In contrast with AR4, a larger range of climate extremes (and assessment on the indices which define such climate extreme) have been dedicated to a separate section. This is also visible in the Technical Summary in the AR5, where progress on understanding of climate extremes is now summarized.

Temperature & Precipitation Extremes

On temperature and precipitation extremes, the AR4 and AR5 largely concur. AR4 found that temperature extremes (hotter days and nights) had likely increased at a global scale; AR5 notches up this likelihood even further:

[N]ew analyses continue to support the AR4 and SREX conclusions that since about 1950 it is very likely that the numbers of cold days and nights have decreased and the numbers of warm days and nights have increased overall on the global scale, i.e., for land areas with sufficient data.

For precipitation extremes the AR5 writes:

It is likely that since about 1950 the number of heavy precipitation events over land has increased in more regions than it has decreased. Regional trends vary but confidence is highest for central North America with very likely trends towards heavier precipitation events.

Heat waves

On heatwaves, AR4, chapter 3, did not elaborate specifically on heatwaves, although it summarized in Table 3.8 that on a global scale heatwaves had likely increased since the middle of the 20th century. (Likely: over 66 percent probability.)

On this topic, AR5, chapter 2, reads:

There is only medium confidence that the length and frequency of warm spells, including heat waves, has increased since the middle of the 20th century mostly due to lack of data or studies in Africa and South America. However, it is likely that heatwave frequency has increased during this period in large parts of Europe, Asia and Australia.

This marks a first deviation with respect to AR4: for regions with sufficient data, science indicates increased numbers of heatwaves since 1950. An assessment on the global scale can not yet be made, but only because of a paucity in data or studies for the remaining regions.

Droughts

Similar to heatwaves, AR4 tallied increasing droughts in several regions but lacked a perspective on global scale, although it did note:

While there are numerous indices and metrics of drought, many studies use monthly precipitation totals and temperature averages combined into a measure called the Palmer Drought Severity Index (PDSI). The PDSI calculated from the middle of the 20th century shows a large drying trend over many Northern Hemisphere land areas since the mid-1950s, with widespread drying over much of southern Eurasia, northern Africa, Canada and Alaska (FAQ 3.2, Figure 1), and an opposite trend in eastern North and South America. In the Southern Hemisphere, land surfaces were wet in the 1970s and relatively dry in the 1960s and 1990s, and there was a drying trend from 1974 to 1998. Longer-duration records for Europe for the whole of the 20th century indicate few significant trends. Decreases in precipitation over land since the 1950s are the likely main cause for the drying trends, although large surface warming during the last two to three decades has also likely contributed to the drying. One study shows that very dry land areas across the globe (defined as areas with a PDSI of
less than -3.0) have more than doubled in extent since the 1970s, associated with an initial precipitation decrease over land related to the El Niño-Southern Oscillation and with subsequent increases primarily due to surface warming.

AR5, TS 4.7, p38:

While the AR4 concluded that it is more likely than not that anthropogenic influence has contributed to an increased risk of drought in the second half of the 20th century, an updated assessment of the observational evidence indicates that the AR4 conclusions regarding global increasing trends in hydrological droughts since the 1970s are no longer supported. Owing to the low confidence in observed large-scale trends in dryness combined with difficulties in distinguishing decadal-scale variability in drought from long-term climate change, there is now low confidence in the attribution of changes in drought over global land since the mid-20th century to human influence.

Floods

In 2007, the first report of the AR4 did not include a scientific assessment on the frequency or magnitude of floods, in fact, it sparsely mentioned floods, although European floods in Germany were briefly listed as an example. In Chapter 9, on attribution, it was observed, though not referenced:

Climate models predict that human influences will cause an increase in many types of extreme events, including extreme rainfall. There is already evidence that, in recent decades, extreme rainfall has increased in some regions, leading to an increase in flooding.

AR5 has a separate section on floods in chapter 2, where it concludes:

While the most evident flood trends appear to be in northern high latitudes, where observed warming trends have been largest, in some regions no evidence of a trend in extreme flooding has been found, e.g., over Russia based on daily river discharge (Shiklomanov et al., 2007). Other studies for Europe (Hannaford and Marsh, 2008; Renard et al., 2008; Petrow and Merz, 2009; Stahl et al., 2010) and Asia (Jiang et al., 2008; Delgado et al., 2010) show evidence
for upward, downward or no trend in the magnitude and frequency of floods, so that there is currently no clear and widespread evidence for observed changes in flooding except for the earlier spring flow in snow dominated regions (Seneviratne et al., 2012).


In summary, there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale.

(Source: NASA)

Tropical Cyclones

The AR4 contained a large section on tropical cyclones, or hurricanes, and on studies for the different ocean basins. For an analysis on the global scale, it read:

Globally, estimates of the potential destructiveness of hurricanes show a substantial upward
trend since the mid-1970s, with a trend towards longer storm duration and greater storm intensity, and the activity is strongly correlated with tropical sea surface temperature. These relationships have been reinforced by findings of a large increase in numbers and proportion of strong hurricanes globally since 1970 even as total numbers of cyclones and cyclone days decreased slightly in most basins. Specifically, the number of category 4 and 5 hurricanes increased by about 75% since 1970. The largest increases were in the North Pacific, Indian and Southwest Pacific Oceans. However, numbers of hurricanes in the North Atlantic have also been above normal in 9 of the last 11 years, culminating in the record-breaking 2005 season.

As detailed previously on ET, most of these conclusions were overturned in 2010 as the hurricane science community moved together to hammer out its differences and the findings of these papers were underlined once more in the assessment of the SREX report. The AR5 concludes:

Current datasets indicate no significant observed trends in global tropical cyclone frequency over the past century and it remains uncertain whether any reported long-term increases in tropical cyclone frequency are robust, after accounting for past changes in observing capabilities (Knutson et al., 2010).

...

However over the satellite era, increases in the intensity of the strongest storms in the Atlantic appear robust (Kossin et al., 2007; Elsner et al., 2008) but there is limited evidence for other regions and the globe.

...

In summary, this assessment does not revise the SREX conclusion of low confidence that any reported longterm (centennial) increases in tropical cyclone activity are robust, after accounting for past changes in observing capabilities. More recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin. Evidence however is for a virtually certain increase in the frequency and intensity of the strongest tropical cyclones since the 1970s in that region.

Abrupt Climate Change - Catastrophes

Chapter 12 in the AR5 summarizes the scientific understanding on a range of climate doom scenario's, from catastrophic methane release to Greenland ice collapse to the shutdown of the 'Gulf Stream', all sensational scenario's which logically attract media attention, Hollywood script writers and even contributors at European Tribune.

In AR4, analysis on abrupt climate change events was scattered throughout the report, and varying degrees of likelihood for these events were expressed. With AR5, the definition of abrupt climate change has been modified:

Synthesis and Assessment Product 3.4 of the U.S. Climate Change Science Program CCSP (CCSP, 2008b). We define abrupt climate change as a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems (see
Glossary). Other definitions of abrupt climate change exist. For example, in the AR4 climate change was defined as abrupt if it occurred faster than the typical time scale of the responsible forcing.

AR5 Table 12.4 summarizes the scientific consensus of a range of abrupt climate change scenarios with one swift stroke, which speaks for itself:

Table 12.4

To summarize. With the AR5, scientific consensus on climate extremes is portrayed more focussed and with richer detail in comparison to AR4. The new IPCC report relies heavily on findings of its special report on extreme events (SREX), published early 2012. Given that the assessment in SREX upset or overturned conclusions of the AR4 on some of the more dramatic climate extremes, it should not be surprising that also AR5 differs with AR4 in several respects.

In line with IPCC's previous reports, the AR5 finds a discernable increasing trend for temperature extremes and extreme precipitation events at the global scale and increasing frequency of heat waves at regional scales since the 1950s. In contrast with findings in AR4, no trend is discerned for droughts, floods, or tropical storms at the global scale.

Furthermore, globally catastrophic doom scenario's such as the abrupt shutdown of the Atlantic thermohaline circulation, dramatic ice sheet collapse or methane release (either from permafrost or oceanic clathrates) are presently considered unlikely. With the exception of the scenario for disappearing Arctic summer ice, other abrupt scenario's have no scientific agreement or evidence, that is, no decisive answer.

With the scientific consensus as captured in the AR5 report, the IPCC puts to rest general yet popular claims that climate change already results in increasing floods, droughts or hurricanes, and invalidates claims that climate change already risks the above listed catastrophic scenario's, as such claims are both scientifically unsupportable and outside the scientific consensus.

Finally, one could observe that the IPCC has commendably increased attention for analysing climate extremes at the regional level, and most likely the second report (to be released next year) will further elaborate on vulnerabilities at that scale.