  Despite major advances in climate science over the last 30 years, persistent
uncertainties in projections of future climate change remain. Climate
projections are produced with increasingly complex models which attempt to
represent key processes in the Earth system, including atmospheric and oceanic
circulations, convection, clouds, snow, sea-ice, vegetation and interactions
with the carbon cycle. Uncertainties in the representation of these processes
feed through into a range of projections from the many state-of-the-art climate
models now being developed and used worldwide. For example, despite major
improvements in climate models, the range of equilibrium global warming due to
doubling carbon dioxide still spans a range of more than three. Here we review
a promising way to make use of the ensemble of climate models to reduce the
uncertainties in the sensitivities of the real climate system. The emergent
constraint approach uses the model ensemble to identify a relationship between
an uncertain aspect of the future climate and an observable variation or trend
in the contemporary climate. This review summarises previous published work on
emergent constraints, and discusses the huge promise and potential dangers of
the approach. Most importantly, it argues that emergent constraints should be
based on well-founded physical principles such as the fluctuation-dissipation
theorem. It is hoped that this review will stimulate physicists to contribute
to the rapidly developing field of emergent constraints on climate projections,
bringing to it much needed rigour and physical insights.
