General questions

• What’s the difference between climate change and global warming?

  “Global warming” refers to the long-term warming of the planet. Global temperature shows a well-documented rise since the early 20th century and most notably since the late 1970s. Worldwide, since 1880 the average surface temperature has gone up by about 0.8 °C (1.4 °F), relative to the mid-20th-century baseline (of 1951-1980). 

  “Climate change” encompasses global warming, but refers to the broader range of changes that are happening to our planet. These include rising sea levels, shrinking mountain glaciers, accelerating ice melt in Greenland, Antarctica and the Arctic, and shifts in flower/plant blooming times. These are all consequences of the warming, which is caused mainly by people burning fossil fuels and putting out heat-trapping gases into the air. The terms “global warming” and “climate change” are sometimes used interchangeably, but strictly they refer to slightly different things.

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  • NASA finds 2013 sustained long-term climate warming trend
  • Five-year global temperature anomalies from 1880 to 2013
• What’s the difference between weather and climate?

  

  Some people say “weather is what you get” and “climate is what you expect.” “Weather” refers to the more local changes in the climate we see around us, on short timescales from minutes to hours to days to weeks. Examples are familiar – rain, snow, clouds, winds, thunderstorms, heat waves and floods. “Climate” refers to longer-term averages (they may be regional or global), and can be thought of as the weather averaged over several seasons, years or decades. Climate change is harder for us to get a sense of because the timescales involved are much longer, and the impact of climate changes can be less immediate. Examples of climate change include several drier-than-normal summers, a trend of, say, winters becoming milder from our grandparents’ childhood to our own, or variations in effects like El Niño or La Niña.

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  • Just 5 questions: Weather vs. climate
  • What’s the difference between weather and climate?
  • Cold snaps plus global warming do add up
• Is the sun causing global warming?

  No. The sun can influence the Earth’s climate, but it isn’t responsible for the warming trend we’ve seen over the past few decades. The sun is a giver of life; it helps keep the planet warm enough for us to survive. We know subtle changes in the Earth’s orbit around the sun are responsible for the comings and goings of the ice ages. But the warming we’ve seen over the last few decades is too rapid to be linked to changes in Earth’s orbit, and too large to be caused by solar activity. In fact, recently (2005-2010) the sun has become less active, while temperatures have marched upwards.

  One of the “smoking guns” that tells us the sun is not causing global warming comes from looking at the amount of the sun’s energy that hits the top of the atmosphere. Since 1978, scientists have been tracking this using sensors on satellites and what they tell us is that there has been no upward trend in the amount of the sun’s energy reaching Earth.

  

  A second smoking gun is that if the sun were responsible for global warming, we would expect to see warming throughout all layers of the atmosphere, from the surface all the way up to the upper atmosphere (stratosphere). But what we actually see is warming at the surface and cooling in the stratosphere. This is consistent with the warming being caused by a build-up of heat-trapping gases near the surface of the Earth, and not by the sun getting “hotter.”

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  • Sun & climate: moving in opposite directions
  • How do we know the sun is not the cause of recent warming?
  • Earth’s energy budget remained out of balance despite unusually low solar activity
• Is it too late to prevent climate change?

  

  Humans have caused major climate changes to happen already, and we have set in motion more changes still. Even if we stopped emitting greenhouse gases today, global warming would continue to happen for at least several more decades if not centuries. That’s because it takes a while for the planet (for example, the oceans) to respond, and because carbon dioxide – the predominant heat-trapping gas – lingers in the atmosphere for hundreds of years. There is a time lag between what we do and when we feel it.

  In the absence of major action to reduce emissions, global temperature is on track to rise by an average of 6 °C (10.8 °F), according to the latest estimates. Some scientists argue a “global disaster” is already unfolding at the poles of the planet; the Arctic, for example, may be ice-free in the summer within just a few years. Yet other experts are concerned about Earth passing one or more “tipping points” – abrupt, perhaps irreversible changes that tip our climate into a new state.

  

  But it may not be too late to avoid or limit some of the worst effects of climate change. Responding to climate change will involve a two-tier approach: 1) “mitigation” – reducing the flow of greenhouse gases into the atmosphere; and 2) “adaptation” – learning to live with, and adapt to, the climate change that has already been set in motion. The key question is: what will our emissions of carbon dioxide and other pollutants be in the years to come? Recycling and driving more fuel-efficient cars are examples of important behavioral change that will help, but they will not be enough. Because climate change is a truly global, complex problem with economic, social, political and moral ramifications, the solution will require both a globally-coordinated response (such as international policies and agreements between countries, a push to cleaner forms of energy) and local efforts on the city- and regional-level (for example, public transport upgrades, energy efficiency improvements, sustainable city planning, etc.). It’s up to us what happens next.

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  • An unrecognizable Arctic
  • If we immediately stopped emitting greenhouse gases, would global warming stop?
  • Antarctic glacier loss is ‘unstoppable,’ study says
• Is the ozone hole causing climate change?

  

  Yes and no. The ozone hole is basically a man-made hole in the ozone layer above the South Pole during the Southern Hemisphere’s spring. The ozone layer, which lies high up in the atmosphere, shields us from harmful ultraviolet (UV) rays that come from the sun. Unfortunately we punched a hole in it, through the use of gases like chlorofluorocarbons (CFCs) in spray cans and refrigerants, which break down ozone molecules in the upper atmosphere.

  While some of the sun’s UV rays slip through the hole, they account for less than one percent of the sun’s energy. So these UV rays cannot explain the global warming of the planet.

  What scientists have uncovered recently, however, is that the ozone hole has been affecting climate in the Southern Hemisphere. That’s because ozone is also a powerful greenhouse gas, and destroying it has made the stratosphere (the second layer of the atmosphere going upwards) over the Southern Hemisphere colder. The colder stratosphere has resulted in faster winds near the pole, which somewhat surprisingly can have impacts all the way to the equator, affecting tropical circulation and rainfall at lower latitudes. The ozone hole is not causing global warming, but it is affecting atmospheric circulation.

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  • Is there a connection between the ozone hole and global warming?
  • Study links ozone hole to broader climate change
• Do scientists agree on climate change?

  

  Yes, the vast majority of actively publishing climate scientists – 97 percent – agree that humans are causing global warming and climate change. Most of the leading science organizations around the world have issued public statements expressing this, including international and U.S. science academies, the United Nations Intergovernmental Panel on Climate Change and a whole host of reputable scientific bodies around the world). The number of peer-reviewed scientific papers that reject the consensus on human-caused global warming is a vanishingly small proportion of the published research. The small amount of dissent tends to come from a few vocal scientists who are not experts in the climate field or do not understand the scientific basis of long-term climate processes.

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  • Consensus: 97% of climate scientists agree
  • “The scientific consensus on climate change,” N. Oreskes, Science, Vol. 306 no. 5702, p. 1686, doi: 10.1126/science.1103618 (2004).
  • “Quantifying the consensus on anthropogenic global warming in the scientific literature,” J. Cook et al., Environ. Res. Lett., 8 024024, doi:10.1088/1748-9326/8/2/024024 (2013).
• What’s NASA got to do with climate change?

  

  When people think of NASA, they think of rovers on Mars, astronauts floating aboard the International Space Station, or probes veering out to the edge of the solar system. They don’t necessarily link NASA with climate research and observations. But Earth is a planet too, and NASA is one of the biggest players in the Earth science arena, with broad expertise on observing our climate, especially from the vantage point of space. Today it spends over a billion dollars a year doing Earth science and has more than a dozen satellites in orbit around the planet watching the oceans, land, ice, atmosphere and biosphere.

  In the 1970s, NASA’s planetary exploration budget fell dramatically. It was then that the agency really got into the business of studying our home planet from orbit. It was also a time when people were beginning to realize that our climate could change relatively fast, on the scale of the human lifespan. Today, we know that our climate is changing at an unprecedented rate and that humans are a key part of that change. NASA continues to launch new satellite missions, and is also relying on aircraft (manned and unmanned), as well as scientists on the ground, to take vital measurements of things like snowpack and hurricanes, augmenting the big-picture view we get from space.

  NASA’s role is to make observations of our climate that can be used by the public, policymakers and to support strategic decisions. Its job is to do rigorous science. However, the agency does not promote particular climate policies.

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  • NASA’s role: Taking a global perspective on Earth’s climate
• What is the greenhouse effect?

  

  The greenhouse effect is the way in which heat is trapped close to the surface of the Earth by “greenhouse gases”. These heat-trapping gases can be thought of as a blanket wrapped around the Earth, which keeps it toastier than it would be without them. Greenhouse gases include carbon dioxide, methane and nitrous oxides.

  Greenhouse gases arise naturally, and are part of the make-up of our atmosphere. Earth is sometimes called the “Goldilocks” planet – it’s not too hot, not too cold, and the conditions are just right to allow life, including us, to flourish. Part of what makes Earth so amenable is the naturally-arising greenhouse effect, which keeps the planet at a friendly 15 °C (59 °F) on average. But in the last century or so, humans have been interfering with the energy balance of the planet, mainly through the burning of fossil fuels that give off additional carbon dioxide into the air. The level of carbon dioxide in Earth’s atmosphere has been rising consistently for decades and traps extra heat near the surface of the Earth, causing temperatures to rise.

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  • The greenhouse effect
• How do we know what greenhouse gas and temperature levels were in the distant past?

  

  Ice cores are scientists’ best source for historical climate data. Every winter, some snow coating Arctic and Antarctic ice sheets is left behind and compressed into a layer of ice. By extracting cylinders of ice from sheets thousands of meters thick, scientists can analyze dust, ash, pollen and bubbles of atmospheric gas trapped inside. The deepest discovered ice cores are an estimated 800,000 years old. The particles trapped inside give scientists clues about volcanic eruptions, desert extent and forest fires. The presence of certain ions indicates past ocean activity, levels of sea ice and even the intensity of the sun. The bubbles can be released to reveal the make-up of the ancient atmosphere, including greenhouse gas levels.

  

  Other tools for learning about Earth’s ancient atmosphere include growth rings in trees, which keep a rough record of each growing season’s temperature, moisture and cloudiness going back about 2,000 years. Corals also form growth rings that provide information about temperature and nutrients in the tropical ocean.

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  • Reconstructing past atmospheres
  • Climate close-up: Coral reefs
  • Global temperatures, volcanic eruptions and trees that didn’t bark
• Why does the temperature record shown on your "Vital Signs" page begin at 1880?

  

  Three of the world’s most complete temperature tracking records – from NASA’s Goddard Institute for Space Studies, the National Oceanic and Atmospheric Administration’s National Climactic Data Center and the UK Meteorological Office’s Hadley Centre – begin in 1880. Prior to 1880, temperature measurements were made with instruments like thermometers. The oldest continuous temperature record is the Central England Temperature Data Series, which began in 1659, and the Hadley Centre has some measurements beginning in 1850, but there are too few data before 1880 for scientists to estimate average temperatures for the entire planet. Data from earlier years is reconstructed from proxy records like tree rings, pollen counts and ice cores. Because these are different kinds of data, scientists generally don’t put proxy-based estimates on the same charts as the “instrumental record.”

  The above-mentioned agencies and others collect temperature data from thousands of weather stations worldwide, including over the ocean, in Antarctica and from satellites. However, instruments are not perfectly distributed around the globe, and some measurement sites have been deforested or urbanized since 1880, affecting temperatures nearby. Each agency uses algorithms to filter the effects of these changes out of the temperature record and interpolate where data is sparse, like over the vast southern ocean, when calculating global averages. Generally, all three datasets agree quite closely (see graph above) and are in agreement on the trend of global warming since the Industrial Revolution. 

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  • Temperature data from NASA’s Goddard Institute for Space Studies
  • Temperature data from the UK Climatic Research Unit (University of East Anglia) / Meteorological Office’s Hadley Centre
  • Temperature data from NOAA’s National Climatic Data Center
• What do volcanoes have to do with climate change?

  

  Overall, volcanoes release about 5 percent of the equivalent amount of CO₂ released by humans. Quite small. However, about once every 20 years there is a volcanic eruption (e.g., Mount Pinatubo, El Chichon) that throws out a tremendous amount of particles and other gases. These will effectively shield us enough from the sun to lead to a period of global cooling. The particles and gases typically dissipate after about 2 years, but the effect is nearly global.

• May I use images and videos from your website? If so, to whom do I credit them?

  For the most part, yes: Since our website is in the public domain, you may use our images and videos without prior permission under the terms of JPL's Image Use Policy. Please credit them to “NASA’s Global Climate Change website,” unless otherwise specified (e.g., “NASA’s Goddard Space Flight Center”).

  This website occasionally features content from stock websites (such as Shutterstock.com), and a special license is needed to acquire these. If you’re unsure if an image on our site is a stock image, you may check using TinEye or email us.

How global temperatures are studied

• What kinds of data do scientists use to study climate?

  Climate researchers use every possible direct and indirect measurement to study the full history of Earth's climate, from the latest satellite observations to samples of prehistoric ice extracted from glaciers. When they focus on changes of the past 100-150 years, they use observations made by modern scientific instruments. Although the mercury thermometer and barometer were both invented in the 1700s, they and other modern instruments were not widely used for weather observations till much later. In the United States, most weather stations were established in the late 1800s. 

• Where do the data come from?

  Modern observations mostly come from weather stations, weather balloons, radars, ships and buoys, and satellites. A surprisingly large number of U.S. measurements are still made by volunteer weather watchers. There are more than 8,700 citizen observers in the National Weather Service's Cooperative Observer Program who log daily weather data. On the oceans, moored and drifting buoys have begun to replace ships in recent decades as the primary method for measuring temperatures at sea.

  The U.S. organization responsible for preserving the global climate record is the National Climatic Data Center (NCDC) of the National Oceanographic and Atmospheric Administration (NOAA). Other nations also maintain archives of global weather and climate observations.

• Can scientists use the data as is?

  No. To understand why not, imagine you're a nurse checking a patient's chart. You find the following temperature readings (Fahrenheit) for the last few hours: 99.2, 99.8, 1000, 101.4. You'd know immediately that the third number was a mistake. To make a realistic assessment of the patient's condition, you'd have to either adjust it or throw it out.

  Weather observers are as human as nurses, and they also make occasional mistakes in recording and transcribing their observations: impossibly high highs and low lows, the exact same temperatures for two months in a row, etc. The first step in data processing is quality control: identifying and eliminating erroneous data.

• Once the mistakes are eliminated, are the data ready to use?

  Not yet. They have to be adjusted to account for all the changes that happen over time:

  •  The landscape has changed dramatically in the last 100-150 years. Villages have become cities; roads have been paved; trees have been planted and cut down.

  •  The observers have changed. Weather watchers have retired and been replaced by people living uphill, downhill or across town.

  •  Daily times of observation have changed.

  •  The technology has changed. Mercury-in-glass thermometers have been replaced by more modern systems. Broken instruments have been replaced by new ones whose readings may not be perfectly consistent.

  None of these changes has anything to do with climate, but they all leave a mark on temperature data. Some changes are known to raise temperatures, like urbanization; others are known to lower them, like the change from liquid-in-glass thermometers to maximum/minimum temperature systems. If these biases are not compensated for, we cannot understand how the climate itself is changing. 

• How do scientists deal with these changes?

  Major climate research organizations worldwide have developed mathematically rigorous, peer-reviewed data processing methods to identify and compensate for changes in observing conditions. Since the research groups are independent, they do not use exactly the same techniques. One example is the way two U.S. research institutions deal with a well-known problem, the urban heat island effect — the fact that cities are warmer than the surrounding countryside. NASA's Goddard Institute for Space Studies (GISS) addresses this problem using satellite images of nighttime lights to classify stations as urban, near-urban or rural, then compares the urbanized records with an average of nearby rural stations to calculate the extent by which urbanization has warmed the data. NCDC, in processing station data for its U.S. climate divisional dataset (nClimDiv) and Global Historical Climatology Network (GHCN), corrects for the urban heat island effect and other changes such as station moves by using a technique that compares each station with multiple neighboring stations to find "guilty" stations and calculate the magnitude of necessary corrections.

  Overviews of the data-processing techniques for NCDC's nClimDiv and GHCN are available here and here. GISS's data analysis methods, with links to papers giving full scientific explanations, are described here.

• How do scientists know their data-processing techniques are reliable?

  The records of global temperatures calculated by U.S. and other major climate research organizations are remarkably similar, despite the different data-processing techniques used. The techniques used by GISS, NCDC and other respected groups such as the British Meteorological Office's Hadley Centre/Climatic Research Unit of the University of East Anglia (CRU) and the Australian Bureau of Meteorology are peer reviewed, as mentioned above, and the processed data sets have undergone many peer-reviewed analyses as well.

  An independent nonprofit organization called Berkeley Earth, funded by educational grants, has devised its own data processing techniques to deal with the changes listed above. Again, the results are very similar to results from NCDC, GISS and the CRU.

• Does data processing make temperature data warmer?

  It can go either way. Almost half of NOAA's corrected data are cooler than the original records. NOAA's corrections of temperatures over the oceans — done to compensate for changes in methods of observing the temperature of water at the surface of the ocean — reduced the warming trend in global temperature.

• Does data processing destroy the original data?

  

  No, the original records are preserved and are available at no cost online. You can access NCDC's U.S. and global records here.

  For more technical answers on this topic, visit GISS's FAQs and NCDC's FAQs.

Less frequently asked, but interesting

• If all of Earth's ice melts and flows into the ocean, what would happen to the planet's rotation?

  

  The Earth rotates about its axis once a day, but it does not do so uniformly. Instead, the rate of rotation varies by up to a millisecond per day. Like a spinning ice skater whose speed of rotation increases as the skater’s arms are brought closer to their body, the speed of the Earth’s rotation will increase if its mass is brought closer to its axis of rotation. Conversely, the speed of the Earth’s rotation will decrease if its mass is moved away from the rotation axis.

  Melting land ice, like mountain glaciers and the Greenland and Antarctic ice sheets, will change the Earth’s rotation only if the meltwater flows into the oceans. If the meltwater remains close to its source (by being trapped in a glacier lake, for example), then there is no net movement of mass away from the glacier or ice sheet, and the Earth’s rotation won’t change. But if the meltwater flows into the oceans and is dispersed, then there is a net movement of mass and the Earth’s rotation will change. For example, if the Greenland ice sheet were to completely melt and the meltwater were to completely flow into the oceans, then global sea level would rise by about seven meters (23 feet) and the Earth would rotate more slowly, with the length of the day becoming longer than it is today, by about two milliseconds.

  Melting sea ice, such as the Arctic ice cap, does not change sea level because the ice displaces its volume and, hence, does not change the Earth’s rotation.

• How do I cite your website?

  It depends on the style you’re using. Here are a few examples:

  MLA format:
  "Global Climate Change: Evidence." NASA Global Climate Change and Global Warming: Vital Signs of the Planet. Jet Propulsion Laboratory / National Aeronautics and Space Administration, 15 June 2008. Web. 14 Jan. 2015. <http://climate.nasa.gov/evidence/>.

  Formula:
  “Webpage Name.” Website Name. Publisher, Publication date. Source type. Date user accessed the webpage. <URL>.

  Chicago style:
  "Global Climate Change: Evidence." NASA Global Climate Change and Global Warming: Vital Signs of the Planet. June 15, 2008. Accessed January 14, 2015. http://climate.nasa.gov/evidence/.

  Formula:
  “Webpage Name.” Website Name. Publication date. Date user accessed the webpage. URL.

  APA format:
  Global Climate Change: Evidence. (2008, June 15). Retrieved January 14, 2015, from http://climate.nasa.gov/evidence/

  Formula:
  Webpage Name. (Publication date). Retrieved [Date user accessed the webpage], from [URL]

  In general, include as much information as possible in your citations. For example, if one of our pieces is dated to “November 2010” (i.e., without a day), then you would need to write it as “Nov. 2010” in MLA format, “November 2010” in Chicago style and so on.

  Bibme.org is also a useful source for creating bibliographies.

• Which is a bigger methane source: cow belching or cow flatulence?

  

  Contrary to common belief, it’s cow belching due to enteric fermentation. (“Enteric fermentation” is the digestive process of converting sugars into simple molecules for absorption into the bloodstream, which produces methane as a by-product.) However, a small percentage of methane is also produced in the cow’s large intestine and then expelled. Settling ponds and lagoons for processing manure also produce copious amounts of this greenhouse gas.

'Ask a climate scientist' videos

• Is there any merit to the studies that show that historical carbon dioxide levels lag behind temperature, and don't lead them?

• Will climate change drastically reduce our food production, or will it change what we produce?

• Does the scientific community feel that global warming is decreasing or increasing?

• Is the frequency of extreme weather events a sign that global warming is gaining pace and exceeding predictions?

• How does climate change affect humans?

• How can Greenland's ice sheets remain more than 10,000 feet thick even with global warming?