Global Warming FAQ

How do we know that humans are the major cause of global warming?


Photo: alohaspirit/iStock

We all know that warming—and cooling—has happened in the past, and long before humans were around. Many factors (called “climate drivers”) can influence Earth’s climate—such as changes in the sun’s intensity and volcanic eruptions, as well as heat-trapping gases in the atmosphere.

So how do scientists know that today’s warming is primarily caused by humans putting too much carbon in the atmosphere when we burn coal, oil, and gas or cut down forests? We know human activities are driving the increase in CO2 concentrations because atmospheric COcontains information about its source. Scientists can tease apart how much CO2 comes from natural sources, and how much comes from burning coal, oil and gas (called fossil fuels).

Compared to other carbon sources, carbon from fossil fuels has a distinctly different “signature,” essentially the relative amount of heavier or lighter atoms of carbon (technically δ13C). The more negative the δ13C, the higher the proportion of carbon from fossil fuels.

Over the years, δ13C has decreased while the overall amount of CO2 has increased. This information tells scientists that fossil fuel emissions are the largest contributor of CO2 concentrations since the pre-industrial era.

Moreover, natural changes alone can’t explain the temperature changes we’ve seen. For a computer model to accurately project the future climate, scientists must first ensure that it accurately reproduces observed temperature changes.  When the models include only recorded natural climate drivers—such as the sun’s intensity—the models cannot accurately reproduce the observed warming of the past half century. When human-induced climate drivers are also included in the models, then they accurately capture recent temperature increases in the atmosphere and in the oceans. When all the natural and human-induced climate drivers are compared to one another, the dramatic accumulation of carbon from human sources is by far the largest climate change driver over the past half century.

When models take into account both natural and human drivers, they better reflect the observed changes in temperature.
IPCC AR5

Explore more about humans' role in global warming.

back to top

Why does CO2 get most of the attention when there are so many other heat-trapping gases?

Global warming is primarily a problem of too much carbon dioxide in the atmosphere.  This carbon overload is caused mainly when we burn fossil fuels like coal, oil and gas or cut down and burn forests. There are many heat-trapping gases (from methane to water vapor), but CO2 puts us at the greatest risk of irreversible changes if it continues to accumulate unabated in the atmosphere. There are two key reasons why.

CO2 has contributed more than any driver to climate change between 1750 and 2011.

The Intergovernmental Panel on Climate Change (IPCC) issued a global climate assessment in 2013 that compared the influence of three changes to the environment resulting from human activity between 1750 and 2011: the emission of key heat-trapping gases and tiny particles known as aerosols, as well as land use change.

By measuring the abundance of heat-trapping gases in ice cores, the atmosphere, and other climate drivers along with models, the IPCC calculated the “radiative forcing” (RF) of each climate driver—in other words, the net increase (or decrease) in the amount of energy reaching Earth’s surface attributable to that climate driver.

Positive RF values represent average surface warming and negative values represent average surface cooling. In total, CO2 has the highest positive RF (see Figure 1) of all the human-influenced climate drivers compared by the IPCC.

Other gases have more potent heat-trapping ability molecule per molecule than CO2 (e.g. methane), but are simply far less abundant in the atmosphere.

Figure 1. How does carbon dioxide compare to other climate drivers? Carbon dioxide (CO2), more than any other climate driver, has contributed the most to climate change between 1750 and 2011. Radiative forcing units are expressed as the power (watts) per square meter surface area of Earth.
IPCC AR5

Explore more about CO2 and other heat trapping gases.

back to top

Does air pollution—specifically particulate matter (aerosols)—affect global warming?

Air pollution occurs when the air contains gases, dust, smoke from fires, or fumes in harmful amounts. Tiny atmospheric particles - aerosols - are a subset of air pollution that are suspended in our atmosphere.

Aerosol can be both solid and liquid. Most are produced by natural processes such as erupting volcanoes, and some are from human industrial and agricultural activities.

Aerosols have a measurable effect on climate change. Light-colored aerosol particles can reflect incoming energy from the sun in cloud-free air and dark particles can absorb it. Over the historic period, the net effect globally was for aerosols to partially offset the rise in global mean surface temperature. Aerosols can modify how much energy clouds reflect and they can change atmospheric circulationpatterns.

Several climate engineering (so-called ‘geoengineering’) strategies for reducing global warming propose using atmospheric aerosol particles to reflect the sun’s energy away from Earth. Because aerosol particles do not stay in the atmosphere for very long—and global warming gases stay in the atmosphere for decades to centuries—accumulated heat-trapping gases will overpower any temporary cooling due to short-lived aerosol particles.

This portrait of global aerosols was produced by a GEOS-5 simulation at a 10-kilometer resolution. Dust (red) is lifted from the surface, sea salt (blue) swirls inside cyclones, smoke (green) rises from fires, and sulfate particles (white) stream from volcanoes and fossil fuel emissions.
Image: William Putman, NASA/Goddard

Explore more about air pollution and global warming.

back to top

Drivers of global average surface temperature “anomalies” (changes away from the 1961-1990 average global surface temperature). (a): Global average surface temperature measurements (in black) and (in red) global average surface temperature modeled by a computer using solar, volcanic, and other natural (internal variability) factors, as well as human (anthropogenic) factors. The extent to which changes in solar radiation (b), volcanoes (c), other internal variability (d) factors, and human (anthropogenic) (e) factors have driven changes in global average surface temperature.
 IPCC AR5

How does the sun affect our climate?

The sun is the source of most of the energy that drives the biological and physical processes in the world around us—in oceans and on land it fuels plant growth that forms the base of the food chain, and in the atmosphere it warms air which drives our weather.

The rate of energy coming from the sun changes slightly day to day. Over many millennia in the Earth-Sun orbital relationship can change the geographical distribution of the sun’s energy over the Earth’s surface. It has been suggested that changes in solar output might affect our climate—both directly, by changing the rate of solar heating of the Earth and atmosphere, and indirectly, by changing cloud forming processes. 

Over the time-scale of millions of years the change in solar intensity is a critical factor influencing climate (e.g., ice ages).  However, changes in solar heating rate over the last century cannot account for the magnitude and distribution of the rise in global mean temperature during that time period and there is no convincing evidence for significant indirect influences on our climate due to twentieth century changes in solar output.

Explore more about the sun's effect on our climate.

back to top

Is there a connection between the hole in the ozone layer and global warming?

Ozone (O3) high in the atmosphere absorbs ultraviolet radiation from the sun, thereby protecting living organisms below from this dangerous radiation. The term ‘ozone hole’ refers to recent depletion of this protective layer over Earth's polar regions. People, plants, and animals living under the ozone hole are harmed by the solar radiation now reaching the Earth's surface—where it causes health problems from eye damage to skin cancer.

The ozone hole, however, is not the mechanism of global warming. Ultraviolet radiation represents less than one percent of the energy from the sun—not enough to be the cause of the excess heat from human activities. Global warming is caused primarily from putting too much carbon into the atmosphere when coal, gas, and oil are burned to generate electricity or to run our cars. These gases spread around the planet like a blanket, capturing the solar heat that would otherwise be radiated out into space. (For more detail on the basic mechanism of global warming, see carbon dioxide FAQ.)

Both of these environmental problems do, however, have a common cause—human activities that release gases into and alter the atmosphere. Ozone depletion occurs when chlorofluorocarbons (CFCs)—formerly found in aerosol spray cans and refrigerants—are released into the atmosphere. These gases, through several chemical reactions, cause the ozone molecules to break down, reducing ozone's ultraviolet (UV) radiation-absorbing capacity.

Because our atmosphere is one connected system, it is not surprising that ozone depletion and global warming are related in other ways. For example, evidence suggests that climate change may contribute to thinning of the protective ozone layer.

Explore more about the ozone hole and global warming.

back to top

What is the best source of scientific information on global warming?

In 1988, the United Nations Environment Programme and the World Meteorological Organization set up the Intergovernmental Panel on Climate Change (IPCC) to examine the most current scientific information on global warming and climate change. Thousands of authors and scientific expert reviewers from 195 member countries contribute to the panel's periodic assessments of climate change and its impacts.  

These scientists review all the published and peer-reviewed scientific information produced during the previous few years to assess what is known about the global climate, why and how it changes, what it will mean for people and the environment, and what can be done about it.

The IPCC Fifth Assessment Report is the most comprehensive evaluation of global warming that serves as the basis for international climate negotiations. 

The most up-to-date comprehensive assessment for the United States was released in 2014 by 13 federal agencies participating in the U.S. Global Change Research Program (USGCRP). 

Explore more about the USGCRP report on US climate change impacts.

Explore more about the scientific consensus on climate change.

back to top

Is global warming already happening

Yes. We know it is getting warmer. Record warm temperatures keep coming up month after month, year after year, locally and globally. However, temperatures are but one of many indicators of global warming. With warmer temperatures come various changes that also point to a steady change in our world.

Examples of observed climatic changes:

  • Increase in global average surface temperature of about 1°C since the pre-industrial age
  • Decrease of snow cover and sea ice extent and the retreat of mountain glaciers in the latter half of the 20th century
  • Rise in global average sea level and the increase in ocean water temperatures
  • Likely increase in average precipitation over the middle and high latitudes of the Northern Hemisphere, and over tropical land areas
  • Increase in the frequency and intensity of extreme precipitation events in some regions of the world

Examples of observed physical and ecological changes:

  • Thawing of permafrost
  • Lengthening of the growing season in middle and high latitudes
  • Poleward and upward shift of plant and animal ranges
  • Decline of some plant and animal species
  • Earlier flowering of trees
  • Earlier emergence of insects
  • Earlier egg-laying in birds

Read more about the signs of global warming. 

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.