Climate change is expected due to increasing CO2. Is there evidence that climate change is happening in the Houston, Texas area’s warmest months?
“Warmest months” is defined as the warmest six months (May thru October). Houston’s weather is dominated in this period by warm, moist flow from the Gulf of Mexico. Weather patterns tend to be redundant – warm, humid, with a chance of showers. There is variation, though, with summer sometimes experiencing periods of drought when upper-air anticyclones dominate.
“Houston area” is defined as NOAA’s Texas Upper Coast climate division (light blue, below), which consists of 13 coastal counties roughly centered on Houston.
First, let’s look at the maximum temperatures since 1895. The data source is NOAA’s NCDC’s Climate at a Glance plotter . The maximum temperatures show variation over time, with a warm 1930-1950s and cooler 1970s and a rise in recent decades, but the direction is not clearly higher. The recent rise might be natural variation only or natural variation plus a CO2-induced rise. Visual examination alone does not provide a clear answer.
Might the variation in summer maximum temperatures be related to soil moisture? It’s established that there is a relationship between soil dryness and maximum summer temperature. I believe the relationship is due to the lack of heat removal by vaporization and possibly because the weather regime which produces droughts in Texas also produces sunny skies and warm air aloft, hindering convective heat removal.
Let’s use a measure of drought (Palmer Modified Drought Index, PMDI) and compare its variation with summer maximum temperature. When comapred, an r-squared value of 0.4 is found, a fairly good correlation. Over the last 30 years the r-squared value has been 0.52.
Below is a plot of the standard deviation of Houston PMDI and max temperature which illustrates the relationship:
An r-squared value of 0.52 over the last 30 years it’s fair to say that most of the recent rise in maximum temperature can be explained by drier warm-weather conditions.
What about minimum temperatures in the warm months? This could be interesting, as the effects of increasing CO2 might be more noticeable at night, when radiative cooling dominates heat transfer. Here’s a plot of Houston area minimum temperatures during May – Oct:
Well, this indeed looks interesting. It appears directionless until the 1980s, at which time it rises to record values. Also interestingly, it bears a resemblance to the global average annual temperature pattern. It’s not correlated to drought (soil moisture), as the r-squared relationship between min temperature and PMDI (not shown) is essentially zero.
My surmise is that this temperature rise includes effects from increasing CO2. The effect might be direct (slower IR loss at nighttime) and/or indirect (higher moisture in the lower troposphere which increases both dewpoint and late-night low clouds). My impression from living along the US Gulf Coast for decades is that the summertime low temperature is mainly determined by the dew point – near the dew point the cooling from IR radiation generates dew rather than lower temperatures.
How about the diurnal temperature spread (max – min)? Here’s a plot of the spread since 1895:
The plot shows a general decline over time, which is generally expected with CO2-driven climate change. However, the decline seems to occur in one step about 1960. I have no explanation for that. The step decline could be random, could be an artifact of the temperature record (which includes adjustments) or it could have a physical basis. It’s something to ponder.
It’s also worth noting that temperature spread is fairly correlated with drought. The r-squared for temperature spread versus drought index (1895-2014) is 0.42. A correlation makes sense as drought conditions are generally associated with less atmospheric water vapor and less water vapor means faster radiation of heat (IR) from the ground into outer space.
Finally, what about Houston area rainfall? Here’s a plot of total May-Oct yearly rainfall since 1895:
The plot shows variation over the years, with perhaps a dry 1930s and wet 1970s and ’80s. The 1950s were also drier than average. The second half of the period is wetter, on average, than the first half. I see no clear trend which would correspond with rising CO2.
So, where does all this leave us? My view:
Maximum temperatures – no CO2-related trend
Minimum temperatures – rising in recent decades, plausibly due to higher CO2, in whole or in part
Max-Min Spread – no clear CO2-related trend
Precipitation – no CO2-related trend