Comment by haberman | Hacker News Reader

Comment by haberman | Hacker News Reader

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> 2. Winters get colder on average

I have never heard this. Do you have a reference?

I am no expert, but I looked for any statement about winter temperatures from the latest IPCC report. I found this in TS.2.6:

    > Warming of the land surface during the period 1971–2018
    > contributed about 5% of the increase in the global energy
    > inventory (TS.3.1), nearly twice the estimate in AR5 (high
    > confidence). It is virtually certain that the average
    > surface warming over land will continue to be higher than
    > over the ocean throughout the 21st century. The warming
    > pattern will likely vary seasonally, with northern high
    > latitudes warming more during winter than summer (medium
    > confidence).  {2.3.1, 4.3.1, 4.5.1, Box 7.2, 7.2.2,
    > Cross-Chapter Box 9.1, 11.3, Atlas 11.2}

    > The frequency and intensity of hot extremes (warm days and
    > nights) and the intensity and duration of heatwaves have
    > increased globally and in most regions since 1950, while the
    > frequency and intensity of cold extremes have decreased
    > (virtually certain). There is high confidence that the
    > increases in frequency and severity of hot extremes are due
    > to human-induced climate change. Some recent extreme events
    > would have been extremely unlikely to occur without human
    > influence on the climate system. It is virtually certain
    > that the further changes in hot and cold extremes will
    > occur throughout the 21st century in nearly all inhabited
    > regions, even if global warming is stabilized at 1.5°C
    > (Table TS.2, Figure TS.12a).

This seems to contradict your claim.

Also the most recent National Climate Assessment says this in its summary finding (https://nca2018.globalchange.gov/):

    > Declines in snow and ice cover caused by warmer winter
    > temperatures are expected to negatively impact the winter
    > recreation industry in the Northwest, Northern Great Plains, 
    > and the Northeast.

I think you're right that 2. is misstated, in my rush to ELI5. Thanks for the correction. I meant to refer to "increase in extreme weather events" not decrease in average winter temp. The IPCC seems pretty clear that on average, we are getting less cold temperatures. (And indeed Scott's article is exploring the implications of there being less cold temperature).

I can find references in the latest IPCC report to increased frequency of droughts, heavy rain, and heat waves, but I can't find much mention of extreme cold events; I was under the impression that these were predicted to be increasing too (i.e. more severe winter weather systems with most non-storm winter days being warmer, or put differently increased variance as the mean increases), but maybe that's not correct.

I stand by my main point which was that you _could_ have an increase in both extreme cold events and extreme hot events while the average temperature is increasing, to correct an oversimplification I believe the GP was making. I think this sibling made that point more concisely though: https://news.ycombinator.com/item?id=28961979.

> I can find references in the latest IPCC report to increased frequency of droughts, heavy rain, and heat waves, but I can't find much mention of extreme cold events

I believe my quoted text above states that the frequency and intensity of cold extremes has decreased, as a "virtually certain" fact.

> I stand by my main point which was that you _could_ have an increase in both extreme cold events and extreme hot events while the average temperature is increasing, to correct an oversimplification I believe the GP was making.

Yes, as a mathematical fact this is undoubtedly true.

I think there is a subtlety in the report there - they seem to use “extremes” to refer to something like the 75th %ile bounds, as in “the edges of the normal range”. Whereas “extreme weather events” is a similar phrase for a different thing, they are more like the 99th %ile outliers. (Made up numbers for discussion, not claiming the actual%, just trying to illustrate a distinction).

Note in your quote how they refer to both:

> The frequency and intensity of hot extremes (warm days and nights) and the intensity and duration of heatwaves

And for cold they say:

> while the frequency and intensity of cold extremes have decreased

But no mention of extreme weather events like storms. This might sound pedantic but I think it’s two different meanings of “extreme”, they discuss the differences a bit in https://www.ipcc.ch/srocc/chapter/chapter-6/.

Anyway, low epistemic confidence on this bit, just my read is a bit less certain on that last point than yours.

This is a very important point because it's very commonly believed that extreme cold weather (99 pct) will be worsened, and it's central to Scott Alexander's piece. Not storms per se, just the inverse to heat waves. Can anyone find any evidence for this from primary sources like the IPCC? My guess is the answer is that they're going to reduced. If they're virtually certain that 75th percentile events will be reduced that usually suggests that 99th percentile events will be reduced too.

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