Competition in Public Procurement Reduces Supply Chain Carbon Intensity

Public procurement represents 12--15\% of GDP in OECD nations but its climate implications remain poorly understood. Here we analyse 21.6 million contracts across 27 countries (2012--2023) and find that single-bidder contracts exhibit 14.8\% higher carbon intensity than competitive contracts ($p < 10^{-300}$). Combined with established price premiums of 7--10\% for non-competitive procurement, this constitutes a dual inefficiency where governments pay more and emit more. The effect follows a U-shaped pattern: small contracts (<\texteuro10,000) show carbon premiums exceeding 50\% (Cohen's $d$ = 0.75), while large contracts show within-sector neutrality. Decomposition analysis reveals the mechanism is entirely allocative---competition shifts procurement from carbon-intensive manufacturing sectors toward lower-carbon services, rather than improving within-sector efficiency. Regression discontinuity analysis at EU transparency thresholds demonstrates that disclosure requirements causally increase bidder participation by 27\%. These findings identify routine procurement as a high-impact target for emissions reduction.