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. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) between the start and the end of the 20th century. The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was very likely caused by increasing concentrations of greenhouse gases resulting from human activity such as fossil fuel burning and deforestation. The IPCC also concludes that variations in natural phenomena such as solar radiation and volcanic eruptions had a small cooling effect after 1950. These basic conclusions have been endorsed by more than 40 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries.
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. Most studies focus on the period leading up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.
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. 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 changes in the frequency and intensity of 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.
Political and public debate continues regarding global warming, its causes and what actions to take in response. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geo engineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions.
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. 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. 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.
Estimates by NASA's Goddard Institute for Space Studies and the National Climatic Data Center show that 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit show 2005 as the second warmest year, behind 1998. 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.
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 discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. Existence of the greenhouse effect as such is not disputed, even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.
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 (CO2), which causes 9–26 percent; methane (CH4), which causes 4–9 percent; and ozone (O3), which causes 3–7 percent. 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 CO2, methane, tropospheric ozone, CFCs and nitrous oxide. The concentrations of CO2 and methane have increased by 36% and 148% respectively since 1750. 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 CO2 values higher than this were last seen about 20 million years ago. Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.
CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. Accordingly, the IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100 (an increase by 90-250% since 1750). Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.
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, but substantial ozone depletion did not occur until the late 1970s. Ozone in the troposphere (the lowest part of the Earth's atmosphere) does contribute to surface warming.
Aerosols and soot
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. James E. Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been driven mainly by non-CO2 greenhouse gases.
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. 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.
Soot may cool or warm, 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
Variations in solar output have been the cause of past climate changes. The consensus among climate scientists is that changes in solar forcing probably had a slight cooling effect in recent decades. This result is less certain than some others, with a few papers suggesting a 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. A recent study concluded that 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.
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.
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, atttributable 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
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 CO2 levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.
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 Hemisphere. Future climate change is expected to particularly affect some sectors and systems related to human activities. These include:
- Water resources in some dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt
- Agriculture in low latitudes
- Low-lying coastal systems
- Human health 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. Some people, such as the poor, young children, and the elderly, are particularly at risk, even in high-income areas.
The first and greatest solution is to simply require current and future man-made vertical surfaces to be painted white or light colored to reflect sunlight back into the upper atmosphere. all roofs of all buildings, all roadways, driveways, and parking areas need to be painted white or light colored. It has already been calculated that this requirement alone in all countries can easily counteract the effects of global warming in a decade or 10 years time. The
The second solution is simple passive and inexpensive foam insulation of all homes and buildings. Sprayed foam, foam panels for walls and movable foam panels for windows at night can easily and dramatically super insulate buildings so that they require 90% less energy to both heat and cool yearround. by insulating only the top half of basements with foam panel insulation you retain the 55degree goethermal ground effect heating in winter and the 55 degree geothermal cooling ground effect in summer. (Add to this the use of only CFS or LED lighting and you can quickly reduce electric energy use by 50% or more.
The third solution to global warming is passive solar panels and solar cooking. if people would use simple passive solar panels which they can build themselves for heating in winter then 30 to 50% of all home and commercial heating can be saved for the entire lifetime of each building. Passive solar panels can also be used to heat most all hot water which is a major energy user in homes. Most coal fired energy plants which are the main CO2 emitters could be retired in a few years time as energy usage and demand in dramatically reduced by these simple conservation measures and the use of these newer technologies of CFS and LED usage.
It is not the very expensive massive government projects that need to be funded to stop and reverse global warming in the next 100 years. It is simply changing our ways to conserve energy with simple easy to apply off the shelf technologies we now have today. Simply enclosing every refrigerator today in the world with an extra 1 inch of foam panel insulation which takes about an hour and cost about $10 each would save enough electrical energy to shut down 25% of all coal fired electrical plants in the world today. and simply switching all lightbulbs in the world today to CFS or LEDs would shut down another 35% or more of all coal fired power plants. Most all of these above energy solutions will pay the consumer back in real immediate savings in a few months time and continue to save big money after this for a lifetime. Each individual must do their part and simply by-pass governments if they are too slow to act and assist in saving the planet from massive disruptions from future inaction.