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Robert Rapier

Robert Rapier

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Explaining The Heat Wave: Separating Weather From Climate Change

  • Individual high temperatures may be weather, but a trend of increasing temperatures is climate change.
  • Data from the National Weather Service indicates a long-term warming trend in Phoenix, with nine of the ten hottest years on record occurring within the past 20 years, and a significant rise in average annual temperatures and nighttime lows.
  • While the current heatwave in Phoenix is amplified by a natural El Niño cycle, the city's baseline climate has become significantly hotter over the past 50 years, a change happening against a backdrop of atmospheric carbon dioxide levels that have risen more than 30% since the 1970s.
Climate Change

I intended for this column to be a discussion about global carbon dioxide emissions. However, I received quite a bit of feedback on my previous column on the Phoenix heat wave, and I want to address that first.

The objections were aimed at my contention that one of the factors involved in the heat wave, is the steadily growing concentrations of carbon dioxide in the atmosphere. This topic always stirs strong emotions in people.

The oddest response to me is always “I don’t believe in climate change”, as if we are talking about a belief in Santa Claus. So, I always ask “Which part do you not believe? That global carbon dioxide concentrations are rising? That it’s getting hotter? Or is it that you believe this is just normal weather fluctuations?”

It turns out that it’s generally the latter two. Multiple people claimed that it’s not any hotter in Phoenix today than it was in the 1970s, and that this is just normal fluctuations. One person insisted that weather is unpredictable, so I spent some time trying to help them understand that weather and climate are not the same thing. So, let’s tackle that first, before addressing whether it is actually getting hotter in Phoenix.

Weather refers to short-term atmospheric conditions over hours to weeks. Weather could be a storm, or a temperature record. One person objected to my article because they asserted that the all-time high temperature record in Phoenix was 122 degrees, and it happened in 1990. That’s weather.

Climate is the long-term prevailing weather patterns over years, decades, or centuries. For example, a daily temperature record would be weather, but a yearly or decade average would be climate. A record storm would be weather, but a record number of storms over years would be climate.

So, that’s the first thing. People who are confused between weather and climate may insist that it has been hotter in the past. Yes, one day in 1990 stands as the all-time record. But it’s a different story when you start looking at patterns over time.

Let’s look at the annual temperature records from the National Weather Service (link). Of the 10 hottest years on record in Phoenix, nine of them have been within the past 20 years. Presently, 2017 is the hottest year on record, with an average annual temperature of 77.3 degrees F. The previous decade is the hottest decade on record.

Now contrast that with annual record low temperatures in Phoenix. The most recent of the Top 10 record lows took place in 1965. Only two of the 10 lowest temperature records in Phoenix took place within the past 100 years.

One of the biggest problems faced by cities like Phoenix is that the nights aren’t cooling off as much as in the past. For example, in the 1970s the average low temperature in Phoenix was 59 degrees F. During the previous decade, that had risen to nearly 65 degrees. That’s a substantial increase for an annual average.

Consider that Phoenix is set to become the first major U.S. city to average more than 100 degrees F for an entire month. We are now approaching an entire month with daily temperatures reaching 110+°F, shattering the previous record that was in fact set in 1974 at 18 days.

During this record heat wave, Phoenix also broke the record for highest daily low temperature, with 97 degrees recorded at Sky Harbor Airport. The all-time highest daily average temperature record was also broken with an average temperature of 108 degrees. (See more records being broken here).

What is happening now is weather fluctuations that are taking place on top of a baseline temperature that has increased significantly in the past few decades. Thus, the high temperatures are not solely climate change. This is climate change in an El Niño year. El Niño years are hotter, but growing carbon dioxide concentrations on top of that leads to rising average temperatures over time — and sometimes individual record temperatures.

Likewise, there are other factors that influence temperature. That’s why global temperature doesn’t scale linearly with rising carbon dioxide temperatures. But it does correlate, especially when considering averages over time (which would smooth out fluctuations due to other factors).


The data clearly shows that Phoenix is experiencing a long-term warming trend even if daily temperature records are variable. While the current heatwave has been amplified by a natural El Niño cycle, the baseline climate in Phoenix has grown significantly hotter over the past 50 years as evidenced by rising annual averages and nighttime lows. This is taking place against a background of atmospheric carbon dioxide levels that have risen more than 30% since the early 1970s.

So, while weather fluctuations will continue, climate change is certainly a key driver of Phoenix’s escalating heat, and that heat will only worsen until carbon emissions are curtailed.

By Robert Rapier 

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  • Terrel Shields on July 31 2023 said:
    That is idiotic nonsense on the face of it. Breaking a record low high next to a radiating air strip? Who woulda thunk it. I worked in AZ, NM, etc. and it is hot hot hot - and you couldn't spray paint during the day because the paint dried before it hit the wall. You worked at night. Take out the heat island effect and it is just what it has always been hot. The whole SW had a 200 year drought a thousand years ago. How hot do you think it got then? No one will ever know.
  • Sam cox on August 01 2023 said:
    How do we normalize ambient temperatures with the growth in buildings and paved surfaces? I have heard that a majority of land based NOAA measurement stations are now near building (HVAC units) and paved surfaces. Most larger cities hold heat longer into the night due to the
    increased thermal mass of man made structures. Is there a factor based on green/dirt space vs. man made heat retaining thermal mass areas?
  • Don Lock on August 01 2023 said:
    Temperatures in Phoenix have unquestionably been rising over recent decades, but not due solely to atmospheric CO2 concentrations. The population of this city's metropolitan area was 221,000 in 1950; now it's 4.7 million. The associated heat island effect is powerful and has been steadily growing, commingled with any genuine climate change.

    Some say that climate scientists have backed-out heat island effects from their global warming reports, but this claim can be fact-checked by asking what the heat island correction factor is for Phoenix or for any city. This figure is not on the NOAA website or anywhere else that I can find, and for good reason, because quantifying the heat island effect requires extensive research at multiple sites over a long period of time.

    This is an especially important factor for obtaining reliable global warming trends because nearly all climate data are taken from airport weather stations, which in most cases have become the center of a vast industrial sprawl since WWII.
  • Jack Kneeland on August 03 2023 said:
    This is a rare and valuable effort to step back without preconceptions and try to figure out a very complex situation that has a huge amount of ideological baggage.

    The analysis is logical and I broadly agree with your conclusions with with regard to the problem and the required solution. I broadly see climate change as a subset of the antropomorphic era and one of many problems, not necessary the single existential crisis, but that is not inconsistent with what you have said.

    I do however think you did seem to miss one of and possibly the most important potential response to the question that you pose: "“Which part do you not believe?"

    That is how much of climate change is natural and not connected with human activity and how much is caused by humans. My understanding is that some of climate change does appear to be natural and some part does appear to be caused by humans. I have followed this issue for a long time and not only do I not have an answer, I am not sure that there is a consensus answer.

    So, I have to fall on a 50/50 split with a 50% margin of error. In this regard, we aloud probably have to accept that the climate is going to change regardless of what we do and that all climate change is not caused by humans.

    This is not a call to inaction. If a large percentage is human caused and the marginal impact of this is significant, then the solution remains the same.

    But this does seem to be a gaping hole in the argument and in my view the single place where the belief chain breaks down.

    I would like to hear your views on what % of climate change is caused by man, if that is the consensus view, and what is the documentation behind that.

    I am also sympathetic with the arguments made in other comments about errors in measurement, but I don't have enough visibility to speculate one way or the other.
  • DoRight Deikins on August 09 2023 said:
    Since my comments on 7/26 and 7/31 on urban sprawl and urban heat islands were not accepted, maybe the 3rd times the charm.

    Urban sprawl that makes a larger and larger urban heat island. Is Phoenix any bigger now than it was in 1970? How much more concrete, asphalt, roads, buildings, airports and runways, and parking lots does it have now than it did then? All those store heat. How many more people and cars and homes are there now than then? All those produce heat.

    But this is the most important factor in my opinion and one that I almost never see mentioned. How many MWs of electricity does Phoenix use in a day. How many did it use in 1970? Physics tells us that all energy is foundationally HEAT. That's why one of the most important courses engineers and physical scientists study is thermodynamics. All energy eventually becomes heat. So if you are collecting 100 GW of electrical power from solar energy and sending it through cables to your city, it eventually all ends up as heat. That super efficient LED light bulb, yep it produces heat. Not as much as an incandescent does, but all the electricity it uses eventually turns in to heat. So whether you have one light bulb that uses 100W or burn 100 LEDs that use 1W each, it produces the same amount of heat. Yes, you can burn only 15 LEDs and get the same amount of light and less heat, but how many MWs are being sent to Phoenix? Because all that power demand, no matter how it is produced, or how it is used, ends up as heat.

    Yes, CO2 and other GHGs have an effect on the climate. Personally as an agriculturalist I love the increase in CO2, but not the other GHGs such as methane, particulate pollution, and the dangerous gases, like nitrous oxides and sulfurous oxides.

    So keep campaigning against those nasty GHGs, but I think that urban heat island in which you live has more to do with your local climate change than CO2 does. And as I mentioned previously, many times are 'solutions' have unintended consequences that make the problems worse.
  • Scott Bulger on August 18 2023 said:
    Los Angeles on track to see longest cold streak in 35 years- winter 2023.
    This is all weather.

    Climate change is measured in hundreds, if not thousands of years. Not 50 years.

    Look at the Maunder and Dalton minimums. The sun/solar activity or the lack of is the biggest factor on temperatures, the weather and the climate.

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