Abstract
We present detailed line-by-line radiation transfer calculations, which were performed under different atmospheric conditions for the most important greenhouse gases water vapor, carbon dioxide, methane, and ozone. Particularly cloud effects, surface temperature variations, and humidity changes as well as molecular lineshape effects are investigated to examine their specific influence on some basic climatologic parameters like the radiative forcing, the long wave absorptivity, and back-radiation as a function of an increasing CO2 concentration in the atmosphere. These calculations are used to assess the CO2 global warming by means of an advanced two-layer climate model and to disclose some larger discrepancies in calculating the climate sensitivity. Including solar and cloud effects as well as all relevant feedback processes our simulations give an equilibrium climate sensitivity of = 0.7°C (temperature increase at doubled CO2) and a solar sensitivity of = 0.17°C (at 0.1% increase of the total solar irradiance). Then CO2 contributes 40% and the Sun 60% to global warming over the last century.
1. Introduction
The Fifth Assessment Report (AR5) [1] of the Intergovernmental Panel on Climate Change (IPCC), a list of all abbreviations is found in the annex, announces new evidence of an anthropogenic climate change based on many independent scientific analyses from observations of the climate system, paleoclimate archives, theoretical studies of climate processes, and simulations using climate models. So, the IPCC classifies the human influence as extremely likely to be the dominant cause of the observed warming since the mid-20th century (AR5-WG1-SPM-D3). Increasing emissions of carbon dioxide (CO2) over the last century especially are made responsible for this change, and the equilibrium climate sensitivity (ECS or ) as a measure for the Earth’s temperature increase at doubled CO2 concentration in the atmosphere is specified to be likely in the range 1.5°C to 4.5°C (high confidence) (AR5-WG1-SPM, p16).
Although in all these fields of climate sciences great progress has been achieved over the last decades and our knowledge about the Earth-atmosphere system (EASy) could significantly be improved, explanations of the observed global warming over the last century in particular the anthropogenic contributions to this warming are still quite contradictorily discussed.
All the more it is surprising(i)that many of the consulted analyses and also the AR5 itself do not better and clearly distinguish between an anthropogenic emission of CO2 and a naturally generated part, where the latter even contributes more than 95% to the overall emission, and its generation rate and the respective absorption rate sensitively respond on global temperature variations;(ii)that the IPCC claims it would be extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings, while contributions from natural forcing and an internal variability both would only likely be in the range of −0.1°C to 0.1°C;(iii)that the meanwhile well known delayed response of CO2 and methane (CH4) to sea and air temperature changes (see, e.g., Petit et al. [2]; Monnin et al. [3]; Caillon et al. [4]; Torn and Harte [5]; Humlum et al. [6]; Salby [7]) are not considered in AR5, and respective consequences for an anthropogenic global warming are not discussed;(iv)that quite uncertain data about cloud feedbacks and studies of the radiative forcing (RF) of greenhouse (GH) gases are referred, which are mostly valid for clear sky conditions, while the introduction of clouds is usually omitted (AR5-WG1- Chap.8.3.1);(v)that the IPCC denies any noticeable solar influence on the actual climate, although strong evidence of an increasing solar activity over the last century exists (see, e.g., Hoyt & Schatten [8]; Willson & Mordvinov [9]; Shapiro et al. [10]; Ziskin & Shaviv [11]; Scafetta & Willson [12]; Usoskin et al. [13]; Zhao & Feng [14]; Soon et al. [15]);(vi)that obviously important effects like convection and evaporation feedback, which can contribute to significant negative feedback (Harde-2014 [16]), are not considered in many analyses.Nevertheless, despite these deficits and simplifications the mean equilibrium climate sensitivity is specified with high confidence, and the GH gases are even assigned with very high confidence (95%) to be responsible for the actual climate changes.
Here we will focus on the assessment of one of the most important quantities in climate sciences and its validation, the ECS, which has to be scrutinized in more detail. Due to its far reaching consequences for future climate predictions it is particularly important to understand and to discover the large discrepancies between different accounting methods applied for this quantity. Also the weighting of some quite different and even counteracting processes which control our climate, but which are not always well understood, has carefully to be investigated with its implications on the climate sensitivity. A quite critical report of actually published ECS values and accounting methods expanded in AR5 has been published by Lewis and Crok [17].
In this contribution we will also retrace the main steps of the IPCC’s preferred accounting system and compare this with our own advanced two-layer climate model (2LCM), which is especially appropriate to calculate the influence of increasing CO2 concentrations on global warming as well as the impact of solar variations on the climate (Harde-2014 [16]). This model describes the atmosphere and the ground as two layers acting simultaneously as absorbers and Planck radiators, and it includes additional heat transfer between these layers due to convection and evaporation. It also considers short wave (sw) and long wave (lw) scattering processes at the atmosphere and at clouds; it includes all common feedback processes like water vapor, lapse rate, and albedo feedback but additionally takes into account temperature dependent sensible and latent heat fluxes as well as temperature induced and solar induced cloud cover feedback.
The objective of our studies is not to present a new only “true ECS” but to identify some of the different assumptions and approximations with their far reaching consequences in climate politics. It is without any doubt that the ECS is the most important measure for the CO2 influence on our climate, but it is also clear that this quantity does not distinguish between anthropogenic and natural CO2 emissions. Therefore, as long as any natural variations in the CO2 concentrations are not accurately known, the ECS cannot be used as a reliable indicator only for an anthropogenic global warming. All this in mind the reader may have his own reservations about the published data for this measure and its significance for a man-made climate change.
For the assessment of the ECS the IPCC favors the concept of radiative forcing (RF), which is supposed to be appropriate to describe the transition of the surface-troposphere system from one equilibrium state to another in response to an externally imposed perturbation. Therefore, in Section 2 we briefly outline some basic relations characterizing this concept, before we present in Section 3 detailed line-by-line radiation transfer (LBL-RT) calculations for the lw up- and downwelling fluxes in the atmosphere (for details see Harde [16, 18, 19]), this for clear sky, global mean cloud cover, full cloudiness, different surface temperatures, humidity, and even different lineshapes. These calculations were performed for the most important GH gases water vapor (WV), carbon dioxide, methane, and ozone and are based on the HITRAN08 database [20]. Since the concentration of the GH gases and the atmospheric pressure are changing with temperature and altitude, the atmosphere has to be segmented into up to 228 sublayers from ground to 86 km height and in some cases additionally into three climate zones. When these computations are repeated for different CO2 concentrations at otherwise same conditions, from the changing fluxes on the one hand the CO2 initiated RF as the main parameter for the IPCC accounting scheme and on the other hand the sw and lw absorptivities as well as the back-radiated fraction of the atmospheric emission as the key parameters in our 2LCM can be derived.
Section 4 summarizes the main features of the 2LCM and its calibration to satellite data, for the radiation and heat fluxes using the well known radiation and energy budget scheme of Trenberth et al. [21], for temperature changes due to cloud cover variations applying the observations within the International Satellite Cloud Climatology Project (ISCCP) [22]. With the respective key parameters of Section 3 integrated in the climate model we simulate the Earth’s surface temperature and the lower tropospheric temperature as a function of the CO2 concentration. The temperature increase at doubled CO2 concentration then directly gives the CO2 climate sensitivity. Such simulations reproduce the direct or basic ECS value (without feedback processes), as specified by the IPCC, within better than 10%. Significant differences, however, can be observed with the different feedback effects included. Our investigations show the largest discrepancies for the WV and cloud feedback, but they also disclose the importance of one of the primary stabilizers of the whole climate system, the evaporation feedback. Therefore, these processes and their contributions are extensively analyzed under different humidity, surface temperature, lapse rate, and cloud cover conditions.
In particular, these studies show that the observed cloud changes within the ISCCP cannot exclusively be explained by pure thermally induced cloud cover changes but obviously are additionally controlled by a further cloud forcing mechanism. Since there exists strong evidence that the solar activity also has a powerful influence on the cloud cover, it is reasonable to postulate such a solar induced cloud feedback (see, e.g., Svensmark [23]; Haigh [24]). This is investigated in detail in Section 5, differentiating between a pure thermal impact of an increasing solar activity and a nonthermally induced solar cloud feedback. An important criterion for a validation, which mechanism might control such cloud changes, can be derived from model simulations, which include the solar anomaly over the last century and compare this directly with the observed global warming over this period. These investigations indicate that due to the strong cloud feedback the observed warming over the last century can only satisfactorily be explained, attributing a significant fraction to the increased solar activity over this period (see also Ziskin & Shaviv [11]; Vahrenholt & Lüning [25]).
Our simulations predict a solar contribution of about 60% and a CO2 induced contribution of 40% to global warming over the last century with an equilibrium climate sensitivity of 0.7°C, which is almost a factor of five smaller than published in AR5.