As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C (2.5 °F–10.4 °F) between 1990 and 2100. They have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors. In addition to having their own characteristic climate sensitivity, models have also been used to derive independent assessments of climate sensitivity.

Climate models can produce a good match to observations of global temperature changes over the last century. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the carbon cycle to the models generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO₂). Some observational studies also show a positive feedback.

Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small-scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considered climate models “to be suitable tools to provide useful projections of future climates”.

In December, 2005 Bellouin et al. suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected, and that therefore some global warming was being masked. If supported by further studies, this would imply that existing models under-predict future global warming.

There are five categories of actions that can be taken to mitigate global warming:

1. Reduction of energy use (conservation)
2. Shifting from carbon-based fossil fuels to alternative energy sources
3. Carbon capture and storage
4. Carbon sequestration
5. Planetary engineering to cool the earth, including screening out sunlight and increasing the reflectivity of the earth.

Strategies for mitigation of global warming include development of new technologies; carbon offsets; renewable energy such as biodiesel, wind power, and solar power; nuclear power; electric or hybrid automobiles; fuel cells; energy conservation; carbon taxes; enhancing natural carbon dioxide sinks; increased use of sulfate aerosols, which exhibit a cooling effect on the Earth; population control; and carbon capture and storage. Many environmental groups encourage individual action against global warming, often aimed at the consumer, and there has been business action on climate change.

The world’s primary international agreement on combating climate change is the Kyoto Protocol. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases.

Although the governments of 163 countries ratified the Kyoto Protocol, (notably excluding the United States and Australia), there is a growing debate about how effective the Kyoto protocol has been. Some politicians, including President of the United States George W. Bush and Prime Minister of Australia John Howard have argued that the cost of mitigating global warming via the Kyoto protocol is too large to be practical. This view may be proving correct, as the signatories of the Kyoto protocol, including Europe and Japan, are currently struggling to meet their targets.[14] After only five years, Canada has given up entirely. Also, of the 163 countries that have signed and ratified Kyoto, only 31 are actually required to lower greenhouse emissions. Notable among those countries that have signed and ratified Kyoto but are not required to reduce greenhouse gas emissions are China and India with their huge populations and rapidly growing economies.

Some segments of the business community have accepted global warming and its attribution to anthropogenic causes as valid, as well as a need for actions such as carbon emissions trading and carbon taxes.

Adaptation strategies accept some warming as a foregone conclusion and focus on preventing or reducing undesirable consequences. Examples of such strategies include defense against rising sea levels or ensuring food security.

The extent and likelihood of these consequences is a matter of considerable political controversy; and in the details, a matter of some scientific uncertainty. A summary of possible effects and our current understanding can be found in the report of the IPCC Working Group II; a discussion of projected climate changes is found in WG I.

Proposed responses to the effects of global warming fall into two categories: mitigation and adaptation.
Projected climate changes due to global warming have the potential to lead to future large-scale and possibly irreversible changes in our climate resulting in impacts at continental and global scales.

Examples of projected climate changes include:

• significant slowing of the ocean circulation that transports warm water to the North Atlantic,
• large reductions in the Greenland and West Antarctic Ice Sheets,
• accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and
• releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments.

The likelihood of many of these changes is uncertain. However, the probability of one or more of these changes occurring is likely to increase with the rate, magnitude, and duration of climate change.

Scientists are unable to accurately predict when various effects of global warming will occur or what the magnitude of the effect will be.

It is not possible to be certain whether there will be any positive benefits of Global Warming. What is known is that some significant negative impacts are projected and these drive most of the concern about global warming and motivates attempts to mitigate or adapt to the effects of global warming.

Most of the consequences of global warming would result from one of three physical changes: sea level rise, higher local temperatures, and changes in rainfall patterns (Figure 1). Sea level is generally expected to rise 50-200 cm in the next century (Dean et al. 1987); such a rise would inundate 7,000 square miles of dry land in the United States (an area the size of Massachusetts) and a similar amount of coastal wetlands; erode recreational beaches 100-200 meters, exacerbate coastal flooding; and increase the salinity of aquifers and estuaries (Titus 1989).

Contrasting with the consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures. Some of these hypotheses (listed here without comment on their validity or lack thereof) include:

• The warming is within the range of natural variation.
• The warming is a consequence of coming out of a prior cool period, namely the Little Ice Age.
• The warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover. It is similar in concept to the operating principles of the Wilson cloud chamber, but on a global scale where earth’s atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of cloud condensation nuclei.
• The observed warming actually reflects the Urban Heat Island, as most readings are done in heavily populated areas which are expanding with growing population.

Adding carbon dioxide (CO₂) or methane (CH₄) to Earth’s atmosphere, with no other changes, will make the planet’s surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who “believe in” and “oppose” the theory that adding carbon dioxide or methane to the Earth’s atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors.

One example of an important feedback process is ice-albedo feedback. The increased CO₂ in the atmosphere warms the Earth’s surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.

Due to the thermal inertia of the earth’s oceans and slow responses of other indirect effects, the Earth’s current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.

Based on estimates by NASA’s Goddard Institute for Space Studies, 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 UK Climatic Research Unit concluded that 2005 was still only the second warmest year, behind 1998.

Depending on the time frame, a number of temperature records are available based on different data sets. The longest perspective is available from various proxy records for recent millennia; see temperature record of the past 1000 years for a discussion of these records and their differences. An approximately global instrumental record of temperature near the earth’s surface begins in about 1860. Global observations of the atmosphere well above the earth’s surface using data from radiosondes began shortly after World War II. Satellite temperature measurements of the tropospheric temperature date from 1979. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available.

Global average near-surface atmospheric temperature rose 0.6 ± 0.2 °Celsius (1.1 ± 0.4 °Fahrenheit) in the 20th century. The prevailing scientific opinion on climate change is that “most of the warming observed over the last 50 years is attributable to human activities.”^[1]

The main cause of the human-induced component of warming is the increased atmospheric concentration of greenhouse gases (GHGs) such as carbon dioxide (CO₂). This leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Greenhouse gases are released by activities such as the burning of fossil fuels, land clearing, and agriculture.

The measure of the response to increased GHGs, and other anthropogenic and natural climate forcings is climate sensitivity. It is found by observational and model studies.^[2] This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO₂ in the atmosphere. The current literature estimates sensitivity in the range 1.5-4.5 °C (2.7-8.1 °F). Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures may increase by between 1.4 and 5.8 °C (2.5 to 10.5 °F) between 1990 and 2100. The uncertainty in this range results from both the difficulty of predicting the volume of future greenhouse gas emissions and uncertainty about climate sensitivity.

An increase in global temperatures can in turn cause other changes, including a rising sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors. Warming is expected to affect the number and magnitude of these events; however, it is difficult to connect particular events to global warming. Although most studies focus on the period up to 2100, warming (and sea level rise due to thermal expansion) is expected to continue past then, since CO₂ has a long average atmospheric lifetime.

Remaining scientific uncertainties include the exact degree of climate change expected in the future, and especially how changes will vary from region to region across the globe. A hotly contested political and public debate has yet to be resolved, regarding whether anything should be done, and what could be cost-effectively done to reduce or reverse future warming, or to deal with the expected consequences.