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.

Global average air temperature near the Earth's surface rose 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), is that most of the temperature increase since the mid-20th century has been very likely caused by increases in atmospheric greenhouse gas concentrations produced by human activity.

Climate models project that avarage global surface temperatures are likely to increase by 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 reportedly “….packs a far greater warming punch than its oxidized sibling [carbon dioxide].” We cannot rule out other effects of global warming that may initiate self-acceleration.

An increase in global temperatures will in turn cause sea level rise, glacier retreat, melting of sea ice, and changes in the amount and pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. These changes to the climate will produce a range of practical effects, such as changes in agricultural yields and impacts on human health.

Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate regarding 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. Template:TOC-right

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

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 in the atmosphere

Existence of the greenhouse effect itself 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 natural greenhouse effect, the average temperature of 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-fourths 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 volcanoes or the oceans.

Future CO₂ concentrations will 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 extensively used. 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.

Feedbacks

The effects of forcing agents on the climate are modified by feedback processes. One of the most important feedbacks is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the Earth's atmosphere and surface. The increased warmth in turn increases the evaporation of water into the atmosphere. Since water vapor itself is a greenhouse gas, this causes still more warming; the warming causes 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; for example, high clouds are at colder temperatures 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 been found to have 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. Stott and coauthors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest 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) relative to 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 studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with some simplifications being necessary because of limitations in computer power. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of fossil fuel consumption and CO₂ emission are used, 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; however, 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-generation models, though progress is being made on this problem. 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 as a result of warmer temperatures 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 prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 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 future climate projections. McLaughlin et al. have documented two populations of Bay checkerspot butterfly being threatened by precipitation change, though they state few mechanistic studies have documented extinctions due to recent climate 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 world's primary 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 United States 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

A variety of issues are often raised in relation to global warming. One is ocean acidification. 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. Since 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.

Another related issue that may have partially mitigated global warming in the late twentieth century is global dimming, the 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 about global climate change and its anthropogenic origin

A fierce discussion is going on about the question whether a change in climate will occur in the 21st century and if so, whether it will be of anthropogenic origin. The latter point is of special importance, because, if it is true, climate change can be avoided by a change in energy supply and consumption patterns of humankind. A future man-made change of climate is accepted as likely by most international scientific organizations; they follow in this the conclusions of the IPCC (Intergovernmental Panel on Climate Change). This panel gives out warnings that the future of the world is at stake and that drastic measures are called for to reduce the emission of carbon dioxide and other greenhouse gases. On the other hand, there is a vocal minority of skeptics who deny the immediate danger and do not see the need for a large reduction in the use of fossil fuels and change of consumption patterns. The dispute will not soon be closed, because of the difficulty of the underlying science and the leeway afforded by the climate data relevant to this discussion.

There is a growing tendency to measure the credibility of IPCC's conclusions by the number of scientists that question IPCC's conclusions; consequently the alleged number of dissenting scientists has become a sensitive issue too. For instance, when an FPS (Forum on Physics & Society) Editor of the APS (American Physical Society) wrote the following comment: "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 this statement does not represent their views. Clearly, the suggestion that there is a "considerable presence" of scientists disagreeing with the IPCC is already politically laden. Yet, the US Senate recently was able to quote more than 400 scientists—with areas of expertise in climate matters— who dispute man-made global warming.

Earlier in this article the point of view of the IPCC was outlined and some scattered attention was paid to the arguments of IPCC's dissidents. Some of the arguments against the IPCC conclusions, carried forward by sceptical scientists, will now be reviewed.

Difficulties in climate change science

All participants in the dispute agree on one fact, namely that climate change science is a field full of difficult problems. Some sceptics state that many of its problems are too difficult to be solvable at all. Climate science itself is already a very hard area of study. In the first place it is an observational science (like astronomy), meaning that one cannot perform experiments to falsify or verify certain hypotheses.

In the second place its object of study, the Earth, is an extremely complex system, much more complex than natural scientists usually dare to tackle. Most of the physical sciences is based on a reductionistic approach, that is, systems of study are reduced to smaller ones that are easier to understand, but still possess their essential characteristics. In climate science, such an approach is impossible, the atmosphere, the oceans, and the landmasses are tightly coupled subsystems and consequently the energy and mass exchanges between the three major subsystems of the Earth must be included simultaneously. Further, it goes without saying that the radiation balance, i.e., insolation (solar irradiation), energy absorption and back radiation by the Earth, plays a crucial role and cannot be omitted, which means that even the Earth itself is not a closed system.

Third, there exists no encompassing theory, like Newton's equations in classical mechanics, that predicts the characteristics of the climate and of which the truth is accepted by all climatologists. Theories are ad hoc and taken from many different—and not the simplest—areas of applied physics: turbulent and dissipative systems, convective and radiative transport phenomena, non-linear (chaotic) systems and their inherent sensitivity to initial conditions (discovered by by Edward Norton Lorenz), and so on. Further, there is a paucity of reliable data to gauge the existing models. Hence, by necessity even models that try to explain the present world climate are based on choices that are subjective and open to criticism. The problems are compounded for predictions of world-wide changes in climate. So, it is no surprise that many workers in the field do not have much faith in climate models' ability to predict the future.

It is also no wonder that global warming, and its possible anthropogenic origin, has been disputed—although, again, to what extent it is disputed is itself a matter of dispute. Some scientists are sceptical about the interpretation of proxy data—indirect data giving information about past temperatures around the world (such as year rings of trees and isotopic composition of arctic and antarctic ice). Proxy data are used to construct historical mean temperature profiles, yielding, for instance, the widely discussed hockey stick shaped graph. The same people questioning the reliability of the past temperature profiles usually have doubts about the uniqueness of the present global warming; they argue that the world has seen warm periods before, even without human intervention. They often refer to the discovery of Greenland by the Vikings around the year 1000, when Greenland was green, and the time before the dinosaurs became extinct.

Others question the validity of the computer models predicting the climate a few decades ahead, referring to the unreliability of computer models in general. One of their arguments is the inability of current computer models to predict the weather for more than 10 days in advance. Also the failure of computer models to forecast the formation of tropical cyclones is pointed to.

From reading the dissenting literature, one gathers the impression that the great majority of sceptical scientists admit a definite increase in CO₂ in the atmosphere, due to the growing use of fossil fuels, but question that this increase in CO₂ concentration will lead to a world-wide catastrophe. The dissidents reject the warnings by the former US vice-president and Nobel Peace Prize winner Al Gore as unscientific and needlessly alarmist.

Open letter to Ban Ki-Moon from sceptical scientists

During the United Nations Climate Conference on the Indonesian island Bali in December 2007, more than a 100 sceptical scientists (climatologists, physicists, biologists, meteorologists, statisticians, and others) took the initiative to write an open letter to Ban Ki-Moon, the Secretary-General of the United Nations. In this letter they express their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction".

They recognize that a climate change is going on but they state that it is a natural phenomenon that is impossible to stop and they express their doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions."

People expressing fears of catastrophic climate change invariably refer to the IPCC Assessment Reports of 2001 and 2007. In the open letter to the UN Secretary-General the sceptical scientists cast doubt on the the procedures leading to these reports. They write that "the reports are prepared by a relatively small core writing team with the final drafts approved line-by-line by ­government ­representatives". Further they write: "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

In a contribution to the APS Forum on Physics & Society of July 2008, Christopher Monckton, a known critic of anthropogenic causes of global warming, takes issue with the 2007 IPCC report. Moncktons' contribution will now be given some attention, because it follows closely the arguments of the IPCC report and yet comes to different conclusions. A review of Monckton's criticism, which is mainly directed at climate sensitivity, gives us a chance to delve deeper into IPCC's scientific reasonings and to give a discussion that is more quantitative than presented earlier in this article. The main purpose of this section is to illustrate that, even when the very same physical effects are accounted for, different estimates of the same parameters lead to different conclusions. Of course, the IPCC is very much aware of this and discusses likelihoods of parameter values wherever possible, but still it is of interest to see how different parameter choices work out in practice. We present Monckton's and IPCC's values vis-à-vis, but we do not phrase opinions about which of the opposing parties is likely to be correct.

Monckton begins with presenting data plots showing that the globally averaged land and sea surface absolute temperature (T_S) has not risen since 1998 and may have fallen since late 2001, in contrast to the prediction of further rapid warming by the IPCC. After having made this introductory point, he directs his criticism at the value of climate sensitivity found by the IPCC.

The concept of climate sensitivity arose when the IPCC members asked themselves the question how much the temperature on Earth would change by an increase of CO₂ in the atmosphere. To give a quantitative answer, it is necessary to define a reference concentration and a quantitative amount of increase of CO₂. The IPCC accepted the following formal 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. That is, as the reference increase in CO₂ they take a doubling with respect to the concentration at the start of the Industrial Revolution (1750).

The IPCC gives an estimate of 3.26 °C for climate sensitivity, the Earth's temperature response to a possible CO₂ concentration doubling since 1750. In contrast, Monckton gives detailed arguments that 0.58 °C is the more reliable value.

The concept of radiative forcing plays an important role in the discussion of climate sensitivity. Basically, this the gross amount of solar radiation absorbed by CO₂ in the atmosphere. As for the climate sensitivity, it is more expedient to give a relative value, ΔF, i.e., an increase or decrease in absorption, rather than the absolute value F of the absorption itself. The IPCC assumes a logarithmic dependence on concentration and relates the concentration C of CO₂ in the atmosphere to radiative forcing, ΔF, by several formulas, the following being the simplest,

$\scriptstyle \Delta F_{\mathrm {CO_{2}} }=5.35\,\ln(C/C_{0}),\qquad \qquad \qquad \qquad (1)$

where C₀ is the CO₂ concentration before the Industrial Revolution. The factor 5.35 has dimension (W/m²) (solar energy absorbed per second per square meter by carbondioxide) and plays a pivotal role in the discussion. Note that ΔF_CO2 represents a gross absorption: back radiation of the Earth into space and feedback effects (evaporation of water, etc., see earlier in this article) are not yet included.

By means of equation (1) changes in CO₂ concentration can be expressed in the unit of ΔF_CO2 (W/m²). At this point Monckton remarks that at 1990 the total ΔF_CO2 was ~27 W/m² and that from 1995-2005, the CO₂ concentration rose from 360 to 378 W/m², with a consequent increase in radiative forcing of 0.26 W/m², which is less than 1% of the 1990 value of ~27 W/m². The 2007 IPCC report states: "The CO₂ radiative forcing increased by 20% in the last 10 years (1995-2005)". Monckton, noticing that the true value is one-twentieth of the value given by the IPCC, states:

The absence of any definition of radiative forcing in the 2007 IPCC Summary, led many to believe that the effect of CO₂ on T_S had increased by 20% in 10 years. The IPCC – despite requests for correction – retained this confusing statement in its report.

By equation (1) a doubling of CO₂ concentration gives a radiative forcing of $\scriptstyle \Delta F_{2x}$ = 5.35 ln2 = 3.71 W/m². When this value is corrected for aerosols etc., it becomes slightly smaller $\scriptstyle \Delta F_{2x}$ = 3.405 W/m² [see Table 1 in Monckton (2008)].

In his explanation of the origin of the factor 5.35 in equation (1), Monckton shows plots of different models considered by the IPCC for relative warming rates of the atmosphere as a function of altitude and latitude. All these plots show a strong dependence of warming rate on altitude and in particular they exhibit a tropical mid-troposphere "hot-spot". Monckton argues that observations from satellites and by radiosondes do not show this hot-spot: they show that not only absolute temperatures but also warming rates decline with altitude. Therefore he concludes: "Since the great majority of the incoming solar radiation incident upon the Earth strikes the tropics, any reduction in tropical radiative forcing has a disproportionate effect on mean global forcings. On the basis of Lindzen (2007), the anthropogenic radiative forcing as established is divided by 3 to take account of the observed failure of the tropical mid-troposphere to warm as projected by the models". Accepting the corrected value of 3.405 he arrives at a readjusted value of radiative forcing: $\scriptstyle \Delta F_{2x}$ = 1.135 W/m². Hence, the allegedly observed absence of hot-spots in the troposphere gives a diminishing of IPCC's climate sensitivity (predicted temperature change) by a factor 3.

Obviously a certain amount of radiation is transmitted back: the Earth is a source of blackbody radiation. Deviating in a non-essential way from the IPCC report, Monckton introduces a parameter κ that gives the fraction of the absorbed solar radiation that is re-emitted by the Earth in the form of black-body (infra-red) radiation. The parameter κ is referred to as climate sensitivity parameter and is introduced as a multiplying factor. If, e.g., κ = 0.3, it means that effectively 70% of the insolation is radiated back into space. Some basic laws of physics state that the energy content of black-body radiation depends on absolute temperature, meaning that κ is a function of absolute temperature [see equation (21) of Monckton].

To give an estimate of the value of κ one may note that at the Earth’s surface, T_S ≈ 288 K (kelvin) ≈ 15 °C, implying the surface value κ_S = 0.185 K·m²/W. At the characteristic altitude at which incoming and outgoing radiative fluxes balance, the temperature T_C ≈ 254 K ≈ −19 °C, giving κ_C = 0.269 K·m²/W. Monckton then writes: "The IPCC’s value for κ is dependent upon temperature at the surface and radiant-energy flux at the tropopause, so that its implicit value κ ≈ 0.313 K·m²/W is considerably higher than either κ_S or κ_C". Then some papers quoted by the IPCC are mentioned by Monckton and he continues: "None of these papers provides any theoretical or empirical justification for a value as high as the κ ≈ 0.313 K·m²/W chosen by the IPCC".

Monckton then proceeds to give an alternative estimate of κ, but since he needs the feedback factor f for this, we digress and consider feedback first.

The effect of solar absorption is strengthened by feedback mechanisms, such as water evaporation, etc., see earlier in this article. To account for these effects the radiative forcing is multiplied by a dimensionless feedback factor f > 1. Typical values of f are 2.095 or 3.077 (roughly doubling or tripling the effect of the solar absorption, as estimated by Monckton and the IPCC, respectively).

The following equation for f is presented in the IPCC report:

$f={\frac {1}{(1-b\kappa )}},\qquad \qquad \qquad \qquad \qquad (2)$

where b is the sum of all climate-relevant temperature feedbacks. Equation (2) is taken from linear feedbacks for electronic circuits, Ref.. Obviously, the value of f is as important as the values of $\scriptstyle \Delta F_{2x}$ and κ. Monckton observes that equation (2) is of "questionable utility because it was not designed to model feedbacks in non-linear objects such as the climate".

Equation (2) gives a singularity (infinite value) for f and a corresponding blow up for the climate sensitivity $\scriptstyle \Delta T_{\lambda }$, when b = 1/κ = 1/0.313 = 3.19 W/(m²·K). (Note that 0.313 is IPCC's implicit value for κ) The IPCC estimates an upper limit b_max = 3.38 W/(m²·K), which is close to the singularity. Monckton argues that it is very unlikely that b will exceed the value 3.19 W/(m²·K), because of its runaway temperature effect that even not has occurred in the Cambrian atmosphere. During that period the CO₂ concentration approached 20 times today’s, with an inferred mean global surface temperature no more than 7 °C higher than today’s. He adds that "a runaway greenhouse effect would occur even in today’s climate when b ≥ 3.2 W/(m²·K), but has not occurred" and: "The IPCC’s high-end estimates of the magnitude of individual temperature feedbacks are very likely to be excessive, implying that its central estimates are also likely to be excessive". After a few more critical comments Monckton concludes that it is "prudent and conservative" to restore f to the 50 % lower value f ≈ 2.08 that is implicit in the 2001 IPCC report. He adjusts its value a little to maintain consistency with his earlier equations and proposes the value f = 2.095.

After this digression to the feedback, we return to the climate sensitivity parameter. The value of κ cannot be directly observed. In order to obtain an empirical estimate, Monckton rewrites κ as

$\kappa ={\frac {\Delta T_{\lambda }}{\Delta F_{2x}+b\,\Delta T_{\lambda }}}\qquad \qquad \qquad \qquad (3)$

by means of equation (2). Now, κ can be computed by equation (3), provided the three parameters, appearing in its right hand side, over a given period are known. Monckton compares the years 1980 and 2005 giving a spread of a quarter of a century. After discussing his choices for these parameters, Monckton arrives at κ = 0.242 K·m²/W. This value is bracketed by the values deduced from atmospheric temperatures: κ_S = 0.185 and κ_C = 0.269 K·m²/W, which adds to its credibility.

Conclusion about climate sensitivity

Following Monckton and slightly deviating from the IPCC, we wrote implicitly the climate sensitivity as a product

$\scriptstyle \Delta T_{\lambda }=\Delta F_{2x}\cdot \kappa \cdot f.$

The climate sensitivity, from the 2007 IPCC parameters that were discussed above, is:

$\scriptstyle \Delta T_{\lambda }=3.405\times 0.313\times 3.077=3.28\,\,^{0}\mathrm {C}$.

The IPCC (2007) reports a likely range of $\scriptstyle \Delta T_{\lambda }$ of 2.0 to 4.5 ºC, with a best estimate of about 3 °C, demonstrating that Monckton has faithfully replicated IPCC’s method with IPCC's parameters. The IPCC report adds that a value of less than 1.5 ºC is very unlikely (probability less than 10%).

Using his own revised values, Monckton gives the following final estimate for the climate sensitivity

$\scriptstyle \Delta T_{\lambda }=1.135\times 0.242\times 2.095=0.58\,\,^{0}\mathrm {C} ,$

which is in the range considered to be very unlikely by the IPCC. Monckton concludes:

If this equation is correct, the IPCC’s estimates of climate sensitivity must have been very much exaggerated. There may, therefore, be a good reason why, contrary to the projections of the models on which the IPCC relies, temperatures have not risen for a decade and have been falling since the phase-transition in global temperature trends that occurred in late 2001. Perhaps real-world climate sensitivity is very much below the IPCC’s estimates. Perhaps, therefore, there is no "climate crisis" at all. At present, then, in policy terms there is no case for doing anything. The correct policy approach to a non-problem is to have the courage to do nothing.

Since Monckton includes faithfully the very same physical effects as the IPCC, but differs in well-argued choices of parameters, this discussion of climate sensitivity illustrates that serious workers can have different interpretations of the same observations on climate.