Natural climate solutions (NCS)—from reforestation and agroforestry to wetland restoration—have long been championed as low-cost, high-benefit pathways for reducing greenhouse gases. In theory, they could provide over a third of the climate mitigation needed by 2030 to stay under 2°C of warming. But in practice, progress is stalling. A sweeping new PNAS Nexus study, led by Hilary Brumberg, reveals why.
Drawing on 352 peer-reviewed papers across 135 countries, researchers cataloged 2,480 documented barriers to implementing NCS. The obstacles are not ecological. Rather, they are human: insufficient funding, patchy information, ineffective policies, and public skepticism. The result is a vast “implementation gap” between what is technically possible and what is politically, economically, or socially feasible.
The analysis found that “lack of funding” was the most commonly cited constraint globally—identified in nearly half of all countries surveyed. Yet it rarely stood alone. Most regions face a tangle of interconnected hurdles. Constraints from different categories often co-occur, compounding difficulties: poor governance erodes trust; disinterest stems from unclear benefits; technical know-how is stymied by bureaucratic confusion.
These patterns vary by region and type of intervention. Reforestation projects, for instance, face particularly high scrutiny over equity concerns—especially in the Global South, where land tenure insecurity and historical injustices run deep. Agroforestry and wetland restoration often struggle with the complexity of design and monitoring. Meanwhile, grassland and peatland pathways remain understudied, despite their importance.
The study’s most striking insight may be spatial. Countries within the same UN subregion tend to share a similar profile of constraints—more so than across broader development regions. This geographic clustering suggests an opportunity: Supranational collaboration, if properly resourced and attuned to local context, could address shared challenges more efficiently than isolated national efforts.
Crucially, the authors argue that piecemeal fixes will not suffice. Because most countries face an average of seven distinct constraints, many from different domains, effective solutions must be integrated and cross-sectoral. Adaptive management—a flexible, feedback-based approach—could help. By identifying which barriers arise at each stage of an NCS project’s lifecycle, it may be possible to design interventions that are not just technically sound, but socially and politically viable.
Natural climate solutions still hold vast potential. But unlocking it will require less focus on where trees grow best—and more on where people can make them thrive.
CITATION: Hilary Brumberg et al 2025. Global analysis of constraints to natural climate solution implementation, PNAS Nexus. https://doi.org/10.1093/pnasnexus/pgaf173
