by Judith Curry

The National Academies has published a new report: Attribution of extreme weather events in the context of climate change.

The authors of the report are:

DAVID W. TITLEY (Chair), Pennsylvania State University

GABRIELE HEGERL, University of Edinburgh, UK

KATHARINE L. JACOBS, University of Arizona, Tucson

PHILIP W. MOTE, Oregon State University, Corvallis

CHRISTOPHER J. PACIOREK, University of California, Berkeley

J. MARSHALL SHEPHERD, University of Georgia, Athens

THEODORE G. SHEPHERD, University of Reading, UK

ADAM H. SOBEL, Columbia University, New York, NY

JOHN WALSH, University of Alaska, Fairbanks

FRANCIS W. ZWIERS, University of Victoria, BC, Canada

This paragraph from the Concluding Remarks of the Summary provides a good overview:

In the past, a typical climate scientist’s response to questions about climate change’s role in any given extreme weather event was “we cannot attribute any single event to climate change.” The science has advanced to the point that this is no longer true as an unqualified blanket statement. In many cases, it is now often possible to make and defend quantitative statements about the extent to which human-induced climate change (or another causal factor, such as a specific mode of natural variability) has influenced either the magnitude or the probability of occurrence of specific types of events or event classes. The science behind such statements has advanced a great deal in recent years and is still evolving rapidly.

Key points from the 14 page summary:

Event attribution is more reliable when based on sound physical principles, consistent evidence from observations, and numerical models that can replicate the event.

Confidence in attribution findings of anthropogenic influence is greatest for those extreme events that are related to an aspect of temperature, such as the observed long-term warming of the regional or global climate, where there is little doubt that human activities have caused an observed change

Confidence in attribution analyses of specific extreme events is highest for extreme heat and cold events, followed by hydrological drought and heavy precipitation. There is little or no confidence in the attribution of severe convective storms and extratropical cyclones.

Attribution of events to anthropogenic climate change may be complicated by low-frequency natural variability, which influences the frequencies of extreme events on decadal to multidecadal timescales.

A definitive answer to the commonly asked question of whether climate change “caused” a particular event to occur cannot usually be provided in a deterministic sense because natural variability almost always plays a role.

Attribution studies of individual events should not be used to draw general conclusions about the impact of climate change on extreme events as a whole.

Unambiguous interpretation of an event attribution study is possible only when the assumptions and choices that were made in conducting the study are clearly stated and uncertainties are carefully estimated.

Bringing multiple scientifically appropriate approaches together, including multiple models and multiple studies helps distinguish results that are robust from those that are much more sensitive to how the question is posed and the approach taken

A focused effort to improve understanding of specific aspects of weather and climate extremes could improve the ability to perform extreme event attribution.

The most illuminating part of the report is the key recommendations:

In particular, this committee recommends research that aims to improve event attribution capabilities, which includes increasing the understanding of:

 the role of dynamics and thermodynamics in the development of extreme events;

 the model characteristics that are required to reliably reproduce extreme events of different types and scales;

 changes in natural variability, including the interplay between a changing climate and natural variability, and characterization of the skill of models to represent low frequency natural variability in regional climate phenomena and circulation;

 the various sources of uncertainty that arise from the use of models in event attribution;

 how different levels of conditioning (i.e., the process of limiting an attribution analysis to particular types of weather or climate situations) lead to apparently different results when studying the same event;

 the statistical methods used for event attribution, objective criteria for event selection, and development of event attribution evaluation methods;

 the effects of non-climate causes—such as changes in the built environment (e.g., increasing area of urban impervious surfaces and heat island effects, land cover changes), natural resource management practices (e.g., fire suppression), coastal and river management (e.g., dredging, seawalls), agricultural practices (e.g., tile drainage), and other human activities—in determining the impacts of an extreme event;

 expected trends in future extreme events to help inform adaptation or mitigation strategies (e.g., calculating changes in return periods to show how the risk from extreme events may change in the future);

 the representation of a counterfactual world that reliably characterizes the probability, magnitude, and circumstances of events in the absence of human influence on climate.

Event attribution capabilities would be improved with better observational records, both near-real time and for historical context. Long homogeneous observed records are essential for placing events into a historical context and evaluating to what extent climate models reliably simulate the effect of decadal climate variability on extremes.

Statements from the authors

The Carbon Brief provides a good overview of the study and interviewed a number of scientists that authored the report and others who were not authors but who have conducted research on this topic.

Heidi Cullen’s NYTimes op-ed, excerpts:

One view holds that no single storm or drought can be linked to climate change. The other argues that all such things are, in some sense, “caused” by climate change, because we have fundamentally altered the global climate and all the weather in it.

While true, this “all in” philosophy doesn’t adequately emphasize the fact that not all of the extreme weather we experience today has changed significantly. Some of it is just, well, the weather.

But some of our weather has changed significantly, and now a new report from the National Academies of Sciences, Engineering and Medicine has outlined a rigorous, defensible, science-based system of extreme weather attribution to determine which events are tied to climate change.

Like the surgeon general’s 1964 report connecting smoking to lung cancer, the report from the National Academies connects global warming to the increased risk and severity of certain classes of extreme weather, including some heat waves, floods and drought.

This is an important development. Climate change can no longer be viewed as a distant threat that may disrupt the lives of our grandchildren, but one that may be singled out as a factor, possibly a critical factor, in the storm that flooded your house last week. The science of extreme weather attribution brings climate change to our doorsteps.

And sometimes it is easier to blame climate change than acknowledge inaction in the face of factors unrelated to the weather. For example, in a severe drought plaguing southeastern Brazil — including São Paulo, with a surrounding metropolitan population of about 20 million — some were quick to blame global warming. But in analyzing the underlying causes of this drought, my colleagues and I found, in a study published last year, that climate change was not a major influence. Instead, population growth, increasing water consumption and leaky pipes were the real culprits.

From Adam Sobel’s WaPo editorial: Dr. Sobel

Though attribution science is advancing quickly, it’s still new, and some scientists are uneasy about it. Some are concerned that it politicizes weather disasters by making them into climate change stories. I have been concerned, on the other hand, that stories focused on attribution in the wake of weather disasters can send misleadingly skeptical messages about climate change as a whole.

So media coverage of attribution studies sometimes ends up focusing more on what we don’t know than what we do. That can leave the impression that we know less than we really do, which is unhelpful in a political climate which already doesn’t take the real one seriously enough.

But attribution studies help to close the gap between the widespread notion of climate change as distant and the real need for us to act on it now. Real extreme weather events get people’s attention. Sometimes, some of that attention lands on broader issues around climate change that are overdue for it.

Ted Shepherd

Co-author Ted Shepherd has just published a very nice paper A common framework for extreme event attribution. From the conclusions:

In climate science, we are accustomed to strive for quantitative answers, but it is important to appreciate that being quantitative is not necessarily the same thing as being rigorous. In particular, it is essential to distinguish between quantifiable uncertainty and Knightian (i.e. deep) uncertainty. Uncertainty associated with sampling variance is quantifiable, e.g. through boot-strapping methods, but many of the uncertainties associated with climate change—especially the deep uncertainties associated with the atmospheric circulation response to climate change—are not easily quantifiable. Examining the sensitivity of a result to the choice of climate model, as is becoming common practice in risk-based ap- proaches, is an important first step in determining robustness. However, model spread is not a quantification of model uncertainty because a multi-model ensemble does not represent a meaningful probability distribution. It has therefore been argued that the quest for more accurate climate model predictions is illusory and that instead we need to be using models for understanding, not prediction. JC reflections While this report contains no surprises or new assessments, it is more cautious about extreme event attribution than is Myles Allen [link]. That said, I remain highly critical of efforts to attribute extreme weather events to human caused climate change. I have written many previous blog posts on this topic [link]. Detection and attribution of extreme weather events requires: a very long time series of historical observations of the extreme event at a particular location (which is rarely available)

an understanding of the variability of extreme weather events associated with multi-decadal ocean oscillations (which requires at least a century of observations)

climate models that accurately simulate both natural internal variability on timescales of years to centuries and the extreme weather events (a pipe dream; we are not even close) IMO the best, and most important, paper on extreme event attribution is a paper by Hall et al. [discussed previously at this link]. This paper by Hall et al. is not even referenced in the new National Academies report. And finally, I find nothing in this report that convinces me that extreme event attribution is feasible. The most plausible attribution is for heat waves; however the recent Sardeshmukh paper [link] finds that the intuitively reasonable attribution of more heat waves to a higher average temperature doesn’t work in most land regions. I will repeat my summary from a previous post: Not sure what the motive is for the attribution of extreme events, other than to build political will for climate change policies. More comprehensive analysis of regional extreme events (including those in the paleo records, of which we need more of) in the context of known modes of natural climate variability is probably the single most useful thing that could be done in this regard.