Global warming

Annual average global warming by the year 2060 simulated and plotted as color differences using EdGCM

Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. There is strong evidence that significant global warming is occurring; this evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades is attributable to human activities, particularly to the burning of fossil fuels and deforestation.

Global average air temperature near the Earth's surface rose by 0.74 ± 0.18 °C (1.33 ± 0.32 °F) from 1906 to 2005. The prevailing scientific view, as represented by the science academies of the major industrialized nations and the Intergovernmental Panel on Climate Change (IPCC), it is very likely that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas concentrations produced by human activity. Climate models predict that average global surface temperatures will increase by a further 1.1 to 6.4 °C (2.0 to 11.5 °F) by the end of the century, relative to 1980–1999. The range of values reflects the use of differing assumptions of future greenhouse gas emissions and results of models that differ in their sensitivity to increases in greenhouse gases.

Scientists have not yet quantitatively assessed the potential self-accelerating effects of global-warming itself, either on threshold or rate. Melting of permafrost, for example, causes increased production and atmospheric release of such newly produced as well as anciently stored methane gas, which “….packs a far greater warming punch than [carbon dioxide] (CO₂),” possibly as much as 25 times that of CO₂ per unit mass.

An increase in global temperatures will cause the sea level to rise, glaciers to retreat, sea ice to melt, and changes in the amount, geographical distribution and seasonal pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events. These changes to the climate will have many practical consequences, including changes in agricultural yields and impacts on human health.

Scientific uncertainties include the extent of climate change expected in the future, and how changes will vary around the globe. There is political and public debate about what, if any, action should be taken to reduce future warming or to adapt to its consequences. The Kyoto Protocol, an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.

Terminology

The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes used when focusing on human-induced changes.

Causes

There is no serious dispute that there has been a large and continuing increase in atmospheric CO₂ concentrations since the middle of the twentieth century; there is no dispute that there has been a parallel increase in fossil fuel use over this time, and there is no dispute that there has been an increase in mean global temperature over the same period. Although some skeptics nevertheless doubt that the rise in CO₂ is a consequence of fuel emissions, all national science academies that have issued statements on the matter accept the IPCC's conclusion that future man-made climate change is likely.

The climate system varies both through internal processes and in response to external forcing. External forcing includes solar activity, volcanic emissions, variations in Earth's orbit , and atmospheric composition. The scientific consensus is that most of the warming observed since the mid-twentieth century is very likely due to increased atmospheric concentrations of greenhouse gases produced by human activity. Some other hypotheses have been offered to explain most of the observed increase in global temperatures but these are not broadly supported in the scientific community. Among these are that the warming is caused by natural fluctuations in the climate, that warming is mainly a result of variations in solar radiation, or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.

The effects of forcing are not instantaneous. Due to the thermal inertia of the oceans and the slow responses of some feedback processes, Earth's climate is never in perfect equilibrium with the imposed forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at present day levels there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.

Greenhouse gases

Existence of the greenhouse effect is not disputed; it is the process by which emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the Earth by about 33 °C (59 °F); without this, the average temperature of the Earth would be about -18 °C (0 °F) making the planet uninhabitable. The major natural greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO₂), which causes 9–26%; methane (CH₄), which causes 4–9%; and ozone, which causes 3–7%.

The present atmospheric concentration of CO₂ is about 383 parts per million (ppm) by volume. From geological evidence, it is believed that CO₂ values this high were last attained 20 million years ago. About three-quarters of man-made CO₂ emissions over the past 20 years have come from the burning of fossil fuels; most of the rest is due to land-use change, mainly deforestation. Measured trends in atmospheric composition and isotope ratios (namely the simultaneous depletion of ¹³C, ¹⁴C, and O₂) confirm that the increased atmospheric CO₂ mainly comes from fossil fuels and not from other sources such as volcanos or the oceans.

Future CO₂ concentrations depend on uncertain economic, sociological, technological, and natural developments. 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. Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or methane clathrates are used extensively. Positive feedback effects such as the 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 not included in climate models cited by the IPCC.

The warming due to atmospheric CO₂ from burning fossil fuels releases methane from the Arctic surface in at least three ways: (1) by forming lakes of melted ice whose waters melts the underlying permafrost, allowing methane-producing microbes to increase methane production by exposing thawing vegetative and animal matter for them to consume; (2) by opening channels in the attenuated permafrost cap for release into the atmosphere of old, trapped methane hydrates; (3) by thawing offshore layers of permafrost capping methane hydrates.

Feedbacks

The effects of forcing agents on the climate are modified by feedback processes, one of the most important of which is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the Earth's atmosphere and surface, which increases the evaporation of water into the atmosphere. As water vapor is itself a greenhouse gas, this causes further warming, causing yet more water vapor to be evaporated, and so on. Eventually a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO₂ alone.

The radiative effects of clouds are a major source of uncertainty in climate projections. Seen from below, clouds emit infrared radiation to the surface, and so have a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so have a cooling effect. The cloud feedback effect is influenced not only by the amount of clouds but also by their distribution; high clouds are colder than low clouds, and thus radiate less energy to space. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by relative humidity rather than the absolute concentration of water vapor. Cloud feedback is second only to water vapor feedback and has a net warming effect in all the models that contributed to the IPCC Fourth Assessment Report.

Another important process is ice-albedo feedback. Warming of the Earth's surface 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.

The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.

Solar variation

It has been hypothesized that variations in solar output, possibly amplified by cloud feedbacks, may have been a secondary contributor to recent warming. Natural phenomena, such as solar variation and volcanoes, probably had a net warming effect from pre-industrial times to 1950 and a small cooling effect since 1950. Some research indicate that the Sun's contribution may have been underestimated. These results suggest that the Sun may have contributed about 40–50% of the global surface warming between 1900 and 2000 and about 25–35% of the warming between 1980 and 2000. Some authors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing, and that the cooling effects of volcanic dust and sulfate aerosols have been underestimated. Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.

Climate change since the Industrial Revolution

According to the instrumental temperature record, mean global temperatures (both land and sea) have increased by 0.75 °C (1.35 °F) since the period 1860–1900. This measured temperature increase is not significantly affected by the urban heat island effect. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). 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 Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Global temperatures in 1998 were exceptionally warm because the strongest El Niño in the instrumental record occurred in that year.

Anthropogenic emissions of other pollutants—notably sulfate 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, though the cooling may also be due in part to natural variability.

Climate models

Scientists have created computer models of the climate, based on physical principles of fluid dynamics, radiative transfer, and other processes. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, the amount of projected warming varies between models and there is a considerable range of climate sensitivity. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects global surface temperatures averaged over 2090-2099 are likely to be 1.1 to 6.4 °C (2.0 to 11.5 °F) hotter than the average temperatures from 1980-1999.

Models have also been 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. Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, but they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Global climate model projections of future climate are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may also include 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.

The representation of clouds is one of the main sources of uncertainty in present models. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

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, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.

Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure. A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records before routine satellite observations. The summary also states that there is no clear trend in the annual number of tropical cyclones.

Other anticipated effects include a sea level rise of 110-770 mm (0.36 - 2.5 ft) between 1990 and 2100, 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% of a sample of 1,103 animal and plant species would be extinct by 2050, based on climate projections. McLaughlin et al. have documented two populations of Bay checkerspot butterfly being threatened by precipitation change.

Mitigation and adaptation

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 or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and (still limited) moves to alternative fuels. One innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The main international agreement on combating global warming is the Kyoto Protocol, an amendment to the UNFCCC, negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions. The USA and Kazakhstan have not ratified the treaty. China and India, two other large emitters, have ratified the treaty but, as developing countries, are exempt from its provisions. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.

The world's primary body for crafting a response is the Intergovernmental Panel on Climate Change (IPCC), a UN-sponsored activity which holds periodic meetings between national delegations on the problems of global warming, and issues working papers and assessments on the current status of the science of climate change, impacts, and mitigation. It convenes four different working groups examining various specific issues.

Related climatic issues

Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans. CO₂ dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO₂. As organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO₂, that could disrupt food webs and impact human societies that depend on marine ecosystem services.

A factor that may have mitigated global warming in the late twentieth century is global dimming - a gradual reduction in the amount of global direct irradiance at the Earth's surface. From 1960 to 1990, human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.

Skepticism

A few skeptical scientists believe that the present climate change, if it exists at all, is not man-made and is unavoidable. Most accept that there has been a large increase of CO₂ in the atmosphere due to the use of fossil fuels, but doubt that there is any immediate danger and dispute any need for large reductions of CO₂ emissions. Their skepticism is based on the complex problems associated with the underlying science and the uncertainty of available climate data.

The Earth's climate is an extremely complex system: the atmosphere, oceans, and land masses are tightly coupled subsystems and consequently the energy and mass exchanges between them must be studied simultaneously. Further, the electromagnetic radiation balance between energy absorption and back radiation by the Earth plays a crucial role, which means that the Earth's climate system is not a isolated system. There is no encompassing theory that predicts the characteristics of the climate which is accepted by all climatologists; theories come from many areas of physics: turbulent and dissipative systems, convective and radiative transport phenomena, non-linear (chaotic) systems and their inherent sensitivity to initial conditions, and so on. Because of this complexity, and, it is argued, the lack of enough reliable data to evaluate existing climate models, some scientists question their predictive validity.

Another problem is the use of proxy data (indirect data such as tree rings and the isotopic content of arctic and antarctic ice). Proxy data are used to construct historical temperature profiles, yielding, for instance, the hockey stick shaped graph.

There is also concern about the lack of transparency of analyses purporting global warming and inaccessibility to data to allow independent analyses. Specifically, skeptics have requested access to data from the University of East Anglia's Climatic Research Unit (CRU) and Penn State Department of Meteorology. The CRU reports that 95% of their data is available for the public

When an FPS (Forum on Physics & Society) editor of the American Physical Society wrote: "There is a considerable presence within the scientific community of people who do not agree with the IPCC conclusion that anthropogenic CO₂ emissions are very probably likely to be primarily responsible for the global warming that has occurred since the Industrial Revolution", the FPS Executive Committee hastened to declare that his statement does not represent their views. Nonetheless, in 2007 a U.S. Senate committee identified more than 400 prominent scientists from more than two dozen countries as objecting to aspects of the "consensus" on man-made global warming. Some of those scientists were participants in the IPCC..

A 2010 assessment of the IPCC report by the Netherlands Environmental Assessment Agency found no significant errors in the report, but its authors reported that they had been unable to find the provenance of some of its conclusions, that the report had not taken account of the positive effects of climate change and that there was a tendency to highlight the upper ends of uncertainty ranges..

Open letter to the UN Secretary-General

During the United Nations Climate Conference on Bali in 2007, more than 100 scientists wrote an open letter to Ban Ki-Moon, the Secretary-General of the United Nations expressing their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction". They recognized that climate change is occurring, but state that it is a natural phenomenon which is impossible to stop and express doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions." The letter criticised the IPCC Assessment Reports of 2001 and 2007, claiming that they "are prepared by a relatively small core writing team with the final drafts approved line-by-line by ­government representatives". It stated that "the great ­majority of IPCC contributors and ­reviewers, and the tens of thousands of other scientists who are qualified to comment on these matters, are not involved in the preparation of these documents. The summaries therefore cannot properly be represented as a consensus view among experts".

Climate sensitivity

The concentration of CO₂ in the Earth's atmosphere at the beginning of the Industrial Age (1750) was about 278 ppm; in 2008, it was about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked how much the temperature on Earth would change by an increase of CO₂ in the atmosphere. To answer that, the IPCC adopted this definition:

Climate sensitivity is the equilibrium temperature change, $\scriptstyle \Delta T_{\lambda }$, in the surface temperature, TS, caused by the doubling of the pre-industrial CO₂ concentration.

Put more simply, the IPCC defined climate sensitivity as the temperature change, in the Earth's surface temperature, $\scriptstyle \Delta T_{\lambda }$, that would be caused by doubling the pre-1750 atmospheric CO₂ concentration of 278 ppm to 556 ppm which is currently expected to occur later in this century.

The IPCC estimated the climate sensitivity to be 3.26 °C. In other words, when the atmospheric concentration of CO₂ reaches 556 ppm (expected later this century), the IPCC predicts that the Earth's surface temperature will be 3.26 °C higher than it was more than 250 years ago (1750).