Global warming is unequivocal, as is manifested in three internationally known global temperature records with the global average temperature rising at a rate of 0.70–0.75°C per 100 years during 1910–2009 [ Wen et al., 2011 ]. In other words, the earth has warmed more than 0.8°C since the Industrial Revolution. If it is true that 2°C above pre-industrial level is a threshold of climate “safety”, we will have to confront a serious situation where less than 1.2°C warming is permissible in the future [ Liu et al., 2011 ]. According to climate system concept [ Wang , 2011 ], the global warming is not only reflected in global temperature change but also in changes of other climate system components, such as widespread melting of snow and ice, rising of global average sea level, and permafrost degradation [IPCC, 2007].
Due to its great influences and potential risks on the society, global warming now attains more and more concerns from governments, scientists and public media. Today, global warming becomes one of the most important issues, which must be taken into consideration in strategic decision making of national development. As a scientific issue, however, global warming has been studied for almost 200 years [ Mitchell, 2002 ; Cullen, 2011 ]. And it now has evolved as an independent discipline closely associated with meteorology, oceanography and environmental sciences. In order to review the development of global warming concisely, in this paper, we shall only focus on the science of global warming itself, excluding impacts, adaptation and policy associated with global warming.
In 2011, Archer and Pierrehumbert  compiled the previous studies on the global warming and added their brief comments. This work offered a great foundation for further investigating global warming. In order to understand global warming systematically and comprehensively, we summarize ten essential issues based on the five cornerstone literatures from Archer’s compilation together with four IPCC Assessment Reports and the First World Climate Conference. These ten issues will be reviewed as follows.
Greenhouse effect Joseph Fourier was the first scientist argued for the greenhouse effect in 1824 [ Archer and Pierrehumbert, 2011 ]. The earth receives energy from the sun and re-radiates to outer space. Calculations based on the planetary energy balance indicate that the temperature of the earth is much lower than 0°C as opposed to the observation of 14°C. This discrepancy is believed to be produced by the atmosphere, which traps more long-wave radiation from earth surface and keeps the earth warmer. A mechanism named greenhouse effect is proposed to describe this process.
Greenhouse gases An Irish scientist Tyndall in 1849 verified through his designed experiments that CO2 has greenhouse effect on earth’s climate [ Tyndall , 1861 ]. He was the first to correctly measure the relative infrared absorptive powers of the gases including nitrogen, oxygen, CO2 , water vapor, methane, ozone, etc. These experiments proved that CO2 , methane, nitrous oxide and water vapor have strong absorption on infrared radiation. These gases are named greenhouse gases owing to their greenhouse effect.
Effects of double CO2 concentration on climate Swedish scientist Arrhenius in 1896 was the first to argue that the earth would become warming due to increased emissions of CO2 from the burning of fossil fuels and other combustion processes [ Arrhenius , 1896 ]. He estimated the global average temperature to rise about 8°F (or 4.5°C) in the scenario of double CO2 concentration in atmosphere. It is usually called equilibrium climate sensitivity (ECS) and very close to the upper limits in the subsequent IPCC Assessment Reports, although this estimation was calculated only with a simple one-layer model at that time.
Atmospheric CO2 concentration rising In 1957, American scientist Keeling set up a station at Mauna Loa the first time to directly observe the CO2 concentration in atmosphere [ Keeling , 1960 ]. The measurements demonstrate atmospheric CO2 concentration continues to increase, which is accorded with the temperature rising in instrumental records. This consistence is believed as strong evidence in favor of attribution of global warming to greenhouse effect. Additionally, the records of CO2 in Antarctic also have similar results.
Climate model The increasing concentration of atmospheric CO2 caused by human activities continues to enhance greenhouse effect in the past century. At the same time, a rising trend is observed in the global average temperature records. Are these two phenomena related? Is there a cause-effect relationship? To understand the relationship between the increasing concentration of anthropogenic greenhouse gases and the modern global warming, climate model is a crucial tool widely used by climate scientists. A pioneering research is Manabe and Wetherald  . This paper investigated effects of double CO2 concentration on the climate using climate models (usually named ECS experiments) and indicated that the global surface temperature would increase 2.93°C. This value locates within the range (1.5–4.5°C) and particularly is very close to the best estimation (3°C) of the First World Climate Conference (FWCC) and the IPCC First Assessment Report (IPCC FAR). Essentially this value has hardly changed in subsequent IPCC Assessment Reports, although the climate models used to make evaluations have undergone a great progress from the early simple atmosphere-ocean coupled models to climate system models later.
First World Climate Conference The FWCC was held in 1979 and sponsored by the World Meteorology Organization (WMO), and it was a landmark in the history of climate science. This conference lasted for two weeks and was attended by more than 400 scientists from around the world. A strong statement was issued at the conference to highlight the risk of climate change. From a scientific point of view, FWCC made a significant contribution, the first time to officially indicate the global average temperature will increase 1.5–4.5°C when the atmospheric CO2 concentration doubles in the future. More importantly, FWCC promoted the development of climate science through establishment of World Climate Programme (WCP), comprised of four components: World Climate Research Programme (WCRP), World Climate Application Programme (WCAP), World Climate Impacts Programme (WCIP), and World Climate Data Programme (WCDP). Among them, WCRP is most active and already attracts a large number of scientists. As a major component of WCRP, Tropical Ocean and Global Atmosphere (TOGA) Program, emphasizing the tropical ocean and their relationship to the global atmosphere, was initiated in 1985, and then successfully implemented for 10 years (1985–1994). Climate Variability and Prediction (CLIVAR) is another program organized by WCRP, which is successful in bringing together the international scientific research community to exploring climate variability and predictability on seasonal to centennial time-scales. The scientists, however, had to confront a frustrating situation when they participated in WCP. On the one hand, they found evidence for aggravation of global warming by human activities, resulting in a series of environmental and ecological problems, such as widespread melting of snow and ice, sea level rising and biodiversity decline, but on the other hand, they had no rights to steer policy to reduce the emissions of greenhouse gases through optimization of energy structure. So, an organization was urgently needed to bring the scientists and policymaker together to deal with the global warming. Under such conditions, IPCC was established in 1988 by WMO and UNEP (United Nations Environment Programme). Subsequently, four IPCC Assessment Reports were published in 1990, 1996, 2001, and 2007 [IPCC , 1990; 1996; 2001; 2007], respectively, and a supplementary report in 1992 [ IPCC , 1992 ].
IPCC FAR and supplementary report The IPCC FAR mainly focused on the theory of greenhouse gases and greenhouse effect [ IPCC , 1990 ]. The report indicated that the global warming would be intensified and its amplitude would exceed the range of natural variability of the past several million years if the atmospheric concentrations of anthropogenic greenhouse gases continually rise. Over the past one hundred years, the global average temperature has already increased 0.5°C in line with the most of model results. The details were described in the supplementary report, which also introduced the first emission scenario of greenhouse gases [ IPCC , 1992 ].
IPCC Second Assessment Report (IPCC SAR) The IPCC SAR, titled Climate Change 1995, was published in 1996 [ IPCC , 1996 ]. This report confirmed further that atmospheric CO2 concentration had been steadily increasing, and now it was at the highest level since 1860. In order to prevent drastic rising in the atmospheric CO2 concentrations in the future, a large number of measures should be taken immediately. In addition, the climate models used for future climate projections have been improved greatly to allow for the effect of sulfate aerosols and O3 in the stratosphere. Another important conclusion in IPCC SAR was to point out that influence of human activity on the climate is discernible, although its signal is hidden in the natural variability.
IPCC Third Assessment Report (IPCC TAR) As the IPCC TAR was published, the global warming became the focus of argument [ IPCC , 2001 ]. The report indicated that 20th century warming was the strongest century, 1990s was the warmest decade and 1998 was the warmest year over the past millennium. This conclusion was made on the reconstructed global temperature records, mainly on the work of Mann et al.  , whose hockey sticky graph emphasizing temperature gradually decreases over the past millennium followed by an abrupt rising since the past century. Extensive research, however, questioned the conclusion of Mann [ Wang et al. , 2005a ; 2005b ; Zhao et al. , 2007 ]. Later, Mann also admitted that the climate is fluctuated in the past millennium with existence of Medieval Warm Period and Little Ice Age. Additionally, in this report, six scenarios were proposed to describe possible future emissions of greenhouse gases based on different assumption of future development. The six scenarios were comprised of A1B, A1T, A1F1, A2, B1, and B2. Among them, A represents high emissions, and B represents low emissions, A1F1 and A2 are the highest emission level while B1 is the lowest emission level. Projections of future temperature change to 2100 in six scenarios are shown in Table 1 , which are results of multi-model ensemble. Most noticeably, IPCC TAR suggests that the change of radiation forcing caused by human activities is one order of magnitude larger than that caused by natural variability, such as solar activity, aerosol. So, projections of future climate changes are mainly determined by future human activities. IPCC AR4 still adheres to this conclusion, and estimation of natural forcing change has little change.
IPCC Fourth Assessment Report (IPCC AR4) The IPCC AR4 was published in 2007 [ IPCC , 2007 ]. It focused on the global warming of the past 50 years, no longer on the 20th century warming, 1990s warming and 1998 warming of the last millennium. The latter were emphasized by IPCC TAR. IPCC AR4 mainly attributed the global warming of the past 50 years to the increase of anthropogenic greenhouse gases concentrations in the atmosphere with more than 90% probability of occurrence, which was more reliable than that of IPCC TAR (66%). Another significant contribution made by IPCC AR4 was to narrow the range of ECS to 2.0–4.5°C. Essentially, the projections of future temperature change to 2100 in six scenarios, however, had little change from IPCC TAR to AR4 (Table 1 ). Estimations of global warming in the scenario of double CO2 concentration are shown in Table 2 , which includes all IPCC Assessment Reports and FAR’s supplementary report. It is obvious that the lower limit of the range of warming has little change, and the upper limit has once decreased due to allowing for the cooling effect of sulfate aerosols (IPCC SAR). But, the upper limit was raised later by accelerating warming in climate models. Objectively, projections of future warming in IPCC Assessment Reports are in substantial agreement with the observations since 1990 (Fig. 1 ).
|IPCC Assessment Report||Best estimation||Range|
|Supplementary report (1992)||1.9–3.8||0.3–5.3|
Model projections of the global mean warming compared to the observed warming for 1990–2005 (IPCC FAR, SAR, and TAR) and for 2000–2005 (IPCC AR4) in four scenarios, black dots and line denote the annual and decadal average of the observations) [IPCC , 2007]
We thank QIN Da-He and DING Yi-Hui for their valuable advice, and are grateful to DING Yi-Hui for sending us the book The Warming Papers . This work was jointly funded by the National Basic Research Program of China (No. 2010CB950104), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. 9-XDA05090104).