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## Natural Variability, Attribution and Climate Models #2

In #1 we looked at some examples of natural variability – the climate changes from decade to decade, century to century and out to much longer timescales.

How sure are we that any recent changes are from burning fossil fuels, or other human activity?

In some scientific fields we can run controlled experiments but we just have the one planet. So instead we need to use our knowledge of physics.

In an attempt to avoid a lengthy article I’m going to massively over-simplify.

## “Simple Physics”

Some concepts in climate can be modeled by what I’ll call “simple physics”. It often doesn’t look simple.

Let’s take adding CO2 to the atmosphere. We can do this in a mathematical model. If we “keep everything else the same” in a given location we can calculate the change in energy the planet emits to space for more CO2. Less energy is emitted to space with more CO2 in the atmosphere.

The value varies in different locations, but we just calculate it in lots of places and take the average.

As less energy is leaving the planet (but the same amount is still being absorbed by the sun) the planet warms up.

In our model, we can keep increasing the temperature of the planet in our model until the energy emitted to space is back to what it was before. The planetary energy budget is back in balance.

So we’ve calculated a new surface temperature for, say, a doubling of CO2.

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## Natural Variability, Attribution and Climate Models #1

In the last set of articles we’ve looked at past trends in extreme weather, following the flow of chapter 11 from the 6th assessment report of the IPCC.

How do we know the cause of any changes?

In recent years in most of the media everything that changes is “climate change” which is implicitly or explicitly equated with burning fossil fuels, i.e., adding CO2 into the atmosphere. It’s a genius catchphrase from a marketing point of view, not so helpful for scientific understanding.

I used to prefer the term “anthropogenic global warming” but it has its flaws as well, as some recent trends are believed to be anthropogenic but not from adding CO2 into the atmosphere. An example is changes that result from reduced aerosols in the atmosphere as a result of burning less biomass.

I’ll generally try and stay with “anthropogenic” or “from more CO2”, but there’s no copy editor, so let’s see.

Lots of changes in past climate metrics are simply natural variability. Understanding and quantifying natural variability is a big topic and our knowledge is always going to be imperfect.

For example, there were multi-decadal megadroughts in North America and Europe in the past 1000 years. They were probably “unprecendented” for their time, but clearly weren’t caused by burning fossil fuels.

Here is a reconstruction of the drought index over 1000 years of western North America..

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## Extreme Weather #14 – Trends in Extreme Hot and Cold Days & Nights

Global temperature has been rising since around 1900, and CO2 is the principal cause. The physics behind the inappropriately-named “greenhouse effect” is certain, so burning fossil fuels, which adds CO2 to the atmosphere, is certain to increase the surface temperature. I’ve written many articles on that topic on the original blog and shown how the equations are derived (see Notes).

So it should be no surprise to find that there are more extreme hot days and less extreme cold days.

If the temperature goes up, then the number of days with a temperature above say 35°C (86°F) or 40°C (95°F) – or whatever number you want to pick – will increase.

Likewise, the number of days with a temperature below say -10°C (14°F) or -20°C (-4°F) – again, pick a number for your region – will decrease.

Here’s a graph of global land and ocean temperature, extracted from a larger graph in chapter 2 of AR6 (see the Notes below for the full map of changes):

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## Summary on Trends in Droughts

In #10-#13 we looked at recent trends in droughts.

After covering Tropical Cyclones in #1-#6 it was worth doing a summary in part because AR6’s summary missed some good news from the report itself.

In #7-#9 we looked at extreme rainfall and floods and the summary was important because AR6’s summary missed some good news.

I’ve included the full text from p. 1575 below in the notes.

If you check the table in the notes section of #11 we can see that there were 8 regions identified with a rainfall deficit and 6 regions identified with a rainfall increase. These are also listed in the report section before the summary. However, the summary section only says:

There is medium confidence in increases in precipitation deficits in a few regions of Africa and South America.

Of course, people can read the section before and find out that there were places with a rainfall increase (a decrease in droughts). But anyone limiting themselves to only the “summary” would miss out on this good news.

On soil moisture droughts – agricultural and ecological droughts – they say:

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## Extreme Weather #13 – Trends in Droughts – More Greening

In #12 we saw that soil moisture droughts – agricultural and ecological droughts – have increased globally.

I’ve been following the flow of AR6 in their discussion of recent trends. They do go on to discuss hydrological droughts without much that’s definitive so perhaps we’ll have a brief look at that in another article, as I’m something of a completionist.

But there’s something important missing from the drought section.

Are plants dying? If not, is there really an increase in soil moisture drought?

Here’s a question from Alexis Berg & Justin Sheffield (2018) to put the problem in a broader context. Here and in all the other papers quoted, bold text is my change:

The notion that a warmer climate leads to a drier land surface, i.e., increased water stress, driven overwhelming by the effect of warmer temperatures on evaporative demand, appears, however, inconsistent with paleo-evidence and vegetation reconstructions for different colder and warmer past climates.

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## Extreme Weather #12 – Trends in Droughts – Soil Moisture – Rainfall & Evaporation

In #11 we looked at “droughts from rainfall”, i.e., rainfall deficits. There were regional variations but no obvious global trend.

In fact, we expect more rainfall as the earth warms (warm air holds more moisture) so a question to return to in a later article is why there hasn’t been an obvious increase in rainfall, i.e., why there hasn’t been a reduction in “droughts from rainfalls”.

We can measure rainfall. Think – a little bucket that fills up with rain and someone comes around every day, takes a measurement, and writes that number down in a notebook. Of course, the measurement is often more sophisticated in recent times. But we can all appreciate it’s a measurement that can easily be taken.

The other side of soil moisture is evaporation. If it’s hotter, all other things being equal, we expect more evaporation and so on the margins, more places in droughts.

The probem? There is no simple instrument we can stick in the ground next to the rainfall bucket to measure evaporation.

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## Extreme Weather #11 – Trends in Droughts – Rainfall

In #10 we saw an overview which included the idea that a rainfall deficit is one part of a soil moisture deficit. But it’s the part we can measure.

AR6 says, p.1573:

Global studies generally show no significant trends in SPI [drought index for rainfall] time series (Orlowsky and Seneviratne, 2013; Spinoni et al., 2014), and in derived drought frequency and severity data (Spinoni et al., 2019), with very few regional exceptions.

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## Extreme Weather #10 – Trends in Droughts – Overview

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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## Summary on Trends in Extreme Rainfall and Floods and Missing Good News

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.

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## Extreme Weather #9 – Trends in Floods – Streamflow

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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