Heat Waves (4) A Climate Case Study:

By: Dr. Ricky Rood , 6:26 AM GMT on July 19, 2011

Heat Waves (4) A Climate Case Study:

In the last article I wrote that the extreme events of 2011 were providing us with the opportunity to think about climate and how to cope with a warming world. The U.S. is experiencing an extreme heat event this week (Masters @ WU). This heat wave is the consequence of a strong, stationary high pressure system over the central U.S., and it will move to the east over the next few days. Back on July 14th The Capital Weather Gang did a nice write up on the forecast of the heat wave. At the end of this blog are links to my previous blogs on heat waves and human health.

When thinking about weather, climate, and extreme events an important idea is “persistence.” For example, a heat wave occurs when there are persistent high temperatures. Persistent weather patterns occur when high and low pressure systems get large and stuck; that is, they don’t move. In the Figure below, you need to imagine North America and the United States. There is a high pressure center over the proverbial Heartland. With blue arrows I have drawn the flow of air around the high pressure system, and in this case moist air. There is moisture coming from the Gulf of Mexico and, in fact on the date when this was drawn, from the Pacific. This is common in the summer to see both the Gulf of Mexico and Pacific as sources of continental moisture.



Figure 1: Schematic of a high pressure system over the central United States in July. While generic, this is drawn to represent some of the specifics of 2011. The green-shaded area is where there have been floods in 2011. The brown-shaded area represents sustained drought in the southern part of the nation.

At the center of this high pressure system there is a suppression of rain, because the air is moving downward. This sets up a situation where the surface heats from the Sun’s energy. There is not much mixing and cooling, because of the suppression of the upward motion that produces rain. Hence, if this high pressure system gets stuck, then there is persistent heat. This is a classic summer heat wave.

Let’s think about it some more. There is lot of moisture being drawn around the edge of the high pressure system, and this moisture contributes to the discomfort of people. People – just a short aside about people: if we think about heat and health, then we are concerned about people’s ability to cool themselves. It is more difficult to cool people when it is humid because sweat does not evaporate. Suppose that in addition to this moisture, there is a region where the ground is soaked with water from flooding. Then on top of already moist air coming from the Gulf, there is local evaporation into the air being warmed by the Sun. If on the interior of the high, where the rain is suppressed, there is hot, wet air, then it becomes dangerous heat.

It’s not easy to derive a number that describes dangerous heat. But in much of the eastern U.S. a number that somehow combines temperature and humidity is useful. Meteorologists often use the heat index. It’s the summer time version of “it’s 98 degrees, but it feels like 105.” For moist climates, the heat index is one version of the “it feels like” temperature. Jeff Masters tells me that in Newton, Iowa yesterday, July 17, 2011, the heat index was 126 degrees F. (see here, and 131 F in Knoxville, Iowa on July 18)

Another measure of heat and humidity is the dew point; that is, the temperature at which dew forms, and effectively limits the nighttime low. The dew points in Iowa, South Dakota, Minnesota, and Wisconsin are currently very high and setting records. Here is a map of dew point for July 19, 2011.



Figure 2: Exceptionally high dew points centered on Iowa.


Now if I was a public health official, and I was trying to understand how a warming planet might impact my life, then here is how I would think about it. First, the Gulf of Mexico and the Pacific are going to be warmer, and hence, there will be more humid air. This will mean, with regard to human health for the central U.S., heat waves will become more dangerous, without necessarily becoming hotter. It is also reasonable to expect heat waves will become more frequent and last longer, because those persistent, stuck high pressure systems are, in part, forced by the higher sea surface temperatures. If I am a public health official here is my algorithm – heat waves are already important to my life, and they are likely to get more dangerous, more frequent, and of longer duration. But by how much? Do I need to know by how much before I decide on a plan for action?

If I think about the air being more humid, then I might expect to see trends in the heat index. I might expect to see trends in dew points, and trends in the nighttime minimum temperatures getting higher. (That’s where a greenhouse effect really matters.) I worry about persistent heat, warm nights, and the inability of people and buildings to cool themselves. I worry about their being dangerous heat in places where people and emergency rooms are not used to dangerous heat – not acclimated to heat – not looking for heat-related illness.

Let’s go back to the figure. Rain is suppressed in the middle of the high pressure system, but around the edge of the high pressure system it will rain; there will be storms. (see Figure 3 at the end) The air around the edge of high is warm and very wet. Wet air is energetic air, and it is reasonable to expect local severe storms. (See Severe Storm on Lake Michigan) And if the high pressure is persistent, stuck, then days of extreme weather are possible. If this pattern sets up, then there is increased likelihood of flooding. If I am that public health official, then I am alerted to the possibility of more extreme weather and the dangers thereof. But, again, can the increase of extreme weather be quantified? Do I need to quantify it before I decide on a plan of action?

Still with the figure - what about that region of extended drought and the heat from the high pressure system? Dehydration becomes a more important issue. As a public health official, I start to see the relation of the heat event to other aspects of the weather, the climate. I see the relation to drought. I see the flood, and it’s relation to the winter snow pack and spring rains.

So what I have presented here is to look at the local mechanisms of the weather – what are the basic underlying physics responsible for hot and cold, wet and dry – for moist air? If I stick to these basic physics, and let the climate model frame the more complex regional and global picture, what can I say about the future? Do I have to have a formal prediction to take action? Here in 2011, I see drought and flood and hot weather and warm oceans that interact together to make a period of sustained, dangerous heat. It does not have to “set a record” to convey the reality of the warming earth. It tells me the type of event that is likely to come more often, of longer duration, and of, perhaps, of greater intensity. If I am a public health planner, then I can know this with some certainty. The question becomes, how do I use that information in my planning?

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Figure 3: Radar loop showing precipitation around the edge of the large high pressure system in the middle of the continent. July 19, 2011.

Previous Blogs on Heat Waves

Hot in Denver: Heat Waves (1)

Heat Waves (2): Heat and Humans

Heat Waves (3): Role of Global Warming




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Quoting cat5hurricane:

Actually, the vast majority of climate scientists initially jumped upon the theory the globe was cooling. It was only until the Earth climate changed before they realized they probably jumped to conclusions.

Silly scientists.

No, they didn't. And no matter how many times that particular fantasy is repeated, it'll never be true. Here, allow me to help:

Strike One: "Not true, climatologist Thomas C. Peterson of the National Climatic Data Center in Asheville, N.C., and his colleagues report in the September Bulletin of the American Meteorological Society. The team’s survey of major journal papers published between 1965 and 1979 found that only seven articles predicted that global average temperature would continue to cool. During the same period, 44 journal papers indicated that the average temperature would rise and 20 were neutral or made no climate predictions."

"When...skeptics mention previous concerns about global cooling, they typically cite media reports from the 1970s rather than journal papers —'a part of their tremendous smoke screen on this issue,' says Peterson. Among major magazines, Time and Newsweek ran articles expressing concern about the previous decades’ cooling trend, juxtaposing the specter of decreased food production with rising global population.

Strike Two: "But even a cursory review of 1970s media accounts shows that there was no consensus about global cooling among journalists, either, Peterson says. In May 1975, the headline of a New York Times article warned that 'major cooling may be ahead.' Three months later, another headline in the same paper — atop a feature written by the same reporter — stated that two recent journal articles 'counter [the] view that [a] cold period is due.'"

Strike Three: "When global warming skeptics draw misleading comparisons between scientists' nascent understanding of climate processes in the 1970s and their level of knowledge today, 'it's absolute nonsense,' Schneider says. Back then, scientists were just beginning to study climate trends and their causes, and the probability of finding evidence to disprove a particular hypothesis was relatively high. Nowadays, he contends, 'the likelihood of new evidence to overthrow the concept of global warming is small. Warming is virtually certain.'"

So, that whole "They predicted global cooling in the 70s" thing? It's out...
Member Since: November 8, 2009 Posts: 4 Comments: 15125
www.ncdc.noaa.gov/indicators


Global Climate Change Indicators
National Oceanic and Atmospheric Administration
National Climatic Data Center

Climate Model Indications and the Observed Climate


Simulated global temperature in experiments that include human influences (pink line), and model experiments that included only natural factors (blue line). The black line is observed temperature change

Global climate models clearly show the effect of human-induced changes on global temperatures. The blue band shows how global temperatures would have changed due to natural forces only (without human influence). The pink band shows model projections of the effects of human and natural forces combined. The black line shows actual observed global average temperatures. The close match between the black line and the pink band indicates that observed warming over the last half-century cannot be explained by natural factors alone, and is instead caused primarily by human factors.

Member Since: Posts: Comments:
Quoting cat5hurricane:
...the last indications are for trending colder winters coupled with hotter summers.


Be careful not to believe or post everything you see. I remember after the brutal winters in the 1970's and 1980's the dawn of the next ice age is upon us. And, well, did that pan out?


Re part 1: if you're talking about North America, that's not what the data show:

uh-oh

So far as part 2, few climatologists believed that an ice age was on its way. But if you have actual proof stating otherwise, I'd love to see it.
Member Since: November 8, 2009 Posts: 4 Comments: 15125
Thank you, Dr. Rood.

First, congrats on reaching 200; that's a whole lotta writing. I think I speak for everyone when I tell you that it is very much appreciated.

Second, those heat indices were even higher yesterday. Here are a few from Iowa:

KNOXVILLE 131
NEWTON 129
ATLANTIC 126
COUNCIL BLUFFS 126

Third, there's every indication that this kind of heat not only will become more and more normal, but likely already is:

Stanford climate scientists forecast permanently hotter summers beginning in 20 years

The tropics and much of the Northern Hemisphere are likely to experience an irreversible rise in summer temperatures within the next 20 to 60 years if atmospheric greenhouse gas concentrations continue to increase, according to a new climate study by Stanford University scientists. The results will be published later this month in the journal Climatic Change Letters.

In the study, the Stanford team concluded that many tropical regions in Africa, Asia and South America could see "the permanent emergence of unprecedented summer heat" in the next two decades. Middle latitudes of Europe, China and North America – including the United States – are likely to undergo extreme summer temperature shifts within 60 years, the researchers found.

"According to our projections, large areas of the globe are likely to warm up so quickly that, by the middle of this century, even the coolest summers will be hotter than the hottest summers of the past 50 years," said the study's lead author, Noah Diffenbaugh, an assistant professor of environmental Earth system science and fellow at the Woods Institute for the Environment at Stanford. The study is co-authored by Stanford research assistant Martin Scherer.

- - - - - - - - - -

To determine the seasonal impact of global warming in coming decades, Diffenbaugh and Scherer analyzed more than 50 climate model experiments –including computer simulations of the 21st century when global greenhouse gas concentrations are expected to increase, and simulations of the 20th century that accurately "predicted" the Earth's climate during the last 50 years. The analysis revealed that many parts of the planet could experience a permanent spike in seasonal temperatures within 60 years.

"We also analyzed historical data from weather stations around the world to see if the projected emergence of unprecedented heat had already begun," Diffenbaugh said. "It turns out that when we look back in time using temperature records, we find that this extreme heat emergence is occurring now, and that climate models represent the historical patterns remarkably well."

Stanford Report Article...

NOTE: that report isn't claiming there haven't been extreme heat waves in the past; it's saying, rather, that they are going to become more frequent and more severe.
Member Since: November 8, 2009 Posts: 4 Comments: 15125

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Dr. Ricky Rood's Climate Change Blog

About RickyRood

I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.

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