Unraveling The Heat Wave: Separating Weather From 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.

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