Environmental Defense Fund
Basic facts are well-understood and accepted by the scientific community
Despite overwhelming scientific evidence, popular myths and misinformation abound. Here are some of the facts of what we know about global warming.
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There is scientific consensus on the basic facts of global warming.
The most respected scientific bodies have stated unequivocally that global warming is occurring, and people are causing it. Read their statements »
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Scientists are certain that the Earth is warming.
Scientists are certain the Earth has been warming for 100 years. Here's how they know »
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Human activity is causing the Earth to get warmer.
Only CO2 and other greenhouse gas emissions from human activities explain the observed warming now taking place on Earth How we know »
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The effects of warming can be seen today.
We can already see the effects of global warming in our world through disappearing habitat, shrinking arctic sea ice and extreme weather. Impacts we can see today »
The greenhouse effect: A natural balance
The atmosphere has a natural supply of "greenhouse gases." They capture heat and keep the surface of the Earth warm enough for us to live on. Without the greenhouse effect, the planet would be an uninhabitable, frozen wasteland.
Before the Industrial Revolution, the amount of carbon dioxide (CO2) and other greenhouse gases released into the atmosphere was in a rough balance with what was absorbed in natural sinks. For example, plants take in CO2 when they grow in spring and summer, and release it back to the atmosphere when they decay and die in fall and winter.
Industrial age increases greenhouse effect
Industry took off in the mid-1700s, and people started emitting large amounts of greenhouse gases. Fossil fuels were burned more and more to run our cars, trucks, factories, planes and power plants, adding to the natural supply of greenhouse gases. The gases—which can stay in the atmosphere for centuries—are building up in the Earth’s atmosphere and, in effect, creating an extra-thick heat blanket around the Earth.
The result is that the globe has heated up by about one degree Fahrenheit over the past century—and it has heated up more intensely over the past two decades.
If one degree doesn't sound like a lot, consider this: the difference in global average temperatures between modern times and the last ice age—when much of Canada and the northern U.S. were covered with thick ice sheets—was only about 9 degrees Fahrenheit. So in fact one degree is very significant—especially since the unnatural warming will continue as long as we keep putting extra greenhouse gases in the atmosphere.
How much is too much?
Today, people have increased by nearly 40 percent the amount of CO2, the chief global warming pollutant, compared to pre-industrial levels.
Today, there is more CO2 in the atmosphere than at any time in the last 800,000 years. Studies of the Earth’s climate history show that even small changes in CO2 levels generally have come with significant shifts in the global average temperature.
Scientists expect that, in the absence of effective policies to reduce greenhouse gas pollution, the global average temperature will increase, on the low end, 2.0 degrees Fahrenheit , and on the high end, 11.5 degrees Fahrenheit by 2100.
That might not sound like a lot, but even if the temperature change is at the small end of the predictions, the changes to the climate are expected to be serious: more intense storms, more pronounced droughts, coastal areas more severely eroded by rising seas. At the high end of the predictions, the world could face abrupt, catastrophic and irreversible consequences.
The science is clear
Scientists are no longer debating the basic facts of climate change. In February 2007, the thousands of scientific experts collectively known as the Intergovernmental Panel on Climate Change (IPCC) concluded that there is greater than 90 percent likelihood that people are causing global warming. (IPCC, 2007)
Scientists are certain the Earth has been warming for 100 years. Here's how they know.
As far back as the 1850s, a small number of weather stations around the world were compiling temperature records. These numbers grew during the 20th century and today there are thousands of land-based weather stations and ocean buoys in every corner of the world monitoring temperatures.
Temperature records since 1850
These temperature records clearly show a warming of the Earth over the past century, with particularly rapid heating over the past few decades.
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Global temperature data
Each of the four agencies that report global temperature trends—NOAA, NASA, HADCRU, and JMA—show the warming trend.
Satellite measurements since 1979
Atmospheric temperature measurements taken from orbiting satellites also show warming. Weather satellites have been monitoring global atmospheric temperatures since 1979.
Sea level rise in the 20th century
During the 20th century, sea level rose an average of 7 inches after 2,000 years of relatively little change.
Before 1850: proxy records
Proxy records are sophisticated ways of inferring surface temperatures over previous centuries and millennia. Taken together, these independent records show widespread warming over the 20th century, with a particularly sharp uptick in temperature over the last few decades.
While proxy records are, by definition, not as accurate or precise as direct measurements, they provide a robust picture of thousands of years of the Earth's history. Three main types of proxy records used to create this picture are:
- Ice cores
One proxy method is to drill into glaciers and ice sheets to extract ice samples. Since the ice was formed from snow that fell over the centuries, the deeper you drill, the farther back in time you are looking.
The chemical composition of the ice correlates very strongly with temperature. Scientists have constructed temperature records from ice cores taken from Tibetan and Andean glaciers, an ice cap in the Canadian Arctic, and the Greenland and Antarctic ice sheets. These records show that, at low latitudes, 20th century climate was unusually warm compared to the previous 2,000 years.
In the Canadian Arctic, warming over the past 150 years is unprecedented compared to the previous millennium. In Greenland and coastal Antarctica, there is clear evidence of warming over the past century. Ice cores from Antarctica's interior do not show warming over the past century.
- Tree rings
In temperate regions, trees generally produce one ring a year, and some tree species are extremely long-lived. (A bristlecone pine, for example, can live more than 4,000 years.) Patterns in the width and density of tree rings provide year-by-year temperature information.
Scientists have tree ring records from more than 2,000 sites on all inhabited continents, though most of the records are from temperate areas of the Northern Hemisphere. These records show that 20th century warming was unusual compared to at least the past 500 years.
- Coral reefs
Corals build their hard skeletons with annual bands of calcium carbonate. The geochemical composition of each annual band varies depending on the temperature of the water at the time the band was formed. Scientists have coral proxy records from the Atlantic, Indian, and Pacific oceans, with most of these going back 400 years. Coral proxy records indicate sea surface warming in most tropical locations over the past century.
We know the planet is warming — scientists have a clear understanding why
The theory of global warming is nothing new. The Nobel Prize-winning chemist Svante Arrhenius first proposed the idea of global warming in 1896. Carbon dioxide, he knew, traps heat in the Earth's atmosphere. He also knew that burning coal and oil releases carbon dioxide (CO2).
Arrhenius speculated that continued burning of coal and oil would increase concentrations of CO2 in the Earth's atmosphere, making the planet warmer. It's called the greenhouse effect.
What warms the Earth?
To determine what is causing today's rapid global warming, scientists have examined all the factors that can affect the Earth's temperature. There are essentially three factors that could be responsible for recent rapid global warming:
- The sun
- Earth's reflectivity
- Greenhouse gases
Which of these is causing our current global warming?
It's not the sun: cause of little warming since 1750, none since 1980s
Ultimately, the climate system is powered by the sun: all else being equal, if you turn up the sun, you'll warm up the Earth. According to IPCC estimates, the sun has accounted for just a small portion of warming since 1750. A study of more recent solar activity has demonstrated that since about 1985 the sun has changed in ways that, if anything, should have cooled the planet—even as global temperatures have been rising. So the sun is not causing global warming.
It's not reflectivity: changes point to cooling, not warming
Around 30% of the sun's energy that reaches the Earth is reflected back into space. Changes in how much sunlight is absorbed, and how much is reflected, can affect global temperatures. Using satellite and land-based observations and computer models, scientists have calculated how Earth's reflectivity has changed over time.
These calculations suggest that human-produced particulate pollution, especially reflective sulfur-containing particles, have had a cooling effect on the climate, masking some of the warming effect of greenhouse gases. In fact, the slight decrease in global temperature between 1945 and 1975 was likely caused by a combination of rising particulate pollution and natural factors. Warming resumed after 1975 when industrialized countries began to clean up their particulate pollution while continuing to increase their greenhouse gas emissions.
As for human land use changes (primarily forest clearing for agriculture), they have on balance brightened the planet since 1750. This would have a cooling effect, yet we've seen warming. Changes in the frequency of volcanic eruptions, which can send reflective particles up into the stratosphere, also cannot explain the observed warming trend. So reflectivity is not causing global warming.
All the evidence points to greenhouse gases
That leaves the greenhouse effect as the only remaining scientific explanation for the rise in global temperatures in recent decades. We have direct measurements of CO2 concentrations in the atmosphere going back more than 50 years, and indirect measurements (from ice cores) going back hundreds of thousands of years. These measurements confirm that concentrations are rising rapidly.
Historic CO2 levels
The amount of carbon dioxide in the atmosphere is
higher than at any time in measurable history, and predicted to increase dramatically this century. Source:
GlobalChange.govThough natural amounts of CO2 have varied from 180 to 300 parts per million (ppm), today's CO2 levels are around 390 ppm. That's 30% more than the highest natural levels over the past 800,000 years. Increased CO2 levels have contributed to periods of higher average temperatures throughout that long record. (Boden, Carbon Dioxide Information Analysis Center)
We also know the additional CO2 in the atmosphere comes mainly from coal and oil, because the chemical composition of the CO2 contains a unique "fingerprint."
As far as scientists are concerned, it's case closed: human activity is causing the Earth to get warmer, primarily through the burning of fossil fuels, with a smaller contribution from deforestation. All other scientific explanations for why the Earth is getting warmer have been eliminated.
World View of Human Impact on Temperature Since 1900
The effects of warming on our world can be seen today
The Earth could warm another 2 to 11.5°F this century if we fail to reduce emissions from burning fossil fuels and deforestation—devastating our livelihoods and the natural world we cherish.
Impacts on the world around us
Biodiversity loss
Thousands of species risk extinction from disappearing habitat, changing ecosystems and acidifying oceans. According to the IPCC, climate change will put some 20% to 30% of species globally at increasingly high risk of extinction, possibly by 2100.
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Decline in polar bears
Arctic sea ice is the polar bear's feeding habitat. As sea ice disappears, bear mortality rises. In 2008, the polar bear became the first animal to be added to the Endangered Species Act list of threatened species because of global warming.
The U.S. Geological Survey has warned that two-thirds of the world's polar bear populations could be lost by mid-century as sea ice continues to retreat.
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Acidifying oceans
About one-third of the CO2 pollution from smokestacks and tailpipes is absorbed by the world's oceans, where it forms carbonic acid. A 2010 study published in Nature Geoscience warns that unchecked greenhouse gas emissions could cause oceans to acidify at a rate unprecedented in at least the last 65 million years.
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Coral bleaching
Coral reefs are highly sensitive to small changes in water temperature. Heat triggers corals to shed the algae that nourish them—a bleaching event that leaves coral white.
In 1998, the world's coral suffered its worst year on record, which left 16% bleached or dead. (ISRS statement [PDF]) Continued warming could cause mass bleachings to become an annual event within the next few decades, wiping out many reef ecosystems.
Photo: Dr. Dwayne Meadows, NOAA/NMFS/OPR.
Coral bleaching from warming waters.
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Shifting habitat
As the mercury rises, plants and animals are shifting their ranges toward the poles and to higher altitudes, and migration patterns for animals as diverse as whales and butterflies are being disrupted.
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Threats to Western forests
The U.S. Geological Survey reports that slight changes in the climate may trigger abrupt ecosystem changes that may be irreversible.
All told, the Rocky Mountains in Canada and the U.S. have seen nearly 70,000 square miles of forest die – an area the size of Washington state – since 2000 due to outbreaks of tree-killing insects.
Thinning ice, rising seas
Rising seas are one of the most certain effects of global warming as warming ocean waters expand and melting glaciers, ice caps and ice sheets add more water to the oceans. The IPCC estimates that melting ice caps and glaciers—which are some of our most visible indicators of climate change—accounted for about 25% of sea level rise from 1993 to 2003.
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Arctic sea ice is shrinking
Satellite images show that the extent of Arctic summer sea ice has decreased by almost 9% per decade since 1979. The Arctic summer could be ice-free by mid-century, according to a study by the National Oceanic and Atmospheric Administration.
The three lowest minimum extents of Arctic sea ice were reached in 2007, 2008 and 2010. Source: NOAAVisualizations
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Sea level rise
During the 20th century, sea level rose an average of 7 inches after 2,000 years of relatively little change. The 2007 IPCC report conservatively predicts that sea levels could rise 10 to 23 inches by 2100 if current warming patterns continue.
In the U.S., roughly 100 million people live in coastal areas within 3 feet of mean sea level. Low-lying cities such as Boston, Miami and New York are vulnerable.
The U.S. Geological Survey, EPA and NOAA issued a joint report in 2009 warning that most mid-Atlantic coastal wetlands from New York to North Carolina will be lost with a sea level rise of 3 feet or more. North Carolina's barrier islands would be significantly breached and flooding would destroy the Florida Everglades.
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Melting glaciers
A 2005 survey of 442 glaciers from the World Glacier Monitoring Service found that 90% of the world's glaciers are shrinking as the planet warms.
Glacier National Park now has only 25 glaciers, versus 150 in 1910. At the current rate of retreat, the glaciers in Glacier National Park could be gone in a matter of decades, according to some scientists.
Photos from 1938, 1981, 1998 and 2009 show the disappearance of Grinnell Glacier. Credit: 1938 T.J. Hileman photo, Courtesy of GNP Archives; 1981 Carl Key photo, USGS; 1998 D. Fagre photo, USGS; 2009 Lindsey Bengtson photo, USGS. Source: USGS
Threats to people around the globe
Extreme weather will become more frequent—and more dangerous.
The World Meteorological Organization reported that 2000-2009 was the hottest decade on record, with eight of the hottest 10 years having occurred since 2000.
It's not just the heat that poses threats. Scientists say global warming is speeding up the cycling of water between the ocean, atmosphere and land, resulting in more intense rainfall and droughts at the same time across the globe.
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A surge in wildfires
Hot, dry conditions create a tinderbox ideal for wildfires. This could have a devastating impact on America's Southwest. -
Increased flooding
The 2007 IPCC report concludes that intense rain events have increased in frequency during the last 50 years and that human-induced global warming has been a factor. -
Increased drought
There have also been increased periods of drought, particularly in famine-stricken areas of Africa and Asia. According to the National Center for Atmospheric Research, the percentage of Earth's surface suffering drought has more than doubled since the 1970s. In Africa alone, the IPCC projects that between 75 and 250 million people will be exposed to increased water stress due to climate change. -
More intense hurricanes
As the oceans warm, scientists predict that hurricane intensity could increase. The associated storm surge poses particular risk to low-lying coastal cities like Miami, Charleston (SC) and Wilmington (NC).
Threats to human health
A warming planet threatens people worldwide, causing deaths, spreading insect-borne diseases and exacerbating respiratory illnesses. Extreme weather will also put more people in harm's way.
The World Health Organization believes that even the modest increases in average temperature that have occurred since the 1970s are responsible for at least 150,000 extra deaths a year—a figure that will double by 2030, according to WHO's conservative estimate.
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Devastating heat waves
Recent studies show extreme heat events that now occur once every 20 years will occur about every other year in much of the country, if current trends continue.
In 1995, Chicago suffered a heat wave that killed more than 700 people. Chicagoans could experience that kind of relentless heat up to three times a year by 2100.
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Spread of disease
Diseases such as malaria and dengue fever could become more difficult to control in areas where it's currently too cold for them to spread year-round. The malaria parasite itself is generally limited to certain areas by cooler winter temperatures since it is not able to grow below 16°C. As temperatures rise, diseases can grow and disease vectors (the carriers that transmit disease, such as mosquitoes) will mature more rapidly and have longer active seasons.
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Worsening air quality
More hot days mean ripe conditions for ground-level ozone, or smog, which forms when pollutants from tailpipes and smokestacks mix in sunny, stagnant conditions. Higher temperatures cause higher emissions of one type of pollutant, namely hydrocarbons and other volatile organic compounds, as well as speeding up the chemical reactions that form ozone smog.
Smog triggers asthma attacks and worsens other breathing problems. The number of Americans with asthma has more than doubled over the past two decades to 20 million. Continued warming will only worsen the problem.
Science community concurs warming is happening — and people are the cause
The most respected scientific bodies have stated unequivocally that global warming is occurring, and people are causing it by burning fossil fuels and cutting down forests.
This conclusion is shared by the national science academies of developed and developing countries (read the statement [PDF]), plus many other organizations, including the Intergovernmental Panel on Climate Change, which was established by the United Nations and the World Meteorological Organization to provide the world with "a clear scientific view" on climate change.
The only real debate is about how fast warming will occur, and how much damage will be done, as a result of human activities that produce heat-trapping CO2 and other greenhouse-gas emissions.
Peer review ensures sound science
Climate scientists, like all scientists, are professional skeptics. They welcome — in fact, rely upon — rigorous challenges to their work from colleagues. Through this process of peer review and independent verification, scientists critique and double- (and triple- and quadruple-) check each others work.
This can lead to debate and controversy, but over time, solid research is validated, errors are discarded, and a body of reliable facts is created. In addition, science advances by focusing on what is not yet known. In the case of climate change, for example, there is an extremely good general understanding of the phenomenon, but many details are not yet understood. These gaps in the research, as they come to light, are systematically tackled by the scientific community.
In this context, the kind of material used by climate-change skeptics to cast doubt on global warming — whether it be a handful of emails stolen from an East Anglian research facility or a few errors in an IPCC report — are meaningless. The mountain of climate data assembled over decades by the scientific community as a whole is irrefutable. The records collected and analyzed by independent scientists from many disciplines and thousands of locations, paint a consistent, verifiable picture of a rapidly warming world.
Make no mistake: Science has given us unequivocal warning that global warming is real. The time to start working on solutions is now.
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