[Vision2020] Realclimate.org 8-9-18: Are the heatwaves caused by climate change?

Ted Moffett starbliss at gmail.com
Tue Aug 14 18:58:20 PDT 2018 

Vision2020 Post: Ted Moffett
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-----------------------------------

I continue to often converse with people influenced by the prevalent junk
science, widespread public misunderstanding of well researched empirical
science, on anthropogenic global warming.

Realclimate.org at least attempts to counter this social political
phenomenon.
----------------------------------------------------------------------------------
Are the heatwaves caused by climate change?
— rasmus @ 9 August 2018

Rasmus E. Benestad
http://www.realclimate.org/index.php/archives/2004/12/rasmus-e-benestad/

http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#more-21575

 get a lot of questions about the connection between heatwaves and climate
change these days. Particularly about the heatwave that has affected
northern Europe this summer. If you live in Japan, South Korea, California,
Spain, or Canada, you may have asked the same question.

*The raindrop analogy*
However, the question is inaccurate and I will try to explain this through
an analogy. Let’s say I go for a walk with a friend and my friend feels a
few drops of water that fall on her. She asks me if it’s raining. But as
long as there was only few drops of water, it could also be something else.

I tell her that we can get some more relevant information in order to get a
more reliable answer. Look at the sky. Are there dark clouds on the sky
above? And what does the weather forecast say?

If there are dark clouds above and the weather forecast suggests showers,
it’s a safe bet to say it is the start of the rain. The rain always start
with a few drops, just the way a climate change starts with a few events.

In the same way as with the observation of the first drops of of water, you
could not be sure whether the heatwave is a freak event or the emerging
pattern of climate change, if you don’t include other relevant information.

There is a range of different pieces of information which are relevant when
it comes to the question about weather events and climate change: (a)
statistical evidence, (b) physical processes connecting different aspects,
and (c) attribution work.

*(a) Statistical evidence *
Heatwaves are becoming more widespread, last longer, and are getting more
extreme (e.g. Keellings and Waylen, 2014)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-0>.
This trend has been predicted and reported in multiple reports, such
as the IPCC
SREX (2013) <http://www.ipcc.ch/report/srex/>, the Proceedings of National
Academy of Sciences (e.g. Palmer, 2009)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-1>,
and European Academy Science Advisory Council (EASAC, 2013
<http://www.dnva.no/binfil/download.php?tid=58783>).

Climate change is equivalent to changing weather statistics, and one line
of evidence includes the nature of record-breaking events
<http://www.realclimate.org/index.php/archives/2011/11/on-record-breaking-extremes/>.
We can find evidence in both the number[a] and the magnitude of new
record-breaking values.

Coumou et al., (2013)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-2>
observed an increase in the global number of monthly heat records that
corresponded to what one should expect if the temperatures increased
everywhere by the same rate as the global mean. They also found that local
monthly records are on average five times as frequent as they would be in a
stationary climate. In other words, four out of five new heat records would
not have occurred without global warming.

Other types of evidence includes how often the events (e.g hurricanes
<http://www.realclimate.org/index.php/archives/2018/05/does-global-warming-make-tropical-cyclones-stronger/>)
take place, their duration and intensity. Standard statistical tests can
also indicate whether a particular event fits in with the expected range of
outcomes.

*(b) Physical processes*
Physical conditions and processes play a role both for the emerging pattern
of precipitation,  the evolution of weather, and their statistical
characteristics. Indeed, we expect the statistics of rainfall and
temperature to respond to an altered physical situation
<http://www.realclimate.org/index.php/archives/2017/09/why-extremes-are-expected-to-change-with-a-global-warming/>
.

Earth’s climate has always changed, and there have always been physical
causes for the changes. This means that the climate is sensitive to altered
conditions, such as greenhouse gases.

It would be difficult to explain why increased concentrations of greenhouse
gases had no effect on the global mean temperature or on the statistics of
extreme weather conditions while other types of forcing clearly have an
effect.

There is no shortage on explanations for why changes in the physical
environment should cause more extreme events. Some of these are:
- Greater temperatures are expected to make heatwaves more widespread in
general.
- Weaker winds circulating the pole make weather episodes such as blocking
high pressure more persistent. This weakening is associated with a polar
amplification
<http://www.realclimate.org/index.php/archives/2006/01/polar-amplification/>
and the retreat of the Arctic sea ice (Francis and Vavrus, 2012;
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-3>Coumou
et al., 2015)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-4>
.
- Changes in the north-south temperature differences, for instance due to
the polar amplification, can increase the prevalence of the phenomenon
known as “quasi-resonant planetary waves”, which is associated with
heatwaves (Petoukhov et al., 2013)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-5>.
Mann et al. (2017)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-6>
identified a specific fingerprint in the zonal mean surface temperature
profile that is associated with conditions that increase the likelihood for
these waves. Both the models and observations suggest that these conditions
only recently have emerged from the background noise of natural variability.

I have also reviewed the greenhouse effect
<http://www.realclimate.org/index.php/archives/2016/02/what-is-the-best-description-of-the-greenhouse-effect/>
and described how convection can be altered by higher concentration of
greenhouse gases. This link with the hydrological cycle may explain why the
rains seem to be concentrated over small area of Earth’s surface (Benestad,
2018)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-7>
.

Diminished area of precipitation explains both more frequent flooding and
more droughts, and dry conditions exacerbate the heat, as moisture restrain
temperatures during evaporation.

We also expect more extreme rainfall in some locations, as higher surface
temperatures boost the evaporation and increase the turn-around rate of the
hydrological cycle. There are also indications of higher cloud tops (Witze,
2016)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-8>
which allow the rain drops to grow further than before.

*(c) Attribution*
It is possible to reproduce extreme weather episodes in computer models,
such as those used for weather forecasting. We can conduct experiments to
see which effects greenhouse gases have for the outcome. In other words,
the models can be used to simulate the same event with and without the
present levels (Schiermeier ,2018)
<http://www.realclimate.org/index.php/archives/2018/08/are-the-heatwaves-caused-by-climate-change/#ITEM-21575-9>
.

The World Weather Attribution (WWA
<https://www.worldweatherattribution.org/>) has carried out such
experiments, and their efforts suggest that recent extreme events have
become more likely with an increased greenhouse effect.

*Individual cases and emergent behaviour of many events*
The planetary system is extremely complex, with interactions between
atmosphere, oceans, ice and land, and taking place over a vast range of
temporal and spatial scales.

It is hard to say that one aspect is directly connected to another, when
there are so many interacting parts and such rich level of complexity.
Understanding the difference between individual versus collective events is
key to making sense of the situation.

Nevertheless, complex systems tend to give rise to emergent behaviour
<https://en.wikipedia.org/wiki/Emergence> (explained in Gavin’s TED-talk
<https://www.ted.com/talks/gavin_schmidt_the_emergent_patterns_of_climate_change>).
And the statistical characteristics of a large number of outcomes is often
predictable. In fact, statistics is remarkably predictable, and we can
often attribute some probability to the causes of some event through
standard statistical tests.

*What is causing what?*
On another level, there is also the more philosophical question of whether
rain drops are caused by the rain or the rain is a result of many rain
drops. Rain is a phenomenon that includes many collective events in the
clouds.

The same way that extra information such as cloud observation and weather
forecast give confidence in our interpretation of the first drops being the
start of the rain, the statistical evidence and our understanding of the
atmospheric physics provide relevant information for judging the connection
between heatwaves and climate change.

*A more relevant question*
I think it makes sense to rephrase the usual question of whether climate
change causes a particular event, since climate and weather are different
aspects of the same earth system.

The bottom line is whether we now are observing the first glimpse of a new
normal, or if the world will return to its old state. In other words, the
question should be whether the recent heatwave is a signs of a new type of
weather patterns we can expect for the future. I think the answer to this
question is “yes”, based on current information and knowledge.

* Footnotes*
[a] *If data is independent and identically distributed (iid), then the
probability of a new record-breaking event diminishes with the number of
measurements ([image: n]) [image: P(X > [x_1, x_2, ... x_{n-1}]) = 1/n]. In
this case, the expected number of records is [image: E(n) =
\sum_{i=1}^{n}(1/i)]. On the other hand, if you count many more records
than [image: E(n)], then that is a sign that upper tail of the statistical
distribution is stretching towards higher levels. In other words, it
indicates that extremes are becoming more frequent.*
References

   1. D. Keellings, and P. Waylen, "Increased risk of heat waves in
   Florida: Characterizing changes in bivariate heat wave risk using extreme
   value analysis", *Applied Geography*, vol. 46, pp. 90-97, 2014.
   http://dx.doi.org/10.1016/j.apgeog.2013.11.008
   2. T.N. Palmer, "Climate extremes and the role of dynamics", *Proceedings
   of the National Academy of Sciences*, vol. 110, pp. 5281-5282, 2013.
   http://dx.doi.org/10.1073/pnas.1303295110
   3. J.A. Francis, and S.J. Vavrus, "Evidence linking Arctic amplification
   to extreme weather in mid-latitudes", *Geophysical Research Letters*,
   vol. 39, pp. n/a-n/a, 2012. http://dx.doi.org/10.1029/2012GL051000
   4. V. Petoukhov, S. Rahmstorf, S. Petri, and H.J. Schellnhuber,
   "Quasiresonant amplification of planetary waves and recent Northern
   Hemisphere weather extremes", *Proceedings of the National Academy of
   Sciences*, vol. 110, pp. 5336-5341, 2013.
   http://dx.doi.org/10.1073/pnas.1222000110
   5. M.E. Mann, S. Rahmstorf, K. Kornhuber, B.A. Steinman, S.K. Miller,
   and D. Coumou, "Influence of Anthropogenic Climate Change on Planetary Wave
   Resonance and Extreme Weather Events", *Scientific Reports*, vol. 7,
   2017. http://dx.doi.org/10.1038/srep45242
   6. R.E. Benestad, "Implications of a decrease in the precipitation area
   for the past and the future", *Environmental Research Letters*, vol. 13,
   pp. 044022, 2018. http://dx.doi.org/10.1088/1748-9326/aab375
   7. A. Witze, "Clouds get high on climate change", *Nature*, 2016.
   http://dx.doi.org/10.1038/nature.2016.20230
   8. Q. Schiermeier, "Droughts, heatwaves and floods: How to tell when
   climate change is to blame", *Nature*, vol. 560, pp. 20-22, 2018.
   http://dx.doi.org/10.1038/d41586-018-05849-9
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