Posted by: David Hone

Extreme hot weather in the USA and climate change

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The current spate of very hot weather across much of the USA (and not forgetting the balmy “winter” days in many states back in the early part of the year) raises the question of the role of climate change in relation to such extremes. Of course long term changes in the climate and the occurrence of daily and weekly weather events are very different things, with the latter being the direct result of chaotic movement within the atmosphere. But weather does follow some pattern and over the longer term these patterns can be influenced by a shift in the climate.

A few months back I commented on a paper written by James Hansen which examined this issue in some detail. As illustrated in the figure below, Hansen showed that the distribution of seasonal temperature has indeed shifted, leading to an increase in anomalous events. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (σ) warmer than the 1951-1980 baseline.  This hot extreme, which covered much less than 1% of Earth’s surface in the base period, now typically covers about 10% of the land area.  He concluded that extreme heat waves, such as that in Texas and Oklahoma in 2011, Moscow in 2010 and now the US heat wave (which has seen thousands of temperature records topple), were “caused” by global warming, because their likelihood was negligible prior to the recent rapid global warming.


Hansen produced an analysis of temperature data which showed this phenomena was indeed happening. Given the current interest in the issue from friends and colleagues in the USA I thought I would try to see what I could establish by doing my own analysis. Long term US temperature data is readily available for download, so I loaded 110  years (1902-2011) of New York Central Park temperature data into Excel and did some number crunching. I looked at the summer period only (June, July and August) and focused on the maximum daytime temperature in °F. I divided the data into two parts, pre 1950 and post 1950. The respective distribution curves are shown in the chart below.


There is certainly a shift in the distribution curves, although it is quite small. The median temperature in the pre-1950 data is 82, whereas in the post 1950 data it is 83. But even such a small shift changes the probability of a very hot day. The chart below shows that while the probability of an 80°F day has barely shifted, the chance of a 95°F day has risen from 1.8% to 2.3%, or about a quarter.


Even such a small shift can have a noticeable impact. A chance of 1.8% means that before 1950 there were about 2 summer days with a maximum temperature of 95°F,  but since 1950 there has been an extra day every two years. Another way of looking at this is to examine the incidence of heat waves in New York. In the chart below, I have counted the number of days where that day and the previous three days were 90°F or above. Adding a trend line to the graph shows that over 110 years the number of “heat wave” days has increased from one to over three, or alternatively a single annual four day heat wave in 1900 to at least two per year by 2010 or one lasting more than a week (and there are other combinations as well).


Finally, a more marked change shows up when comparing the period from 1902-1925 with the period 1990-2011. Now the chance of a 95°F day in the summer has about doubled!! The Post 1990 distribution is also starting to show the appearance of very hot days (more than 100°F), which hardly existed prior to 1925.

Authored by:

David Hone

David Hone serves as the Chief Climate Change Advisor for Royal Dutch Shell. He combines his work with his responsibilities as a board member and Chairman of the International Emissions Trading Association (IETA). Additionally, he works closely with the World Business Council for Sustainable Development and has been a lead contributor to many of its recent energy and climate change ...

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July 10, 2012

Rick Engebretson says:

David, I don't understand why most "climate change" proponents seem to exclude land use as causal.

The very land area now experiencing record heat and drought in North America had over 50 million bison roaming just 150 years ago. The beaver was trapped from their wetlands.

Deep rooted perennial plants that self manage water and solar absorbtion, and grow fastest in early spring are gone. They are replaced by shallow rooted annual plants on naked mineral soil, and don't usually grow until late spring or early summer.

NOAA's map clearly shows where this "climate change" is happening. Exactly the same place newly invented tractor tillage was considered a cause of the dust bowl in the 1930s.

I'm not saying we can return to earlier agriculture. But if we want to discuss a growing problem let's discuss it with open minds. We don't want to convict the wrong suspects.

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July 6, 2012

Michael Jones says:

Thank you for the study and though it was limited it provided some insight. Of course, other factors need to be addressed that were not expressed in your paper, such as, regional location impacts, ocean warming temperatures, and supports Dr James Hansen's "loaded dice" explaination. Also, increase carbon levels have additional ways of changing ecosystems, such as, lowering the ph of ocean waters.

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July 6, 2012

David Hone says:


I agree, its a bit limited as it stands and in hindsight I should have said that in the posting. For example, the only reason I picked NYC Central Park was that the full dataset was available on the web. As you note, James Hansen did the more complete analysis of the shift in distribution in the paper that I pointed to in my post. My only point here was to show that within the day to day weather which is a chaotic process, there are underlying trends that point to certain externalities in play and that those externalities do impact events such as heat waves. In this case the externality might simply be the urban heat island and not the global impact of rising CO2 level, or both.

Thanks for the comment.


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July 5, 2012

Geoffrey Styles says:


As a participant in the current eastern-US heat wave (and experiencing part of it without electricity over the weekend) I find your analysis very interesting, though hardly reassuring.  Assessing altered probabilities of occurrence seems like a much more scientifically honest and productive avenue than either the categorical claim of climate change influence on Colorado's fires that I saw a well-known climate scientist (and Climategate figure) make on a recent network news broadcast, or the typical weathercaster's comment that we can't attribute any individual event to climate change.   

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