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In the last article we looked at trends in extreme rainfall. Now we’ll take a look at what the 6th Assessment Report (AR6), chapter 11, says on floods.

Here’s the plain English version:

The Special Report on Extremes in 2012 and the 5th Assessment Report in 2013 didn’t know whether floods were getting worse globally. There have been lots of studies since but they are still regional and sub-regional so it’s still not possible to measure whether floods are getting worse on a global level.

This isn’t bad news or good news, it’s uncertainty. But if you learnt about climate from the media this may be surprising.

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In #1 – #6 we looked at trends in tropical cyclones from chapter 11 on extreme weather of the IPCC 6th Assessment Report (AR6), with a summary article.

Now we’ll take a look at Extreme Rainfall. It’s needed to understand changes in floods.

There are a number of ways to characterize extreme rainfall – so it’s more complicated than something like annual rainfall which only has one number.

The idea is that even without annual rainfall changing there can be a shift towards more rainfall falling in a given day or a short period – more dry days, more intense rainfall on fewer rainfall days. If you have more extreme rainfall you have more chance of floods.

Warm air holds more moisture, so in a warmer climate we expect more rainfall. The actual physics is more complicated as another factor pushes the other way. A topic for another day. This article is about trends – what do we observe?

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In parts #1 through #6 of this series we’ve followed the six metrics on tropical cyclones (TCs) that are discussed in chapter 11 on Extreme Weather of the IPCC 6th Assessment Report.

The conclusion of this section of chapter 11 on TC trends says:

In summary, there is mounting evidence that a variety of TC characteristics have changed over various time periods.

It is likely that the global proportion of Category 3–5 tropical cyclone instances and the frequency of rapid intensification events have increased globally over the past 40 years. It is very likely that the average location where TCs reach their peak wind intensity has migrated poleward in the western North Pacific Ocean since the 1940s. It is likely that TC translation speed has slowed over the USA since 1900.

Here’s my summary. It’s a little different..

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In #1 we looked at the trends in intensity and frequency of landfalling tropical cyclones (TCs) over 120+ years. In #2 we looked at the same metrics out over the ocean using satellite data, which is available for about the last 40 years. In #3 we looked at “translation speed” or changes in the speed at which the overall TCs are moving. In #4 we looked at trends in rainfall in TCs – missing in action from the IPCC report. And in #5 something called “intensification rates” of TCs.

This is the last trend in TCs that we’ll look at from the 6th Assessment Report.

Here’s the plain English version of the report:

Tropical Cyclones are moving away from the equator towards the poles, at the rate of about 50-60 km (30-40 miles) per decade.

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We’ve been looking at various aspects of Tropical Cyclones (TCs) in the section “Observed Trends” from the latest IPCC report, AR6 (the 6th Assessment Report). Chapter 11 is all about extreme weather.

The report says, p.1585:

..there is evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era.

The satellite era is 1980 to present, so we have about 40 years of global data from satellites.

What is “intensification rate”?

I’m moving to Substack. It’s a great publishing platform. See the rest this article (for free) at Science of Doom on Substack.

In #1 we looked at the trends in intensity and frequency of landfalling tropical cycles (TCs) over 120+ years. In #2 we looked at the same metrics out over the ocean using satellite data, which is available for about the last 40 years. And in #3 we looked at “translation speed” or changes in the speed at which the overall TCs are moving.

We’re looking primarily at what the IPCC 6th Assessment Report (AR6) has to say, from section 11.7.1.2 “Observed Trends”.

I was expecting to review what this section said about trends in TC rainfall. The simple climate science idea is that warmer air holds more moisture. As the planet warms we expect more rainfall. This idea will be explained in more detail in future articles on floods.

Here’s the plain English version of trends in TC rainfall from the report:

.

.

It’s not mentioned in the section on Observed Trends. I was surprised.

– I’m moving to Substack. It’s a great publishing platform. See the rest this article (for free) at Science of Doom on Substack.

In #1 and #2 we looked at trends in frequency and intensity of Tropical Cyclones (TCs) and found that the IPCC 6th Assessment Report (AR6) contained good news. Not in the executive summary, and not particularly clearly even in the body of the report. But still, it is good news.

This article looks at the speed of TCs – how fast they move overall, not how fast the winds are swirling around. This is important, because as they hit land if they go more slowly there will be more rain and therefore more flooding.

This was going to be a short article, but as long time readers of this blog will know, brevity was never my strong point.

Here’s the plain English summary of the report:

Translation speed of TCs has reduced over the last 70 years, leading to more flooding as TCs hit the coast.

This is bad news. The actual text, from p. 1587, is in the Notes at the end of this article.

James Kossin’s paper from 2018 is the main idea of this section of the report. Two papers are noted as questioning his conclusion. Kossin replied in 2019, confirming his original conclusion. The report essentially agrees with Kossin.

One of the lead authors of this chapter 11 on Extreme Weather is also James Kossin.

The main focus of this series of articles is the conclusions of the IPCC 6th Assessment Report, but it seems the question is still open, so read on for more analysis.

I’m moving to Substack. It’s a great publishing platform. See the rest this article (for free) at Science of Doom on Substack.

In Extreme Weather #1 we looked at trends in landfalling tropical cyclones (TCs), where data goes back over 100 years. Way more TCs form over the ocean and don’t hit land, thankfully. Trends on these would be informative – are they getting worse?

There isn’t much quality data before satellites started going up around 1980, so we have good data for over 40 years. More coverage was added around 1990 so we have even better data over the last 30 years.

What does the latest IPCC report say? Chapter 11 of AR6 covers extreme weather.

Here’s the simple version:

There are significant positive global trends in TC intensity.

The actual text, from p. 1585, is in the Notes at the end of this article.

This seems like bad news but it’s actually good news.

The Executive Summary for the chapter includes the “bad news”, p. 1519:

It is likely that the global proportion of Category 3–5 tropical cyclone instances has increased over the past four decades.. The global frequency of TC rapid intensification events has likely increased over the past four decades. None of these changes can be explained by natural variability alone (medium confidence).

I was confused when I read this section of the report and the paper referenced – Kossin et al., “Global increase in major tropical cyclone exceedance probability over the past four decades”, 2020. I’ve read a number of papers on TCs in the satellite era and “getting worse” didn’t seem correct. I found a paper from Klotzbach et al 2022 in my files and reread it. Both Kossin and Klozbach are heavily cited in this field, including by this IPCC report and the previous report (AR5).

Here’s Klotzbach 2022:

This study investigates 1990–2021 global tropical cyclone (TC) activity trends, a period characterized by consistent satellite observing platforms. We find that fewer hurricanes are occurring globally and that the tropics are producing less Accumulated Cyclone Energy—a metric accounting for hurricane frequency, intensity, and duration.

Here’s Kossin 2020:

Here we address and reduce these heterogeneities and identify significant global trends in TC intensity over the past four decades. The results should serve to increase confidence in projections of increased TC intensity under continued warming.

I emailed Phil Klotzbach asking for clarification – different dataset? different time period? looking at a different metric? and he very kindly replied within 24 hours explaining. (I’ve emailed a number of climate scientists during the years of writing this blog and have found them to be exceptionally responsive, courteous and helpful).

Now it’s clear. And I should have figured it out myself. Here is my plain English version:

The number of category 4-5 TCs (the most extreme) hasn’t changed. The number of category 1-3 TCs has reduced.

So this seems like good news. We can express it as “the percentage of the most extreme TCs has increased” but that’s just another way of saying the same thing.

For people still confused, like a couple of friends I explained this to.. suppose the number of murders is flat but the number of other violent offences has reduced. We could say “violent crime is down”, or we could say “extreme violence has increased (as a percentage of overall violent crime)”. The first one is the plain English version.

Now, we’re looking at a short duration – 30-40 years. Is the trend due to climate variables like La Nina? Will the trend continue? Reverse? All good questions, perhaps to be considered in a future article.

This aim of this article is about the simpler question of what has been observed about trends in tropical cyclones out over the oceans. We’ll let Phil have the last word:

We find that fewer hurricanes are occurring globally and that the tropics are producing less Accumulated Cyclone Energy—a metric accounting for hurricane frequency, intensity, and duration

Notes

Text of AR6 on TC trends in the satellite era, from p. 1585:

There are previous and ongoing efforts to homogenize the best-track data (Elsner et al., 2008; Kossin et al., 2013, 2020; Choy et al., 2015; Landsea, 2015; Emanuel et al., 2018) and there is substantial literature that finds positive trends in intensity-related metrics in the best-track during the ‘satellite period’, which is generally limited to around the past 40 years (Kang and Elsner, 2012; Kishtawal et al., 2012; Kossin et al., 2013, 2020; Mei and Xie, 2016; Zhao et al., 2018; Tauvale and Tsuboki, 2019).

When best-track trends are tested using homogenized data, the intensity trends generally remain positive, but are smaller in amplitude(Kossin et al., 2013; Holland and Bruyère, 2014).

Kossin et al. (2020) extended the homogenized TC intensity record to the period 1979–2017 and identified significant global increases in major TC exceedance probability of about 6% per decade.

In addition to trends in TC intensity, there is evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era (Kishtawal et al., 2012; Balaguru et al., 2018; Bhatia et al., 2018). The increase in intensification rates is found in the best-track and the homogenized intensity data.

References

Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

Global increase in major tropical cyclone exceedance probability over the past four decades, Kossin et al, PNAS (2020)

Trends in Global Tropical Cyclone Activity: 1990–2021, Philip J. Klotzbach et al, GRL (2022)

The latest IPCC report, AR6, was released in draft form in 2021 and in what seemed like an approved released form early 2022. You can download each chapter from ipcc.ch (Working Group 1 is “the physical science basis”).

Chapter 11 covers extreme events – “Weather and Climate Extreme Events in a Changing Climate”.

Here’s the simple version of what they say about long term trends in tropical cyclones (severe tropical storms):

For long term trends of landfalling tropical cyclones, we have a data quality issue. We do have good data for the USA going back to 1900 and there’s been no increase. We do have good data for Australia going back to the late 1800s and there’s been a decrease. On a global basis the data quality isn’t good enough to have any confidence in trends in intensity or frequency.

The actual text, from p. 1585-1586, is in the Notes at the end of this article.

This how the executive summary for the chapter captures the essence of this apparently good news, p.1519:



That’s the main dish.

Here’s an extract from Thomas Knutson et al 2019:

One of their summaries:

In summary, no detectable anthropogenic influence has been identified to date in observed TC- landfalling data, using type I error avoidance criteria. From the viewpoint of type II error avoidance, one of the above changes (decrease in severe landfalling TCs in eastern Australia), was rated as detectable, though not attributable to anthropogenic forcing (9 of 11 authors), with one dissenting author expressing reservations about the historical data quality in this case.

It’s important to note that landfalling TCs are only a small subset of TCs that form out over the ocean, and in the next article we’ll look at this.

Notes

Text of AR6 from p. 1585-1586 about long term trends in tropical cycles:

Identifying past trends in TC metrics remains a challenge due to the heterogeneous character of the historical instrumental data, which are known as ‘best-track’ data (Schreck et al., 2014). There is low confidence in most reported long-term (multi-decadal to centennial) trends in TC frequency- or intensity-based metrics due to changes in the technology used to collect the best-track data. This should not be interpreted as implying that no physical (real) trends exist, but rather as indicating that either the quality or the temporal length of the data is not adequate to provide robust trend detection statements, particularly in the presence of multi-decadal variability..

..A subset of the best-track data corresponding to hurricanes that have directly impacted the USA since 1900 is considered to be reliable, and shows no trend in the frequency of USA landfall events (Knutson et al., 2019)…

..A similarly reliable subset of the data representing TC landfall frequency over Australia shows a decreasing trend in Eastern Australia since the 1800s (Callaghan and Power, 2011), as well as in other parts of Australia since 1982 (Chand et al., 2019; Knutson et al., 2019). A paleoclimate proxy reconstruction shows that recent levels of TC interactions along parts of the Australian coastline are the lowest in the past 550–1500 years (Haig et al., 2014).

References

Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

Knutson, T.R. et al., 2019: Tropical Cyclones and Climate Change Assessment: Part I: Detection and Attribution. Bulletin of the American Meteorological Society

In VI – Australia CanESM2, CSIRO, Miroc and MRI compared vs history we looked at how each model thought rainfall had changed in Australia over about 100 years, and we compared that to observations. We did this for annual rainfall, also for Australian summer (Dec, Jan, Feb) and Australian winter (Jun, Jul, Aug).

Here we will look at two of the four emissions scenarios. We compare 2081-2100 vs 1979-2005.

Note that we are not comparing the end of the 21st century from the model with observations at the end of the 20th century. That produces much different results – the model’s view of recent history doesn’t match observations very well. We are comparing the model future with the model past. So we are asking the model to say how it sees rainfall changing as a result of different amounts of CO2 being emitted.

The two scenarios are:

  • RCP4.5 – with current trends continuing we are something like RCP6. I think of RCP4.5 as being “what we are doing now” but with some substantial reductions in CO2 emissions. But it’s nothing like RCP2.6, which is more “project Greta” where emissions basically stop in a decade
  • RCP8.5 – extreme CO2 emissions. Often described as “business as usual” perhaps to get people’s attention. Think – most of Africa moving out of abject poverty, not passing through the demographic transition (so population going very high) and burning coal like crazy with the efficiency of 19th century Europe.

Each pair of graphs is future RCP4.5 as % of recent past, and RCP8.5 as % of recent past. The four models, clockwise from top left – MPI (Germany), Miroc (Japan), CSIRO (Australia) and CAN (Canada):

Figure 1 – Click to expand

And now the same, but only looking at Australian summer, DJF:

Figure 2 – Click to expand

Depending on which model you like, things could be really bad, or really good, or about the same with “climate change”.

Note that the color scale I’m using here is the same as the last article, but different from all the earlier articles, the % range is from 50% to 150% (rather than 0% to 200%).

References

An overview of CMIP5 and the experiment design, Taylor, Stouffer & Meehl, AMS (2012)

GPCP data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/

GPCC data provided from https://psl.noaa.gov/data/gridded/data.gpcc.html

CMIP5 data provided by the portal at https://esgf-data.dkrz.de/search/cmip5-dkrz/