Heat Waves (3): Role of Global Warming

Heat Waves (3): Role of Global Warming: In the last two blogs I talked about heat waves. I use heat waves as an example of the relationship of a “societal impact” sensitive to climate change. The impact of environmental heat on humans is a problem that has been around forever. It depends not only on how hot it is, but also the characteristics of the built environment (is the pavement black?), and the ability of a person to respond to the high heat (can you cool down?). It is a problem that is likely to be amplified by global warming. This blog will highlight some of the impacts that climate change will have on heat waves.

In 1998 Dian Gaffen and Rebecca Ross (Gaffen and Ross, Nature, 1998) analyzed station data in the United States and found that in the last half of the twentieth century there was an increase in the heat stress in the eastern and western parts of the U. S. The heat stress is a quantity that combines temperature and humidity observations; it has been used to indicate the impact of heat and humidity on people. (There is a heat index in the Weather Stations part of the WU website.). In the Gaffen and Ross study heat waves were determined by the stress index being higher than the historic 85th percentile for three days. The authors state that the increase was possibly related to urbanization, but the regional clustering of the trends were suggestive of a climate signal. A follow up paper the next year, explored the issues of urbanization more thoroughly (Gaffen and Ross, J. Climate, 1999).

Jerry Meehl and Claudia Tebaldi (Meehl and Tebaldi, Science, 2004) and David Easterling and co-authors (Easterling et al., Amer. Meteor. Soc., 2007) have investigated the increase of heat waves in the future. In these studies models were used. Simulations of the past 100 years were used to establish that the models could reproduce the observations. These studies use a variety of temperature-based definitions for heat waves and find that heat waves will be more intense, more frequent, and of longer duration. There are a couple of important points from these studies. The occurrence of heat waves has a definitive spatial pattern related to the distribution of quasi-stationary high and low pressure systems. (A relevant blog from the past: Records and Patterns) For the U.S., these studies show, both in the observations and the models, heat waves increase preferentially in the western U.S. The studies of Easterling et al. look at warm spells all year around. They point out a seasonal signal with the greatest increase in the spring. (Remember the blogs on springtime snow coverage: Getting Ready for Spring (3)) There is also an interesting result that after about 2050, in large parts of the U.S., the number of heat waves deceases. Why? We are always in a heat wave – it’s like one long hot spell.

The geographical distribution of heat waves is studied further by Noah Diffenbaugh and colleagues using regional climate models (Diffenbaugh et al., Geophys. Res. Lett., 2007). Regional climate models run at higher resolution than global models; they are, essentially, high resolution models embedded in a lower resolution global model. The resolution used in these studies is 20 km horizontal resolution. With this strategy, topography and land-coast interfaces are better resolved. Low-level jet streams, which are responsible for the flux of moisture to the interior of continents, are far better represented. It is possible to better represent local feedbacks, such as role of water vapor, land surface type, and vegetation. In the paper Diffenbaugh et al. show that in some “hot spots” there can be 2-5 times as many high heat events. They also show that if we were to reduce carbon dioxide, it would matter. Here is a figure and caption from the Purdue University website, where Diffenbaugh does his research.




Figure 1. Caption from original Purdue University web site: This image illustrates heat stress in the 21st century for two greenhouse gas emissions scenarios. The top panel shows the expected intensification of the severity of extreme hot days given accelerating increases in greenhouse gas concentrations. The bottom panel shows the expected decrease in intensification associated with decelerated increases in greenhouse gas concentrations. (Purdue University image/Diffenbaugh Laboratory) (Rood: In the paper the model runs were for the last 30 years of the 21st century.)

This series of blogs show the nature of an “impact” that is amplified by climate change. There are a number of things to remember. The “impact,” the problem, exists even in the absence of global warming, but it is amplified by global warming. In the short-term, the impact is lessened by education, increased preparedness, and engineering solutions, such as air conditioning, building materials, and cooling centers. In the short-term, controlling CO2 emissions has little impact, but looking out 50 years, decisions to reduce greenhouse gas emissions today will matter a lot.

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Some previous heat wave blogs

Hot in Denver: Heat Waves (1)
Heat Waves (2): Heat and Humans

Letter from India
Heat, Flood, and Fires
Records and Patterns

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