Source: Earth System Research Laboratory (ESRL) / National Oceanic and Atmospheric Administration (NOAA)


Climate change science is the study of the Earth’s changing atmospheric composition and the resulting climate and environmental impacts worldwide. Since the Industrial Revolution, the planet has experienced a massive spike in emissions of heat-trapping greenhouse gases (GHGs) produced by human activities, from about 280 parts per million, to more than 400 ppm. This rise in emissions has led to a corresponding rise in the global atmospheric concentrations of these GHG pollutants. As humans increase the volume of climate pollutants in the atmosphere, average global temperatures rise. This is because of the greenhouse effect: carbon dioxide (CO2), methane, and other greenhouse gases trap heat from the sun, making the atmosphere warmer than it otherwise would be. The greenhouse effect makes life as we know it possible: without it, the average temperature on Earth would be a mere 0 degrees Fahrenheit (instead of the 58.3 degrees Fahrenheit recorded in 2013). But too much greenhouse gas in the atmosphere can destabilize its balance, leading to much higher global temperatures and disruptive climate change.

Carbon dioxide is the primary greenhouse gas driving global climate change, though methane, hydrofluorocarbons (HFCs) and other gases also have a major hand in warming the planet. Greenhouse gases are not the only form of climate-changing pollutants—particulates, or aerosols, also play a large role. Some particulates, such as black carbon, absorb heat and contribute to warming. Other particulates, primarily sulfates, temporarily cool the planet by reflecting solar energy back into space, but the overall effect of pollution is to warm the atmosphere.

We already are seeing the many impacts of rising global temperatures, including changes in precipitation patterns, sea levels, ice mass, extreme weather events, and many more indicators. The world’s top scientific institutions have been studying these impacts for decades through rigorous data collection and state-of-the-art models. Scientific societies, national science academies, government agencies, and intergovernmental bodies are researching, assessing, and compiling their findings into reports providing important information on the causes, effects, and ramifications of climate change. Their reports not only examine current climate changes and impacts, but also predict what the world will experience in the future.


An Overwhelming Scientific Consensus

A Global Consensus: the United Nations Intergovernmental Panel on Climate Change

The leading international organization for the study of climate change is the Intergovernmental Panel on Climate Change (IPCC), which operates under the auspices of the United Nations (UN). The IPCC, established in 1988, has 195 member governments, all of which participate in the review of its reports and endorse its findings. IPCC reports are policy-neutral, offering knowledge without prescribing solutions. The IPCC does not conduct its own research; instead, more than one thousand experts review and assess available science from many thousands of peer-reviewed studies conducted world-wide in many different disciplines. The IPCC publishes a report every five to seven years which collects the latest science on global climate change. The fifth and most recent report was finalized in 2014. Each IPCC report contains three assessment reports: Working Group I (WGI) assesses the physical science of climate change; Working Group II (WGII) assesses the threats climate change presents to natural and socio-economic systems, as well as adaptive options; and Working Group III (WGIII) assesses mitigation options, such as limiting or avoiding greenhouse gas emissions. After these three assessment reports are released, the material is condensed into a Synthesis Report, which includes a Summary for Policymakers. In April 2014, IPCC WGIII published its contribution to the Fifth Assessment Report, following the publication of WGII and WGI in March 2014 and September 2013, respectively. The Fifth Assessment Synthesis Report was released in November 2014.

According to multiple studies of the scientific literature on climate change, 97 percent of climate scientists are convinced that the warming trends over the past century are most likely due to human activity. And they are increasingly confident in their assessment. The United Nations Intergovernmental Panel on Climate Change (IPCC), whose reports are considered the global benchmark in climate change understanding, stated in September 2013: "Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased." Critically, the report concludes, "It is extremely likely [95 percent confidence] that human influence has been the dominant cause of the observed warming since the mid-20th century." This was the strongest, most emphatic language yet used by the IPCC to describe its confidence in the human causes of climate change.

In 2009, 18 American scientific societies, including the American Physical Society, American Chemical Society, and The Geological Society of America, came together to release a statement on climate change: “Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research demonstrates that the greenhouse gases emitted by human activities are the primary driver.” Science academies such as the U.S. National Academy of Sciences concur, citing evidence from “direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems.”


The Hard Numbers Behind the Consensus


An American Perspective: the U.S. Global Change Research Program and the National Climate Assessment

The United States government regularly conducts a wide-ranging analysis to understand the domestic effects of climate change, as mandated by the Global Change Research Act of 1990. To fulfill this legal requirement, the U.S. Global Change Research Program (USGCRP) was established in 1989 and given the task of producing a National Climate Assessment (NCA) every four years. USGCRP coordinates with the 13 federal agencies that fund climate change research to produce the NCA.

The Third National Climate Assessment, released in May 2014, breaks down climate trends in eight regions of the United States: the Northwest, Midwest, Northeast, Southwest, Great Plains, Southeast and Caribbean, Alaska and the Arctic, and Hawaii and U.S.-affiliated Pacific Islands. The report covers an extensive range of climate issues, such as forecasts of drought in the South, increased wildfires in the Southwest, and the increased intensity, frequency, and duration of North Atlantic hurricanes. Other report findings include predictions of negative impacts on human health due to climate change, as well as decreased air and water quality, disruptions to agriculture, the spread of diseases, and damages to infrastructure. More than 300 scientists from across public, private, nonprofit, and academic sectors worked on the report.

The leading U.S. climate science research agencies are the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). In 1956, the National Oceanic and Atmospheric Administration initiated construction of the Mauna Loa Observatory on the Big Island of Hawaii to continually monitor atmospheric CO2 levels. At 11,141 feet, the high-altitude observatory is remotely located and mostly free of local pollution, allowing for atmospheric data that closely represents the global baseline. When the station first went online in March 1958, its atmospheric CO2 reading was 315.70 parts per million (ppm). In May 2015, the station recorded 403.77 ppm, the highest monthly level ever recorded. The average annual gain since measurement began is 1.4 ppm, and year-to-year gains are on an upward trend (see Figure 1).

Figure 1: The rise in atmospheric carbon dioxide concentrations at Mauna Loa, Hawaii, from 1958 to 2015 (Credit: NOAA).

The average annual increase in the 1960s was 0.85 ppm; for the past ten years it has been 2.05 ppm. This trend will likely continue to accelerate, leading to increasingly dangerous levels of CO2 in the atmosphere, and to increasingly disruptive climate change.

As the concentration of carbon dioxide in the atmosphere has increased, so have global temperatures. According to the NASA Goddard Institute for Space Studies (GISS), the average global temperature has increased by about 1.4 degrees Fahrenheit (°F) since 1880, when the average annual global temperature was roughly 56.71 °F. Global temperatures hovered around 56.7 °F up until 1910, but have since seen a rapid increase. In 1980, the average annual global temperature was 57.52 °F, and the latest studies show that global temperatures have reached 58.12 °F. A rise of 1.4 degrees Fahrenheit may sound trivial, but it is already having significant impacts. Arctic ice is melting, and glaciers and mountain snow packs around the world have greatly diminished. Coral reefs, which are especially sensitive to temperature changes, have been dying due to the added stress. We are also seeing an increase in the frequency and severity of wildfires, heat waves, and tropical storms. These impacts are why the United Nations' Intergovernmental Panel on Climate Change (IPCC), the world's leading authority on climate change (see box), has called on nations to limit the temperature increase to 3.6 °F (2 degrees Celsius). But, if current trends continue, temperatures are expected to warm another 3.6 °F within the next century (so 5 °F in all since 1910).

One of the most noticeable and troublesome effects of climate change is global sea level rise. According to NOAA, annual sea levels are currently increasing at a rate of about 3 mm (0.12 inches) per year, which “is a significantly larger rate than the sea-level rise averaged over the last several thousand years." This is particularly alarming for the more than one billion people who live in low-lying coastal regions and whose homes are at risk. Over the past century, the global average sea level has risen over 7.7 inches, with no signs of falling. The United Nations Intergovernmental Panel on Climate Change (IPCC), predicted in 2013 that global sea levels will rise another 20-40 inches within the next century under a high emissions scenario (see Figure 2). A 2014 analysis carried out by U.S. scientific agencies, the Third National Climate Assessment (see box), predicted that the sea level will rise 1-4 feet by 2100.


Figure 2: Past and future sea-level rise. "For the past, proxy data are shown in light purple and tide gauge data in blue. For the future, the IPCC projections for very high emissions (red) and very low emissions (blue) are shown."
Source: Intergovernmental Panel on Climate Change AR5 Fig. 13.27.


Troublingly, as models and data-gathering techniques have improved over the years, scientists have not only found that their past projections were correct, but found that some trends are outpacing their current projections, as greenhouse gases in the atmosphere reach critical levels.


For more information see:


The United Nations Intergovernmental Panel on Climate Change (IPCC)

IPCC Fifth Assessment Report on Climate Change (2013-2014)

National Aeronautics and Space Administration (NASA): "Climate change: How do we know?"

Environmental Protection Agency (EPA): Climate Economic Modeling

Environmental Protection Agency (EPA): Frequently Asked Questions About Global Warming and Climate Change

U.S. Global Change Research Program (USGCRP)

USGCRP Third National Climate Assessment (2014)

National Aeronautics and Space Administration (NASA): Scientific consensus: Earth's climate is warming

National Geographic: Global Warming Fast Facts


And learn more about Climate Change Science:

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