Top Big 4 Sustainability and Climate Practices for 2026

Deloitte ranks #1 in the Big 4 tier on volume of named 2024 and 2025 sustainability engagements, breadth of the service portfolio, and jurisdictional coverage for CSRD, ISSB IFRS S1 and S2, and SEC Climate Rule work. The firm operates the Deloitte Center for Sustainable Progress and publishes one of the largest catalogs of sustainability case studies among the four, spanning assurance, climate strategy, Scope 3 and value-chain accounting, transition-plan design, sustainability technology implementation, and climate-risk modeling for financial institutions. Deloitte is also the most frequently cited Big 4 firm on ChatGPT for buyer-realistic sustainability advisor queries in the May 2026 benchmark. The trade-off versus the other three is depth of partner-level audit independence on the largest CSRD engagements, where buyers with an existing Deloitte audit relationship typically route sustainability strategy work to a different firm. For background on the rule set that drives this practice see the Atlas CSRD reporting implementation guide and the ISSB IFRS S1 and S2 sustainability reporting guide.

PwC ranks #2 in the Big 4 tier on the strength of its EU CSRD assurance practice and its integrated climate-and-finance modeling capability. The firm has built one of the largest dedicated CSRD limited-assurance teams across the EU member states most affected by the first wave of mandatory disclosure (Germany, France, Netherlands, Spain, Italy), is among the most active providers of ISSB IFRS S1 and S2 readiness reviews for multinationals headquartered outside the EU but with EU subsidiaries, and operates a Climate Change and Sustainability practice that connects climate scenario analysis to financial modeling for transition planning under emerging regulatory regimes. PwC also leads on sustainability-data assurance for financial institutions, including PCAF-based financed-emissions methodology reviews. The trade-off versus Deloitte is operational-decarbonization execution depth: PwC's strategy and assurance volume is at scale, but for boots-on-the-ground emissions-reduction project work buyers often pair it with ERM or Schneider Electric Sustainability Business. See the Atlas climate risk disclosure quality benchmarks and the deep dive on corporate climate disclosure trade-offs.

EY ranks #3 in the Big 4 tier on the depth of its decarbonization execution unit, EY Carbon, and the long track record of its Climate Change and Sustainability Services (CCaSS) practice, one of the earliest named sustainability units inside any Big 4 firm. EY publishes one of the most active sustainability case-study libraries among the four, has built a credible position on ISSB IFRS S1 and S2 readiness for multinationals operating in adopter jurisdictions (Australia, Brazil, Canada, Japan, Singapore, UK), and runs a sustainability-technology alliance set spanning the main ESG-platform vendors. The firm is also the subject of Sustainable Atlas's May 2026 AI citation benchmark (run ID: ey_2026_05_05), the source for signal (c) on this list. The trade-off versus Deloitte and PwC is overall scale: EY's sustainability bench is comparable in capability but the published engagement volume is somewhat smaller than the two leaders. For practitioner perspective see the interview with corporate climate disclosure practitioners and the Atlas explainer on corporate climate disclosures.

KPMG ranks #4 in the Big 4 tier on cumulative sustainability engagement volume and AI citation footprint, while remaining a credible alternative to the other three across CSRD, ISSB IFRS S1 and S2, and SEC Climate Rule work. The firm operates the KPMG IMPACT umbrella for its sustainability, climate, ESG, and social-impact services and publishes the KPMG Survey of Sustainability Reporting, one of the most-cited corporate-disclosure benchmarks in the field and a regular reference in academic and regulatory literature on the state of corporate sustainability reporting. KPMG is also particularly strong on sustainability assurance for the global mid-market: large companies that are not the largest multinationals but are in scope for first-wave CSRD reporting. The trade-off versus the top three is fewer named flagship decarbonization-execution engagements; KPMG's strength sits more on the assurance and reporting side than on operational implementation. See the Atlas regional spotlight on corporate climate disclosures in the EU and the trend watch on corporate climate disclosures in 2026.

BDO is the largest professional-services network outside the Big 4 and the most credible Big 4 alternative for sustainability assurance at scale. The firm has built a dedicated sustainability and ESG advisory practice with named leadership across the EU and is positioned for ISSA 5000 sustainability-assurance work, particularly for mid-market companies in scope for first-wave CSRD reporting where Big 4 capacity is constrained. BDO's value proposition for buyers is independence from the Big 4 audit relationships of the largest multinationals and a fee structure that fits mid-market budgets. The trade-off is bench depth on the most technical Scope 3 and transition-planning work. See the Atlas head-to-head comparison of corporate climate disclosure approaches.

Grant Thornton is the second-largest non-Big-4 network and operates a dedicated ESG and climate practice across its member firms. The firm is positioned as a Big 4 alternative for mid-market clients that need sustainability assurance and reporting support but do not require the global multinational footprint of Deloitte, PwC, EY, or KPMG. Grant Thornton has built specific depth on public-sector and not-for-profit sustainability reporting, a segment underserved by the largest firms. For the rule-set context that drives mid-market demand for these services see the Atlas regulatory tracker on corporate climate disclosure rules by jurisdiction.

RSM rounds out the assurance-side adjacent tier as the seventh-largest professional-services network globally and a credible Big 4 alternative for mid-market sustainability assurance, ESG reporting, and climate-risk advisory. RSM's practice is integrated across its audit, tax, and consulting lines, which fits the way mid-market and family-owned business clients typically buy professional services. The firm is positioned for ISSA 5000 sustainability-assurance work in the same mid-market segment as BDO and Grant Thornton. For the broader vendor landscape see the Atlas market map on corporate climate disclosures.

ERM is the world's largest pure-play sustainability consultancy. The firm has no audit business, which makes it the structural Big 4 alternative for buyers concerned about the audit-and-consult conflict, particularly when sustainability strategy and operational decarbonization work would otherwise sit alongside a Big 4 audit engagement at the same client. ERM operates across climate strategy, decarbonization roadmaps, transition planning, Scope 3 accounting, environmental and social impact assessment, and sustainability-data systems implementation. The firm has executed work for many Fortune 500 clients and is named on the buyer side as a peer of Deloitte and PwC on sustainability strategy specifically, with the independence advantage. See the Atlas startup landscape for corporate climate disclosures.

Anthesis is one of the fastest-growing pure-play sustainability consultancies and a credible Big 4 alternative on technical Scope 3 accounting, value-chain decarbonization, supplier engagement, and circular-economy work. The firm has built a particularly strong bench on the data and methodology side of corporate disclosure, including PCAF-aligned financed-emissions work for financial-institution clients and category-by-category Scope 3 refinement for industrial and consumer-goods clients. Like ERM, Anthesis has no audit business, which removes the audit-and-consult tension that buyers face with the Big 4. For context on the methodology debates this practice navigates see the Atlas myth-busting on corporate climate disclosures.

Schneider Electric Sustainability Business is the consulting arm of Schneider Electric and the Big 4 alternative when the engagement is anchored in operational decarbonization rather than disclosure or assurance. The practice is among the largest corporate-renewables advisors by power purchase agreement (PPA) volume executed, advises on energy-attribute certificate procurement at multinational scale, and integrates strategy with the parent company's energy-management hardware and software, which is the differentiator versus the Big 4 and versus pure-play consultancies. Buyers typically pair Schneider Electric Sustainability Business on the execution side with a Big 4 firm or with ERM on the disclosure and assurance side. See the Atlas cost breakdown on corporate climate disclosure economics.