What are the main scientific uncertainties in climate projections and how do they affect policy choices?
Executive summary
Climate projections are robust in the broad conclusion that continued greenhouse-gas emissions will warm the planet, but key scientific uncertainties—cloud and aerosol feedbacks, climate sensitivity, ocean circulation, regional responses, and socioeconomic scenario choices—leave wide ranges in how fast, where, and how severely impacts will unfold [1] [2] [3]. Those scientific and scenario uncertainties interact with economic and damage–estimation uncertainties to shape policy trade-offs: they complicate target-setting, adaptation timing, and investment in long-lived infrastructure, yet they do not justify deferring action because uncertainty can raise as well as lower risk [4] [5] [2].
1. The physics everyone cites: climate sensitivity, clouds and aerosols
A dominant scientific uncertainty is climate sensitivity—the warming produced by a doubling of CO₂—because it depends heavily on feedbacks such as cloud formation, water‑vapour amplification and aerosol interactions; cloud processes alone remain the largest single source of spread in model warming for a given emissions pathway [1] [6] [2]. Aerosols complicate the picture further by both cooling and modifying clouds, and incomplete knowledge of their net effects translates to material differences in projected warming and regional precipitation patterns [2] [7].
2. The ocean and slow components: circulation, heat uptake and ice sheets
Long-term uncertainty is amplified by slow components: ocean circulation and heat uptake modulate how quickly surface temperatures rise and where heat is stored, while ice‑sheet dynamics and glacier loss feed into uncertain projections of sea‑level rise; imperfect representation of these processes in models produces substantial variance in century-scale outcomes relevant to infrastructure planning [1] [8] [9].
3. Model structure and regional projections: “global confidence, local fuzziness”
While global mean temperature projections have become better constrained—recent work shows uncertainties in projected global warming have fallen over the past decade—translating that improved global signal into reliable local and regional projections remains challenging because models differ in how they represent circulation changes and local feedbacks, limiting the utility of projections for site-specific adaptation decisions [10] [11] [12].
4. Scenario uncertainty: human choices that science cannot predict
A separate, irreducible source of uncertainty is the future emissions pathway itself: decisions by societies about energy, land use, and policy determine concentrations and therefore warming trajectories; scenario uncertainty is policy‑driven and orthogonal to climate‑system uncertainty, meaning projections must be conditioned on plausible socioeconomic futures rather than presented as a single forecast [3] [4].
5. How uncertainty translates to policy dilemmas and choices
Uncertainty changes policy framing: it widens cost–benefit ranges in integrated assessment models and can push optimal policy toward either precaution (to avoid low‑probability high‑impact outcomes) or delay (if costs now are judged large relative to uncertain benefits); studies show that accounting for uncertainty generally argues for stronger near‑term mitigation to hedge tail risks, but it also requires iterative adjustment of targets as scientific understanding improves [5] [13] [4].
6. Practical responses: design for robustness, update as science refines
Policymakers facing these uncertainties use three recurring strategies in the literature: adopt no‑regrets measures and flexible, adaptive infrastructure; prioritize actions that reduce the most consequential uncertainties (observations and modelling) because uncertainty ranges have shrunk with better data; and stress-test investments against a range of scenarios so that long‑lived systems remain resilient if climate sensitivity or regional impacts land at the worse end of current bounds [14] [8] [11].
7. Politics, communication and the misuse of uncertainty
Uncertainty is frequently invoked by actors seeking delay; historical analyses warn that emphasizing scientific limits can serve political agendas that prefer inaction, while the scientific community and policy reviews stress that uncertainty is not symmetric and can imply both lower and higher risk—thus responsible policy treats uncertainty as a management problem, not an excuse [9] [2] [7].