The Science of Doom

In #9 we looked at trends in peak streamflow. As there isn’t a global database of floods this is the best proxy for flood risk from rivers and waterways. The result was a surprising decrease globally – the trend was down in some stations and up in others, but the down outweighed the up. This is good news.

And it’s the opposite of the trend in extreme rainfall, which was bad news.

On the subject of extreme rainfall, AR6 summarises it like this, p. 1560:

In summary, the frequency and intensity of heavy precipitation have likely increased at the global scale over a majority of land regions with good observational coverage

On the subject of peak streamflow, here is the AR6 summary, p. 1568:

In summary, the seasonality of floods has changed in cold regions where snowmelt dominates the flow regime in response to warming (high confidence).

There is low confidence about peak flow trends over past decades on the global scale, but there are regions experiencing increases, including parts of Asia, Southern South America, north-east USA, north-western Europe, and the Amazon, and regions experiencing decreases, including parts of the Mediterranean, Australia, Africa, and south-western USA.

[Emphasis added].

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In #8 we looked at the overview on floods. The IPCC report, AR6, says:

Flooded area is difficult to measure or quantify and, for this reason, many of the existing studies on changes in floods focus on streamflow. Thus, this section assesses changes in flow as a proxy for river floods, in addition to some types of flash floods.

As we saw in #7, extreme rainfall has increased in more places than it has decreased around the world. So we would expect rivers and waterways to reflect that and cause more flooding.

Here’s what the report says in plain English:

Peak streamflow has reduced overall – decreased in more places than it has increased. This is opposite to the trend in extreme rainfall.

The text is below in the notes.

This is good news.

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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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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.

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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)