Scientists know that certain gases trap heat and act like a blanket to warm the planet. One of the most important is carbon dioxide (CO2), which we release into the atmosphere when we burn fossil fuels — oil, coal, and natural gas — to generate electricity, power our vehicles, and heat our homes.

As we overload our atmosphere with carbon dioxide, more and more heat is trapped — and Earth steadily warms up in response. How do we know? The scientific evidence is overwhelming.

The planet's temperature is rising

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Trends in temperature readings from around the world show that global warming is taking place.

Every one of the past 40 years has been warmer than the 20th century average. 2016 was the hottest year on record. The 12 warmest years on record have all occurred since 1998.

Over the past 130 years, the global average temperature has increased 1.5 degrees Fahrenheit, with more than half of that increase occurring over only the past 35 years.

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Carbon dioxide levels are increasing in the atmosphere

Detailed measurements of atmospheric carbon dioxide (CO2) levels have been taken continuously since the late 1950s. The data show that CO2 levels have steadily increased every year. In 2017, they were 28 percent higher than in 1959, the year CO2 measurements began at the Mauna Loa Observatory in Hawaii.

What's more, scientists have detailed records of past CO2 levels from ice core studies, which show that CO2 levels are higher today than at least any point in the last 800,000 years ago.

Increased CO2 is the primary driver of global warming

hot sun over skyscrapers

CO2 absorbs heat reflected from the Earth’s surface — heat that would otherwise pass freely into space. The CO2 then releases that heat, warming the Earth’s atmosphere .

As CO2 levels increase, the pace of warming accelerates. Satellite measurements confirm that less heat is escaping the atmosphere today than 40 years ago. Though other heat-trapping gases also play a role, CO2 is the primary contributor to global warming.

The climate has changed many times in the geologic past due to natural causes — including volcanic activity, changes in the sun’s intensity, fluctuations in Earth's orbit, and other factors — but none of these can account for the current rise in global temperatures.

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We are responsible for the increase in CO2

Coal power plant smokestacks

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Scientists can conclusively identify that human activity is responsible for the observed increase in CO2. How? The carbon dioxide emitted by burning coal, natural gas, and oil has a unique chemical “fingerprint" — and the additional CO2 in the atmosphere bears that signature.

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An overwhelming majority of scientists agree

IPCC Meeting

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Scientific societies and scientists have released numerous statements and studies showing the overwhelming consensus that global warming is happening and that human activity is the primary cause.

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The consequences of rising temperatures

Flooded houses

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Global warming has serious implications for our health, environment, and economy. Dangerous heat waves are increasing in severity and frequency. Sea level rise is accelerating. Extreme rainfall are on the rise in some areas. More severe droughts are occurring in others. Collectively, these effects pose a threat to the entire planet — including you, your community, and your family.

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The National Climate Assessment

map of projected precipitation increaseImage: Greg Shirah

Produced on a regular basis by the U.S. Global Change Research Program, the National Climate Assessment provides a comprehensive assessment of the current understanding of climate science, including an overview of likely impacts in the United States on a region-by-region basis.

The IPCC

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess climate change based on the latest science.

Through the IPCC, thousands of experts from around the world synthesize the most recent developments in climate science, adaptation, vulnerability, and mitigation every five to seven years.

The IPCC has issued comprehensive assessments in 1990, 1996, 2001, 2007 and 2013, plus methodology reports, technical papers, and periodic special reports assessing specific impacts of climate change (the latest ones in the works: oceans and ice cover, land degradation, and impacts of 1.5°C warming).

Understanding climate science

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Scientists use specific terms to describe how well something is known, which can lead to confusion among non-scientists about important scientific findings on global warming. Knowing this terminology is key to understanding what is known about climate change.

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We Need Your Support
to Make Change Happen

We can reduce global warming emissions and ensure communities have the resources they need to withstand the effects of climate change—but not without you. Your generous support helps develop science-based solutions for a healthy, safe, and sustainable future.