11 Topics from Chapter 11: Weather and Climate Extreme Events in a Changing Climate

TEMPERATURE EXTREMES

Mechanisms and Drivers

Temperature extremes in this context refer to the “increase in the intensity, frequency, and duration of warm extremes and the decrease in those of cold extremes.” To simplify, the increase in hot extremes refers to a collective “increase in the magnitude of extreme day and/or night temperatures, in the number of warm day and/or nights, and in the number of heat wave days”, and this definition applies similarly to cold extremes. When looking at temperature extremes, the concluded main driver is greenhouse gas forcing.

Additionally, these changes in extreme event magnitude are usually larger than the overall changes in global surface temperature, and this occurs due to a larger warming effect on land than on the ocean surface and feedbacks. The specific feedbacks that are of interest in this context are snow/ice- albedo-temperature feedbacks and soil moisture- evapotranspiration-temperature. Local forcings, such as changes in land use or aerosol release, also affect the global-scale temperature extremes.

The effect of land-atmosphere feedbacks is most prevalent in the mid-latitude regions. This particularly attributed to the soil- evapotranspiration feedback (described in the box), and the increase in sensible heat flux. When the temperature increases and soil warms, there is an increase in heat fluxed from soil to the surrounding environment, amplifying warming. Feedbacks relating to soil moisture have affected past as well as presently occurring heat waves in modeled projections and actual observations on a local and regional scale.

Additionally, the lack of evaporative cooling from reduced plant transpiration (due to increased heat) and stomata
resistance from plants that are under enhanced CO₂ conditions is a direct CO₂ forcing, resulting in higher warming on land. Another feedback that plays a significant role in amplifying temperature extremes and variability in high latitudes is the snow/ice-albedo feedback. In mid and high latitudes (particularly in the Northern Hemisphere), it contributes most to the expedited warming of colder areas.

Regional and local forcings, particularly land-use change and anthropogenic aerosol release, significantly supplement variability in temperature extremes in certain regions with high confidence. In terms of land use, wide-spread deforestation to convert land to other uses has contributed to almost 33% of increased warming some mid- latitude regions when compared to the pre-industrial era. Additionally, land in densely populated city areas lends itself to the urban heat island (UHI) effect, resulting in increased warming local to these areas when compared to their surrounding regions. However, certain aspects of agricultural land use practices can lead to the opposite effect—cooling of hotter temperature extremes. Practices like irrigation, crop intensification, and no-till farming have been proven to reduce temperature percentiles in the US Midwest as well as in mid-latitude regions in our present climate scenario. Another forcing that encourages cooling is the emission of aerosols. A reduction in the anthropogenic aerosol emissions over Europe since the 1980’s led to an expedited increase in summer warming innortheast Asia and western Europe, showing how regional emissions can affect larger areas.

Observed Trends

Overall trends based on past and current observations support an increase in the frequency and intensity of warmer extremes and a likewise decrease for colder extremes. Though these temperature-related trends vary depending on spatial-temporal factors, hot extremes are almost always correlated with an increase in the hottest temperatures recorded and the number of heat waves.

As referenced in Figure 11.2 in the IPCC report, global trends depict an increase in warm days/nights and a decrease in cold days/nights as well as an increase in the coldest and hottest temperature extremes.

Fig. 11.2.

This figure depicts a timeseries of globally-averaged annual maximum daily maximum (TXx) and the annual minimum daily minimum (TNn) over land. This shows a clear increase in the mean land temperature since the 1900’s and moving forward (projections), and as mentioned in the previous section, this correlates with an overall increase in temperature extremes (annual hottest temperature and annual coldest temperature).

Figure 11.9.

Specific geographic trends, specifically the TXx, TNn, and frequency of warm days exceeding the 90th percentile for that region (TX90p), are shown in Figure 11.9 for the years 1960 – 2018. These trends display an overall warming in almost all regions (as indicated by the dark burnt umber colour), where some of the most warming is shown for TXx is shown in Europe, South America, and the Arctic, which is consistent with the feedbacks and forcings we discussed earlier (specifically deforestation, snow/ice-albedo feedback, soil evapotranspiration, and reduced aerosol emission). As we can see, some of the most prominent changes are seen by the increase of frequency for TX90p, but not depicted in Figure 11.9 is how the frequency of colder nights (TN10p) has significantly reduced since the1950’s in almost all land regions. One important fact to note is that some areas do not have sufficient data to evaluate observed and predicted variability in temperature extremes (for example, in certain areas of Africa), but the areas where there is sufficient data availability robustly reinforce the trends of increased hot extremes and decreased cold extremes.

Overall, these reported extreme temperature variabilities are virtually certain on a global scale, and on a region scale, these observed trends are very likely. Both Africa and South America have lower confidence in these trends, but that occurrence relates to fewer studies and a lack of a data availability. Warming manifests with strong intensity in the Arctic as well.

Model Evaluation

The models used to project extreme temperature events tend to match observed occurrences; however, there is a tendency to overestimate heat extremes and underestimate cold extremes. Some potential reasons for this discrepancy hypothesize that certain land forcings are not adequately represented in climate models (such as the effect of deforestation/land use) and some mitigative activities (specifically irrigation) are not integrated into models. Though deforestation of temperate areas correlates with summer warming and winter cooling, several models predict warmed daytime temperatures. Additionally, as mentioned earlier, irrigation contributes to regional cooling, and it is not always accounted for in current models.

Detection and Attribution

When looking at where to attribute the primary cause of these temperature extremes, anthropogenic activities stand out as the most likely culprit. The previous IPCC report issued that same conclusion, stating that human influence likely increased the probability of extreme heat events. The mechanism for the increase in extreme temperatures, as stated in the IPCC report, is primarily due to greenhouse gas forcing. However, forcing from certain aerosols offsets the heating, and their cooling effects can be detected over Asia and Europe. As mentioned earlier, land tends to heat faster than water and 75% of moderate daily hot extremes over land mass attribute their cause to anthropogenic warming. The signals that indicate anthropogenic causes are distinct from natural forcing, and this observation is backed by different models. Additionally, models indicate that that without anthropogenic influence, these probability for these extreme temperature events falls close to zero.

The strength of these anthropogenic forcing influences on temperature events varies per region, and a part of this is due to the feedbacks and forcings we discussed earlier. Local forcings can attenuate or amplify the warming effects of greenhouse gases, but conscious agriculture techniques, such as irrigation and crop intensification can lead to cooling, particularly in North America, Europe, and India. Despite many regional variations and increases in uncertainties on smaller, more local scales, almost all studies conclude that human influence is a significant contributor to extreme temperature events.

Projections

The increase in warm temperature extremes on a global and region overall scales linearly with global warming matters, but this observation depends on the region. When looking at the mid-latitude areas, the warming of hotter extremes can double the rate of global warming. However, the biggest increase for temperature of the coldest days will occur in the Arctic regions at a rate that is triple global warming projections. When looking at thresholds for the highest temperature extremes, the probability of exceeding the warmer extremes will increase while the cold extremes will likewise decrease with global warming projections in a non-linear fashion. Overall, when looking at trends on land, the intensity and frequency of hot extremes will increase while the intensity and frequency of cold extremes will decrease when compared to historical baselines, and the confidence of this prediction increases as the mean global warming projections increase. In fact, temperature extremes are projected to increase more strongly that global mean temperatures, indicating that things are about to get a lot warmer than they already are for us.

COMPOUND EVENTS

Overview

The broadest, most accepted definition of compound events is “the combination of multiple drivers and/or hazards that contributes to societal or environmental risk”. This definition encompasses the risk framework utilized by the IPCC, and it is what compound events will refer to in the context of this report. Compound events can be classified into four different types:

1. Preconditioned events: “A weather-driven or climate-driven precondition aggravates the impacts of a hazard.”
2. Multivariate events: “Multiple drivers and/or hazards lead to an impact.”
3. Temporally compounding events: “A succession of hazards leads to an impact.”
4. Spatially compounding events: “Hazards in multiple connected locations cause an aggregated impact.”

The two contributing categories to compound events are drivers and hazards. Drivers are any variables, processes, and occurrences in the realm of climate and weather that happen over multiple and/or various spatial and temporal domains. Many drivers are linked to anthropogenic activities, though there are naturally occurring drivers, both of which will be mentioned in later sections. Hazards refer to extreme events, such as wildfires, droughts, heat waves, floods, etc., that precede and often cause negative impacts both societally and environmentally, though they can seldom have positive effects.

When these weather/climate events occur at the same time, in close succession, or concurrently in various regions, it can lead to synergistic impacts. The extreme events can be similar types (like clustered floods) or different types (like a simultaneous drought and heat wave), and this synergistic impact occurs because multiple events can exceed the coping capacity of a system (which refers to the adequacy of available resources to alleviate the impacts of a hazard). This phenomenon has been documented for multiple hazards, ranging from droughts to wildfires to coastal flooding. These compound occurrences can cause local weather impacts, such as extreme rainfall or extreme winds, but they can also instigate disruptions in human systems, such as food prices because of the affect on crop yields.

Additionally, we have learned that climate extremes tend to be more apparent in land than over sea, especially when paired with an increase of global mean temperatures to 1.5^oC or 2^oC. This occurs because the heat capacity of land is less than water, meaning that land requires less heat to raise its temperature. Therefore, an increase in emerging climate extremes over land increases the probability of compounds events as well as the amount of land area that faces impacts. Concurrent extreme events that take place at different events can still impact the same sector (as mentioned with the food prices and agriculture example above), and this will also become more likely with increase in global mean temperatures.

Concurrent Extremes in Coastal and Estuarine Regions

Coastal and estuarine ecosystems are some of the most vulnerable zones out there. There are many drivers at play that determine the health of these ecosystems, such as precipitation levels, river flow, sea level rise, storm surging, wave and tide flow, and river flow. Imbalances in these factors lead to coastal flooding, and compound floods occur when multiple drivers interact to cause extreme coastal flooding events.

The United States coast has taught us that there is a positive correlation between heavy precipitation and storm surging, indicating that heavier rainfall leads to an increase in storms. Due to this dependence, the likelihood of compound flood occurrence can be ascertained via the relationship between storm surge and river flow. A similar relationship between higher sea levels (a consequence of sea level rise) and high river discharge where those synergistic drivers tend to precede compound flooding in the coasts of North America, Australia, Europe, and Japan. In fact, the synergistic relationship of the drivers is crucial to modeling flood occurrences, where single driver analyses that consider each factor (i.e., storm surge or high stream flow) as independent can underestimate flood probabilities, potentially leaving a community unprepared.

Coastal ecosystems also bear the brunt of compound precipitation and wind extremes, often causing damage to built and natural environments. These compounds wind/heavy rainfall events occur mostly in coastal regions as well as areas prone to tropical cyclones. Finally, coastal erosion can be expedited due a combination of extreme wave height and duration.

These concurrent, compound extremes have only increased in frequency and magnitude, and coastal regions in the United States and Europe see an increase in compound flooding probability mostly due to an intensification in extreme precipitation and sea level rise. When projecting under high emission scenarios, the meteorological drivers leading to compound flooding are expected to increase by 25% by 2100. Additionally, this increase in sea level rise and storm surging is projected to not only increasing the frequency but also the intensity of compound coastal flooding. These predictions are made with medium confidence.

Concurrent Drought and Heat Waves

When looking at trends over land, we see that precipitation and temperature have a strong negative correlation, and this is mostly due to land-atmosphere feedbacks. This phenomenon forms the basis of the correlation between heat waves and droughts. Compound drought and heat wave events characterized by high temperatures and reduced rainfall have been seen in California, Australia, and large parts of Europe (specifically Germany where crop growing seasons have been particularly impacted by record-breaking heat), and projections for these events predict an increased probability of occurrence under unmitigated global warming.

Out of the two co-occurring events, heat waves constitute the dominant signal, and they are credited to anthropogenic forcing, implying that compound hot/dry events will continue in frequency and magnitude even if droughts are not projected to increase. This is because the main anthropogenic forcings are tied to the burning of fossil fuels and land use change, and those emissions will not drastically decrease or cease in the immediate future (on a global scale), meaning that heat waves will also continue as they are linked.

The same land conditions associated with drought and heat waves (high temperatures→low soil moisture→low humidity) are also correlated with fire weather, which is defined as “weather conditions conducive to triggering and sustaining wildfires…”. These compound hot/dry events also exacerbate wildfire conditions, and this can be seen in California when looking at the increase of wildfire burnt areas. Since a primary driver of wildfires is vapor pressure deficit, compound burn events are linked to anthropogenic factors (as described in the box above). On a global scale, the mean length of the fire-weather season increased by 19%, and we have experienced an increase in the frequency and magnitude of wildfires as Pacific Northwest residents. Projections back up these lived experiences as compound hot and dry events are expected to increase with high confidence, and future climate variability will enhance the occurrence of severity of wildfires in various biomes.

Wrap Up

Overall, compound events are linked to the underlying mechanisms of other trends discussed in Chapter 11, such as extreme temperature events and droughts or flooding. As global mean temperatures rise and anthropogenic forcings continue, we will continue to see amplification of extreme events. However, steps can be taken to attenuate the impacts we see and promote cooling, such as the agricultural practices that were mentioned earlier. To learn more about the observations and mechanisms underlying other extreme events (like drought or storms), please read the sections done by my classmates. Thank you!