Global warming

Global warming is the observed increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.

Global average air temperature near Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the last century. The Intergovernmental Panel on Climate Change (IPCC) concludes, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations,"^[1] which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Other phenomena such as solar variation and volcanoes have probably had a warming effect from pre-industrial times to 1950, but a cooling effect since 1950.^[1] These conclusions have been endorsed by at least 20 scientific societies and academies of science, including all of the national academies of science of the G8 states. Some scientists disagree with parts of this conclusion as does the American Association of Petroleum Geologists.^[2]

Models referenced by the IPCC predict that global temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100.^[1] The range of values reflects the use of differing scenarios of future greenhouse gas emissions as well as uncertainties regarding climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if no further greenhouse gases are released after this date.^[1] This reflects the long average atmospheric lifetime of carbon dioxide (CO₂).

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. There may also be increases in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. Other consequences include changes in agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.

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 also 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. Most national governments have signed and ratified the Kyoto Protocol aimed at combating greenhouse gas emissions.

Terminology

The term global warming is a specific example of the broader term climate change, which can also refer to global cooling. In principle, global warming is neutral as to the period or causes, but in both common and scientific usage the term generally refers to recent warming and implies a human influence.^[3] The UNFCCC uses the term "climate change" for human-caused change, and "climate variability" for other changes.^[4] The term "anthropogenic climate change" is sometimes used when focusing on human-induced changes.

History of warming since mid-1800s

Main article: Temperature record

Global temperatures on both land and sea have increased by 0.75 °C (1.4 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade).^[5] Temperatures in the lower troposphere have increased between 0.12 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 possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

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 the second warmest year, behind 1998.^[6][7]

Anthropogenic emissions of other pollutants—notably sulphate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,^[8] though the cooling may also be due in part to natural variability.

Causes

Main articles: Attribution of recent climate change and scientific opinion on climate change

The climate system varies through natural, internal processes and in response to variations in external "forcing" factors including solar activity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. The detailed causes of the recent warming remain an active field of research, but the scientific consensus^[9][10] identifies increased levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available.

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, so that Earth would be uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the greenhouse effect as such. Rather, the debate concerns the net effect of the addition of greenhouse gases when allowing for positive or negative feedback.

Contrasting with the consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures, including: 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; and the warming is primarily a result of variances in solar radiation.

One example of an important feedback process is ice-albedo feedback.^[11] 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 this 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 about 0.5 °C (0.9 °F) would still occur.^[12]

Greenhouse gases in the atmosphere

Main article: Greenhouse effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which emission of infrared radiation by atmospheric gases warms a planet's surface. On Earth, the major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect (not including clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%.

The atmospheric concentrations of CO₂ and methane (CH₄) have increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO₂ values this high were last attained 20 million years ago.^[13] About three-quarters of the anthropogenic (man-made) emissions of CO₂ to the atmosphere during the past 20 years are due to fossil fuel burning. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation.^[14]

Future CO₂ levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO₂ scenarios,^[15] ranging from 541 to 970 parts per million by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.^{[citation needed]}

Carbon dioxide sink ecosystems (forests and oceans)^[16] are being degraded by pollutants.^[17] Degradation of major carbon sinks results in higher atmospheric CO₂ levels.

Positive feedback effects such as the expected release of methane from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions^[18] not included in IPCC's climate models.^[19]

The measure of the temperature response to increased greenhouse gas concentrations and other anthropogenic and natural climate forcings is climate sensitivity. It is found by observational and model studies.^[20] 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 of 1.5 to 4.5 °C (2.7 to 8.1 °F).

Solar variation

Main article: Solar variation

Variations in solar output, possibly amplified by cloud feedbacks, have been suggested as a possible cause of recent warming. The debate is complicated by the lack of reliable measures of solar output, even over the 30 years of satellite record; further back requires proxies such as sunspot count or cosmogenic isotopes, which are believed to (partly) correlate to solar output. In general, the IPCC describes the level of scientific understanding of the contribution of variations in solar irradiance to historical climate changes as "low."^[1]

The present level of solar activity is high in the context of the last 8,000 years.^[21] However, most records say that there has been no increase over the last 30 years.^{[citation needed]} Since 1750, solar variation is estimated to be less than one-tenth of the forcing from greenhouse gases.^[1]

Estimates of recent solar forcing vary. Modeling studies indicate that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been cooling since then.^[22] Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period.^[23] However, in 2005, researchers at Duke University have found that 10–30% of the warming over the last two decades may be due to increased solar output.^[24] Stott et al conclude in 2003 that climate models overestimate the relative effect of greenhouse gases compared to other forcings, and that solar forcing may account for 16% or 36% of the recent warming due to the greenhouse effect. They also estimate that climate sensitivity with respect to the cooling effect of volcanic dust and sulphate aerosols has been underestimated, and that the absolute value of greenhouse warming is likely to be even larger than previously assumed.^[25]

It appears likely that solar variations are too small to directly explain a significant fraction of the observed warming. Various researchers, notably Nigel Marsh and Henrik Svensmark, have proposed that feedback from clouds or other processes enhance the direct effect of solar variation.^[26] A warming of the stratosphere, which has not been observed, would be expected if there were a significant increase in solar activity.^[27]

Attributed and expected effects

Main article: Effects of global warming

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, increased intensity and frequency of extreme weather events, are being attributed in part to global warming.^[28] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming.

The World Meteorological Organization^[29] has linked increasing extreme weather events to global warming, as have Hoyos et al. (2006), writing that the increasing number of category 4 and 5 hurricanes is directly linked to increasing temperatures.^[30] Kerry Emmanuel in Nature writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming.^[31] Thomas Knutson and Robert E. Tuleya of the NOAA stated in 2004 that warming induced by greenhouse gas may lead to increasing occurrence of highly destructive category-5 storms.^[32]

Some anticipated effects include sea level rise of 110 to 770 mm (0.36 to 2.5 feet) by 2100,^[33] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18 to 35 percent of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.^[34] Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change.^[35] Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species"^[36] and McLaughlin et al. agree "few mechanistic studies have linked extinctions to recent climate change."^[35]

The extent and probability of these consequences has caused controversy, as is a matter of uncertainty. A summary of probable effects and recent understanding can be found in the report of the IPCC Working Group II;^[28] the newer AR4 summary reports, "There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones."^[1] Two British scientists supporting the mainstream scientific opinion on global warming criticize what they call the "catastrophism and the 'Hollywoodisation'" of some of the expected effects. They argue that sensationalized claims cannot be justified by science.^[37]

Mitigation

Main articles: Mitigation of global warming and adaptation to global warming

The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations and individuals to implement actions to try to curtail global warming. Some of the strategies that have been proposed for mitigation of global warming include development of new technologies; carbon offsets; renewable energy such as wind power, and solar power; nuclear power; electric or plug-in hybrid electric vehicles; non-fossil fuel cells; synthetic hydrocarbon fuel; energy conservation; carbon taxes; improving natural carbon dioxide sinks; deliberate production of sulfate aerosols, which produce a cooling effect on the Earth; population control; carbon capture and storage; nanotechnology; and environmental vegetarianism. Many environmental groups encourage individual action against global warming, often aimed at the consumer, and there has been business action on climate change.

Kyoto Protocol

Main article: Kyoto Protocol

The world's primary international agreement on combating global warming 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 CO₂ and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases. Developing countries are exempt from meeting emission standards in Kyoto. This includes China and India, the second and third largest emitters of CO₂, behind the United States. The International Energy Agency predicts China will exceed total U.S. emissions before 2010.^[38]

Climate models

Main article: Global climate model

Scientists have studied global warming with computer models of the climate. These models predict that the net effect of adding greenhouse gases will be a warmer climate in the future. However, even when the same assumptions of fossil fuel consumption and CO₂ emission are used, the amount of predicted warming varies between models and there still remains a considerable range of climate sensitivity.

Including model and future greenhouse gas uncertainty, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °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.

Climate models can produce a good match to observations of global temperature changes over the last century.^[39] 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.

Most global climate models, when run to predict future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this 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.^[40]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.^[41] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Other related issues

Ocean acidification

Main article: Ocean acidification

Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans.^[42] Carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid resulting in ocean acidification. Since biosystems are adapted to a narrow range of pH, this is a serious concern directly driven by increased atmospheric CO₂ and not global warming.

Relationship to ozone depletion

Main article: Ozone depletion

Although they are often interlinked in the mass media, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:

• The same CO₂ radiative forcing that produces near-surface global warming is expected (perhaps surprisingly) to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in ozone (O₃) depletion and the frequency of ozone holes.

• Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects: Reduced ozone causes the stratosphere to absorb less solar radiation, thus cooling the stratosphere while warming the troposphere; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere. Overall, the cooling dominates; the IPCC concludes that "observed stratospheric O₃ losses over the past two decades have caused a negative forcing of the surface-troposphere system"^[43] of about −0.15 ± 0.10 watts per square meter (W/m²).^[44]

• One of the strongest predictions of the greenhouse effect theory is that the stratosphere will cool. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory show that above 20 km (12.4 miles), the greenhouse gases dominate the cooling.^[45]

• Ozone depleting chemicals are also greenhouse gases, representing 0.34 ± 0.03 W/m², or about 14% of the total radiative forcing from well-mixed greenhouse gases.^[44]

Relationship to global dimming

Main article: Global dimming

Scientists have stated with 66-90% confidence that the effects of volcanic and human-caused aerosols have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for this effect.^[1]

Pre-human global warming

Further information: Paleoclimatology and temperature record

The earth has experienced natural global warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles with interglacial warming periods much hotter than current temperatures. The chart also shows the time of the last glacial maximum about 20,000 years ago.

It is thought by some geologists^{[attribution needed]} that a rapid buildup of greenhouse gases caused the Earth to experience global warming in the early Jurassic period, with average temperatures rising by 5 °C (9.0 °F). Research by the Open University indicates that this caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO₂ levels dropped back to normal over roughly the next 150,000 years.^[46][47]

Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other past global warming events, including the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last glacial period is thought not to be due to methane release.^[48] Instead, natural variations in the Earth's orbit (Milankovitch cycles) are believed to have triggered the retreat of ice sheets by changing the amount of solar radiation received at high latitude and led to deglaciation.

Using paleoclimate data for the last 500 million years, Veizer et al. (2000, Nature 408, pp. 698–701) concluded that long-term temperature variations are only weakly related to CO₂ variations. Most paleoclimatologists believe this is because other factors, such as continental drift and mountain building have larger effects in determining very long-term climate. Shaviv and Veizer (2003) proposed that the largest long-term influence on temperature are variations in the flux of cosmic rays received by the Earth as the Solar System moves around the galaxy.^[49] They argued that over geologic time-scales a change in CO₂ concentrations comparable to doubling pre-industrial levels results in about 0.75 °C (1.35 °F) warming, less than the 1.5–4.5 °C (2.7–8.1 °F) reported by climate models.^[50] Shaviv and Veizer (2004) acknowledge that this conclusion may only be valid on multi-million year time scales when glacial and geological feedback have had a chance to establish themselves. Rahmstorf et al. argue that Shaviv and Veizer arbitrarily tuned their data, and that their conclusions are unreliable.^[51]

See also: Snowball Earth

Pre-industrial global warming

Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.^[52] He contends that forest clearing explains the rise in CO₂ levels in the current interglacial that started 8,000 years ago, contrasting with the decline in CO₂ levels seen in the previous three interglacials. He further contends that the spread of rice irrigation explains the breakdown in the last 5,000 years of the correlation between the Northern Hemisphere solar radiation and global methane levels, which had been maintained over at least the last eleven 22,000-year cycles. Ruddiman argues that without these effects, the Earth would be nearly 2 °C (3.6 °F) cooler and "well on the way" to a new ice age. Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.^[53]

References