The Wayback Machine -
Global Warming:Early Warning Signs


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US Climate Impacts


The map identifies events that foreshadow the types of impacts likely to become more frequent and widespread with continued warming. These "harbingers" of climate change are indicated with red icons.

For these events, evidence for a direct link to long-term climate change cannot be confirmed or ruled out at this time.

The following events are identified as global warming harbingers:

Spreading disease

Spreading disease

Warmer temperatures allow mosquitoes that transmit diseases such as malaria and dengue fever to extend their ranges and increase both their biting rate and their ability to infect humans. The map highlights locations of mosquito-borne disease outbreaks in previously unexposed highland communities where temperatures have risen during the past century, and other indicators of health impacts.

Hotspot Selection Criteria: The majority of the spreading disease events came from articles by Dr. Paul Epstein, Center for Health and the Global Environment, Harvard Medical School. The most recently added points come from the published, peer-reviewed literature. We also spoke with Dr. Epstein directly to confirm some of the wording and use of certain locations. The goal was to find high elevation areas where the vector-borne diseases had not previously been found. (Note: mosquitoes are limited to certain temperatures and therefore elevations.) The focus was on the spread of these diseases (or the mosquitoes that carry them) to highland areas and previously unexposed populations. We also confirmed that the areas chosen have shown a warming trend according to the data presented by IPCC. Although some of these estimates are based on limited numbers of weather stations probably not at high elevations, they do give some indication of the temperature trend in that region.

Earlier spring arrival

Early spring

Spring now arrives earlier in many parts of the world. Evidence of this comes from earlier thaw dates for rivers and lakes; earlier dates for plant blooming and leafing; and earlier animal egg-laying, spawning and migration. An earlier spring may disrupt animal migrations, alter competitive balances among species, and cause other unforeseen problems.

Hotspot Selection Criteria: These events were confirmed by published journal articles or personal communications with the researchers. Sites for which we could not find confirmation from a source other than a newspaper were not included. A clear documentation of a direct or indirect link to climate change -- such as an increase in temperature or change in precipitation -- was necessary for final inclusion.

Plant and animal range shifts and population changes

Plant and animal range shifts

Plants and animals generally react to consistently warmer temperatures by moving to higher latitudes and elevations. Recent studies reveal that some species have already started to shift their ranges, consistent with warming trends. Many populations and species may become more vulnerable to declining numbers or extinction if warming occurs faster than they can respond or if human development presents barriers to their migration.

Hotspot Selection Criteria: These events were confirmed by published journal articles or personal communications with the researchers. Sites for which we could not find confirmation from a source other than a newspaper were not included. A clear documentation of a direct or indirect link to climate change—such as an increase in temperature or change in precipitation—was necessary for final inclusion.

Coral reef bleaching

Coral reef bleaching

Reefs in 32 countries experienced dramatic bleaching in 1997-98. Bleaching results from the loss of microscopic algae that both color and nourish living corals. Water that is warmer than normal by only 2 to 3F (1.1-1.6C) has been linked to bleaching. Other factors that contribute to coral reef bleaching include nutrient and sediment runoff, pollution, coastal development, dynamiting of reefs, and natural storm damage.

Hotspot Selection Criteria: We used NOAA press releases and information from their coral bleaching website to compile a list of areas that suffered bleaching episodes during the past year and a half, making sure that in every case they were associated with warm ocean temperatures. Although human activities such as pollution strongly influence bleaching, we did not attempt to separate out the relative effects of humans for different regions. Instead, we clearly state the impacts of humans on reefs in our category description.

Downpours, heavy snowfalls, and flooding

Downpours, snowfalls, and flooding

A warmer climate will bring an increase in precipitation worldwide, especially during winter and in mid- to high latitudes, according to climate model projections. In addition, more precipitation is expected to fall in downpours and heavy snowstorms leading to increased flooding and damages. The area of the U.S. affected by extreme rainfall has increased significantly since 1910. Heavy rainfalls have also increased in Japan, the former Soviet Union, China, and Australia. As climate change increases the risk of flooding, human changes in land use and land cover can also contribute to the growing risk of flooding.

Hotspot Selection Criteria: Observed events in this category are consistent with the projections for a more intense hydrologic cycle as outlined by the IPCC (2001), including more winter precipitation at mid to high latitudes and a greater portion of precipitation from heavy downpours. They are considered examples of events that would become more frequent and/or more widespread with continued global warming. For the rainfall events we confirmed that the region has been experiencing either an increase in heavy precipitation or an increase in annual precipitation, using journal articles or Annex A of the IPCC Regional Impacts report (IPCC, 1998). For flooding, we did not include events for which other factors (e.g., land clearing or wetland filling) were clearly major contributors, such as the summer 1998 flooding in the Yangtze Valley in China.

Droughts and fires

Droughts and fires

As the climate heats up, droughts are expected to become more frequent and severe in some locations. Sustained drought makes wildfires more likely, and crops and trees more vulnerable to pest infestations and disease. Generally, local land use and land cover changes can exacerbate the climate change-driven increase in drought risk. For example, in the tropics, "slash-and-burn" land clearing practices can trigger large fires during extended droughts.

Hotspot Selection Criteria: Droughts and fires highlighted on the map are examples of events that are likely to become more frequent and widespread with global warming. Although the projections for rainfall vary considerably by region, droughts are expected to be longer-lasting and more severe because of higher temperatures and increased evaporation, even without a decrease in precipitation. Wildfires are also likely to become more frequent due to the combination of higher temperatures and decreased soil moisture in some areas, although other factors such as the availability of fuel and frequency of igniting events will also play a role. We only included on the map those wildfires that were associated with severe drought conditions, and it is clearly stated in the category description that human activities can trigger the start of fires.

The following organizations produced GLOBAL WARMING: Early Warning Signs:
Environmental Defense
Natural Resources Defense Council
Sierra Club
Union of Concerned Scientists
U.S. Public Interest Research Group
World Resources Institute
World Wildlife Fund
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