions: purchased goods and services, business travel, employee commuting, end-of-life treatment of sold products, and more. CDP data shows supply chain emissions run 26 times higher than a company’s own operational footprint (CDP, 2024).

Only 56% of CDP-disclosing companies reported any Scope 3 data as of the most recent cycle (GHG Protocol). So nearly half of companies that voluntarily disclose still haven’t figured out Scope 3. Now it’s mandatory.

Related: Not sure where to start? Read our step-by-step SB 253 compliance preparation guide.

What Are the SB 253 Reporting Deadlines?

The first SB 253 filing is due August 10, 2026, covering Scope 1 and Scope 2 emissions for the prior reporting year. Scope 3 reporting starts in 2027 (PwC, March 2026). Assurance requirements phase in gradually: limited assurance from 2027 through 2029 for Scope 1 and 2, then reasonable assurance from 2030 onward.

Let’s map this out clearly, because the phased approach confuses a lot of teams.

Assurance Requirements

Limited assurance is a lighter review. Think of it as a plausibility check. Reasonable assurance, which kicks in for Scope 1 and 2 starting 2030, is closer to a financial audit in rigor. For Scope 3, limited assurance starts in 2030, giving companies a few extra years to clean up their supply chain data.

What Are the Penalties for SB 253 Non-Compliance?

Companies that fail to comply with SB 253 face administrative penalties of up to $500,000 per reporting year (California Legislature). For SB 261 (the climate risk disclosure companion law), penalties cap at $50,000 per year. CARB also charges an annual filing fee of approximately $3,106 per reporting entity (Inside Energy & Environment, Dec 2025).

But here’s the thing: the financial penalty is just the visible cost. What about investor confidence? Customer trust? If your competitors file clean emissions reports and you’re hit with a CARB enforcement action, the reputational damage could cost far more than half a million dollars.

CARB has also indicated it will consider good-faith efforts when assessing penalties. Companies that demonstrate genuine attempts to comply but fall short on specific Scope 3 categories may receive more lenient treatment than companies that simply ignore the requirement. That said, “we didn’t know” won’t hold up as a defense.

Why Is Scope 3 the Biggest Challenge Under SB 253?

A staggering 83% of companies report difficulty accessing relevant Scope 3 data, and 53% cite a lack of standardized methodologies as a primary barrier (TCFD Status Report, cited in WRI/GHG Protocol analysis). Scope 3 covers the full value chain, from raw material suppliers to end-of-life product disposal. For most companies, it represents 70-90% of total emissions, yet it remains the least measured and least understood category.

Why is it so hard? Because you’re measuring emissions you don’t control. Your suppliers’ factories, your customers’ product usage, your employees’ commutes. The data sits in hundreds or thousands of different organizations, each with different tracking capabilities, different fiscal years, and different definitions of what counts.

We’ve heard this firsthand from companies preparing for compliance. One chemicals manufacturer described Scope 3 data collection as a “herculean task,” pointing to multi-tier supplier data scattered across fragmented Excel sheets with no ERP integration. That’s not unusual; it’s the norm.

That statistic should alarm anyone responsible for SB 253 compliance. Spreadsheets introduce version control problems, formula errors, and zero audit trail. When assurance providers show up, they need traceable, verifiable data. An Excel file emailed between three departments doesn’t meet that standard.

Here’s what most compliance guides miss. The bottleneck isn’t methodology. It’s infrastructure. ESG teams at mid-to-large enterprises spend roughly 70% of their time collecting and cleaning data, not analyzing it. One compliance advisory firm described ESG data as “chaotic, fragmented, and often inaccurate” when collected via Excel. Fixing that pipeline is the real challenge, not understanding which Scope 3 category your business travel falls into.

How Does SB 253 Compare to the SEC Climate Disclosure Rule?

The SEC abandoned its defense of the federal climate disclosure rule on March 27, 2025 (SEC.gov). That decision effectively killed the only federal competitor to California’s approach. SB 253 now stands as the most comprehensive mandatory emissions reporting requirement in the U.S., and it goes significantly further than the SEC rule ever would have.

So what’s different? Almost everything that matters.

The practical implication? Companies that were waiting for the SEC rule to set the standard now face California’s stricter version instead. And since SB 253 applies based on revenue and California nexus, not stock exchange listing, the affected pool is actually broader.

What Is the Difference Between SB 253 and SB 261?

SB 253 and SB 261 are companion laws, but they serve different purposes. SB 253 requires GHG emissions reporting for companies exceeding $1 billion in revenue, while SB 261 targets climate-related financial risk disclosures for firms above $500 million (California Legislature). Importantly, SB 261 remains enjoined by the Ninth Circuit as of early 2026, while SB 253 is fully active.

Bottom line: if you’re above $1 billion in revenue, SB 253 is your immediate priority. SB 261 may come back into play if the injunction is lifted, but SB 253 is the law with teeth right now.

Related: See our deep dive into SB 261 and how it compares to SB 253.

How Should Companies Prepare for SB 253 Compliance?

CARB estimates annual compliance costs between $135,000 and $152,000 per reporting entity, with third-party assurance adding $150,000 to $500,000 more (CARB; EcoVadis). Starting early is the single most effective way to reduce those costs. Companies that begin preparation 12+ months before deadlines consistently report smoother, cheaper compliance cycles.

How Credibl ESG Supports SB 253 Compliance

Credibl ESG is an AI-powered platform built to help companies measure, manage, and report greenhouse gas emissions across 20+ frameworks, including SB 253, CSRD/ESRS, IFRS S1/S2, BRSR, GRI, and CDP. The platform’s Framework Crosswalks Engine maps one set of data inputs to multiple reporting outputs, eliminating duplicate data entry.

For Scope 3, Credibl maintains a library of 90,000+ emission factors and uses an AI-powered feature called Magic Drive to extract data from PDFs, scanned invoices, and even handwritten bills. The platform holds GHG Protocol conformity certification under ISO 14067, and currently serves 180+ enterprise customers across 11 countries.

Frequently Asked Questions About SB 253

Related: See the full SB 253 and SB 261 reporting timeline for year-by-year deadlines through 2030.

What Happens Next?

SB 253 is no longer a future concern. It’s an active law with a firm August 2026 deadline, enforceable penalties, and no materiality loopholes. For the ~4,160 entities on CARB’s preliminary list, the clock is ticking.

The companies that will find compliance manageable are the ones starting now: auditing their emissions data, mapping Scope 3 categories, selecting platforms that can handle GHG Protocol calculations with an audit trail, and engaging assurance providers before the market gets saturated. Those that wait until Q2 2026 will face higher costs, rushed data, and real compliance risk.

The SEC climate rule is gone. CSRD applies to EU-facing operations. SB 253 is the U.S. standard now. Whether that feels like progress or pressure depends entirely on how prepared you are. Start today.