I'm not 100% sure if we're talking at cross-purposes here, so I've tried to be very explicit in the following about dividing the question into its two parts:
1. Is there a trend?
2. Can it be attributed to anthropogenic climate change?
The second question is concerned with trying to link observed changes in whatever climatic trend of interest in directly to anthropogenic radiative forcings. So whether or not something is attributed to climate change is a different question - perhaps counter-intuitively - from whether or not that aspect of the climate is observed to be changing.
IvanV wrote: ↑Tue Mar 01, 2022 9:24 pm
There are smaller numbers of green and yellow tiles, and they generally have only one dot in them - low confidence. This bears out what I was saying in the other thread. It has long been hard to demonstrate a statistically clear climate change signal for unpleasant weather events other than heat. A lot of places don't even show that. Where we see it, the quantity of data is such that we have low statistical confidence in the conclusion.
Note that the dots represent confidence specifically in
human contribution to the observed trends - it's the colours that tell you if there's agreement on the direction of the trend. As you can see there are quite a lot of regions with well-characterised trends, and
IvanV wrote: ↑Tue Mar 01, 2022 9:24 pm
Though maybe the collective data is giving us overall statistical confidence in a direction of change, even if each individual locality shows, roughly equally divided, either no change, or low confidence of a unpleasant change.
Yes, the piece I linked to goes into that in more detail. It's often the case that there are observable global trends, but pinning them down locally is trickier. Similarly, there are fewer studies assessing anthropogenic causation of trends at regional scales than globally - as I say, attribution is a very young field. (When I looked at that map I thought "Wow! They've done that many regional studies already?" because even in AR5 there was still a lot of uncertainty attributing extreme weather to climate globally). There are also still serious issues with a lack of good data in the most vulnerable regions.
So for instance, for
trends in heavy precipitation we have:
The frequency and intensity of heavy rainfall events “have likely increased at the global scale over a majority of land regions with good observational coverage”, the IPCC says. Regional increases in the frequency and/or intensity of heavy rainfall “have been observed with medium confidence for nearly half” of regions, the report adds.
Since 1950, the annual maximum amount of rainfall falling in a day or over five consecutive days “has likely increased” for land areas with sufficient observations, the report notes.
In general, the evidence for increasing rainfall extremes for Europe, North America and Asia is the most robust, the report says, and heavy precipitation has likely increased on the continental scale over these three continents.
For example, “there is a significant increase in heavy rainfall” in central India during the South Asian monsoon season, the report says, which has occurred alongside “a significant decrease in moderate rainfall”.
Whereas across Africa, for example, there is a “general lack of continent-wide systematic analysis” and a “sporadic nature” of available data, the report says. So, while “increases in the frequency and intensity of extreme precipitation have been observed over the well-gauged areas during 1950-2013”, this only covers “15% of the total area”.
Similarly, in Central and South America, “evidence shows an increase in extreme precipitation, but in general there is low confidence”, the report says. And in small island states, “there is a lack of evidence showing changes in heavy precipitation overall”.
But then for the
attribution of those trends to human activity, we get:
For the increases that are observed, human influence – particularly through greenhouse gas emissions – “is likely the main driver of the observed scale intensification”, the report says, adding:
“In particular, detection and attribution analyses have provided consistent and robust evidence of human influence on extreme precipitation of one- and five-day durations at global to continental scales.”
Evidence for human influence on extreme precipitation at regional scales is limited, the report notes, but “new evidence is emerging”. For example, studies have shown that “the increase in widespread extremes over the South Asian Monsoon during 1950-2015 is due to the combined impacts of the warming of the Western Indian Ocean (Arabian Sea) and the intensification of irrigation water management over India”.
The picture is broadly similar for other types of extremes. (I won't bother quoting everything, but if you are interested in the science here I suggest you take a look at the article.) In terms of trend vs. attribution:
- Generally speaking, flooding has more data issues (as people rarely measure the extent of a flood, and we're lucky if there's good data on river flow). Obviously floods themselves are also an interaction between weather and infrastucture making generalisation trickier. They are confident attributing increased flooding to climate change in well-studied regions like the UK, fwiw.
- Droughts can be defined in multiple ways, which is part of the reason for the disagreement about trends. When using an index that combines plant physiology along with rainfall they find increases “on all continents and several regions” - for some reason they can tie drought to evapotranspiration, and evapotranspiration to heating, but have lower confidence tying drought to heating directly. Science is hard, I guess.
- Tropical storms are deffo getting heavier, moving polewards and spending more time in the same places over land. Reasonably likelihood of anthropogenic attribution in this case too - perhaps unsurprisingly as they're basically the result of SST(?), but they seem happy that ENSO et al. isn't sufficient explanation now.
- The area affected by "compound threats" is going up. This is stuff like where drought plus a heatwave causes fires. There are some attribution studies of individual fires upthread, but nothing big-picture mentioned yet.
So (TLDR) I'd say that there are very often clear signals of trends even at regional levels, as long as the variable is defined consistently in the literature, the data exists, and there are enough climate scientists working on the area (sorry, African agronomists). Attributing those observed trends to human activity has come on leaps and bounds since AR5 in 2013, but seems to be much easier when the chain of causation is short, as it is with heatwaves, storms and evaporative stress, compared with precipitation and flooding.
We have the right to a clean, healthy, sustainable environment.