Reduced Complexity Model Intercomparison Project Phase 2: Synthesising
Earth system knowledge for probabilistic climate projections
Abstract
Over the last decades, climate science has branched out into many
smaller expert communities across the carbon cycle, radiative forcings,
climate feedbacks or ocean heat uptake domains. Our best tools to
capture state-of-the-art knowledge are the increasingly complex fully
coupled Earth System Models (ESMs). However, computational limitations
and the structural rigidity of ESMs mean that the full range of
uncertainties are difficult to capture with multi-model ESM ensembles or
single ESM perturbed parameter ensembles. The tools of choice are hence
more computationally efficient reduced complexity models (RCMs), which
are structurally flexible and can span the response dynamics across a
range of domain-specific models and/or ESM experiments. Here, we provide
the first comprehensive intercomparison of multiple RCMs that are
probabilistically calibrated to key benchmark ranges from specialised
research communities. This exercise constitutes Phase 2 of the Reduced
Complexity Model Intercomparison Project (RCMIP Phase 2). We find that
even if RCMs perform similarly against historical benchmarks, their
future projections can still diverge. Under the low-emissions SSP1-1.9
scenario, across the RCMs, median 2081-2100 warming projections range
from 1.1 to 1.4{degree sign}C while median peak warming projections
range from 1.3 to 1.7{degree sign}C (relative to 1850-1900, using an
observationally-based historical warming estimate of 0.8{degree sign}C
between 1850-1900 and 1995-2014). Our findings suggest that users of
RCMs should carefully evaluate the RCM they are using, specifically its
skill against key benchmarks and consider the need to include future
projections benchmarks either from ESM results or other assessments to
reduce such divergence.