Climate Change Facts: Answers to Common Questions

The global average temperature increased by more than 1.4°F over the last century. ^[2] In fact, according to the National Oceanic and Atmospheric Administration (NOAA), the decade from 2000 to 2010 was the warmest on record, and 2010 was tied with 2005 as the warmest year on record. ^[3] Rising global temperatures have also been accompanied by other changes in weather and climate. Many places have experienced changes in rainfall resulting in more intense rain, as well as more frequent and severe heat waves. The planet's oceans and glaciers have also experienced changes: oceans are warming and becoming more acidic, ice caps are melting, and sea levels are rising. ^[4] All of these changes are evidence that our world is getting warmer.

» Learn more about the indicators of climate change.

The Earth does go through natural cycles of warming and cooling, caused by factors such as changes in the sun or volcanic activity. This has been closely examined, and the warming we have seen in the past 50 years cannot be explained by natural factors alone. ^[5] This figure illustrates one piece of evidence that shows that recent global warming is primarily a result of greenhouse gas emissions from human activities.

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The sun has natural periods of warming and cooling. With satellites, scientists have measured fluctuations in the sun's energy and found that these recent variations have been small in comparison to human influences in the last several centuries, with no increase in solar energy in the past 50 years. ^[2] Thus, changes in the sun's energy cannot explain the warming we have seen over the past several decades. In contrast, the warming we are observing is consistent with the warming properties of carbon dioxide and other heat-trapping gases that we are adding to the atmosphere.

» Learn more about the causes of climate change.

» Learn more about greenhouse gases.

Carbon dioxide is a necessary ingredient for plants to perform photosynthesis, and a critical component of our atmosphere. However, you can have too much of a good thing. The excess carbon dioxide we are adding to the atmosphere increases global temperatures, leading to climate changes that can harm plants, animals, and humans.

» Learn more about the impacts of climate change on society and ecosystems

Changing the average global temperature by even a degree or two can lead to serious consequences around the globe. For about every 2°F of warming, we can expect to see

• 5—15% reductions in the yields of crops as currently grown
• 3—10% increases in the amount of rain falling during the heaviest precipitation events, which can increase flooding risks
• 5—10% decreases in stream flow in some river basins, including the Arkansas and the Rio Grande
• 200%—400% increases in the area burned by wildfire in parts of the western United States ^[6]

Global average temperatures have increased more than 1.4 degrees Fahrenheit over the last 100 years. ^[2] Many of the extreme precipitation and heat events that we have seen in recent years are consistent with what we would expect given this amount of warming. ^[5] Scientists project that Earth's average temperatures will rise between 2 and 12 degrees Fahrenheit by 2100. ^[1]

» Learn more about the future of climate change.

A few extra cold or snowy winters in your hometown doesn't mean that global warming isn't happening. We know thatglobal average temperatures are rising. However, even with this global warming, at the local or regional level, we can expect to have some colder-than-average seasons or even colder-than-average years. For example, in the Eastern United States, the winters of 2010 and 2011 were colder than the average winters from the previous decades. In fact, extra snowy winters can be expected. In a warmer climate, more water vapor is held in the atmosphere causing more intense rain and snow storms. As the climate warms, we do expect the duration of the snow season to decrease — however, as long as it is still cold enough to snow, a warming climate can lead to bigger snowstorms. ^[5]

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Higher concentrations of carbon dioxide and other greenhouse gases in the atmosphere cause Earth to warm. Warmer temperatures increase the amount of water vapor in the atmosphere. Because water vapor is a greenhouse gas this leads to even further warming. In this way, water vapor actually magnifies the warming caused by excess carbon dioxide and other greenhouse gases. ^[5]

» Learn more about the causes of climate change.

Plants, oceans, and soils release and absorb large quantities of carbon dioxide as a part of the Earth's natural carbon cycle. These natural emissions and absorptions of carbon dioxide on average balance out over time. However, the carbon dioxide from human activities is not part of this natural balance. Ice core measurements reveal that carbon dioxide levels in the atmosphere are higher than they have been for at least 800,000 years. ^[5] The global warming that has been observed in recent decades was caused by elevated levels of carbon dioxide and other greenhouse gases in the atmosphere, due primarily to human activities. ^[1]

» Learn more about the recent role of the greenhouse effect.

There were times in the distant past when Earth was warmer than it is now. However, human societies have developed and thrived during the relatively stable climate that has existed since the last ice age. Due to excess carbon dioxide pollution, the climate is no longer stable and is instead projected to change faster than at any other time in human history. This rapid climate change will expose people to serious risks. Sea level rise, increasing droughts and wildfires in some regions and increasing flooding in others, more heat waves, and other effects of climate change all pose risks to human health, infrastructure critical to our homes, roads, and cities, and the ecosystems that support us. ^[5]

» Learn more about the impacts of climate change on society and ecosystems.

The ozone hole and climate change are essentially two separate issues. The "ozone hole" refers to the destruction of a layer of ozone molecules found high in Earth's atmosphere. When healthy, this ozone layer helps to shield Earth from the sun's harmful ultraviolet rays. The ozone layer has become thinner because of chemicals called chlorofluorocarbons that were once commonly used in products ranging from spray cans to foam furniture cushions. A thinner ozone layer allows more ultraviolet rays to reach Earth, increasing the risk to humans from skin cancer, cataracts, and other health impacts. This, however, has only minor effects on climate change.

» Learn more about the science of stratospheric ozone depletion.

A small rise in sea level will affect many people, even in the United States. The amount of sea level rise expected to occur as a result of climate change will increase the risk of coastal flooding for millions to hundreds of millions of people around the world, many of whom would have to permanently leave their homes. ^[7] Global sea level has risen approximately 9 inches, on average, in the last 140 years. ^[4] This has already put some coastal homes, beaches, roads, bridges, and wildlife at risk. ^[5] By the year 2100, sea level is expected to rise another 1.5 to 3 feet. ^[6] Rising seas will make coastal storms and the associated storm surges more frequent and destructive. For example, in New York City what is currently termed a once-in-a-century coastal flooding event could occur as frequently as once per decade. ^[5]

» Learn more about the impacts of climate change on coastal areas.

Multiple temperature records from all over the world have all shown a warming trend, and these records have been deemed reliable by the National Aeronautics and Space Administration (NASA), and the National Oceanic and Atmospheric Administration (NOAA), among others. ^[8] Other observations that point to higher global temperature includes: warmer oceans, melting arctic sea ice and glaciers, sea level rise, increasing precipitation, and changing wind patterns. ^[4]

» Learn more about climate change indicators.