Introduction

The Earth's climate is changing. Scientists are highly confident that many of the observed changes can be linked to the climbing levels of carbon dioxide and other greenhouse gases in our atmosphere, which are caused by human activities. Current and future emissions will continue to increase the levels of these gases in our atmosphere for the foreseeable future.

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

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U.S. Greenhouse Gas Emissions

In the United States, greenhouse gas emissions caused by human activities increased by 6 percent from 1990 to 2013. However, since 2005, total U.S. greenhouse gas emissions have decreased by 9 percent. Carbon dioxide accounts for most of the nation's emissions and most of the increase since 1990. Electricity generation is the largest source of U.S. greenhouse gas emissions, followed by transportation.

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Global Greenhouse Gas Emissions

Worldwide, net emissions of greenhouse gases from human activities increased by 35 percent from 1990 to 2010. Emissions of carbon dioxide, which account for about three-fourths of total emissions, increased by 42 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. Historical measurements show that current levels of many greenhouse gases are higher than any levels recorded for hundreds of thousands of years, even after accounting for natural fluctuations.

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Climate Forcing

Climate forcing refers to a change in the Earth's energy balance, leading to either a warming or cooling effect. An increase in the atmospheric concentrations of greenhouse gases produces a warming effect. From 1990 to 2014, the total warming effect from greenhouse gases added by humans to the Earth's atmosphere increased by 36 percent. The warming effect associated with carbon dioxide alone increased by 28 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. Seven of the top 10 warmest years on record have occurred since 1998. Average global temperatures show a similar trend, and the top 10 warmest years on record worldwide have all occurred since 1998. Within the United States, temperatures in parts of the North, the West, and Alaska have increased the most.

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High and Low Temperatures

Many extreme temperature conditions are becoming more common. 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 decade from 2000 to 2009 had twice as many record highs as record lows.

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Total Precipitation

Total annual precipitation has increased in the United States and over land areas worldwide. Since 1901, precipitation has increased at an average rate of 0.15 inches per decade in the contiguous 48 states and 0.09 inches per decade over land areas worldwide. However, shifting weather patterns have caused certain areas, such as the Southwest, to experience less precipitation than usual.

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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 2014, roughly 20 to 70 percent of the United States experienced drought at any given time.

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Tropical Cyclone Activity

Tropical storm activity in the Atlantic Ocean, the Caribbean, and the 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 in storm activity has occurred.

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Ocean Heat

Three separate analyses show 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 clear, and sea surface temperatures have been higher during the past three decades than at any other time since reliable observations 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 six-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 land vary by region. Along the U.S. coastline, sea level has risen the most along the Mid-Atlantic and Gulf coasts, in some places by more than 8 inches.

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Ocean Acidity

The ocean has become more acidic over the past few centuries because of increased levels of atmospheric carbon dioxide, which dissolves in the water. Higher acidity affects the balance of minerals in the water, which can make it more difficult for certain marine animals to build their skeletons and shells.

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Arctic Sea Ice

Part of the Arctic Ocean is covered by ice year-round. The area covered by ice is typically smallest in September, after the summer melting season. The minimum extent of Arctic sea ice has decreased over time, and in September 2012 it was the smallest on record. Arctic ice has also become thinner, which makes it more vulnerable to additional 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 has 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

Most lakes in the northern United States are freezing later and thawing earlier compared with the 1800s and early 1900s. Freeze dates have shifted later at a rate of roughly half a day to one day per decade, while thaw dates for most of the lakes studied have shifted earlier at a rate of half a day to two days per decade.

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Snowfall

Total snowfall—the amount of snow that falls in a particular location—has decreased in most parts of the country since widespread records began in 1930. One reason for this decline is that nearly 80 percent 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 average portion of North America covered by snow has decreased at a rate of about 3,100 square miles per year 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 or thickness of snow on the ground (snowpack) in early spring decreased at more than 90 percent of measurement sites in the western United States between 1955 and 2015. The average change across all sites for this time period amounts to about a 23 percent decline.

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Heating and Cooling Degree Days

Heating and cooling degree days measure the difference between outdoor temperatures and the temperatures that people find comfortable indoors. As the U.S. climate has warmed in recent years, heating degree days have decreased and cooling degree days have increased overall, suggesting that Americans need to use less energy for heating and more energy for air conditioning. In this map, "warmer" colors indicate an increase in temperatures, leading to less of a need to turn on the heat—that is, fewer heating degree days. "Cooler" colors indicate the opposite.

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Heat-Related Deaths

Since 1979, more than 9,000 Americans were reported to have died as a direct result of heat-related illnesses such as heat stroke. The annual death rate is higher 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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Lyme Disease

Lyme disease is a bacterial illness spread by ticks that bite humans. Tick habitat and populations are influenced by many factors, including climate. Nationwide, the rate of reported cases of Lyme disease has approximately doubled since 1991. Lyme disease is most common in the Northeast and the upper Midwest, where some states now report 50 to 100 more cases of Lyme disease per 100,000 people than they did in 1991.

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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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Ragweed Pollen Season

Warmer temperatures and later fall frosts allow ragweed plants to produce pollen later into the year, potentially prolonging the allergy season for millions of people. The length of ragweed pollen season has increased at 10 out of 11 locations studied in the central United States and Canada since 1995. The change becomes more pronounced from south to north.

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Wildfires

Since 1983, the United States has had an average of 72,000 recorded wildfires per year. Of the 10 years with the largest acreage burned, nine have occurred since 2000, with many of the largest increases occurring in western states. The proportion of burned land suffering severe damage each year has ranged from 5 to 21 percent.

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Streamflow

Changes in temperature, precipitation, snowpack, and glaciers can affect the rate of streamflow and the timing of peak flow. Over the last 73 years, minimum, maximum, and average flows have changed in many parts of the country—some higher, some lower. Nearly half of the rivers and streams measured show peak winter-spring runoff happening at least five days earlier than it did in the mid-20th century.

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Great Lakes Water Levels and Temperatures

Water levels in most of the Great Lakes have declined in the last few decades. Water levels in lakes are influenced by water temperature, which affects evaporation rates and ice formation. Since 1995, average surface water temperatures have increased by a few degrees Fahrenheit for Lakes Superior, Michigan, Huron, and Ontario. Less of a temperature change has been observed in Lake Erie.

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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 more than 40 miles since 1966, with several species shifting by hundreds of miles.

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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 dates and bloom dates show a great deal of year-to-year variability. Leaf and bloom events are generally happening earlier throughout the North and West but later in much of the South.

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Visit EPA's climate change indicators website (www.epa.gov/climatechange/indicators) to see all 30 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.