Here Comes the Sun
The Sun (1): Some time ago I promised to write some blogs about the Sun and solar variability in climate change. Recently, there has been a lot of discussion about how the future variability of the Sun is accounted for in climate change predictions. This is the first in a series of blogs about the Sun.
First, however, I want to go back to fundamentals. Early on in this blog I did a series on the basics of climate science. The links to those blogs are given at the end, and here is a link of a collection of these blogs that I have put together. Briefly, the source of energy for the Earth and its climate is the Sun. Most of the energy of the Sun comes to the Earth as visible radiation, but there is energy in other parts of the radiative spectrum, for example, infrared and ultraviolet. The radiative energy that makes it to the Earth is either absorbed at the Earth or it is reflected back to space.
Here is an update of one of the iconic figures of climate change.
Figure 1: This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.
Most of us were probably taught that the energy that came from the Sun was “constant.” From grade school through graduate school I was taught that the solar constant was a number that was something like 1366 Watts/meter^2. There were various measurements that gave a “range” of this constant.
As our education got more sophisticated, we were ultimately taught that the solar constant was not, in fact, constant. One, we knew that the Sun’s output varied, with the most widely known source of variability correlated with the sunspot cycle. Two, in as much as the solar constant was the amount of radiative energy that fell upon the Earth, the various asymmetries and wobbles in the Earth’s orbit around the Sun caused some variability. Still, given the known (or hoped for?) stability of the Earth’s climate, the output from the Sun was commonly viewed as “constant.”
To climate scientists, solar physicists, and astrophysicists the Sun has never been constant. Here is a paper from the astrophysical community that has been cited many times in the climate community. (It happens to be by my big brother. Newman and Rood, “Implication of Solar Evolution for the Earth’s Early Atmosphere”, Science, 1977) This paper looks at very long time scales, over the lifetime of the Earth and the lifetime of the Sun (billions of years). Over this length of time, the energy coming from the Sun has increased by about 25%. This paper talks about the consistency between the observations of the climate of the Earth in the presence of this increasing energy from the Sun. Briefly, the surface of the Earth is warmer than it “should” be, and it is the fact that there is an atmosphere with greenhouse house gases that keeps the Earth warm (This is the part called “back radiation” on the right hand side of the figure.). Long ago, when the Sun was faint and young, the composition of the atmosphere had to be different, to contain more greenhouse gases, in order for the Earth’s temperature to be as high as it is known to have been.
The warmth and habitability of the Earth comes from the Sun, but the climate of the Earth is largely determined by the concentration of greenhouse gases in the atmosphere and ice on the surface of the Earth. In the absence of greenhouse gases, Earth would be frozen and white (super reflector!). Because of the large impact that greenhouse gases have on the Earth’s climate, the variability that comes from the Sun is buffered. Again, looking at the figure, a 1% change in the “incoming solar radiation” would be about 3.5 Watts/meter^2. Under the assumption that a 1% change would partition itself throughout the system in a proportional way, the 3.5 Watts/meter^2 change at the top of the atmosphere would translate to about 1.65 Watts/meter^2 at the Earth’s surface. This would then be portioned throughout the atmosphere, land, and ocean. The change from the Sun stands in stark contrast to the 333 Watts/meter^2 of “back radiation” that comes from the greenhouse effect of the atmosphere and clouds. From this sort of estimation, often called scaling arguments, if the Sun varies on the order of 1-2 %, then the impact on Earth’s surface is expected to be small. Variability from the Sun is spread throughout the Earth, and the 1% is, effectively, diluted.
That said: there are many signals that are observed in the Earth’s climate that are correlated with variability of solar energy. Some of these correlations are likely to be coincidence, but some of the correlations are quite compelling. (Remember correlation does not lead to a conclusion of cause and effect.) In order to establish the cause and effect a physical mechanism needs to be established. Often, in solar-terrestrial physics for the physical causality to be established between the Sun and temperature and precipitation at the surface of the Earth requires something that “amplifies” a relatively weak solar signal. The likely mechanisms to amplify a solar signal reside in, again, ice and greenhouse gases. More next time.
r
Blogs on radiative balance
Absorbing
Reflections
Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm
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Updated: 11:09 PM GMT del 29 Agosto 2008
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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.
r
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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Updated: 10:47 PM GMT del 21 Agosto 2008
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Heat Waves (2): Heat and Humans
The long spell of hot days broke in Denver, and my garden is starting to look happier. Still, I want to write one more about heat waves now. (At the bottom, I have a couple of off topic question. One's for teachers, one is for gardeners?)
From a human health point of view, heat waves are often listed as an urban problem. There have been a number of events in the past few years that have brought this message to the forefront. One of the most prominent heat waves was the sustained heat in Europe in 2003, which has been associated with more than 20,000 deaths. (Here is an old USA Today link to the impact in Paris.) The heat wave in Chicago in 1995 motivated much activity in the United States to develop better heat-health warning systems. (Here are a couple of articles from the Bulletin of the American Meteorology Society. Tom Karl and Richard Knight (Chicago Heat Wave) Larry Kalkstein et al. Philadelphia Heat Health Warning System)
In the previous blog the role of environmental , geographical and population information (Figure 1). It would seem to be a simple thing to calculate "when it is hot;" that is, the environmental information.
Figure 1. A schematic of the types of information needed in order to evaluate and respond to environmental heat threats.
Looking at the recent period of prolonged heat in Denver, however, it was not a period of with high human health impact. Perhaps this is because it was well reported, and Denver is an area used to summer heat. Examination of past heat waves give more information.
First, if you look at the Karl and Knight paper referenced above, they use a simple formula for identifying a heat wave. That definition is the nighttime minimum for three days. And, in fact, sustained high nighttime minima are one of the best predictors of human heat health threats. This is an indicator of both the inability of buildings to cool down and people to cool down. In Colorado, where it is dry and often not so cloudy, it continued to cool down at night.
Looking further, a hot spell in April or May in, say, Saint Louis, has a much greater health impact than the same or higher temperatures in August. Hence, there is an element of preparedness, acclimation, or expectation that has a large impact. 95 degrees F in Montreal or Paris is much more dangerous than 95 degrees F in Houston. 95 degrees F in a city where air conditioning is uncommon is more dangerous than where air conditioning is common. Hence the calculation of environmental heat that will be a health threat is far more difficult than calculating the temperature, or even a temperature-humidity combination, like the heat stress index.
(BTW, is you are in an enclosed building, blowing a fan at yourself, a stream of hot air, can make things worse because it dehydrates you faster. Remember that!)
The situation mentioned above is derived primarily from cities, which has been where the greatest public health impact is found. This threat is stongly correlated with chronic day and night high temperatures as well as some measure of "preparedness." There is another type of heat threat which is associated with people who are exercising or who are exposed to extremely high heat. In this case there is a temperature, some place around 105 degrees F, where the mortality rate jumps up. Here is a paper by Sam Keim who works in an emergency room in Arizona.
The human heat health problem indicates the complexity that is realized when trying to address environmental conditions, their impact on health, and how to address those impacts. Next time a little about heat waves and climate change.
Remember my gardening and teaching question below.
r
Some previous heat wave blogs
Hot in Denver: Heat Waves (1)
Letter from India
Heat, Flood, and Fires
Records and Patterns
1) Gardening: I know that some of you are avid gardeners of the organic flavor. I have been trying to find out information about irrigation systems. In particular, have you read anything about PVC pipe and chemicals placed in water or soil?
2) Teaching: I have been contacted by several people who are using my notes to teach climate change across disciplines. Some have expressed interest in starting a "community." If you teach or know of other courses that are teaching problem solving in climate change, let me know. (rbrood@umich.edu)
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Updated: 04:09 PM GMT del 11 Agosto 2008
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Hot in Denver: Heat Waves (1)
In the summer I live in Boulder, Colorado and this summer has been hot and dry. In fact in both Denver and Boulder the record for number of days above 90 degrees F has been broken. This is a sustained heat wave, not a heat wave where daily records fall one right after another.
Heat waves that impact human health are generally listed as the environmental condition which causes the most deaths annually. From a human health point of view, they are primarily an urban problem. From the point of view of the climate scientist, heat waves are expected to increase in intensity, frequency and duration. That is, it will be hotter, more often, and the heat will last longer.
Examination of the problem of heat waves is useful for understanding how climate change fits in. Human health is currently impact by excess environmental heat. Therefore a warming climate is likely to amplify the risk, but the fact that there is heat related health impacts is NOT a consequence of climate change. Heat waves, like many other impacts, represent a class of problem that already exists that is likely to be amplified by climate change.
Also like many other problems if your job was to reduce the deaths associated with excess environmental heat, then your motivation to reduce carbon dioxide emissions would be very low. Any impact on heat waves from reducing CO2 emissions would be realized far in the future. The most effective ways to lessen the health threats associated with heat waves are improved heat-health warning systems, better communication of threat conditions and proper responses to affected communities, and provision of a way to stay cool. Therefore, if your job was to reduce the deaths associates with heat waves, you would concentrate your efforts on developing the societal capabilities to warn, taking action, and protecting vulnerable communities.
Figure 1 provides a schematic for thinking about how to improve our abilities to address heat related health threats. There are three basic types of information. The first type is environmental information that informs that there is the likelihood that heat-related health threats are present. The second type of information is geographical information. An important ingredient of determining where heat is a threat is the built environment, city or country, park or parking lot? Knowing the characteristics of the built environment is important, as is a way to determine, for instance, just how hot it might it be in a particular neighborhood. Then once it is known how hot it is, then the health impact is strongly related to knowing if the population is vulnerable. This is often related to wealth and education. Do people have air conditioners or a way to get to a cooling center? Do people see themselves as vulnerable? Do people get the information that dangerous heat is likely? Are people acclimated to high heat?
Figure 1. A schematic of the types of information needed in order to evaluate and respond to environmental heat threats.
From the perspective of someone concerned about climate change and heat waves, then you anticipate how climate change will impact the margins. Will heat health warnings be needed at more northern cities? Will they need to be initiated earlier in the year? Will more facilities be needed to cool people? Strategies to build resilience follow from urban planning, for instance, the use of parks, and policy such as use of roofing materials to moderate urban warming. These strategies are all called for in the absence of climate change; climate change is an additional increment.
r
Some previous heat wave blogs
Letter from India
Heat, Flood, and Fires
Records and Patterns
