Global warming will affect human health through an increase in the incidence of disease. Global warming is likely to increase the incidence of infectious disease by increasing the abundance and geographic distribution of organisms which cause or carry disease, reducing availability of water in some regions, and causing economic and political problems which will reduce resources allocated to health care. Increases in extreme weather and changes in rainfall patterns are likely to disrupt agriculture, leading to malnutrition and disease susceptibility in many areas (Khasnis, 2005). Insect species (such as mosquito species which can carry diseases ranging from malaria to yellow fever) could spread to new latitudes and altitudes. Greater flooding caused by global warming will increase the spread of certain diseases such as cholera (Patz, 2002). Higher temperatures increase the growth of weeds, molds, and fungi which are contributing to the increase in asthma rates in children (Ault, 2004).


Climate change will affect plants and animals. A number of amphibian species have become extinct because of diseases which have spread in the aftermath of climate change. Global warming poses a threat to earth’s biodiversity (Pounds, 2006).Climate change poses a serious threat to biodiversity. Models have estimated that by 2050, between 15% and 37% of modern species will be “committed to extinction.” (Thomas, 2004). Warming temperatures affect the distribution of animals. As temperatures increase, species are exposed to new pathogens. Global warming has been implicated in the extinction of several species of amphibian (Pounds, 2006). For example, robins have extended their range to 250 miles north of the Arctic Circle (Blatt, 2005). The distributions of marine species adapted to cold water are changing around Great Britain and some species have almost disappeared (Williams, 2007c). Increased temperatures would worsen the prevalence of tick-born diseases (Gray, 2007). The impact of pests (such as the bark beetle) on forests will worsen as climate change becomes more serious (Seidl, 2008).

Global warming seems to be responsible for earlier flowering dates of plants (including crops like wheat) in the U.S. (Hu, 2006). The global wheat crop represents a fifth of the world’s food supply and requires 200 million hectares for cultivation.  As global temperatures increase, some areas will experience better growing conditions for wheat while others (such as South Asia) will experience a reduction of agricultural potential (Ortiz, 2008). The decrease of Antarctic ice, combined with changing water temperatures, currents, and salinity are affecting marine life (Laidre, 2005). Global warming threatens to allow non-indigenous species to spread into new areas where they can increase at the expense of native plants and animals. The increase in maximum and minimum temperatures in an area seem to have a greater effect than increases in the annual average temperature (Stachowicz, 2002). The habitat change which would result from global warming would threaten many species with extinction (Hilbert, 2004). Even common species could experience significant decrease in range (Meynecke, 2004). Even the remotest parts of the arctic seem to be displaying effects caused by global warming such as earlier summers, the reduction of glaciers, a change in biological species, and thinning of sea ice (Smol, 2005). Global warming threatens coastal mangrove swamps through sea level rise, increase in severe storms, change in weather patterns, and loss of surrounding coral reefs (Gilman, 2008).

In 2007, the U.S. Department of the Interior proposed adding the polar bear to the endangered species list because of the habitat change caused by global warming. The report cited that in late summer, the amount of arctic ice has decreased by 25% in the past 50 years and the amount perennial sea ice had decreased 9.8% per decade since 1978. As sea ice breaks up earlier in the year, polar bears have shorter hunting seasons, are thinner, and cub survival rates have dropped. Many populations are in decline such as those of the western Hudson Bay and southern Beaufort Sea, which have declined 22% and 17% in the past two decades (Williams, 2007b).

When the temperature of water increases, less gas can be dissolved in it. Increased global temperatures will decrease the amount of oxygen which can be dissolved in the world's oceans (Keeling, 2002).


Global warming is being increasingly viewed as a security risk.  In addition to the risk from severe weather and rising sea levels, the effects of drought and famine are likely to impact security scenarios.  For example, in East Timor, 86% of the population is involved in agriculture (compared to 1-2% of those living in developed regions) and 46% of those living in rural areas live on less than 55 cents per day.  The failure of agricultural production can increase the numbers of young men who are frustrated and willing to join armed groups.  States which cannot provide for the needs of the population are more likely to experience civil strife (Barnett, 2007; Reuveny, 2007).

Mass migrations often follow agricultural crises (such as the migration of 2.5 million Americans away from the Great Plains during the Dust Bowl or the migration of up to 17 million Bangladeshis to India since the 1970s following droughts and other environmental problems.  Other areas which have experienced mass migrations following environmental stresses include Ethiopia, Rwanda, southern Mexico, Somalia, Nigeria, and Pakistan (Barnett, 2007; Reuveny, 2007)


At first glance, it might seem that the greenhouse effect is a fairly simple phenomenon: Carbon dioxide absorbs heat, fossil fuel burning produces more carbon dioxide and, as a result, global temperatures rise and the poles melt. Of course, we all know that predicting the weather from day to day is difficult enough, predicting global trends and identifying the causal factors is much more challenging. There are a number of complicating factors.
1) Not all scientists agree.
Do all scientists agree on an accepted model of the greenhouse effects, the severity of its effects, and the cost of moderating these effects? Of course not. The data on this incredibly complex system is still being processed and new hypotheses are constantly being tested. This is true of any healthy science. While it is true that not all scientists agree, it should also be emphasized that the basic aspects of our understanding of the greenhouse effect are agreed upon by the vast majority of climate experts, such as the more than 2,500 climate experts of more than 60 nations which compose the International Panel on Climate Change, IPCC. Waiting for 100% consensus before implementing plans to control emissions would make the situation more difficult. For example, emission reductions of 60% would be required in the year 2020 to accomplish what 17% emission reductions could accomplish if implemented in the year 2000.
2) The Changing Amounts of CO2 Absorbed
Estimating the effects of CO2 is complex since every change in atmospheric levels can change the amounts that forests, phytoplankton, and the ocean can absorb (remember that most of the earth's carbon is already stored in the deep ocean). Increased carbon dioxide may enhance plant productivity although any such increases would likely be more than balanced by the difficulties plants would face due to changes in climate and rainfall. One study indicated that increase in tree growth due to increasing carbon dioxide levels was minor.
3) Aerosols and Particulates are Cooling the Planet
Even if the climate is changing, can we be sure that human activity (anthropogenic forces) are the cause? It is certain that human activity is responsible for some of the observed climate changes. There are natural phenomena such as the solar sunspot cycle and methane release from wetlands which could increase global temperature and other natural phenomena such as volcanic eruptions which would have a cooling effect. Aerosols (which are liquid droplets so small that they can be suspended in the air such as sulfate aerosols produced from fossil fuel combustion) have a cooling affect on the atmosphere by blocking solar radiation and through their effects on clouds (such as increasing cloud brightness, cloud cover, and longevity). Including aerosol data with that of carbon dioxide better explains the changes that using data from carbon dioxide alone.
Particulates are small pieces of solid materials dispersed into the atmosphere. Many particulates block sunlight and thus have a cooling effect. Large volcanic eruptions can cause measurable drops in global temperature.

4) Other Greenhouse Gases
Some have argued that while CO2 does cause global warming, most of the recent increase in global temperature has been due to other gases such as chlorofluorocarbons (CFCs) and N2O. CFCs are discussed in the next unit; N2O levels are increasing slowly but steadily through the use of fertilizers and the burning of biomass. Methane is a greenhouse gas which, in addition to that which is produced naturally, is released into the atmosphere by rice agriculture, domestic cattle, landfill and sewage decomposition, and leakage from natural gas pipelines. Seventy-three million metric tons of methane are produced through cattle belching and flatulence alone. It may be easier and cheaper to reduce the amount of these other greenhouse gases than to reduce carbon dioxide emissions.


--after Kemp, 2004

After carbon dioxide, methane is the most significant greenhouse gas, accounting for 15-20% the anthropogenic sources which promote global warming. Between 15-20% of the methane released into the atmosphere each year originate in the soil of rice paddies. Agricultural practices, such as occasional draining of rice paddies, can both cut the methane emissions and improve crop yield (Xu, 2007).

More people depend on rice than any other food source. In the past 50 years, the land area devoted to rice crops increased from 104 million hectares to 148 million hectares. Most of this land (131 million hectares) is flooded and the resulting anaerobic conditions cause the production of methane instead of carbon dioxide (Xu, 2007).


--after Kemp, 2004

Since 1970, methane emissions have increased 40% and nitrous oxide emissions by 40% (IPCC, Document III, 2007).