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University of Colorado Boulder

The Science

Climate change is real, and human caused.


Climate change - a short overview

Sometimes people make climate change more complex than it is, and portray it as new unproven science. It may surprise you to know that way back in 1859, John Tyndall, a scientist, discovered that CO2 was a powerful greenhouse gas. Greenhouse gases (which include CO2, methane and water vapor) trap heat in the atmosphere by absorbing infrared radiation.

A small amount of greenhouse gases are essential to our survival. Without any greenhouse gases the world would be a very frigid place. Greenhouse gases trap heat, like a blanket around the earth, thereby providing a livable atmosphere. The problem is that we are increasing the amount of these gases in the atmosphere, and thus increasing the heat being trapped. We went from 280 parts per million (ppm) CO2 in the atmosphere before the industrial revolution, to over 400 ppm now. We know this extra CO2 was generated by human activity because it has a unique chemical signature (carbon isotopes) that ties it back to the burning of fossil fuels.

Greenhouse gases come from a number of different sources [carbon percentages & terminology]. The interactive chart above shows the primary sources of emissions in the United States in 2016, where Com = Commercial and Res = Residential (EPA data). An explanation of each category is provided on the EPA’s web site. You can calculate your own family’s contribution to greenhouse gas emissions using a carbon footprint calculator. We like the Berkeley CoolClimate calculator.

What happens when this increased heat is trapped in the atmosphere is the complex part of climate change – how hot will it get and how fast, how bad will droughts get, how fast will the glaciers and ice sheets melt and sea levels rise, how much more intense or frequent will storms become? These are some of the questions that the thousands of scientists who work on climate change are striving to answer.

What's the Deal with Carbon? (3 min). Watch an animation of the carbon cycle and how it is affected by human activity (Bell Museum of Natural History and the Center for Sustainable Building Research).


Weather vs. climate change (2 min). If we cannot predict the weather 2 weeks from now, how can we predict the climate? Neil deGrasse Tyson explains the difference in this video clip (Cosmos TV show, National Geographic).

What is Global Weirding (1 min). Katharine Hayhoe, climate scientist, has a series of short videos called Global Weirding, covering many aspects of climate change (KTTZ Texas Tech Public Media, PBS Digital Studios). See all her videos.


Decoding the Weather Machine (2 hrs). Discover how Earth’s intricate climate system is changing in this comprehensive, up-to-date documentary (NOVA, PBS).

Is there consensus among scientists on the causes of climate change?

pie chart showing more than 97 percent of scientists and their papers endorse the consensus view on climate change.

Scientists overwhelmingly agree that climate change is real and human caused. The consensus project did a study that concluded 97.1% of peer-reviewed climate change papers and 98.4% of climate scientists agree that humans are causing climate change. Their results are supported by previous studies also showing 97% of papers agree on this.

NASA has a list of American Scientific Associations that confirm human-caused climate change. A California government site lists numerous agencies worldwide that support that view.

Learn more

NASA - Global Climate Change provides an illustrated overview of the evidence, causes, effects, scientific consensus, and vital signs.

C2ES - Changes in the Climate is a short overview page assembled by the Center for Climate and Energy Solutions.

The National Climate Assessment (NCA4) is a collaborative report by 13 U.S. Federal agencies and departments, and is mandated by the Global Change Research Act of 1990. The latest report is the fourth assessment, divided into two volumes. Volume I concentrates on the science (released November 2017) and Volume II on impacts, risks, and adaptation (released November 2018).

Global Warming of 1.5 °C is a special report by the United Nations Intergovernmental Panel on Climate Change (IPCC). It summarizes the worldwide effects of 1.5°C (2.7°F) of warming and shows emission pathways that can stop further warming. See: Vox article | Brookings Institution article | IPCC press release.

Real Climate is a site written by scientists and whose goal is to “provide a quick response to developing stories and provide the context sometimes missing in mainstream commentary”

Skeptical Science is a great web site for understanding the truth behind climate change myths. It provides various levels of explanation for each issue on different tabs, from Basic to Advanced, so you can get just the right amount of detail you want.

The Teacher-Friendly Guide™ to Climate Change is a comprehensive book written for high-school teachers but useful to a much wider audience. It covers the basics of climate change science as well as providing perspectives on a subject that has become socially and politically polarized.

Climate Literacy and Energy Awareness Network (CLEAN) supports teaching and learning about climate and energy topics with over 700 free, peer-reviewed, and scientifically accurate resources for secondary through higher education classrooms.

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