The Science of Doom

In #1-6 we looked at trends in Tropical Cyclones. In #7-#9 we looked at trends in extreme rainfall and floods.

Now we move onto droughts.

The simplest idea about droughts is they occur when there’s a drop in rainfall over some time period.

Rainfall is relatively easy to measure. However, as a caveat, the world is a big place and different datasets have differences.

Now suppose the rainfall in a given region is the same this year as 50 years ago, but it’s 2°C warmer. More water will evaporate from the surface. In some cases this will lead to more droughts than 50 years ago.

So we can’t just look at the easy measurement of rainfall. We need to measure evaporation. That’s a lot harder as we don’t have measurements.

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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.

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