The Wonderful Issues of Climate Change - Post #6 - Extreme Weather

Original Date: January 25, 2024; Revised February 25, 2024

Author: Tim Platnich

It is often reported in the media that a given 'extreme weather event' was caused by climate change. Indeed, ironically, the recent (mid-January, 2024) extreme cold experienced across Canada and the US has been attributed to global warming. See, however the recent Nature article that finds 'no detectable trend in mid-latitude cold extremes during the recent Arctic amplification'.

The attribution of weather events to global warming is part of an area known as 'attribution science'. Attribution science is another area where modelling is used. Modelling was discussed generally in the The Wonderful Issues of Climate Change - Post #5. In this post, we will focus on modelling in the context of attribution science.

“Attribution science concerns the identification of causes for changes in characteristics of the climate system (e.g., trends, single extreme events).”[IPCC, AR6 WGI (2023) Chapter 11, p.28]

“… since AR5, the attribution of extreme weather events has emerged as a growing field of climate research with an increasing body of literature … including the number of approaches to examining extreme events.”[AR6] Approaches include the ‘probability-based approach’ with statements like – climate change has made this event type twice as likely or 15% more intense. [AR6]

“The outcome of event attribution is dependent on the definition of the event … and uncertainties in observations and modelling.”[AR6] Results of attribution studies can be sensitive to the choice of climate variables.

Regarding ‘attribution’, Tim Palmer [In his book, "The Primacy of Doubt"] explains it this way. Some models are run that assume no warming. Other models are run that show different degrees of warming. Within the models, predictions will be made about, say, the number of droughts that will occur in California over a 100 year period. If the models show a substantial increase in number, one then is able to say that droughts are more likely with warming than not. Palmer concedes that an individual drought event may not be attributed to warming but he argues that the probability of drought can and should be used for mitigation purposes, if nothing else. e.g. if a 1/1000 year drought become as 1/10 year drought, different mitigative measures are warranted.

Palmer makes several good points but one that sticks out, and should be kept in mind by critical thinkers when reading media reports: an individual event (like his example of drought) may not be attributed to warming but rather may be part of natural variability.

An extreme weather event is described in IPCC, AR6 as an event that is rare at a particular place and time of year. [AR6, Chapter 11, p. 10]. Particular examples of extreme weather events are considered to include: record temperatures (heat waves); cyclones; severe convective storms - storms associated with tornadoes, hail, heavy precipitation, strong winds and lightning [AR6, s. 11.7.3]; extreme precipitation events; dought; flooding; and wildfires.

Further to Tim Palmer's statement above, according to the IPCC, “Scientists cannot answer directly whether a particular event was caused by climate change, as extremes do occur naturally, and any specific weather and climate event is the result of a complex mix of human and natural factors.”[AR6 Chapter 11, p.99]. Notwithstanding this, it is theorized, based on models, that some extreme events are becoming more probable and more intense.

The tricky issue surrounding attribution of severe weather events is this: are the severe weather events a result of global warming or are they merely part of natural variability?

If there is a causal connection between global warming and severe weather events, one would at least expect to see an increase in severe weather events correlated with the warming. By increase in severe weather events, two things are often meant: frequency and/or intensity. What does the data say?

The data, and interpretations thereof, are mixed at best. Following are some examples.

Regarding heat waves, according to IPCC AR5, since about 1950 globally it is very likely:

a.     the number of cold days and nights have decreased; and

b.     the number of warm days and nights have increased.

Furthermore, there is medium confidence that globally the length and frequency of warm spells, including heat waves have increased since 1950.

AR6 is to the same effect but the likelihood has gone from ‘very likely’ to ‘virtually certain’.

In the US, the world’s most extensive and highest-quality weather data indicates that record lows have become less common but that record high temperatures are no more frequent than they were a century ago ["Unsettled", pp. 100 and 102]. In fact, empirical data suggests that heatwaves in the US were more common in the past, for example in the 1930s when atmospheric CO2 was 75% of what it is today.[ "Impacts of Climate Change: Perception and Reality", p.2]

An analysis of ‘absolute records” of maximum highs in the US from 1895 to present shows no significant trend over the 120 years of observation or even since 1980 [Unsettled, p.106] The same analysis shows that the numbers of record daily cold temperatures have decreased over 120 years with that trend accelerating after 1985 [Unsettled, pp 106 and 107]

The difference between global trends and the US trends illustrates the regional aspects of climate change.

The IPCC’s AR5 WGI (2014) report indicated a low level of confidence regarding any observed trends: in increased flooding, drought or dryness on a global scale; in increased small scale weather phenomena such as hail storms or thunderstorms; or in the increased intensity of cyclones [as reported in Koonin, "Unsettled", p. 98]

There is little evidence that recent warming has worsened extreme events such as floods, droughts, wildfires and hurricanes. [ Curry, "Climate Uncertainty and Risk", p. 10]

Regarding cyclones, the generic term is tropical cyclones. They are called hurricanes in the Atlantic Ocean and in the eastern Pacific. They are called typhoons in the western Pacific and just cyclones in the Bay of Bengal and northern Indian Ocean [Unsettled, p. 112].

Despite headlines to the contrary, the underlying science shows that there have been no significant trends beyond natural variability in the global number of tropical cyclones, in their intensity or in their related rainfall or storm surge flooding. ["Unsettled", pp. 116-117, p.120]. Between 1870 and 2017 there has been no significant increase in either the frequency or intensity of hurricanes in the US which is consistent with global statistics ["Impact", supra at p.3].

In its AR6 WG1 report, released in August 2021, the IPCC wrote, “[i]dentifying past trends in TC [tropical cyclones] metrics remains a challenge …. and [t]here is low confidence in most reported long-term trends in TC frequency or intensity-based merits….” [AR6, Chapter 11, p.1585].

Regarding severe convective storms, AR6 concludes that “there is low confidence in observed long-term (40 years or more) trends in TC (tropical cyclone) intensity, frequency, and duration, and any observed trends in phenomena such as tornadoes and hail … .”[AR 6, Chapter 11, s.11.7, p.1583].

The above referenced data is current until about 2022. Data from 2023 will need to be added into the mix. Regarding 2023 , this link is to NOAA's Annual 2023 Global Climate Report of Climate Anomalies and Events. The question becomes: was 2023 an outlier (as suggested by the prior data referenced above) or evidence of a trend. If an outlier, was 2023 evidence of the effects of global warming, or just part of natural variability? Let's wait to see what 2024 brings. It should be noted that 2023 was affected by a strong El Nino which is predicted to continue through the first 4-5 months of 2024.