Climate Change

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Past Climate Change

 

The Earth's climate has changed throughout history. From glacial periods (or "ice ages") where ice covered significant portions of the Earth to interglacial periods where ice retreated to the poles or melted entirely - the climate has continuously changed.

Scientists have been able to piece together a picture of the Earth's climate dating back decades to millions of years ago by analyzing a number of surrogate, or "proxy," measures of climate such as ice cores, boreholes, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in the Earth's orbit around the sun.

This page contains information about the causes of climate change throughout the Earth's history, the rates at which the climate has changed, as well as information about climate change during the last 2,000 years.

Causes of Change Prior to the Industrial Era (pre-1780)

Known causes, “drivers” or “forcings” of past climate change include:

  • Changes in the Earth's orbit: Changes in the shape of the Earth's orbit (or eccentricity) as well as the Earth's tilt and precession affect the amount of sunlight received on the Earth's surface. These orbital processes -- which function in cycles of 100,000 (eccentricity), 41,000 (tilt), and 19,000 to 23,000 (precession) years -- are thought to be the most significant drivers of ice ages according to the theory of Mulitin Milankovitch, a Serbian mathematician (1879-1958). The National Aeronautics and Space Administration's (NASA) Earth Observatory offers additional information about orbital variations and the Milankovitch Theory.
  • Changes in the sun's intensity: Changes occurring within (or inside) the sun can affect the intensity of the sunlight that reaches the Earth's surface. The intensity of the sunlight can cause either warming (for stronger solar intensity) or cooling (for weaker solar intensity). According to NASA research, reduced solar activity from the 1400s to the 1700s was likely a key factor in the “Little Ice Age” which resulted in a slight cooling of North America, Europe and probably other areas around the globe. (See additional discussion under The Last 2,000 Years.)
  • Volcanic eruptions: Volcanoes can affect the climate because they can emit aerosols and carbon dioxide into the atmosphere.
    • Aerosol emissions: Volcanic aerosols tend to block sunlight and contribute to short term cooling. Aerosols do not produce long-term change because they leave the atmosphere not long after they are emitted. According to the United States Geological Survey (USGS), the eruption of the Tambora Volcano in Indonesia in 1815 lowered global temperatures by as much as 5ºF and historical accounts in New England describe 1816 as “the year without a summer.”
    • Carbon dioxide emissions: Volcanoes also emit carbon dioxide (CO2), a greenhouse gas, which has a warming effect. For about two-thirds of the last 400 million years, geologic evidence suggests CO2 levels and temperatures were considerably higher than present. One theory is that volcanic eruptions from rapid sea floor spreading elevated CO2 concentrations, enhancing the greenhouse effect and raising temperatures. However, the evidence for this theory is not conclusive and there are alternative explanations for historic CO2 levels (NRC, 2005). While volcanoes may have raised pre-historic CO2 levels and temperatures, according to the USGS Volcano Hazards Program, human activities now emit 150 times as much CO2 as volcanoes (whose emissions are relatively modest compared to some earlier times).

These climate change “drivers” often trigger additional changes or “feedbacks” within the climate system that can amplify or dampen the climate's initial response to them (whether the response is warming or cooling). For example:

  • Changes in greenhouse gas concentrations: The heating or cooling of the Earth's surface can cause changes in greenhouse gas concentrations. For example, when global temperatures become warmer, carbon dioxide is released from the oceans. When changes in the Earth's orbit trigger a warm (or interglacial) period, increasing concentrations of carbon dioxide may amplify the warming by enhancing the greenhouse effect. When temperatures become cooler, CO2 enters the ocean and contributes to additional cooling. During at least the last 650,000 years, CO2 levels have tended to track the glacial cycles (IPCC, 2007). That is, during warm interglacial periods, CO2 levels have been high and during cool glacial periods, CO2 levels have been low (see Figure 1).

This graph shows CO2 concentrations from 647,000 BC to 2006 AD, and Antarctic temperatures from 421,000 BC to 2000 AD. (Antarctic temperature is measured as the change from average conditions for the period 1850 AD to 2000 AD.) The graph shows a fairly close relationship between CO2 concentrations and temperature during the period when both CO2 and temperature are available, and shows a sharp increase in CO2 concentrations during the 20th century.

Figure 1: Fluctuations in temperature (red line) and in the atmospheric concentration of carbon dioxide (yellow) over the past 649,000 years. The vertical red bar at the end is the increase in atmospheric carbon dioxide levels over the past two centuries and before 2007. Click on thumbnail for a full-size image and references.

  • Changes in ocean currents: The heating or cooling of the Earth's surface can cause changes in ocean currents. Because ocean currents play a significant role in distributing heat around the Earth, changes in these currents can bring about significant changes in climate from region to region.

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Rates of Change

Studies of the Earth's previous climate suggest periods of stability as well as periods of rapid change. Recent climate research suggests:

  • Interglacial climates (such as the present) tend to be more stable than cooler, glacial climates. For example, the climate during the current and previous interglacials (known as the Holocene and Eemian interglacials) has been more stable than the most recent glacial period (known as the Last Glacial Maximum). This glacial period was characterized by a long string of widespread, large and abrupt climate changes (NRC, 2002).
  • Abrupt or rapid climate changes tend to frequently accompany transitions between glacial and interglacial periods (and vice versa). For example, a significant part of the Northern Hemisphere (particularly around Greenland) may have experienced warming ratesof 14-28ºF over several decades during and after the most recent ice age (IPCC, 2007).

While abrupt climate changes have occurred throughout the Earth's history, human civilization arose during a period of relative climate stability.

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The Last 2,000 Years

During the last 2,000 years, the climate has been relatively stable. Scientists have identified three departures from this stability, known as the Medieval Climate Anomaly (also referred to as the Medieval Warm Period), the Little Ice Age and the Industrial Era:

  • The Medieval Climate Anomaly: Between roughly 900 and 1300 AD, evidence suggests Europe, Greenland and Asia experienced relative warmth. While historical accounts and other evidence document the warmth that occurred in some regions, the geographical extent, magnitude and timing of the warmth during this period is uncertain (NRC, 2006). The American West experienced very dry conditions around this time.
  • The Little Ice Age: A wide variety of evidence supports the global existence of a "Little Ice Age" (this was not a true "ice age" since major ice sheets did not develop) between about 1500 and 1850 (NRC, 2006). Average temperatures were possibly up to 2ºF colder than today, but varied by region.
  • The Industrial Era: An additional warm period has emerged in the last 100 years, coinciding with substantially increasing emissions of greenhouse gases from human activities (see Recent Climate Change for more information).

Prior to the Industrial Era,  the Medieval Climate Anomaly and Little Ice Age had defined the upper and lower boundaries of the climate's recent natural variability and are a reflection of changes in climate drivers (the sun's variability and volcanic activity) and the climate's internal variability (referring to random changes in the circulation of the atmosphere and oceans).

The issue of whether the temperature rise of last 100 years crossed over the warm limit of the boundary defined by the Medieval Climate Anomaly has been a controversial topic in the science community. The National Academy of Sciences recently completed a study to assess the efforts to reconstruct temperatures of the past one to two millennia (see Figure 2) and place the Earth's current warming in historical context (NRC, 2006).

Figure 2. This graph provides reconstructions of Northern Hemisphere average or global average surface temperature variations over the last 1,100 years from six research teams, along with the instrumental record of global average surface temperature. Overall, the curves show a warming around 1000 AD followed by a long general cooling trend that continues until the early 1900s. Each curve illustrates a somewhat different history of temperature changes, with a range of uncertainties that tend to increase backward in time.

Figure 2: Reconstructions of (Northern Hemisphere average or global average) surface temperature variations from six research teams (in different color shades) along with the instrumental record of global average surface temperature (in black). Each curve illustrates a somewhat different history of temperature changes, with a range of uncertainties that tend to increase backward in time (as indicated by the shading). Reference: NRC, 2006. (Figure reprinted with permission from Surface Temperature Reconstructions© (2006) by the National Academy of Sciences, Courtesy of the National Academies Press Exit EPA Disclaimer, Washington, D.C.)

According to the study Exit EPA Disclaimer(NRC, 2006):

  • There is a high level of confidence that the global average temperature during the last few decades was warmer than any comparable period during the last 400 years.
  • Present evidence suggests that temperatures at many, but not all, individual locations were higher during the past 25 years than any period of comparable length since A.D. 900. However, uncertainties associated with this statement increase substantially backward in time.
  • Very little confidence can be assigned to estimates of hemisphere average or global average temperature prior to A.D. 900 due to limited data coverage and challenges in analyzing older data.

 

Recent Climate Change

Since the Industrial Revolution (around 1750), human activities have substantially added to the amount of heat-trapping greenhouse gases in the atmosphere. The burning of fossil fuels and biomass (living matter such as vegetation) has also resulted in emissions of aerosols that absorb and emit heat, and reflect light.

The addition of greenhouse gases and aerosols has changed the composition of the atmosphere. The changes in the atmosphere have likely influenced temperature, precipitation, storms and sea level (IPCC, 2007). However, these features of the climate also vary naturally, so determining what fraction of climate changes are due to natural variability versus human activities is challenging.

The following pages provide a summary of the atmosphere and climate changes observed during the Industrial Era and, where possible, current understanding of why the changes have occurred:

The release of greenhouse gases and aerosols resulting from human activities are changing the amount of radiation coming into and leaving the atmosphere, likely contributing to changes in climate.

Greenhouse Gases

Greenhouse gas concentrations in the atmosphere have historically varied as a result of many natural processes (e.g. volcanic activity, changes in temperature, etc). However, since the Industrial Revolution humans have added a significant amount of greenhouse gases in the atmosphere by burning fossil fuels, cutting down forests and other activities. Because greenhouse gases absorb and emit heat, increasing their concentrations in the atmosphere will tend to have a warming effect. But the rate and amount of temperature increase is not known with absolute certainty. Changes in the atmospheric concentration of the major greenhouse gases are described below:

 

Carbon dioxide (CO2) concentrations in the atmosphere increased from approximately 280 parts per million (ppm) in pre-industrial times to 382 ppm in 2006 according to the National Oceanic and Atmospheric Administration's (NOAA) Earth Systems Research Laboratory, a 36 percent increase. Almost all of the increase is due to human activities (IPCC, 2007). The current rate of increase in CO2 concentrations is about 1.9 ppmv/year. Present CO2 concentrations are higher than any time in at least the last 650,000 years (IPCC, 2007). See Figure 1 for a record of CO2 concentrations from about 420,000 years ago to present. For

Figure 1: Atmospheric Concentrations of Carbon Dioxide in Geologic Time and in Recent Years: This diagram is in three sections. The first, using data from ice cores in Antarctica, shows CO2 concentrations from 647,426 B.C. to 337 B.C., with a clear cyclical pattern of peaks and valleys. The second, using data from other Antarctic ice cores, shows CO2 concentrations from 8947 B.C. to 1975 A.D. The diagram shows a slight upward trend in concentrations until the 20th century, when they shoot up rapidly. The third section, using data from CO2 monitoring stations around the world, shows CO2 concentrations from 1959 to 2006. The trend shows a steady increase in concentrations from about 320 ppm in 1959 to approximately 380 ppm in 2006.

Figure 1 - Carbon Dioxide: Click on Thumbnail for full size image

Methane (CH4) is more abundant in the Earth’s atmosphere now than at any time in at least the past 650,000 years (IPCC, 2007). Methane oncentrations increased sharply during most of the 20th century and are now 148% above pre-industrial levels. In recent decades, the rate of increase has slowed considerably (see Figure 2). For more information on CH4 emissions and sources, and actions that can reduce emissions, see EPA’s Methane Site.

Figure 2: Atmospheric Concentrations of Methane in Geologic Time and in Recent Years: This diagram is in three sections. The first, using data from ice cores in Antarctica and Greenland, shows methane concentrations from 648,679 B.C. to 346 B.C. Concentrations during the period vary widely, from as high as 800 ppb to as low as less than 100 ppb. The second, using data from other ice cores, shows methane concentrations from 8945 B.C. to 1980 A.D. The diagram shows a relatively flat trend in concentrations until the 20th century, when they shoot up rapidly. The third section, using data from several atmospheric monitoring stations around the world, shows methane concentrations from 1985 to 2001. The trend shows an increase in concentrations during most of the period, with an apparent leveling off in the later years.

Figure 2 - Methane: Click on Thumbnail for full size image

 

Nitrous oxide (N2O) has increased approximately 18 percent in the past 200 years and continues to increase (see Figure 3). For about 11,500 years before the industrial period, the concentration of N2O varied only slightly. It increased relatively rapidly toward the end of the 20th century (IPCC, 2007). For more information on N2O emissions and sources, see EPA’s Nitrous Oxide Site .

Figure 3: Atmospheric Concentrations of Nitrous Oxide in Geologic Time and in Recent Years: This diagram is in three sections. The first, using data from ice cores in East Antarctica and Greenland, shows nitrous oxide concentrations from 104,301 B.C. to 1871 A.D. Concentrations during the period varied widely, ranging from 180 ppb to more than 280 ppb, with an upward trend toward the end of the period. The second, using data from a variety of sources, shows nitrous oxide concentrations from 9000 B.C. to 1976 A.D. The diagram shows a relatively flat trend in concentrations until the 20th century, when they shoot up rapidly. The third section, using data from several atmospheric monitoring stations around the world, shows nitrous oxide concentrations from 1977 to 2005. The trend shows a steady increase in concentrations, rising from around 300 ppb in 1997 to 320 ppb in 2005.

Figure 3 - Nitrous Oxide: Click on Thumbnail for full size image

How are Greenhouse Gas Concentrations from Thousands of Years Ago Determined?

Portions of the Antarctic ice sheet are several miles deep, consisting of ice that has accumulated over hundreds of thousands of years or longer. Paleoclimatologists (scientists who study the history of the Earth's climate) drill holes in this ice to extract what are called "cylindrical cores," or "ice cores."

Ice cores can provide valuable information about the Earth’s past. For example, the cores contain trapped air bubbles that can be analyzed to obtain snapshots of the composition of the atmosphere at the time the ice accumulated. Through this analysis, concentrations of greenhouse gases (CO2, CH4, N2O) dating back thousands of years or longer can be obtained with a high level of confidence. See the National Aeronautics and Space Administration’s (NASA) Earth Observatory feature "Paleoclimatogy: The Ice Core Method" for more information.

  • Tropospheric ozone (O3) is created by chemical reactions from automobile, power plant and other industrial and commercial source emissions in the presence of sunlight. It is estimated that O3 has increased by about 36% since the pre-industrial era, although substantial variations exist for regions and overall trends (IPCC, 2007). Besides being a greenhouse gas, ozone can also be a harmful air pollutant at ground level, especially for people with respiratory diseases and children and adults who are active outdoors. Measures are being taken to reduce ozone emissions in the U.S. (through the Clean Air Act) and also in other countries.
  • Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are used in coolants, foaming agents, fire extinguishers, solvents, pesticides and aerosol propellants. These compounds have steadily increased in the atmosphere since their introduction in 1928. Concentrations are slowly declining as a result of their phaseout via the Montreal Protocol on Substances that Deplete the Ozone Layer.
  • Fluorinated gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are frequently used as substitutes for CFCs and HCFCs and are increasing in the atmosphere. These various fluorinated gases are sometimes called "high global warming potential greenhouse gases" because, molecule for molecule, they trap more heat than CO2. For more information, visit EPA’s High Global Warming Potential Gases Site.

Aerosols

The burning of fossil fuels and biomass (living matter such as vegetation) has resulted in aerosol emissions into the atmosphere. Aerosols absorb and emit heat, reflect light and, depending on their properties, can either cool or warm the atmosphere. NASA’s Earth Observatory describes how aerosols can also affect how clouds form.

  • Sulfate aerosols are emitted when fuel containing sulfur, such as coal and oil, is burned. Sulfate aerosols reflect solar radiation back to space and have a cooling effect. These aerosols have decreased in concentration in the past two decades resulting from efforts to reduce the coal-fired power plant emissions of sulfur dioxide in the United States and other countries.
  • Black carbon (or soot) results from the incomplete combustion of fossil fuels and biomass burning (forest fires and land clearing) and is believed to contribute to global warming (IPCC, 2007). Though global concentrations are likely increasing, there are significant regional differences. In the United States and many other countries, efforts to reduce particulate matter (of which black carbon is a part) are lowering black carbon concentrations.
  • Other aerosols emitted in small quantities from human activities include organic carbon and associated aerosols from biomass burning. Mineral dust aerosols (e.g., from deserts and lake beds) largely originate from natural sources, but their distribution can be affected by human activities.

Radiative Forcing

Radiative forcing is the change in the balance between solar radiation entering the atmosphere and the Earth's radiation going out. On average, a positive radiative forcing tends to warm the surface of the Earth while negative forcing tends to cool the surface. Radiative forcing is measured in Watts per square meter, which is a measure of energy. For example, an increase in radiative forcing of +1 Watt per square meter is like shining one small holiday tree light bulb over every square meter of the Earth.

Greenhouse gases have a positive radiative forcing because they absorb and emit heat. Aerosols can have a positive or negative radiative forcing, depending on how they absorb and emit heat and/or reflect light. For example, black carbon aerosols - which have a positive forcing - more effectively absorb and emit heat than sulfates, which have a negative forcing and more effectively reflect light. The following are estimates of the change in radiative forcing in the year 2005 relative to 1750 for different components of the climate (IPCC, 2007):

  • The radiative forcing contribution (since 1750) from increasing concentrations of well-mixed greenhouse gases (including CO2, CH4, N2O, CFCs, HCFCs, and fluorinated gases) is estimated to be +2.64 Watts per square meter - over half due to increases in CO2 (+1.66 Watts per square meter), strongly contributing to warming relative to other climate components described below.
  • The radiative forcing contribution from increasing tropospheric ozone, an unevenly distributed greenhouse gas, is estimated to be +0.35 Watts per square meter (on average), resulting in a relatively small warming effect. This forcing varies from region to region depending on the amount of ozone in the troposphere at a particular location.
  • The radiative forcing contribution from the observed depletion of stratospheric ozone is estimated to be -0.05 Watts per square meter, resulting in a relatively small cooling effect.
  • While aerosols can have either positive or negative contributions to radiative forcing, the net effect of all aerosols added to the atmosphere has likely been negative. The best estimate of aerosols’ direct cooling effect is -0.5 Watts per square meter; the best estimate for their indirect cooling effect (by increasing the reflectivity of clouds) is -0.7 Watts per square meter, with an uncertainty range of -1.8 to -0.3 Watts per square meter. Therefore, the net effect of changes in aerosol radiative forcing has likely resulted in a small to relatively large cooling effect.
  • Land use change (including urbanization, deforestation, reforestation, desertification, etc) can have significant effects on radiative forcing (and the climate) at the local level by changing the reflectivity of the land surface (or albedo). For example, because farmland is more reflective than forests (which are strong absorbers of heat), replacing forests with farmland would negatively contribute to radiative forcing or have a cooling effect. Averaged over the Earth, the net radiative forcing contribution of land use changes, while uncertain, is estimated to be -0.2 Watts per square meter (IPCC, 2007), resulting in a relatively small cooling effect.
  • Based on a limited, 25-year record, the effect of changes in the sun's intensity on radiative forcing is estimated to be relatively small, or a contribution of about +0.12 Watts per square meter, resulting in a relatively small warming effect.

How Is Radiative Forcing Determined?

For well-mixed greenhouse gases, mathematical equations are used to compute radiative forcing based on changes in their concentration relative to 1750 (or 1990 for NOAA's AGGI) and the known radiative properties of the gases. Confidence in these calculations is high due to reliable current and historic concentration data and well-established physics.

Due to limited measurements and regional variation, changes in tropospheric ozone, aerosols, land use and the sun’s intensity are much more uncertain. In the case of aerosols, uncertainty is increased due to an incomplete understanding of how aerosols interact with clouds and the effects the interactions have on aerosol radiative forcing.

For more information, see Working Group I’s contribution to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (2007), Chapter 2, “Changes in Atmospheric Constituents and Radiative Forcing,” pp. 133-134 (PDF, 8.6 MB, 106 pp

Future Climate Change

Greenhouse gas concentrations in the atmosphere will increase during the next century unless greenhouse gas emissions decrease substantially from present levels. Increased greenhouse gas concentrations are very likely to raise the Earth's average temperature, influence precipitation and some storm patterns as well as raise sea levels (IPCC, 2007). The magnitude of these changes, however, is uncertain.

The amount and speed of future climate change will ultimately depend on:

  • Whether greenhouse gases and aerosol concentrations increase, stay the same or decrease.
  • How strongly features of the climate (e.g. temperature, precipitation and sea level) respond to changes in greenhouse gas and aerosol concentrations.
  • How much the climate varies as a result of natural influences (e.g. from volcanic activity and changes in the sun ’s intensity) and its internal variability (referring to random changes in the circulation of the atmosphere and oceans).

Climate Models

Virtually all published estimates of how the climate could change in the future are produced by computer models of the Earth’s climate system. These models are known as general circulation models (GCMs). According to the IPCC (2007):

 

“[C]onfidence in models comes from their physical basis, and their skill in representing observed climate and past climate changes. Models have proven to be extremely important tools for simulating and understanding climate, and there is considerable confidence that they are able to provide credible quantitative estimates of future climate change, particularly at larger scales. Models continue to have significant limitations, such as in their representation of clouds, which lead to uncertainties in the magnitude and timing, as well as regional details, of predicted climate change. Nevertheless, over several decades of model development, they have consistently provided a robust and unambiguous picture of significant climate warming in response to increasing greenhouse gases.”

It is important to recognize that projections of climate change in specific areas are not forecasts comparable to tomorrow’s weather forecast. Rather, they are hypothetical examples of how the climate might change and usually contain a range of possibilities as opposed to one specific high likelihood outcome.

 

 

State of Knowledge

As with any field of scientific study, there are uncertainties associated with the science of climate change. This does not imply that scientists do not have confidence in many aspects of climate science. Some aspects of the science are known with virtual certainty1, because they are based on well-known physical laws and documented trends. Current understanding of many other aspects of climate change ranges from “very likely” to “uncertain.”

What's Known

Scientists know with virtual certainty that:

  • Human activities are changing the composition of Earth's atmosphere. Increasing levels of greenhouse gases like carbon dioxide (CO2) in the atmosphere since pre-industrial times are well-documented and understood.
  • The atmospheric buildup of CO2 and other greenhouse gases is largely the result of human activities such as the burning of fossil fuels.
  • An “unequivocal” warming trend of about 1.0 to 1.7°F occurred from 1906-2005. Warming occurred in both the Northern and Southern Hemispheres, and over the oceans (IPCC, 2007).
  • The major greenhouse gases emitted by human activities remain in the atmosphere for periods ranging from decades to centuries. It is therefore virtually certain that atmospheric concentrations of greenhouse gases will continue to rise over the next few decades.
  • Increasing greenhouse gas concentrations tend to warm the planet.

What's Very Likely?

The Intergovernmental Panel on Climate Change (IPCC) has stated "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations" (IPCC, 2007). In short, a growing number of scientific analyses indicate, but cannot prove, that rising levels of greenhouse gases in the atmosphere are contributing to climate change (as theory predicts). In the coming decades, scientists anticipate that as atmospheric concentrations of greenhouse gases continue to rise, average global temperatures and sea levels will continue to rise as a result and precipitation patterns will change.

What's Not Certain?

Important scientific questions remain about how much warming will occur, how fast it will occur, and how the warming will affect the rest of the climate system including precipitation patterns and storms. Answering these questions will require advances in scientific knowledge in a number of areas:

  • Improving understanding of natural climatic variations, changes in the sun's energy, land-use changes, the warming or cooling effects of pollutant aerosols, and the impacts of changing humidity and cloud cover.
  • Determining the relative contribution to climate change of human activities and natural causes.
  • Projecting future greenhouse emissions and how the climate system will respond within a narrow range.
  • Improving understanding of the potential for rapid or abrupt climate change.

References

1 Throughout the science section of this Web site, use of "virtual certainty" (or virtually certain) conveys a greater than 99% chance that a result is true. Other terms used to communicate confidence include “extremely likely” (greater than 95% chance the result is true), "very likely" (greater than 90% chance the result is true), "likely" (greater than 66% chance the result is true), “m