Thread by @Weather_West

Some brief thoughts from a climate & Earth system scientist on natural disasters & #ClimateChange. This is coming from someone who grew up watching PBS specials on natural disasters and then went on to obtain a PhD focused on extreme events in the natural world. (1/n)

Various entities & individuals use different definitions regarding what, exactly, constitutes a "natural disaster." Some focus on physical characteristics of event itself; others focus on event's impacts on human systems. Both definitions can be useful in specific contexts. (2/n)

A key element in any definition of a natural disaster, though, is that the causative event must arise from a natural processes of the Earth--in other words, they stem from an atmospheric, geologic, hydrologic, or biospheric process of some kind. (3/n)

Colloquially, and even more formally in the geosciences, most folks would therefore consider things like earthquakes, hurricanes, floods, wildfires, and tsunamis to be "natural disasters." I certainly do, and most of my colleagues do. (4/n)

It's important to recognize that societal impacts of natural disasters are not just function of severity of event itself: in most cases, economic and human tolls are strongly context dependent. Certain interventions can greatly decrease (or increase) these societal impacts. (5/n)

This is succinctly summarized in a framework posed by the IPCC, which separates contributions to natural disaster "risk" from the actual physical hazard (i.e., the flood, wildfire, or storm) and the broader societal context (i.e., "vulnerability" and "exposure"). (6/n)

What's critical to understand here is that the physical themselves hazards and the vulnerability/exposure variables that shape subsequent societal impacts can change largely independently of one another. (7/n)

There are many real-world examples of how human interventions have decreased vulnerability (i.e.,development of effective storm warning & ocean surge evacuation infrastructure in Bangladesh) and increased exposure (i.e., higher population in flood zones due to urban sprawl).(8/n)

At the same time, there is now abundant and growing evidence that #ClimateChange is increasing many types of atmosphere-related physical hazards (like heatwaves, intense rainfall, storm surges, and wildfire severity). (9/n)
sciencedirect.com/science/articl…

Therein lies the critical distinction: for many atmospheric physical hazards, climate change is increasing the physical hazard component of the risk equation *irrespective* of whether humans are amplifying or reducing societal exposure and/or vulnerability. (10/n)

For some event types, successful exposure and vulnerability reductions may have historically balanced out increasing physical hazards brought about by climate change from a *net* disaster risk perspective. There is indeed evidence for this in certain regions. HOWEVER... (11/n)

Opposite is true for other events: climate change has increased physical hazard *at same time that societal factors have increased exposure*. Stark example of this is California wildfire risk: both climate & urban expansion play large role. (12/n)
iopscience.iop.org/article/10.108…

(We explored this confluence of aggravating human and climate-related factors with respect to California wildfire in a perspective piece two years ago:)
theguardian.com/environment/20… cc @pyrogeog @climate_guy (13/n)

This complexity is precisely why it is impossible to quantify the #ClimateChange contribution to natural disasters using metrics that conflate shifts in vulnerability/exposure and shifts in the underlying climate-related physical hazards (e.g., economic losses). (14/n)

Climate scientists work to isolate "physical hazard" component of disaster risk, since that's the component most directly affected by climate change. This is the primary goal of the emerging climate science sub-field of "extreme event attribution." (15/n)

https://twitter.com/Weather_West/status/1274007004072210433?s=20

One challenge when assessing overall natural disaster risk & climate change is that many *human* systems have non-linear impact thresholds. A seawall or levee, for example, may be an effective risk-mitigating strategy--right up until point flood waters flow over the top. (16/n)

As #ClimateChange worsens & associated physical hazards increase, human adaptations to reduce vulnerability & exposure often won't be able to keep pace. This is especially true in resource-limited contexts--not just Global South,but also poor communities in wealthy nations.(17/n)

All of this is to say: I think it's fundamentally misleading to say that #ClimateChange is not making natural disasters worse. From physical science perspective, it certainly is. That disaster *risk* can be strongly modulated by societal factors does not change this fact. (18/18)

Addendum: okay, maybe not so brief! But given the recent discourse, I think it's worth clarifying that climate scientists are fully aware that there's more to disaster risk than just physical events themselves, & that we explicitly account for that in our work and public claims.