Hydro-meteorological hazards vs climate extremes

Question

I always saw the definitions of "hydro-meteorological hazards" and "climate extremes" as almost interchangeable in the climate change and natural hazards literature. The other day I was discussing with a colleague claiming that "climate extremes" would be a sort of subset of the natural hazards.

Although, I can find reasonable that, somehow, not all the hazards are climate extremes, I still find formally acceptable to define the most common ones (floods, droughts, heat waves, storms ...) both as climate extremes and hydro-meteorological hazards. Articles in literature (as this, for instance), and also the IPCC Glossary do not seem to make a clear distinction.

Any specialist here that can help me to shed light on this?

Answer 1

The UN International Strategy for Disaster Reduction gives a good definition of a "hydrometeorological hazard": Natural processes or phenomena of atmospheric, hydrological or oceanographic nature, which may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation..

The IPCC define and "Extreme weather event" as follows: "An extreme weather event is an event that is rare at a particular place and time of year".

The debate around extremes is strongly focused on extremes which are of interest to society because of their harmful effect (extreme wind, extreme heat, extreme cold, etc), but the term itself could be interpreted as having a broader meaning. We might, for instance, experience extremes of the surface pressure difference between the Azores and Iceland (a statistic which is studied by climatologist because changes in this statistic have interesting links to weather patterns in Europe) -- but this pressure difference is not usually considered as a hazard.

An earlier answer by haresfur has given several examples of hydro-meteorological hazards which are not climate extremes. To these I would like to add:

• Coastal erosion: this is a continuous process caused by tides and storms. It is hazardous for anyone living in a coastal region;
• Lightning strikes: thunderstorms always pose a hazard. You could, of course, call every thunderstorm "extreme", but this is not the common usage. Thunderstorms can be common in a given region, but still be a hazard.

Hence, though the bulk of public and scientific debate occurs at the intersection of extremes and hazards, there can be hazards which are not extreme and extremes which are not hazardous.

Answer 2

I would agree that not all hydro-meteorological hazards are directly related to extreme events if you look at the whole hydrological cycle. In particular, vadose zone and groundwater processes damp out extreme events in the subsurface but can produce hazards. A few examples:

• Slope failure induced by saturation of the sediments. The conditions leading to the failure can take years to develop and the failure process can be very slow. For example, recently slope movement above Union Gap in Washington State caused closure of the roads and evacuation of houses until enough data were collected to determine it had stabilized for now
• Soil salinisation in Australia. When land was cleared for farming the trees were replaced by grasses which lead to greater infiltration and recharge of groundwater. Over years, this caused the watertable to rise to the surface in some places. Since the groundwater is often saline, the result was salt scalds killing the grass, erosion, and higher salt loads in the rivers
• Groundwater dependent ecosystems get much of their water from the subsurface. Long term changes in the flow system, often exacerbated by pumping or land use change, can cause water table declines that either directly affect the terrestrial ecosystem (eg wetlands) or reduce baseflow to rivers

There are also hazards associated with weather events that are not particularly extreme. If it snows, there will be car accidents. The focus on hazards from extreme events is more because they are unusual and tend to affect what events we consider extreme.