Debates on the Causes of Global Warming

Abstract

The controversy between the IPCC and Non-governmental IPCC (NIPCC) on the attribution of global warming are reviewed. IPCC holds that today’s global warming is mainly due to anthropogenic activities rather than natural variability, which is emphasized by NIPCC. The surface temperature observations since the mid-20th century support the hypothesis of anthropogenic impact, but for the last one hundred years or so, natural forcings such as solar activity, volcanic eruptions and thermohaline circulation variations also have had great influences on the Earth’s climate, especially on inter-decadal timescales. In addition, evidence suggests that the Medieval Warm Period (MWP) and Little Ice Age (LIA) are closely associated with the solar activity over the past 1 thousand years. Over the past 10 thousand years, the North Atlantic cold events and solar activity are closely correlated. Nevertheless, the physical mechanisms of the solar-climate variability and interrelation are not well understood, yet. Notably, a prevailing view recently indicates that galactic cosmic rays may result in climatic cooling through modulating global low cloud cover. However, its process and mechanism need to be further investigated.

Keywords

global warming ; causes ; greenhouse effect ; solar activity ; galactic cosmic rays

1. Introduction

Global warming is one of today’s most popular and controversial topics. Beyond our early expectation, the controversy almost involved every aspects of global warming, including its nature, causes, consequences, and even whether the global warming has really happened. Three key issues of the current controversy covering the current status of global warming, its impacts and anthropogenic effect, were reviewed by Wang et al. [2010] , who pointed out that the global warming is the result of combined effects of human activity and natural variability. In this paper, two apparently diametrically opposed views on the global warming from Intergovernmental Panel on Climate Change (IPCC) and Non-governmental International Panel on Climate Change (NIPCC) will be reviewed.

2. Focus of the debates

The IPCC Fourth Assessment Report (AR4) [ IPCC, 2007 ] pointed out that most of the observed global temperature rising since the mid-20th century is “very likely” due to the increase of anthropogenic greenhouse gases (GHGs) concentrations in atmosphere. This conclusion made a great progress in attribution of global warming as compared to the IPCC Third Assessment Report (TAR) [ IPCC , 2001 ], in which the global warming is “likely” induced by anthropogenic emissions. In IPCC AR4, “very likely” means above 90% in probability of occurrence, while “likely” means just more than 66%. So, this advance implies that IPCC has a higher confidence in the conclusion of anthropogenic global warming than before. However, NIPCC, as a strong opponent of anthropogenic global warming, takes an entirely opposite standpoint, which is expressed clearly in the title of their summary for policymakers [ Singer, 2008 ], “Nature, not human activity, rules the climate”. In order to understand this discrepancy between IPCC and NIPCC, it is necessary to review these two opposite views and their respective arguments.

3. Evidence for the greenhouse effect hypothesis

From the First Assessment Report released in 1990, IPCC still has been insisting on its conclusion which attributes modern climate warming to the increase of anthropogenic GHGs concentrations in the atmosphere. This conclusion appears to be reinforced continuously by emerging scientific evidence in recent years. Generally, the scientific evidence for the IPCC’s standpoint of human-made global warming comes from two sources: the observations and model simulations. They will be reviewed in the following paragraphs.

3.1. Observations

It is required by the greenhouse effect hypothesis that the increase of temperature (such as daily average, maximum and minimum temperature) owing to increased concentrations of GHGs in the atmosphere, should occur globally in each season, and more noticeably in high latitudes than low latitudes, especially in winter. Additionally, the minimum temperature should rise more drastically than the maximum temperature if the global warming is induced by increased GHGs in the atmosphere. So far, all of these theoretical assumptions associated with greenhouse effect hypothesis have been almost proved by instrumental records. Over the past four decades, the global surface temperature variations, as shown in Figure 1 , are characterized by significant warming, especially over the continental high latitudes [Hansen et al., 2010]. The warming is highest in the first decade of the 21st century with global averaged temperature rising about 0.51°C relative to 1951–1980. The trends of seasonal surface temperature are comparable in each season with significant upward trend ranging from 0.60 to 0.65°C ( Fig. 2 ) over the past 60 years [Hansen et al., 2010]. The increase of boreal winter temperature is most noticeable over the continent of Eurasia and the northern part of North America, while in austral winter (June-July-August) distinctive warming appears over the coastal area of Antarctica.

Figure 1. Globally decadal surface temperature anomalies (°C) over the period of 1970s–2000s relative to 1951–1980 [ Hansen, 2010 ]

Figure 2. The trends in global seasonal temperature (°C per 60 years) for the period of 1950–2009 [ Hansen, 2010 ]

In addition, instrumental observations show that the diurnal temperature range (DTR) has a downward trend [ IPCC, 2007 ], which also meets the requirements of the greenhouse effect theory. About 80% of this decreasing trend in the DTR, which is slighter in IPCC AR4 than in IPCC TAR, can be attributed to changes in cloud cover and rainfall. Satellite datasets over the period of 1979–2005 demonstrate that warming has been globally occurring in the troposphere from the Earth’s surface to the level of 10 km high, while cooling in the stratosphere [IPCC, 2007]. This observed phenomenon is also in agreement with the greenhouse effect hypothesis. Climate models are additional useful and powerful tools to provide evidence for anthropogenic global warming. One discrepancy, however, exists between model simulations and observations. Driven by increased anthropogenic GHGs, model simulations’ results show much warmer conditions in the tropical upper troposphere than at surface level. However, this phenomenon is not verified in observations and becomes an evidence for NIPCC against IPCC’s viewpoint on anthropogenic warming. But, Allen and Sherwood [2008] made a calculation of the temperature of the tropical (20°N–20°S) upper troposphere with observational data based on the thermal wind theory recently. The results show that the temperature increase is larger at the tropical upper troposphere than at the Earth’s surface. In addition, Thorne [2008] also pointed out that temperature at the level of 200 hPa increased by about (0.40±0.29)°C per 10 years over the period of 1979–2005, which is much larger than that of surface temperature (0.13°C per 10 years). It needs to be further investigated why this phenomenon only appears both in model simulations and theoretical calculations, but not in observations.

3.2. Model simulations

If the global averaged temperature variations could be reproduced in model simulations forced by the prescribed anthropogenic GHGs, it would be convinced with high confidence that the warming of the 20th century was caused principally by human activities. For a more realistic climate in the models, the climate of the 20th century were simulated (summarized in IPCC AR4) using atmosphere-ocean general circulation models (AOGCMs) integrated with forcings of GHGs, aerosol, volcanic eruptions, solar activity, ozone, etc. These climate models have good performance on the global temperature variations from 1960s, although they do not capture some striking features before 1960s, such as one peak of the global temperature series from late 1930s to mid-1940s, and two valleys in the early and middle of 1950s, respectively. Particularly, without forcings of anthropogenic emissions, the warming starting from the late 20th century cannot be realized in model simulations. Hence, these experiments imply that the increase of GHGs concentrations in atmosphere play a principal role in the warming of the late 20th century. Additionally, the IPCC AR4 also summarized model simulations of the past millennial climate changes, which were performed in the period of 2002–2007 with 12 models comprising of 6 global climate models (GCMs), 4 Earth-system models of intermediate complexity (EMICs) and 2 energy balance models (EBMs). These models were driven only by natural forcings of insolation and volcanic eruptions, and succeed in reproducing the warming of the 12th–14th century and cooling in the 15th, 17th, and 19th century. After taking account of anthropogenic emissions, these models can also reproduce the warming in both the 1940s and the latter 30 years of the 20th century. As a result, these modeling experiments suggest that anthropogenic activities dominate the warming of the 20th century, while natural forcings, such as volcanic and solar activities, are the principal drivers of climate change over the past millennium.

However, on the issue of anthropogenic global warming, NIPCC took an entirely different position from IPCC and vigorously criticized IPCC for their exaggerating the effect of human-generated GHGs on climate change. They pointed out that the global temperature records cited by the IPCC is unreliable because of their poor geographic distribution and sampling, and contamination from urban heat-island effect. In terms of climate models, a crucial source for predominance of anthropogenic GHGs in global warming, their mismatch with observations is taken by the NIPCC as an important evidence for the current warming of natural rather than anthropogenic origin [Singer, 2008].

4. Evidence for solar activity impacts

The relationship of solar activity with climate started to attract attention of scientists from the late 19th to the early 20th century. A hypothesis on the influence of solar activity on the global climate was proposed, but it still has to face challenges mainly from two aspects: 1) no persistent 11-year cycles associated with the sunspot cycle are observed in long-term climate indices; 2) how solar variability influences the Earth’s climate. Initially, the total solar irradiance (TSI) was believed to be a constant, but later this “constant” is proved to be a variable. The amplitude of the TSI varies in an 11-year cycle by only about 0.1%, which induces that the solar irradiance change is not enough to entirely explain the past climate variability. Therefore, a new mechanism needs to be established for the solar-climate variability.

4.1. Medieval Warm Period (MWP) and Little Ice Age (LIA)

As there is no persistent 11-year cycle in the longterm Earth’s climate, we mainly focus on climate variability of decadal to centennial timescales. Over the past one thousand years, the MWP and LIA are two important epochs for the global climate. Prior to the industrial revolution, the MWP and LIA do almost not involve the influence of human activity and are attributed to natural forcings such as solar activity by most scientists. Kikby [2007] summarized the relationship of the surface temperature with solar variations over the past thousand years ( Fig. 3 ). Compared with temperature variations (Fig. 3a ), it is obvious in Figure 3b that galactic cosmic rays (GCRs) are weak (corresponding to strong solar activity) in the MWP, while the condition is reversed in the LIA. Additionally, the GCRs variability also matches well with the advance and retreat of the tropical Andean glacier. These evidences suggest a close relationship of GCRs with Earth’s climate.

Figure 3. (a) The surface temperature, (b) galactic cosmic rays, and (c) tropical Andean glacier changes during the past millennium (After Figure 2 in Kikby [2007])

4.2. Abrupt climate changes in the Holocene

Based on ice rafting debris (IRD) records, Bond et al. [1997] indicated that the North Atlantic has experienced ice-rafting events (also named cold events) 8 times, numbered 1–8. Later, as result of having similar features with the preceding cold events, the LIA is also regarded as a cold event and numbered the 0 event. Consequently, there are altogether 9 cold events in the Holocene. Substantial studies [ Mayewski et al., 2004  ; Morrill et al., 2003 , Staubwasser, 2006  and Wang et al., 2005 ] show that cold events in the North Atlantic are closely associated with climate over the Northern Hemisphere, even with the global climate changes. Wang [2009] analyzed the chronology of the North Atlantic cold events in the Holocene and its impacts, and pointed out that when cold events happen in the North Atlantic, it is cold and dry over the boreal high latitudes, especially over Northwest Europe, while it is wet in Central and Western Europe. Additionally, evidently reduced summer monsoon precipitation over the Africa-Asian monsoon regions, droughts occurred over the areas extending from the coastal area of Eastern Africa through the Arabian Sea to South Asia and East Asia, and droughts in South China recorded in stalagmite records are also closely associated with the Northern Atlantic cold events [ Wang et al., 2005 ].

Denton and Karlén [1973] proposed that the weakening solar activity may be the reason for the Holocene abrupt climate changes. Bond et al. [2001] reconstructed the solar variations using¹⁴ C and ¹⁰ Be records, and established a composite index with 4 sea sediment records to represent the North Atlantic cold events in the Holocene. These reconstructed series were removed from linear trends firstly, and then filtered by 70 years. It is obvious that the solar activity is closely correlated with the composite index (Fig. 4 ), which the correlation coefficients are 0.44 and 0.56, respectively. In addition, 9 times of Northern Atlantic cold events match well with the valley of the solar intensity (corresponding peak in GCRs series). These suggest that the Northern Atlantic cold events may be induced by the weakening of the solar intensity.

Figure 4. Correlation of GCRs with North Atlantic cold events (ice-rafted debris events) during the Holocene [Kikby, 2007]

4.3. Mechanisms of solar-climate variability

The solar activity exerts great influence on the Earth’s climate in three ways [ Carslaw et al., 2002 ]: TSI, ultraviolet radiation, and GCRs changes. With respect of TSI, the observations over the past two and a half 11-year cycles proved that the “solar constant” does not keep a constant, and varies with an amplitude of 0.1% [ Fröhlich and Lean, 2004 ]. This amplitude can only induce Earth’s temperature change of 0.1°C, which is not nearly enough to explain the observed climate variability [ Wigley and Raper, 1990 ]. Even though the variation of TSI is larger in the past several hundred years than at present, it has just increased by 0.5 W m^–2 (equivalent to a radiative forcing enhancing about 0.08 W m^–2  at the top of the atmosphere), and induced temperature rising only 0.06°C since the 18th century, almost smaller than the observed (0.6°C) by an order of magnitude, presuming climate sensitivity is 0.7°C (W m^–2 )^–1 [ Foukal et al., 2006 ]. Consequently, TSI variations are not believed as the major way of solar activity to influence the Earth’s climate currently by more and more climatologists and astronomers.

Drastic change of ultraviolet radiation in the 11-year solar cycle is the second way of solar activity to influence the Earth’s climate [ Haigh, 1996 ]. The ultraviolet radiation variation is estimated to be about 7% from the maximum year (M year) to the minimum year (m year) in an 11-year cycle, although different scientists have different estimates. When the solar activity intensifies, ultraviolet radiation becomes stronger, O₃  concentration increases and absorbs more ultraviolet radiation leading to a warming in the stratosphere. Thereafter, through the way of radiation and dynamic processes, the stratosphere exerts great influence on the troposphere. However, the process and mechanism by which solar activity affects the Earth’s climate are hardly understood and require more attention.

On the third way of solar activity affecting the Earth’s climate, numerous researches have already been carried out [Carslaw et al., 2002; Kikby, 2007  and Svensmark, 2007 ]. In an 11-year cycle, about 15% of GCRs variation is measured, and at the same time about 1.77% of low cloud amount change is also observed, which is equivalent to radiative forcing varying by about 1 W m^–2 . Compared with the radiative forcing change (1.66 W m^–2 ) due to the increase of CO₂  in the atmosphere, the influence of GCRs on radiative forcing (1 W m^–2 ) cannot be negligible [Svensmark, 2007]. The mechanism and process by which GCR affects the Earth’s climate cause great attention. Three hypotheses are proposed to explain how GCRs affect the low cloud cover: 1) ionization from GCRs influences the number of cloud condensation nuclei [ Dikinson, 1975 ]; 2) ionization from GCRs modulates the cloud physical processes through thunderstorm currents [ Markson and Muir, 1980 ]; 3) ionization from GCRs affects the nucleation and/or growth rate of ice crystals in high-level clouds in winter cyclone [ Tinsley and Deen, 1991  ; Tinsley, 1996 ]. In addition, Carslaw et al. [2002] pointed out that the bottleneck of the influence of GCR on the climate is to modulate the formation of cloud particles and ice crystals, and proposed two kinds of mechanisms for this process: “ionaerosol clear-air” and “ion-aerosol near-cloud”. However, these mechanisms are not entirely proved by the observations. To what extent the solar activity influence the Earth’s climate is not clear, and to what extent to strengthen or offset the greenhouse effect on the Earth’s climate need to be investigated further.

5. Conclusions

The hypothesis of global warming since the mid-20th century owing to the increase of CO₂  in the atmosphere is supported observationally by the spatial and seasonal characteristics of global temperatures variations. Additionally, climate warming over the past 30 years can be reproduced by climate models forced by increased GHGs concentrations in the atmosphere. Over the last hundred years, however, the warming in the 1940s and cooling in the period of 1950–1970 cannot be reproduced by climate models driven only by increased concentrations of GHGs. This discrepancy between model simulations and observations imply that natural forcings, such as solar activity, volcanic activity and thermohaline circulation in global deep oceans, also play important roles in climate variability on decadal timescales.

Substantial evidences indicate that the MWP and the LIA are closely associated with solar activity, and the Northern Atlantic cold events in the Holocene was also closely correlated with the weakening solar activity recorded in ¹⁴ C and ¹⁰ Be. The variations of TSI are not enough to explain the contemporaneous climatic changes. At present, most scientists are aware of the importance of GCRs’s impacts on climate variability. When solar irradiance weakens, increased GCR induces more low clouds leading to the climatic cooling. However, the details of the mechanism are poorly understood, and need to be investigated further. To what extent, the solar activity and natural forcings strengthen or offset the greenhouse effect also needs to be studied further, too.

Although NIPCC insists on the current warming of natural rather than anthropogenic origin, accumulated evidence demonstrates that the global warming is due to the combined effects of anthropogenic and natural forcing, and mainly due to man-generated GHGs especially from the mid-20th century. The influence of GCR on the radiation received by the Earth through modulating the low cloud cover is considered the major mechanism. Therefore, how solar activity affects the Earth’s climate is urgently needed to be explored further.

Acknowledgements

The study was supported by National Basic Research Program of China (No. 2010CB950104). The authors also thank the anonymous reviewers and the editor whose comments and suggestions are helpful for improving this paper.

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