Global warming is expected to have far-reaching, long-lasting and, in many cases, devastating consequences for planet Earth.
For some years, global warming — the gradual heating of Earth's surface, oceans and atmosphere — was a topic of heated debate in the scientific community.
But the overwhelming consensus of researchers today is that global warming is real and is caused by human activity, primarily the burning of fossil fuels that pump carbon dioxide (CO2), methane and other greenhouse gases into the atmosphere. In fact, a major report released Sept. 27, 2013, by the Intergovernmental Panel on Climate Change (IPCC) stated that scientists are more certain than ever of the link between human activities and global warming.
Additionally, global warming is having a measurable effect on the planet right now.
Increase in average temperatures
One of the most immediate and obvious impacts of global warming is the increase in temperatures around the world. The average global temperature has increased by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past 100 years, according to the National Oceanic and Atmospheric Administration (NOAA).
Since recordkeeping began in 1895, the hottest year on record for the 48 contiguous U.S. states was 2012. Worldwide, 2012 was also the 10th-warmest year on record, according to NOAA. And nine of the warmest years on record have occurred since 2000.
Extreme weather events
Scientists have found that the number and severity of extreme weather events — record-breaking high or low temperatures, high rainfall events or intense storms — are an effective measure of climate change and global warming.
The U.S. Climate Extremes Index (CEI) was established in 1996 to track these events. The number of extreme weather events that are among the most unusual in the historical record, according to the CEI, has been rising over the last four decades, according to NOAA.
Scientists project that extreme weather events, such as heat waves, droughts, blizzards and rainstorms will continue to occur more often and with greater intensity due to global warming, according to Climate Central.
Shift in climate patterns
Climate models forecast that global warming will cause climate patterns worldwide to experience significant changes. These changes will likely include major shifts in wind patterns, annual precipitation and seasonal temperatures variations.
And because high levels of greenhouse gases in the atmosphere are likely to remain high for many years, these changes are expected to last for several decades or longer, according to the Environmental Protection Agency (EPA).
In the northeastern United States, for example, climate change is likely to bring increased annual rainfall, while in the Pacific Northwest, summer rainfall is expected to decrease.
Snow and ice
Since 1970, the area of snow cover in the United States has steadily decreased, according to the EPA, and the average temperature of permafrost (soil that's at or below freezing temperature) has grown warmer.
One of the most dramatic effects of global warming is the reduction in Arctic sea ice: In 2012, scientists saw the smallest amount of Arctic ice cover ever recorded. Most analyses project that, within a matter of years, the Arctic Sea will be completely ice-free during the summer months.
Glacial retreat, too, is an obvious effect of global warming. Only 25 glaciers bigger than 25 acres are now found in Montana's Glacier National Park, where about 150 glaciers were once found, according to the U.S. Geological Survey. A similar trend is seen in glacial areas worldwide.
Rising sea levels
Melting polar ice in the Arctic and Antarctic region, coupled with melting ice sheets and glaciers across Greenland, North America, South America, Europe and Asia, are expected to raise sea levels significantly. And humans are mostly to blame: In the IPCC report released on Sept. 27, 2013, cimate scientists said they are at least 95 percent certain that humans are to blame for warming oceans, rapidly melting ice and rising sea levels, changes that have been observed since the 1950s.
Global sea levels have risen about 8 inches since 1870, according to the EPA, and the rate of increase is expected to accelerate in the coming years. If current trends continue, many coastal areas — where roughly half of the Earth's human population lives — will be inundated.
Researchers project that by 2100, average sea levels will be 2.3 feet higher in New York City, 2.9 feet higher at Hampton Roads, Va., and 3.5 feet higher at Galveston, Texas, the EPA reports. According to the Sept. 27 IPCC report, if greenhouse gas emissions remain unchecked, sea levels could rise by as much as 3 feet (0.9 meters) by 2100. That estimate is an increase from the estimated 0.9 to 2.7 feet (0.3 to 0.8 meters) that was predicted in the 2007 IPCC report for future sea level rise.
As levels of atmospheric carbon dioxide (CO2) increase, the oceans absorb some of that CO2, which increases the acidity of seawater. Since the Industrial Revolution began in the early 1700s, the acidity of the oceans has increased about 25 percent, according to the EPA.
Because acids dissolve calcium carbonate, seawater that's more acidic has a deleterious effect on organisms with shells made of calcium carbonate, such as corals, mollusks, shellfish and plankton.
If current ocean acidification trends continue, coral reefs are expected to become increasingly rare in areas where they are now common, including most U.S. waters, the EPA reports.
Plant and animal impacts
The effects of global warming on the Earth's ecosystems are expected to be profound and widespread. Many species of plants and animals are already moving their range northward or to higher altitudes as a result of warming temperatures, according to a report from the National Academy of Sciences.
Additionally, migratory birds and insects are now arriving in their summer feeding and nesting grounds several days or weeks earlier than they did in the 20th century, according to the EPA.
Warmer temperatures will also expand the range of many disease-causing pathogens that were once confined to tropical and subtropical areas, killing off plant and animal species that formerly were protected from disease.
These and other impacts of global warming, if left unchecked, will likely contribute to the disappearance of up to one-half of the Earth's plants and one-third of animals from their current range by 2080, according to a 2013 report in the journal Nature Climate Change.
As dramatic as the effects of climate change are expected to be on the natural world, the projected changes to human society may be even more devastating.
Agricultural systems will likely be dealt a crippling blow: Though growing seasons in some areas will expand, the combined impacts of drought, severe weather, lack of snowmelt, greater number and diversity of pests, lower groundwater tables and a loss of arable land could cause severe crop failures and livestock shortages worldwide.
This loss of food security might, in turn, create havoc in international food markets and could spark famines, food riots, political instability and civil unrest worldwide, according to a number of analyses from sources as diverse as the U.S Department of Defense, the Center for American Progress and the Woodrow Wilson International Center for Scholars.
The effect of global warming on human health is also expected to be serious: The American Medical Association has reported an increase in mosquito-borne diseases like malaria and dengue fever, as well as a rise in cases of chronic conditions like asthma, are already occurring, most likely as a direct result of global warming.
Many of these expected impacts are the result of exhaustive scientific research and climate models, and the fact that most of them are already being observed gives additional credibility to the projected effects of global warming and climate change.
This reference page was first published on May 31, 2013.
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