Updated assessment of climate tipping elements suggests
>1.5oC global warming could trigger multiple tipping points
Abstract
Over the past 15 years climate tipping points (CTPs) – which are
reached when change in a climate subsystem (the ‘tipping element’)
becomes self-perpetuating independent of the original forcing and
results in a regime shift to a new subsystem state – have emerged as a
source of scientific and public concern. Some CTPs are estimated to be
reachable within the 1.5-2oC Paris Agreement range, with many more
accessible by the ~3-4oC of warming possible on current
policy trajectories. Recent work has also hypothesised that CTPs could
‘cascade’ – with the impacts of triggering one tipping element
sufficient to trigger the next and so on – resulting in an emergent
global tipping point. However, much discussion relies on a tipping
element characterisation that is now over a decade old, which itself was
based on an expert elicitation exercise in 2005. Since then there have
been substantial advances in our understanding of CTP dynamics based on
results from coupled and offline models, observations, and palaeoclimate
studies. The tipping cascade hypothesis has also not yet been rigorously
tested, with the suggestion of 2oC as a global tipping point remaining
speculative. Furthermore, CTP definitions are often inconsistent, with
some purported globally-impactful CTPs more accurately represented as
either localised CTPs or even threshold-free feedbacks. Here we
undertake an updated review of CTPs based on a wide range of recent
literature. We estimate ranges for each proposed element’s tipping
threshold, timescales, and impact on global and regional warming, as
well as if evidence exists for self-perpetuation, rate-dependence, or
hysteresis. Each proposed element is then catalogued with reference to a
clear tipping point definition, separating global ‘core’ and regional
‘impact’ tipping elements from threshold-free feedbacks. Our estimates
confirm that current global warming (~1.2oC) already
lies within the lower end of some CTP threshold ranges, and several CTPs
become likely or possible within the 1.5-2oC Paris range. In further
work we use these estimates to test the potential for and impact of
tipping cascades in response to global warming scenarios using a
stylised model.