USC Dana and David Dornsife College of Letters, Arts & Sciences > Blog

April 16, 2012

Climate change and the hydrological cycle

Filed under: climate,Water — admin @ 9:21 pm

A severe change in the hydrological cycle is expected, and it is expected to hit snow- or ice-dominated areas most severely. This change is expected because of an increase in greenhouse gases. This change at first was expected to increase the amount of potable water but now the dynamics of the changes have been analyzed more closely. We have found out that as temperatures increase less precipitation will fall as snow and snowmelt will occur sooner in early spring and not in the summer or autumn when the water is needed most. The snowmelt and rain will cause an overflow in rivers and causing loss of potable water to the oceans when there are not sufficient reservoirs.

And it is not necessarily changes in precipitation that causes all this because the amount of precipitation generally remains the same. It is the change in temperature that changes the seasonal runoff patterns in these snowmelt-dominated areas because less water falls as snow and more falls as rain, preventing the normal release of water as snowmelt and the quick flowing of rainwater.

The Colorado River of the western United States was determined to be one of the four snowmelt-dominated rivers that also do not have sufficient reservoir capacity to prevent overflow and loss to the ocean. To determine these, first the snowmelt-dominated areas were determined by the ratio of accumulated annual snowfall to annual rainfall and those with R greater than 0.5 were considered snowmelt-dominated. Next, to determine reservoir capacity, the runoff was compared to the reservoir capacity. These determined areas underestimate the area and population affected because populations downstream and other farther areas also depend on the water that comes from snowmelt-dominated areas.

The aspect of the most importance is water supply. In the Western United States, the Colorado River is the most important contributor of water supply. There are no predicted changes in precipitation, only a change in seasonal snowpack and snowmelt, as discussed earlier. The winter snow is expected to decrease and the melting is expected to occur a whole month earlier. On top of that, there is currently not enough reservoir capacity to prevent water loss to the ocean.

The Colorado River, along with the Rio Grande and San Joaquin, supply water to Wyoming, Colorado, New Mexico, Arizona, Nevada, California, Utah, Texas, and parts of Mexico. These rivers, especially the Colorado River, were determined by the Interior Department in 2011 to deplete by 8 to 14 percent over the next 40 years. But in a more optimistic short-term study done in 2009 at the University of Colorado at Boulder, the risk of the Colorado River depleting its reservoirs remains below 10 percent at least through 2026. It was also said as a result of this study that even if the worst drought scenario were to occur, we wouldn’t feel the effects immediately because we have a great storage capacity along the Colorado River, storing almost four times the annual flow of the river. But in between 2026 and 2057, the risk of reservoir depletion increases seven times.

These studies on the Colorado River can comfort us because we know we are relatively safe until 2026, but 2026 is approaching fast and we cannot get comfortable. Large scale changes such as shift in seasonal snowmelt and decreasing amounts of snowfall took decades to develop and will take decades to reverse, if it is even at all possible. The most plausible solution for now is that we must find ways to direct and store this precipitation so we do not lose it to the ocean.


This post was authored by Alejandra Rocha ’12, a senior majoring in Environmental Studies.

Heat Waves and the Growing Threat Posed by Extreme Heat Events

Filed under: climate — admin @ 9:00 pm

The potential for climate change to drastically alter the weather is by no means a new topic. We often hear of the potential for the formation of more severe hurricanes, widespread drought in some areas and widespread flooding in others. However, one type of event which often serves as a footnote to this discussion is the potential for climate change to drastically increase the incidence and severity of extreme heat events (EVEs), more commonly known as heat waves. Already, heat waves account for more deaths in the United States than any other weather phenomena. In fact, heat waves account for more deaths annually than hurricanes, tornadoes, floods and earthquakes combined (CDC). Despite this fact, heat waves are commonly overlooked as a major threat to a population – they kill silently and leave little or no physical destruction in their wake, leaving few lasting reminders of the danger that exist (Luper et al, 2008). Aside from the impact in human lives, extreme heat events put severe stress on healthcare services and energy supply and distribution networks, which can, in severe cases, result in major social and economic problems. With experts predicting more intense and frequent heat waves in the future, increased awareness and preparation will be crucial to mitigating the dangers associated with these extreme heat events.

While the nature of conditions which characterize a heat waves differs between organizations, the World Meteorological Organization recommends as a definition a situation where the daily maximum temperature of more than five consecutive days exceeds the average temperature by 5oC (9oF). Of particular note in this definition is the reference to five consecutive days of high temperature. This temporal element is what separates a heat wave from a situation when you may have one or two days of extremely hot weather – and this extended period of heat exposure is also what makes heat waves so dangerous.

Temperature map showing mean temperature and degrees above average during the 2003 European heat wave, which killed over 70,000 people.

Although local conditions (i.e. warm winds like the Santa Ana winds) can cause localized heat events and even regional heat waves, large scale heat waves like the one which struck Europe in 2003, killing over 70,000 people (wiki), are caused by a specific situation. The majority of heat waves occur when a high pressure air mass remains over a region for several days or even months. This high pressure sinks, warming as it does so, and also acts essentially as a ‘cap’ – trapping heat and stagnant air close to the ground and preventing warm air from rising (National Weather Service). The high pressure zone also limits convection, preventing convective clouds from forming and minimizing the chances for rainfall. This effect is particularly noticeable in cities, which are often significantly warmer than the surrounding area due to the urban heat island effect (largely caused by widespread use of heat-retaining materials). The extra heat generated by cities exacerbates the build-up of heat due to the atmospheric conditions, and can result in extremely high sustained temperatures and significant risk to the population (Luber et al. 2008).

From a physiological standpoint, heat waves and related sustained heat exposure can have a variety of effects, ranging in severity. Heat cramps, fainting, dizziness and heat exhaustion are common in heat wave scenarios and if steps are not taken to mitigate these effects, they can progress to nausea, cardiovascular problems and ultimately hyperthermia, also known as heat stroke (Luber et al. 2008). Heat stroke is an extremely dangerous condition which occurs when core body temperature reaches or exceeds 40.6oC (105oF). In this condition, the body looses the ability to regulate its temperature and severe central nervous system problems such as delirium, convulsions and coma may occur, and if steps are not made to immediately cool the victim, death will result. Naturally, individuals who have compromised ability to regulate body temperature due to old age, chronic diseases, or use of certain medications as well as individuals with cardiovascular problems are at extreme risk during these extreme heat events (Luber et al. 2008).

CDC diagram showing trends towards a warmer, but more variable climate in the future, with higher risk of severe heat events.

While heat waves themselves and the aforementioned risks are not new phenomena, there is a consensus in the scientific community that the temperature shift caused by climate change will result in much more frequent extreme temperature events, and also much more intense events (CDC, Luber et al. 2008, Huang et al. 2011). Using Los Angeles as an example, Hayhoe et al. (2004) predicts that heat waves and other extreme events will occur with a frequency four times greater than the current level (roughly 12 days annually) by 2100 using the more conservative B1 emissions scenario, and with a frequency up to 8 times greater than the current level using the more pessimistic A1fi emissions scenario. These predictions are consistent with predictions of increase extreme heat events and related mortality in other regions of the United States and the world. For example, Huang et al. (2011) predicts an increase of heat-related fatalities to increase from the current rate of roughly 700 people annually by 70-100% by the middle of the century, while Takahashi eta l. predicts that globally, deaths due to heat exposure may increase anywhere from 100-1000% of the current rate by the end of this century. Clearly, warmer, more frequent heat waves will have significant impact on human lives and society in this century and beyond.


One very important sector that will be impacted by more extreme heat events is the healthcare sector. The serious health effects of heat exposure are discussed above, and dramatic events like the 2003 European heat wave serve as testament to the dangers posed by the events, yet the fact remains that many cities, including those in the U.S. are simply not prepared to deal with the health impacts of a prolonged heat wave (Luber et al.). While the number of fatalities for severe events may be significant, the fact is that the total amount of people requiring medical attention due to heat exposure may be enormous. Public health and emergency services would likely be overwhelmed, which could result in further fatalities and problems. Climate change makes this scenario more likely to occur, while at the same time, demographic shifts in the developing world towards an aging population increases the percentage of the population at significant risk. Ultimately, steps must be taken by public health and emergency response officials to revise current response plans and develop new methods of dealing with, and making the public more aware of the dangers associated with future intense heat wave events.


In addition to the direct on health and health services that can result from extreme heat, there are heat related illnesses and deaths that will increase with the rising temperatures.  Climate change will cause human health problems related to dirtier air and water, more flood-related accidents and injuries, threats to food supplies, stress on native and domesticated ecosystems that either purify our air and water or provide food.

There may be an increase in infectious diseases due to less availability of clean water and sanitary conditions for medicine and standard living.  This is much more of a concern in parts of the world other than the United States where public health systems are not as structured and available.  Even in the US, there will be a stress on public health facilities in areas that are particularly vulnerable to extreme heat conditions.  In areas of the southern United States there are high levels of humidity coupled with extreme temperatures that put these people at risk.  Also, in areas prone to flooding will be increased indirect health problems that will be exacerbated by the increased temperatures and more erratic weather patterns.  These indirect health risks can be mitigated by preventative measures against climate warming, which will surely be less costly than trying to fix the problems after they’ve happened.

Agriculture is another sector of society that will be greatly affected by climate warming and weather extremes.  Farming productivity depends on steady climate with steady characteristics such as temperature, rainfall, levels of carbon dioxide, and ground level ozone.  These levels have already been changed by human activity and with increased climate and more erratic weather; they will continue to become more intense and potentially harmful.  In addition to the heat affecting the farmers, the climate will affect water supply and soil moisture, which in certain areas can have a ripple effect on wastewater run-off and sewage treatment—both of which can become significant health risks.

In developed countries such as the US, the most common method for coping with extreme heat comes in the form of electric Air Conditioning.  This is not a technology wide spread in less developed countries, so is generally limited to developed countries as a widespread method for dealing with heat related health risks.  The increase of climate variability and more extreme heat waves will come with an increase of the use of air conditioning, which will put some stress on the energy sector.  With rising energy costs, there will be some incentive to avoid the preventative measure.  Just like when rising gas prices force drivers off the street, some people may choose not to use air conditioning if it becomes more costly.  This could put people at risk especially in particularly vulnerable areas.

While there is definitely a continuing pattern of increased temperature and more erratic weather patterns, there is also a decreasing vulnerability to heat and heat related risks.  These risks are certainly more of an issue in poorer countries, but will become more of an issue in the US as the climate intensifies.  There will be a balance between energy conservation and other preventative measures to prevent further man-induced climate change with how we choose to deal with the risks that are already present and will worsen with climate change.

This post was written by Daniel Sugar ’12 who is majoring in Environmental Studies and Nick Horsburgh ’12 a double major in Environmental Studies and Psychology.


Takahashi K, Honda Y, Emori S. Assessing mortality risk from heat stress due to global warming. J Risk Res. 2007;10(3):339–354.

Luber G., McGeehin M. (2008)Climate Change and Extreme Heat Events. American Journal of Preventive Medicine – November 2008 (Vol. 35, Issue 5, Pages 429-435, DOI: 10.1016/j.amepre.2008.08.021)

Huang C., Gerard Barnett A., Wang X., Vaneckova P., FitzGerald G., Tong S. (2011) Projecting Future Heat-Related Mortality under Climate Change Scenarios: A Systematic Review. Environmental Health Perspective. December 119(12): pg 1681-1690.

Heat waves, Centers for Disease Control and Prevention.



Climate Change and the Importance of Biodiversity

Filed under: climate — admin @ 8:54 pm

As humans burn more fossil fuels, we emit more greenhouse gases, and bring ourselves closer and closer to the impending doom of global warming. It may seem dramatic and over-played when portrayed in the media, but the consequences are real. Climate can affect global water supplies, increasing our need to conserve water and find more reliable sources especially in desert and temperate environments where water is already scarce. Continuous population growth only aggravates the problem. The problem, however, is not only a matter of water resources for our consumption; water allows for all forms of life on earth. With ever changing water and precipitation patterns in the midst of climate change, fauna and flora globally too will have to reestablish themselves or make other adaptations to be able to survive such a tumultuous time.

Changes in climate will also affect plants worldwide, thereby influencing the distribution of most other forms of life. As plants are at the first trophic level, they convert sunlight into chemical bonds of energy that are made available for next trophic level to use. If climate change stresses water supplies of plants, these effects will easily carry on to all the species that rely on those plants for survival. The natural ecosystems and biomes, or those that remain, will be hard-hit with this accelerating climate change. Plants are adapted to the regions they live in right now, but climate variations can disturb these native plants as well as crops. In this way, climate change has potential to make long lasting and calamitous effects on wildlife all over the world. As humans, we naturally first think of our own food needs, largely coming from agricultural crop and livestock production. Climate change has the potential to cause havoc not just for natural systems but for our industrial farming systems as well. As industrialized as our farming has become, humans still rely on the weather to stay stable and within a temperate range so that their crops can grow. Monoculture planting found in agricultural landscape are especially susceptible to failure under extreme conditions. A few of the possible effects of these climate changes are shown in the graphic below.

These are just some of the effects of global climate change on agriculture. Full effects are hard to predict with the earth’s complex feedback systems.

Scientific models have attempted to predict how the locations of the earth’s biomes, including agricultural lands, would change. At the rate that carbon dioxide is being released into the atmosphere, plants have little time to be able to evolve to better fit their new habitats. Instead of evolving, the dispersed seeds of flora will now germinate in places where they previously have not been able to because conditions were not compatible with their needs. Plants, biomes, and whole ecosystems will move towards the poles where conditions are more favorable for their growth and they have better access to water, while places closer to the equator will be become hotter and drier. Times like these prove the importance of biodiversity and species richness. The more diverse a certain species, the greater the chance that they will be able to survive a set of extreme conditions that climate change brings. Increased diversity means more alleles, and rare alleles can increase fitness of a species. The benefits of this are only often realized during disturbances. Consider the following graphics that demonstrate such a trend of drifting ecosystems and biomes.


Above show the “measured global vegetation biomes.” Below are the “future global vegetation biomes” showing biome locations if carbon dioxide levels doubled their present levels. Vegetation moves closer to the poles where conditions for growth will be more favorable. Places previously covered in ice will allow for lush forests, as deserts continue to get hotter and drier.

When species find it necessary to migrate, we discover the usefulness and practicality of diverse patches of natural areas even among the most developed urban areas. Connected patches and networks will allow for animals and, more passively, plants to find their way to more suitable environments. This movement also gives species opportunities to mate with other populations thereby increasing their gene diversity and chances to share rare alleles. Biodiversity and allele diversity within species makes those plant or animal populations more stable and resilient following a time of disturbance, like extreme temperatures and precipitation that climate change brings.

Scientists have tried their best with the latest technology to try to incorporate each of the variables and feedback systems to predict specific consequences on various species. For instance, it may be that plants prosper with the excess of carbon dioxide, maximizing plant growth potential and mitigating climate changes. Or, on the other hand, it may turn out that the extirpation of animal species around the world may release 15-20% as much carbon as that coming from anthropogenic causes. Two possible effects are shown below. It is difficult to incorporate the complexities and diverse state of the natural world into a computer model, as well as include slow vegetation responses and species and population interactions. Specific effects of climate change are questionable, but we need to ask ourselves if we are really willing to risk our current systems (agricultural and otherwise) that have been working so well for us.


Specific consequences of climate change on our agricultural supplies are unknown. Shown here are two different possibilities.

Of course, this is all assuming that global climate change is inevitable. Scientists are predicting what would happen if we continue to emit at current rates. It is not inevitable; we can help change the projections. We all need to realize the gravity of our choices and actions, especially in regards to energy sources. In the process of preparing for such a series of events, we need to allow for a diverse amount of species to flourish so that disastrous events will not lead to extirpation, or even extinction, of a species. Although indicator species and species that have a narrow range of tolerance will be the first hit, some plants and animals will be able to migrate as a means of adapting to the new set of environmental conditions. Before such dramatic possibilities are considered though, we should first reevaluate our lifestyles and reflect on the long term impacts of our actions on people and life in general all over the world. This way we might be more willing to make necessary adjustments to our lives in order to ensure food and water availability and presence of natural spaces for future generations to enjoy.

This post was authored by Marisa Spinella ’12, who is majoring in Environmental Studies (BS) with a minor in Architecture.

Works referenced

Adams, Richard M., Brian H. Hurd, Stephanie Lenhart, and Neil Leary. “Effects of Global Climate Change on Agriculture: An Interpretive Review.” Climate Research 11 (1998): 19-30. Inter-Research Science Center. 17 Dec. 1998. Web. 9 Apr. 2012. <>.

Forman, R.T.T. Land Mosaics: The Ecology of Landscapes and Regions. New York:

Cambridge University Press, 1995