Introduction

The Earth's climate is changing. Scientists are confident that many of the observed changes in the climate can be linked to the increase in greenhouse gases in the atmosphere, caused largely by people burning fossil fuels to generate electricity, heat and cool buildings, and power vehicles. Current and future emissions will continue to increase the levels of these gases in our atmosphere for the foreseeable future.

One way to track and communicate the causes and effects of climate change is through the use of climate change indicators.

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U.S. Greenhouse Gas Emissions

In the United States, greenhouse gas emissions caused by human activities increased by 8 percent from 1990 to 2011. Carbon dioxide accounts for most of the nation's emissions and most of this increase. Electricity generation is the largest source of greenhouse gas emissions in the United States, followed by transportation.

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Global Greenhouse Gas Emissions

Worldwide, emissions of greenhouse gases from human activities increased by 26 percent from 1990 to 2005. Emissions of carbon dioxide, which account for nearly three-fourths of the total, increased by 31 percent over this period.

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Atmospheric Concentrations of Greenhouse Gases

Concentrations of carbon dioxide and other greenhouse gases in the atmosphere have increased since the beginning of the industrial era. Almost all of this increase is attributable to human activities.

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Climate Forcing

Climate or "radiative" forcing is a way to measure how substances such as greenhouse gases affect the amount of energy that is absorbed by the atmosphere. An increase in radiative forcing means a heating effect, which leads to warming, while a decrease in forcing produces cooling. From 1990 to 2011, the total radiative forcing from greenhouse gases added by humans to the Earth's atmosphere increased by about 30 percent.

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Average Temperature

Average temperatures have risen across the contiguous 48 states since 1901, with an increased rate of warming over the past 30 years. Average global temperatures show a similar warming trend, and 2001–2010 was the warmest decade on record worldwide. Within the United States, parts of the North, the West, and Alaska have seen temperatures increase the most.

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High and Low Temperatures

If the climate were completely stable, one would expect to see the number of record highs and record lows each accounting for about half of the records set in a year. However, since the 1970s, unusually hot summer temperatures have become more common in the United States, and heat waves have become more frequent. Record-setting daily high temperatures have become more common than record lows. The most recent decade had twice as many record highs as record lows.

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Total Precipitation

Average precipitation has increased in the United States and worldwide. Since 1901, precipitation has increased at an average rate of nearly 5 percent per century in the contiguous 48 states and more than 2 percent per century worldwide.

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Heavy Precipitation

In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events. Nationwide, eight of the top 10 years for extreme one-day precipitation events have occurred since 1990. The occurrence of abnormally high annual precipitation totals has also increased.

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Drought

Average drought conditions across the nation have varied since records began in 1895. The 1930s and 1950s saw the most widespread droughts, while the last 50 years have generally been wetter than average. However, specific trends vary by region. A more detailed index developed recently shows that between 2000 and 2012, roughly 30 to 70 percent of the U.S. land area experienced drought conditions at any given time.

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Tropical Cyclone Activity

Tropical storm activity in the Atlantic Ocean, Caribbean, and Gulf of Mexico has increased during the past 20 years. This increase is closely related to variations in sea surface temperature in the tropical Atlantic. However, changes in observation methods over time make it difficult to know for sure whether a long-term increase has occurred.

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Ocean Heat

Several studies have shown that the amount of heat stored in the ocean has increased substantially since the 1950s. Ocean heat content not only determines sea surface temperature, but also affects sea level and currents.

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Sea Surface Temperature

Ocean surface temperatures increased around the world over the 20th century. Even with some year-to-year variation, the overall increase is statistically significant, and sea surface temperatures have been higher during the past three decades than at any other time since widespread measurement began in the late 1800s.

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Sea Level

When averaged over all the world's oceans, sea level has increased at a rate of roughly seven-tenths of an inch per decade since 1880. The rate of increase has accelerated in recent years to more than an inch per decade. Changes in sea level relative to the height of the land vary widely because the land itself moves. Along the U.S. coastline, sea level has risen the most relative to the land along the Mid-Atlantic coast and parts of the Gulf Coast, while sea level has decreased relative to the land in parts of Alaska and the Northwest.

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Ocean Acidity

The ocean has become more acidic over the past 20 years because of increased levels of atmospheric carbon dioxide, which in turn dissolves in the water. Higher acidity has led to decreased availability of minerals such as aragonite, which is an important form of calcium carbonate that many marine animals use to build their skeletons and shells.

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Arctic Sea Ice

Part of the Arctic Ocean stays frozen year-round. The area covered by ice is typically smallest in September, after the summer melting season. Arctic sea ice fell to its lowest extent in the satellite record in September 2012, having declined to 49 percent below the 1979 to 2000 historical average. In addition, since 1982, the proportion of older, long-term sea ice has decreased, making it more vulnerable to melting.

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Glaciers

Glaciers in the United States and around the world have generally shrunk since the 1960s, and the rate at which glaciers are melting appears to have accelerated over the last decade. The loss of ice from glaciers has contributed to the observed rise in sea level.

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Lake Ice

Lakes in the northern United States generally appear to be freezing later and thawing earlier than they did in the 1800s and early 1900s. The length of time that lakes stay frozen has decreased at an average rate of one to two days per decade.

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Snowfall

Total snowfall has decreased in most parts of the country since widespread records began in 1930. One reason for this decline is that more than three-fourths of the locations studied have seen more winter precipitation fall in the form of rain instead of snow.

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Snow Cover

The portion of North America covered by snow has decreased somewhat since 1972, based on weekly measurements taken throughout the year. However, there has been much year-to-year variability.

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Snowpack

The depth of snow on the ground (snowpack) in the early spring decreased between 1950 and 2000 at most measurement sites across the United States and Canada—some by more than 75 percent. A few locations in the western United States and Canada saw an increase.

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Streamflow

Changes in temperature, precipitation, snowpack, and glaciers can affect the amount of water carried by rivers and streams and the timing of peak flow. Over the last 70 years, minimum and maximum flows have changed in many parts of the country—some higher, some lower. Three-fifths of the rivers and streams measured show peak winter-spring runoff happening at least five days earlier than it did in the past.

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Ragweed Pollen Season

Warmer temperatures and later fall frosts allow ragweed plants to produce pollen later into the year, potentially prolonging allergy season for millions of people. The length of ragweed pollen season has increased at eight out of 10 locations studied in the central United States and Canada since 1995. The change becomes more pronounced from south to north.

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Length of Growing Season

The average length of the growing season in the contiguous 48 states has increased by nearly two weeks since the beginning of the 20th century. A particularly large and steady increase has occurred over the last 30 years. The observed changes reflect earlier spring warming as well as later arrival of fall frosts.

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Leaf and Bloom Dates

Leaf growth and flower blooms are examples of natural events whose timing can be influenced by climate change. Observations of lilacs and honeysuckles in the contiguous 48 states suggest that first leaf growth is now occurring a few days earlier than it did in the early 1900s. Lilac and honeysuckle bloom dates vary greatly from year to year, which makes it difficult to determine whether a statistically meaningful change has taken place.

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Bird Wintering Ranges

Some birds shift their range or alter their migration habits to adapt to changes in temperature or other environmental conditions. Long-term studies have found that bird species in North America have shifted their wintering grounds northward by an average of 35 miles since 1966, with a few species shifting by several hundred miles.

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Heat-Related Deaths

Over the past three decades, more than 7,000 Americans were reported to have died as a direct result of heat-related illnesses, such as heat stroke. The annual death rate rises when accounting for other deaths in which heat was reported as a contributing factor. Considerable year-to-year variability in the data and certain limitations of this indicator make it difficult to determine whether the United States has experienced long-term trends in the number of deaths classified as "heat-related."

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Climate Change Indicators and Human Health

Climate change impacts associated with human health include expected increases in heat-related illness and death, worsening air quality, which can aggravate lung diseases and lead to premature death, and likely increases in the frequency and strength of certain extreme events such as floods, droughts, and storms. Climate change may also allow some diseases to spread more easily, increasing the risk of exposure to infection. People most vulnerable to health impacts include the poor, the elderly, those already in poor health, the disabled, and indigenous populations.

In the future, EPA plans to explore opportunities to expand the suite of health-related climate change indicators.

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Visit EPA's climate change indicators website (www.epa.gov/climatechange/indicators) to see all 26 indicators, read the full report, download graphs and maps from the report, and access supporting technical documentation.

Visit EPA's broader climate change website (www.epa.gov/climatechange/) to learn more about the impacts of climate change and the steps that society can take to adapt.