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 Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of current climate change, and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming.
There were prehistoric periods of global warming, but observed changes since the mid-20th century have been much greater than those seen in previous records covering decades to thousands of years.
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century the global surface temperature is likely to rise a further 0.3 to 1.7 °C (0.5 to 3.1 °F) in a moderate scenario, or as much as 2.6 to 4.8 °C (4.7 to 8.6 °F) in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects.These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing.


Effects:

The effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of Surface temperature increases are greatest in the Arctic, which has contributed to the retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead. Climate change threatens to diminish crop yields, harming food security, and rising sea levels may flood coastal infrastructure and force the abandonment of many coastal cities.Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately the environments of coral reefs, mountains, and the Arctic. Because the climate system has a large "inertia" and greenhouse gases persist in the atmosphere, climatic changes and their effects will continue for many centuries even if greenhouse gas emissions are stopped.
Response:
Mitigation of and adaptation to climate change are two complementary responses to global warming. Successful adaptation is easier if there are substantial emission reductions. Many of the countries that have contributed least to global greenhouse gas emissions are among the most vulnerable to climate change, which raises questions about justice and fairness with regard to mitigation and adaptation.

Driver of greenhouse gas emission:

Over the last three decades of the twentieth century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions. CO2 emissions are continuing to rise due to the burning of fossil fuels and land-use change. Emissions can be attributed to different regions. The attribution of emissions from land-use change is subject to considerable uncertainty.
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased
Global warming is defined in this report as an increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Unless otherwise specified, warming is expressed relative to the period 1850–1900, used as an approximation of pre-industrial temperatures in AR5..

Climate Forcings

Figure 1 shows estimated climate forcings since 1850, measured in Watts per square meter (W/m2). We separate carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and ozone (O3) in Figure 1, because they are produced by different processes and have different growth rates. We associate with CH4 its indirect effects on tropospheric O3 and stratospheric H2 to make clear the importance of CH4 as a climate forcing.

Fig. 1: Estimated climate forcings between 1850 and 2000.
Climate forcing by CO2 is the largest forcing, but it does not dwarf the others. Forcing by CH4 (0.7 W/m2) is half as large as that of CO2 and the total forcing by non-CO2 GHGs (1.4 W/m2) equals that of CO2. Moreover, in comparing forcings due to different activities, note that the fossil fuels producing most of the CO2 are also the main source of atmospheric aerosols, especially sulfates, black carbon, and organic aerosols.
Aerosols cause a climate forcing directly by reflecting sunlight and indirectly by modifying cloud properties. Forcing by atmospheric aerosols is uncertain, but research of the past decade indicates that it is substantial (IPCC 1996). The aerosol forcing that we estimate has the same magnitude (1.4 W/m2) but opposite sign of the CO2 forcing. Fossil fuel use is the main source of both CO2 and aerosols, with land conversion and biomass burning also contributing to both forcings. Although fossil fuels contribute to growth of some of the other GHGs, it follows that the net global climate forcing due to processes that produced CO2 in the past century probably is much less than 1.4 W/m2.
A corollary following from Figure 1 is that climate forcing by non-CO2 GHGs (1.4 W/m2) is nearly equal to the net value of all known forcings for the period 1850-2000 (1.6 W/m2). Thus, assuming only that our estimates are approximately correct, we assert that the processes producing the non-CO2 GHGs have been the primary drive for climate change in the past century.
Observed heat storage in the ocean provides a fundamental consistency check on the estimated climate forcing. The ocean is the only place that the energy from a planetary radiation imbalance can accumulate, because of the low thermal conductivity of land and the limit on ice melting implicit in observed sea level rise. Global ocean data (Levitus et al. 2000) reveal that ocean heat content increased 2*1023 joules between the mid-1950s and the mid-1990s. The simplest interpretation is that the change of ocean heat content, and the implied planetary energy imbalance, are a reflection of the net global climate forcing. Observed heat storage (Levitus et al. 2000) is in good agreement with results in global climate models that use the forcings of Figure 1, thus providing empirical evidence for the sign and approximate magnitude of the net climate forcing of Figure 1.

Summary

Business-as-usual scenarios, which have an additional human-made forcing of about 3 W/m2 in the next 50 years, provide a useful warning about the potential for human-made climate change. Our analysis of climate forcings suggests, as a strategy to slow global warming, an alternative scenario focused on reducing non-CO2 GHGs and black carbon (soot) aerosols. Investments in technology to improve energy efficiency and develop non-fossil energy sources are also needed to slow the growth of CO2 emissions and expand future policy options. The increase of climate forcing would be less than or about 1 W/m2.
A key feature of this strategy is its focus on air pollution, especially aerosols and tropospheric ozone, which have human health and ecological impacts. If the World Bank were to support investments in modern technology and air quality control in India and China, e.g., the reductions in tropospheric ozone and black carbon would not only improve local health and agricultural productivity, but also benefit global climate and air quality.
Non-CO2 greenhouse gases are probably the main cause of observed global warming, with CH4 causing the largest net climate forcing. There are economic incentives to reduce or capture CH4 emissions, but global implementation of appropriate practices requires international cooperation. Definition of appropriate policies requires better understanding of the CH4 cycle, especially CH4 sources.
Climate forcing by CFCs is still growing today, but, if Montreal Protocol restrictions are adhered to, there should be no net growth of the CFC forcing over the next 50 years. A small decrease of the CFC forcing from today's level is possible.
Tropospheric O3 increases in business-as-usual scenarios, which assume that CH4 increases and that there is no global effort to control O3 precursors. The human health and ecological impacts of O3 are so great that it represents an opportunity for effective international cooperation. At least it should be possible to prevent O3 forcing in 2050 from exceeding that of today.
Carbon dioxide will become the dominant climate forcing if its emissions continue to increase and aerosol effects level off. Business-as-usual scenarios understate the potential for CO2 emission reductions from improved energy efficiency and decarbonization of fuels. Based on this potential and current CO2 growth trends, we argue that limiting the CO2 forcing increase to 1 W/m2 in the next 50 years is plausible.
Indeed, CO2 emissions from fossil fuel use declined slightly in 1998 and again in 1999, while the global economy grew. However, achieving the level of emissions needed to slow climate change significantly is likely to require policies that encourage technological developments to accelerate energy efficiency and decarbonization trends.
Climate forcing due to aerosol changes is a wild card. Current trends are uncertain even in the sign of the effect. Unless climate forcings by all aerosols are precisely monitored, it will be difficult to define optimum policies.
We argue that black carbon aerosols, via several effects, contribute significantly to global warming. This suggests one antidote to global warming, if its impacts begin to increase. As electricity plays an increasing role in future energy systems, it should be relatively easy to strip black carbon emissions at fossil fuel power plants. Stripping and disposing of CO2, though more challenging, provides an effective backup strategy.


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