commentary
Acts of God, human influence
and litigation
Sophie Marjanac, Lindene Patton and James Thornton
Developments in attribution science are improving our ability to detect human influence on extreme
weather events. By implication, the legal duties of government, business and others to manage
foreseeable harms are broadening, and may lead to more climate change litigation.
A
dvances in the science of extreme
weather event attribution have the
potential to change the legal landscape
in novel ways. Identifying the human
influence in events once known as ‘acts of
God’ is likely to inform litigation relating to
claims and liability for damages. Attribution
science is also leading to better predictions
of the expected severity of certain types of
weather-related natural disasters. Such a shift
in our understanding of extreme events could
have legal implications for decision-makers
with a duty to manage foreseeable harm and
plan for the future.
Litigation may play an important role in
reducing greenhouse gas emissions in the
absence of enforceable commitments from
government1. Despite the shifting sands
of climate politics — or perhaps because
of them — the courts are being asked to
play an increasing role in apportioning
responsibility for loss and damage resulting
from climate change2.
Improvements in attribution science are
affirming the foreseeability of certain climatic
events and patterns in specific locations,
and in identifying increasing risks of
consequential impacts on property, physical
assets and people. Such improvements are
key from a legal point of view, because
foreseeability of damage is an important
requirement to establish a duty of care in
many legal systems.
Determining the foreseeability of an
action, event or loss may therefore inform
common-law-based litigation related to
directors’ and officers’ liability, professional,
sovereign, premises and product liability,
and more. The question is not whether
there will be another wave of climate-related
litigation — the wave is already in motion.
The question instead is whether it will be
more successful than previous efforts3.
We expect that evidence from attribution
science will catalyse future climate change
litigation. Such cases are likely to involve
616
actors such as local government agencies,
built-environment professionals, and
companies and their directors alleged to have
had duties of care or special knowledge about
specific climate-related risk (Box 1).
Claims are likely to arise when those actors
fail to share or disclose relevant knowledge,
or fail to take adaptation actions that would
have protected those to whom they owed
a duty of care. Such litigation may become
an important driver of both mitigation of
greenhouse gas emissions and adaptive action
by both public and private sectors.
Attribution science
Probabilistic event attribution is the science
of seeking to determine the extent to which
anthropogenic climate change has altered
the probability or intensity of a particular
weather event or class of weather events,
with an assignment of statistical confidence4.
Existing methodologies have been deemed
robust by the National Academy of Sciences,
though uncertainties remain and confidence
is far higher in studies of extreme heat and
cold events5.
Event attribution is a relatively new
discipline that developed in response
to interest from outside the scientific
community in the extent to which damaging
extreme events can be attributed to humaninduced climate change or natural climatic
variability, or both.
The primary approach used is to compare
the changes in the observed record over
time with climate model simulations. The
‘real world’, defined through observations
and models, is compared to a ‘counterfactual
world’ modelled without human forcings
(greenhouse gases and aerosols), an
approach that allows isolation and analysis
of the influence of anthropogenic factors.
In 2004, an attribution study analysed
the link between anthropogenic climate
change and the 2003 European heatwave
(Fig. 1)6. Since then, the conclusions of
event attribution studies have become
more confident — qualitatively and
quantitatively — in their expression of the
(probabilistic, not determinative) causal
relationship between anthropogenic
greenhouse gas emissions and certain
extreme weather events. In addition, the
ability of scientists to differentiate between
natural and human-caused drivers of
temperature extremes, droughts and heavy
rain events has improved markedly7.
The soundness of the scientific
conclusions are evaluated based on the three
pillars of attribution science: the quality
of the observational record; the ability of
models to simulate the event being studied;
and scientific understanding of the physical
processes that drive the event and how they
are being impacted by climate change.
It should be noted that all attribution
studies express their findings in probabilistic
terms, as scientists reject the notion that
deterministic attribution of weather events
is ever possible — because it is impossible
to say that the event would ‘never’ have
occurred in the ‘counterfactual’ world8. We
wish to emphasize, however, that this does
not diminish the utility of attribution science
for the law and liability.
In the UK, courts considering
occupational exposure to toxic substances
have accepted probabilistic evidence as
proving causation when such evidence
demonstrates that the risk of the event
occurring was increased by a factor of
2:1, known as the doubling-of-the-risk
test9–12. In the US, toxic tort litigation has
adopted similar tests for situations where
deterministic causation is impossible, known
as proximate cause13–16.
In addition, the law in civil cases accepts
as proven any evidence that is shown to be
correct ‘on the balance of probabilities’ or
‘more likely than not’ (that is, with certainty
of >50%). Indeed, a British judge has stated:
“…in the event that the epidemiological
NATURE GEOSCIENCE | VOL 10 | SEPTEMBER 2017 | www.nature.com/naturegeoscience
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