1880-2010 global annual
mean surface air temperature change relative to the 1951–1980 average. The red line is the 5-year running mean (temperature averaged over 5 years). Source:
NASA GISS Comparison of surface based (blue) and satellite based (red:
UAH; green:
RSS) records of global temperature change since 1979. Linear trends plotted since 1982.
2000-2009 global mean land-ocean surface temperature anomalies relative to the 1951-1980 average. Source:
NASA Earth Observatory Global warming is the increase in the
average temperature of
Earth's near-surface air and oceans since the mid-
20th century and its projected continuation. According to the 2007
Fourth Assessment Report by the
Intergovernmental Panel on Climate Change (IPCC), global surface temperature increased by 0.74 ± 0.18 °
C (1.33 ± 0.32 °
F) during the 20th century.
Most of the observed temperature increase since the middle of the 20th century has been
caused by increasing concentrations of
greenhouse gases, which result from
human activities such as the burning of
fossil fuel and
deforestation.
Global dimming, a phenomenon of increasing atmospheric concentrations of man-made
aerosols, which affect cloud properties and block sunlight from reaching the surface, has partially countered the effects of warming induced by greenhouse gases.
Climate model projections summarized in the latest IPCC report indicate that the global surface
temperature is likely to rise a further
1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.
The uncertainty in this estimate arises from the use of models with differing
sensitivity to greenhouse gas concentrations and the use of differing
estimates of future greenhouse gas emissions. An increase in global temperature will cause
sea levels to rise and will change the amount and pattern of
precipitation, probably including expansion of
subtropical deserts.
[5] Warming is expected to be
strongest in the Arctic and would be associated with continuing
retreat of glaciers,
permafrost and
sea ice. Other likely effects include more frequent and intense
extreme weather events,
species extinctions, and changes in
agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain.
As a result of contemporary increases in atmospheric carbon dioxide, the oceans have become
more acidic, a result that is predicted to continue.
The
scientific consensus is that anthropogenic global warming is occurring.
Nevertheless, skepticism amongst the wider public remains. The
Kyoto Protocol is aimed at stabilizing greenhouse gas concentration to prevent a "dangerous anthropogenic interference". As of November 2009,
187 states had signed and ratified the protocol.
Proposed responses to climate change include
mitigation to reduce emissions,
adaptation to the effects of global warming, and
geoengineering to remove greenhouse gases from the atmosphere or block incoming sunlight.
Temperature changes
Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale, with the instrumemtal temperature record overlaid in black.
Evidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.
[14][15][16][17] The most common measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a
linear trend, this temperature rose by 0.74 ± 0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13 ± 0.03 °C per decade, versus 0.07 °C ± 0.02 °C per decade). The
urban heat island effect is estimated to account for about 0.002 °C of warming per decade since 1900.
[18] Temperatures in the lower
troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to
satellite temperature measurements. Temperature is believed to have been relatively stable over the
one or two thousand years before 1850, with regionally varying fluctuations such as the
Medieval Warm Period and the
Little Ice Age.
[19]
Estimates by
NASA's
Goddard Institute for Space Studies (GISS) and the
National Climatic Data Center show that 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding the previous record set in 1998 by a few hundredths of a degree.
[20][21] Estimates prepared by the
World Meteorological Organization and the
Climatic Research Unit show 2005 as the second warmest year, behind 1998.
[22][23] Temperatures in 1998 were unusually warm because the strongest
El Niño in the past century occurred during that year.
Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade)
Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.
The
Northern Hemisphere warms faster than the
Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to
ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.
The
thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing.
Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about
0.5 °C (0.9 °F) would still occur.
External forcings
External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as
radiative forcing due to changes in atmospheric composition (mainly
greenhouse gas concentrations), changes in
solar luminosity,
volcanic eruptions, and
variations in Earth's orbit around the Sun.
[31] Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.
Greenhouse gases
Greenhouse effect schematic showing energy flows between space, the atmosphere, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m2).
Recent atmospheric
carbon dioxide (CO
2) increases. Monthly CO
2 measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the
Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO
2.
The
greenhouse effect is the process by which
absorption and
emission of
infrared radiation by gases in the
atmosphere warm a
planet's lower atmosphere and surface. It was proposed by
Joseph Fourier in 1824 and was first investigated quantitatively by
Svante Arrhenius in 1896
Naturally occurring
greenhouse gases have a mean warming effect of about 33 °C (59 °F).
The major greenhouse gases are
water vapor, which causes about 36–70 percent of the greenhouse effect;
carbon dioxide (CO
2), which causes 9–26 percent;
methane (CH
4), which causes 4–9 percent; and
ozone (O
3), which causes 3–7 percent.
[34][35][36] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have
different effects on radiation from water vapor.
Human activity since the
Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased
radiative forcing from CO
2,
methane, tropospheric
ozone,
CFCs and
nitrous oxide. The
concentrations of CO
2 and methane have increased by 36% and 148% respectively since 1750.
[37] These levels are much higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from
ice cores.
Less direct geological evidence indicates that CO
2 values higher than this were last seen about 20 million years ago.
Fossil fuel burning has produced about three-quarters of the increase in CO
2 from human activity over the past 20 years. Most of the rest is due to land-use change, particularly
deforestation.
Over the last three decades of the 20th century,
GDP per capita and
population growth were the main drivers of increases in greenhouse gas emissions. CO
2 emissions are continuing to rise due to the burning of fossil fuels and land-use change.
:71 Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic,
sociological,
technological, and natural developments. In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced.
These emission scenarios, combined with carbon cycle modelling, have been used to produce estimates of how atmospheric concentrations of greenhouse gases will change in the future. Using the six IPCC
SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO
2 could range between 541 and 970 ppm.
[49] This is an increase of 90-250% above the concentration in the year 1750. Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if
coal,
oil sands or
methane clathrates are extensively exploited.
[50]
The destruction of
stratospheric ozone by
chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few
areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence.
[51] Substantial ozone depletion did not occur until the late 1970s.
[52] Ozone in the troposphere (the lowest part of the
Earth's atmosphere) does contribute to surface warming.
[53]
Aerosols and soot
Ship tracks over the
Atlantic Ocean on the east coast of the United States. The climatic impacts from aerosol forcing could have a large effect on climate through the indirect effect.
Global dimming, a gradual reduction in the amount of global direct
irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present
The main cause of this dimming is aerosols produced by volcanoes and
pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion—CO
2 and aerosols—have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO
2 greenhouse gases such as
methane.
Radiative forcing due to aerosols is temporally limited due to
wet deposition which causes aerosols to have an
atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosol concentrations will only delay climate changes due to carbon dioxide.
In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget. Sulfate aerosols act as
cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds
reflect solar radiation more efficiently than clouds with fewer and larger droplets.
[58] This effect also causes droplets to be of more uniform size, which reduces
growth of raindrops and makes the cloud more reflective to incoming sunlight.Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Aerosols, particularly their indirect effects, represent the largest uncertainty in radiative forcing
Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric
soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by
atmospheric brown clouds.When deposited, especially on glaciers or on ice in arctic regions, the lower surface
albedo can also directly heat the surface.
The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.
Solar variation
Solar variation over thirty years.
Variations in solar output have been the cause of past
climate changes.
The effect of changes in solar forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effectwhile others studies suggest a slight warming effect.
Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the
troposphere, an increase in solar activity should warm the
stratosphere while an increase in greenhouse gases should cool the stratosphere. Observations show that temperatures in the stratosphere have been cooling since 1979, when satellite measurements became available.
Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.
A related hypothesis, proposed by
Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.
Other research has found no relation between warming in recent decades and
cosmic rays.
The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.
Feedback
Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first.
Positive feedback increases the change in the first quantity while
negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process. The main positive feedback in global warming is the tendency of warming to increase the amount of
water vapor in the atmosphere, a significant
greenhouse gas. The main negative feedback is
radiative cooling, which
increases as the fourth power of temperature; the amount of heat radiated from the Earth into space increases with the temperature of Earth's surface and atmosphere. Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming. A wide range of potential feedback process exist, such as
Arctic methane release and
ice-albedo feedback. Consequentially, potential
tipping points may exist, which may have the potential to cause
abrupt climate change.
Climate models
The geographic distribution of surface warming during the 21st century calculated by the
HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
The main tools for projecting future climate changes are
mathematical models based on physical principles including
fluid dynamics,
thermodynamics and
radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact
combinations of models for different parts of the Earth. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature,
salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes.
Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes. Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (
climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.
Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC
Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the
carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.
[78][79][80] Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of
1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.
[2]
Models are also used to help investigate the
causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.
The physical realism of models is tested by examining their ability to simulate current or past climates.Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.
Not all
effects of global warming are accurately predicted by the
climate models used by the
IPCC. Observed
Arctic shrinkage has been faster than that predicted.
Precipitation increased proportional to atmospheric humidity, and hence significantly faster than current global climate models predict.
Attributed and expected effects
Global warming may be detected in
natural,
ecological or
social systems as a change having statistical significance.
Attribution of these changes e.g., to natural or human activities, is the next step following detection
Natural systems
Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the
WGMS and the
NSIDC.
Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on
temperature changes.
Rising sea levels and observed decreases in snow and ice extent are consistent with warming.
Most of the increase in global average temperature since the mid-20th century is, with high probability,attributable to human-induced changes in greenhouse gas concentrations
Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades Over the course of the 21st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20th century.
In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.
Changes in regional climate are expected to include greater warming over land, with most warming at high northern
latitudes, and least warming over the
Southern Ocean and parts of the North
Atlantic Ocean.
Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.
Ecological systems
In terrestrial
ecosystems, the earlier timing of
spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. Future climate change is expected to particularly affect certain ecosystems, including
tundra,
mangroves, and
coral reefs.
It is expected that most ecosystems will be affected by higher atmospheric CO
2 levels, combined with higher global temperatures.
[90] Overall, it is expected that climate change will result in the
extinction of many species and reduced diversity of ecosystems.
Social systems
There is some evidence of regional climate change affecting systems related to human activities, including agricultural and
forestry management activities at higher latitudes in the Northern HemisphereFuture climate change is expected to particularly affect some sectors and systems related to human activitiesLow-lying
coastal systems are vulnerable to sea level rise and storm surge. Human
health will be at increased risk in populations with limited capacity to adapt to climate change. It is expected that some regions will be particularly affected by climate change, including the
Arctic,
Africa, small
islands, and
Asian and African
megadeltas. In some areas the effects on agriculture, industry and health could be mixed, or even beneficial in certain respects, but overall it is expected that these benefits will be outweighed by negative effects.
Responses to global warming
Mitigation
Reducing the amount of future climate change is called
mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of
carbon sinks to absorb GHGs from the atmosphere. Many countries, both
developing and
developed, are aiming to use cleaner, less polluting, technologies.
192 Use of these technologies aids mitigation and could result in substantial reductions in CO
2 emissions. Policies include targets for emissions reductions, increased use of
renewable energy, and increased
energy efficiency. Studies indicate substantial potential for future reductions in emissionsSince even in the most optimistic scenario,
fossil fuels are going to be used for years to come, mitigation may also involve
carbon capture and storage, a process that traps CO
2 produced by factories and
gas or
coal power stations and then stores it, usually underground.
Adaptation
Other policy responses include
adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention.The
ability to adapt is closely linked to
social and
economic development.Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.
Geoengineering
Another policy response is engineering of the climate (
geoengineering). This policy response is sometimes grouped together with mitigation.
] Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published.
Geoengineering encompasses a range of techniques to
remove CO2 from the atmosphere or to
block incoming sunlight. As most geoengineering techniques would affect the entire globe, the use of effective techniques, if they can be developed, would require global public acceptance and an adequate global legal and regulatory framework.
UNFCCC
Most countries are Parties to the
United Nations Framework Convention on Climate Change (UNFCCC).
The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system. As is stated in the Convention, this requires that GHGs are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change,
food production is not threatened, and economic development can proceed in a sustainable fashion.
The UNFCCC recognizes differences among countries in their responsibility to act on climate change In the
Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the treaty) took on legally binding commitments to reduce their emissions
] Policy measures taken in response to these commitments have reduced emissions. For many developing (non-Annex I) countries, reducing
poverty is their overriding aim.
At the
15th UNFCCC Conference of the Parties, held in 2009 at
Copenhagen, several UNFCCC Parties produced the
Copenhagen Accord.
[106] Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2 °C The
16th Conference of the Parties (COP16) was held at
Cancún in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet the 2 °C goal. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.
Views on global warming
Total greenhouse gas emissions in 2000, including land-use change.
There are different views over what the appropriate policy response to climate change should be.These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.
Politics
Developed and
developing countries have made different arguments over who should bear the burden of
economic costs for cutting emissions. Developing countries often concentrate on
per capita emissions, that is, the total emissions of a country divided by its population. Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries
This is used to make the argument that the real problem of climate change is due to the profligate and unsustainable lifestyles of those living in rich countries
On the other hand, commentators from developed countries point out that
total carbon emissions,
carrying capacity,
efficient energy use and
civil and political rights are very important issues.
World population is the number of humans per unit area. However the land is not the same everywhere. Not only the quantity of fossil fuel use but also the quality of energy use is a key debate point. For example,
efficient energy use supporting
technological change might help reduce excess
carbon dioxide in Earth's atmosphere. The use of fossil fuels for
conspicuous consumption and excessive
entertainment are issues that can conflict with
civil and political rights. People in developed countries argue that history has proven the difficulty of implementing fair
rationing programs in different
countries because there is no global system of
checks and balances or
civil liberties.
The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries.
Developing countries are not subject to limitations. This exemption led the U.S. and Australia to decide not to ratify the treaty, although Australia did finally ratify the treaty in December 2007Debate continued at the
Copenhagen climate summit and the
Cancún climate summit.
Public opinion
In 2007–2008
Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in
developed, and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite beliefIn the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of
Cardiff University said that "results show the different stages of engagement about global warming on each side of the Atlantic", adding, "The debate in Europe is about what action needs to be taken, while many in the U.S. still debate whether climate change is happening."
A 2010 poll by the
Office of National Statistics found that 75% of UK respondents were at least "fairly convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006.
A January 2011
ICM poll in the UK found 83% of respondents viewed climate change as a current or imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll asking the same question, though there had been a slight polarisation of opposing views.
A survey in October, 2009 by the
Pew Research Center for the People & the Press showed decreasing public perception in the United States that global warming was a serious problem. All political persuasions showed reduced concern with lowest concern among Republicans, only 35% of whom considered there to be solid evidence of global warming.
The cause of this marked difference in public opinion between the United States and the global public is uncertain but the hypothesis has been advanced that clearer communication by scientists both directly and through the media would be helpful in adequately informing the American public of the scientific consensus and the basis for it.
Other views
Most scientists accept that humans are contributing to observed climate change.
National science academies have called on world leaders for policies to cut global emissions. However, some scientists and non-scientists question aspects of climate-change science.
Organizations such as the libertarian
Competitive Enterprise Institute, conservative commentators, and some companies such as
ExxonMobil have challenged IPCC climate change scenarios, funded scientists who disagree with the
scientific consensus, and provided their own projections of the economic cost of stricter controls.
[129][130][131][132] In the finance industry,
Deutsche Bank has set up an institutional climate change investment division (DBCCA)which has commissioned and published research
on the issues and debate surrounding global warming.Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductionsSome fossil fuel companies have scaled back their efforts in recent years,
or called for policies to reduce global warming.
Etymology
The term
global warming was probably first used in its modern sense on 8 August 1975 in a science paper by
Wally Broecker in the journal
Science called "Are we on the brink of a pronounced global warming?".
Broecker's choice of words was new and represented a significant recognition that the climate was warming; previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going The National Academy of Sciences first used
global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible." The report made a distinction between referring to surface temperature changes as
global warming, while referring to other changes caused by increased CO
2 as
climate change.
Global warming became more widely popular after 1988 when NASA scientist
James Hansen used the term in a testimony to Congress.
He said: "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming."
His testimony was widely reported and afterward
global warming was commonly used by the press and in public discourse.
