by Judith Curry

“The current thinking and approaches guiding this conceptualization and description have been shown to lack scientific rigour, the consequence being that climate change risk and uncertainties are poorly presented. The climate change field needs to strengthen its risk science basis, to improve the current situation.” – Norwegian risk scientist Terje Aven

For decision-makers, climate change is a problem in risk assessment and management.  Climate change is a risk because it may affect prosperity and security in a negative way, and because its consequences are uncertain.

Global climate change policy has been dominated by a specific strategy of risk management – the Precautionary Principle as a justification for setting specific targets for the elimination of manmade emissions of carbon dioxide.  In the early 1980s, the United Nations Environmental Program (UNEP) became bullish on the idea that fossil fuels would produce dangerous climate change. The prospect of eliminating fossil fuels was congruent with UNEP’s broader interests in environmental quality and world governance. At Villach in 1985 at the beginning of the climate treaty movement, the policy movement to eliminate fossil fuels became detached from any moorings in the science – the rhetoric of precaution argued that we should act anyway to eliminate fossil fuels, just in case. This perspective became codified by the 1992 UN Framework Convention on Climate Change (UNFCCC) Treaty in 1992, the Kyoto Protocol in 1997 and the 2015 Paris Climate Agreement.

Instead of framing the IPCC assessments around risk assessment, the IPCC reports narrowly framed its assessments to support the UNFCCC policies, focusing on dangerous climate change associated with fossil fuel emissions.  The torquing of climate science and the manufacture of a consensus around dangerous human-caused climate change not only oversimplified the scientific and social challenges, but led to the adoption of a “predict then act” strategy to manage and control, supporting decisions about elimination of fossil fuel emissions that were begun in the 1980’s.  The congruence of the IPCC assessments and UNFCCC policies enforces the belief that climate change is a simple or tame problem, with science trumping all practical questions and conflicting values and purposes.

This strategy of risk management implies that climate change is a simple, tame problem of “dose-response.”  This characterization has led to the relative neglect of climate risk in formal assessment processes such as the IPCC.  It is only in their most recent assessment report, AR6, that a consistent risk framing of climate change was attempted (it will be interesting to see what this looks like in the forthcoming WGII, III reports).  As a result of the early adoption of a preferred risk management strategy, we are far from a complete assessment of full climate risk.

By characterizing climate change as a well-understood problem with a strong consensus, traditional risk management approaches assume that climate change can and ought to be rationally managed, or at the very least contained, and preferably eliminated.  However, the diversity of climate-related impact drivers and their complex linkages, various inherent and irreducible uncertainties, ambiguities about the consequences of climate change, and the unequal distribution of exposure and effects across geography and time, confound any simple or uncontested application of traditional risk management approaches. As a result, the policy process that has evolved over the past several decades is not only inadequate to deal with the risks associated with climate change, but has fueled societal controversies around climate risk.

Risk has often been characterized as some type of statistical expected value – the product of the likelihood of occurrence and the impact.  However, such a characterization is appropriate only for simple, or tame, problems.  Broader definitions of risk integrate specified consequences of an event or actions, a measure of uncertainty associated with the consequences, and the strength of the knowledge base that supports the assessment.

Accepting the IPCC’s assessments as the “best available” knowledge base is not inconsistent with acknowledging significant weaknesses in the knowledge base in context of climate risk analysis.  An important element of characterizing risk is evaluating the strength of the knowledge base.  Concerns about strength of the knowledge base are raised by people questioning aspects of the IPCC’s assessment that are used to infer climate risk.  The IPCC approach is based on judgement of the available evidence and agreement among experts.  More sophisticated knowledge characterizations for risk management (Aven 2017b) include:

  • the degree to which the assumptions made are reasonable/realistic – growing concern about the focus on implausible emissions scenarios RCP8.5/SSP5-8.5.
  • the degree to which data/information exists and are reliable and relevant – the historical and paleo data base is inadequate for a full, global characterization of natural climate variability on multi-decadal to millennial time scales
  • the degree to which there is disagreement among experts (including those from different environments) – attempts to suppress disagreement and alternative perspectives among experts
  • the degree to which the phenomena involved are understood and accurate models exist – concerns about the fidelity and utility of climate models.
  • the degree to which the knowledge has been thoroughly examined with respect to unknown knowns (i.e. others, but not the analysis group, have the knowledge) – neglect of the unknown knowns associated with natural climate variability.

The politics of international climate governance has produced systematic biases in the kinds of expertise and evidence that are deemed appropriate for consideration. (Lucas)  The UNFCCC and IPCC have characterized climate change as an environmental and economic problem, and geoscientists and economists have dominated the assessment and policy making process.

However, the issues with the current CO2 increase and warming are social, not environmental. The Earth has undergone geological periods of higher temperatures and atmospheric CO2 concentrations, during which life thrived.  Characterization of climate change as an environmental problem has downplayed the cultural and political dimensions of the issue. Many social scientists have argued that the disciplinary constrictions imposed by the IPCC and UNFCCC have neglected many important insights arising from a wide range of expert and unaccredited sources.

A risk assessment for a problem such as climate change – with high levels of complexity, uncertainty and ambiguity – must include the following elements (King et al. 2015):

  • Clarify objectives of the risk analysis – the dangers or values at risk
  • Take a holistic view of all relevant factors
  • Identify the biggest risks – the plausible worst-case scenarios
  • Be explicit about value judgements

Values and dangers

One of the biggest problems associated with climate change risk assessments is that there is no simple way to articulate danger associated with a warmer climate.  However, in attempting to build political will for the international treaties, the adverse impacts of fossil-fuel driven warming have been exaggerated – severe weather/climate events, sea level rise and many adverse ecosystem, health, economic and geopolitical impacts, with all of their complex causes, were confounded with fossil-fuel driven warming.  Further, the risks from fossil fuel emissions have not been placed in the appropriate context of other global and regional risks.

A key element of risk assessment is to judge whether activities are acceptable, tolerable, or intolerable.  Activities are tolerable if they are considered as worth pursuing for the associated benefits. For tolerable risks, efforts for risk reduction or coping are welcomed provided that the benefits of the activities are not lost.  Burning fossil fuels has historically been considered a tolerable risk.  Genuinely intolerable risks include existential threats – such as portrayed by the earth-impacting comet in the move Don’t Look Up – or “ruin” problems.  For less dire threats that are considered intolerable, notwithstanding the benefits, risk management should be focused on banning or phasing out the activity creating the risk or, if that is not possible, to mitigate or fight the risk in other ways or to increase societal resilience.

How to draw the lines between “acceptable”, “tolerable” and “intolerable’’ is one of the most controversial tasks in the risk governance process for complex risks. Ambiguity results from divergent and contested perspectives on the justification, severity or wider meanings associated with a perceived threat (Stirling 2003).  Climate change risks have been characterized as acceptable, tolerable and intolerable by different individuals and constituencies – clearly an ambiguous situation.  “Ambiguity” means that there are different legitimate viewpoints from which to evaluate whether there are, or could be, adverse effects and whether these risks are tolerable. Ambiguity results from divergent and contested perspectives on the justification, severity or wider meanings associated with a perceived threat (Stirling 2003).

Subjective value judgments are inherent both in identifying what constitutes a risk, and in deciding how much we care about it. All formal climate change risk assessments are structured by underlying values and normative goals that are sometimes explicit but often hidden. These values include societal attitudes to the intrinsic value of nature, misperceptions of risk, and implicit judgements on the acceptability or aversion to inequality in society.

Judgments of intolerable risks from climate change relate to mistakenly conflating the slow creep of global warming with consequences associated with extreme weather and climate events, concerns about inequitable risk exposure to poorer populations, and concerns about future generations.

Climate change risk includes elements of both incremental risk (e.g. the slow creep of sea level rise) and emergency risk.  Emergency risks are associated with extreme weather events; technically these are weather risks and not climate risks, even if global warming could be shown to incrementally worsen the weather hazard. Weather risk can become climate risk if global warming causes the event to exceed a vulnerability threshold that otherwise wouldn’t have been exceeded by the weather event. Attempts are also made to assess incremental costs/damages associated with extreme weather events.  Such assessments are very challenging to make against the background of natural weather and climate variability.

Removing the risk from most extreme weather and climate events from the consequences of global warming diminishes the perceived urgency for reducing fossil fuel emissions. Mischaracterizing incremental risks as urgent has led to policies that are not only costly and suboptimal, but also arguably reduce resilience.  The poorest populations would benefit far more from access to grid electricity and help in reducing vulnerability from extreme weather events, than from reductions to the amount of CO2 in the atmosphere.

Moralizing surrounding the issue of climate change has regarded the climate change problem as a simple, righteous values choice:  Are you for the planet or against it? This moralizing neglects to understand that people engage in activities which are of value to them that happen to emit carbon as a byproduct. Further, this narrow moralizing systematically excludes important ethical values, such as improving the lives of the billion people presently living in unacceptable poverty or protecting other aspects of the environment.

The question of intergenerational equity (concerns about the grandchildren) is of special importance as there is a lag between the emissions of greenhouse gases and the occurrence of the damage.  There is no simple way to decide what duty of care we owe to future generations, but the IPCC’s socioeconomic pathways for the 21st century all have the world being better off by 2100, even under the most extreme emissions scenarios.

With this context, we need a broader ethical debate about what the consequences of climate change will be for what we humans have reason to value so that we can take credible actions to protect them.  This requires an equally careful consideration of beneficial as well as adverse consequences.

Holistic view of plausible worst-case scenarios

The IPCC assessments have focused on the likely range of warming, sea level rise and other impact drivers.  As I have discussed in many previous blog posts, the IPCC scenarios of 21st century climate do not provide a holistic perspective on 21st century climate change – they neglect a range of plausible scenarios of solar variability, volcanic eruptions, and multi-decadal to millennial natural internal variability.  Their interpretation of extreme weather and climate events is drawn from data since 1950 – ignoring longer historical data sets and paleo climate data sets.

The bottom line is that the IPCC has not provided a complete set of plausible scenarios of 21st climate change outcomes.  While models can be useful for understanding complex systems, factors that fall outside the consideration of a model should not be ignored. When a system is impossible to model in a meaningful way, scenarios may be developed to imagine its possible future states.

In addition to the risks associated with increased CO2 concentrations and the risks of rapidly transitioning away from fossil fuels, there is a need to recognized that natural climate variability and change along with extreme weather and climate events have equally important societal impacts. Further, human-caused climate change also includes emissions of non-CO2 greenhouse gases and aerosol particles plus land use changes.

For the first time in the AR6, attention is given to identifying worst case outcomes, beyond its misguided focus on the implausible RCP8.5/SSP5-8.5 emissions scenarios.  The AR6 treatment of sea level rise is exemplary in this regard, clarifying the strength of the knowledge base associated with different extreme scenarios.  The AR6 focus on regional climate change rightfully pulls away from the previous strategy of climate model generated scenarios as being adequate for this purpose, with a growing emphasis on physically-based storyline scenarios.  The historical data record, especially when it extends back into the 19th century, is arguably the richest source of extreme weather and climate scenarios for the 21st century.

How to assess the plausibility of scenarios involving a high level risk is a topic that has received too little attention.

Transition risk

Social amplification of risk can occur via responses to perceived outcomes, either in anticipation or in reaction.

The UNFCCC in its urgent drive for NETZERO emissions ignores transition risk. Consequences of a rapid transition to renewable energy include the economic costs of the transition, adverse environmental impacts associated with wind and solar energy and biofuels, impacts of the intermittency of renewable energy on energy reliability and cost, more complex and extensive electricity transmission infrastructure with a larger number of failure nodes, decrease in energy security, the extensive need for rare earth minerals and the associated changes in geopolitics.  These consequences of the transition are associated with a fairly solid knowledge base, leading many people to be more concerned about transition risks than they are about the more uncertain risks from climate change itself, with a far weaker knowledge base. The debate is then between tolerable but potentially unnecessary imposition of risks from the rapid transition away from fossil fuels, versus the highly uncertain impacts from climate change that are assessed as ranging from acceptable to intolerable by different individuals, countries and organizations.

The biggest risk from a rapid transition away from fossil fuels is arguably an opportunity cost – we are at risk of squandering our resources on effort that may not change the climate in a meaningful way, so that we do not have resources available for better solutions that improve human well-being in both the short and long terms.  Further, we are ignoring other risks that are arguably more important to near-term human well being that could be more productively addressed with the same resources.


The UNFCCC is promoting a solution to an exceedingly complex, uncertain and ambiguous problem, without the context of an adequate risk assessment that references the wider ethical issues and political and practical feasibility. As a result, we have neglected to truly understand the the climate system and the broader causes of vulnerabilities of human and natural systems, and to systematically and broadly evaluate the feasible policy space.

The end result is that after 30 years of the UNFCCC/IPCC, we are fixated on the minutiae of greenhouse gas emissions levels and the abstract and impossible problem of constraining atmospheric CO2 concentration – while ignoring natural climate variability and drastically simplifying the human side. As long as the current situation prevails, the IPCC’s assessments of anthropogenic climate change and the UNFCCC recommendations for action will remain seriously inadequate.