Most Of Warming Trend Since the 1980s Is Naturally Driven, Not CO2-Driven
According to a new statistical analysis of centennial-scale surface temperature changes, half (0.5 °C) of the warming trend over the last 135 years (0.95 °C) can be explained by both (a) the existence of commonly-occurring natural (non-anthropogenic) variations of temperature that can reach the same amplitudes of the modern trend (see above image), and (b) external factors such as solar activity and greenhouse gases, with the latter factor accounting for “less than is commonly believed”.
Scientist Dr. Maxim Ogurtsov and his colleagues cite extensive evidence that any external forcing of the modern trend that falls outside the range of natural variability can be appreciably attributed to non-greenhouse factors.
It is widely accepted also that this global warming is caused primarily by an anthropogenic increase of greenhouse gases concentration. However, debates on this question still continue.
Some experts maintain that current warming does not exceed the natural fluctuations of climate. Evidence of appreciable contribution to global warming of non-greenhouse factors has been obtained by many authors.
1. Soon et al., 2015 noted that if the urbanization effect is properly taken into account, one can conclude that solar variability is the dominant factor of Northern Hemisphere long-term temperature changes since at least 1881.
2. Zhao and Feng, 2014 reported that variations in solar activity play an important role in changes of climate over global scale during the last more than 100 years.
3. According to Harde (2014), the Sun is the main contributor to global warming of the last century.
4. Lüdeсke et al. (2014) showed that variations of Central European temperature after 1757 were likely governed by periodic oscillations resulted from intrinsic climatic dynamics.
5. Scafetta (2010) and Scafetta (2012) claimed that the global climate oscillations from 1950 to 2011 were appreciably influenced by astronomic planetary cycles, particularly by the motion of Jupiter and Saturn.
6. Swanson and Tsonis, 2009 noted that in the period 1900-2000 Northern Hemisphere climate variability might be partly explained by chaotic dynamics.
7. Privalsky and Fortus (2011) modeled variations of global temperature during 1850- 2009 as an autoregressive process of the fourth order. They arrived at a conclusion that global warming of the last 150 years could be fully explained by natural climatic variability without any external forcing.
[I]t is reasonable to regard the global warming as a phenomenon exceptional from the point of view of intrinsic climatic oscillations, which need an additional external forcing factor for an explanation. On the other hand, the statistical experiments showed that an appreciable part of the global warming might be a result of natural fluctuations of the climatic system. … [O]ur results show that the contribution of these external factors (including greenhouse effect) to the global warming could be less than is often believed.
Changes in the solar radiation at the Earth’s surface (global brightening) might be an important source of the warming of the last decades (Ogurtsov et al., 2012).
Surface Incident Solar Radiation Trend Since The 1980s Can Explain AllRecent Warming (And More)
As noted in their conclusion above, Ogurtsov and colleagues have previously published a paper that establishes surface incident solar radiation (SSR) – solar radiation absorbed (or not) by the Earth’s surface (oceans) via decadal-scale reductions (or increases) in cloud cover – can account for all of the radiative forcing of temperature changes during the 1983 to 2001 period, when surface temperatures increased by about 0.5 °C. In fact, the intensity of the direct, shortwave forcing during that 18-year period – 3 W m–2 to 6-7 W m–2 – was larger than the resulting temperature change itself.
Changes in the climate of the Earth depend evidently on the background solar irradiance, i.e. on the amount of shortwave solar radiation incoming into the atmosphere and the fraction of this radiation, which is reflected back to space. Recent evidence shows that solar radiation incident on the Earth’s surface has increased appreciably in the end of 20th century (Pallé et al, 2006). The phenomenon is often called a global brightening.
Change in background solar radiation through 1983- 2001 causes a positive radiative forcing ranging from 3 W × m–2 to 6 – 7 W × m–2. If we take a value of climatic sensitivity adopted by IPCC, we obtain that increase of the global temperature by 1.5˚C – 3.6°C is a result of the radiative forcing of 3 W × m–2.
In Contrast, CO2 Forcing Contributes Just 0.2 W m–2 Per Decade To Modern Warming
According to climate models, the total climate forcing effect of the roughly 120 parts per million (ppm) increase in atmospheric CO2 during the ~165 years since 1750 is 1.8 W m–2.
As assessed in a 2015 paper published in the journal Nature, the CO2 concentration increased by 22 ppm during the first 10 years of the 21st century. The radiative forcing (warming) effect of this 22 ppm CO2 increase was modeled to be 0.2 W m–2. So of the 1.8 W m–2 of total CO2 radiative forcing since 1750, 0.2 W m–2 was added during the first decade of this century.
So if CO2 forcing accounts for roughly 0.2 W m–2 per decade of the globe’s radiative forcing with an increase of 22 parts per million (ppm), and if surface incident solar radiation (SSR) accounts for 3 to 6-7 W m–2 for the 18-year period (~2.5 W m–2 per decade)between 1983 and 2001, it could be reasonably concluded that surface incident solar radiation could account for at least 10 times more of the modern climate forcing as CO2 increases have. Graphed, the difference in trends may look like this:
In sum, not only is the variation in temperature of the last 135 years not remarkable or outside the range of what can be achieved naturally or internally, but the magnitude of the external forcing from surface incident solar radiation for recent decades far exceeds the reputed attribution from CO2 concentration changes. Therefore, it could reasonably be said that there is no clear anthropogenic signal detectable in the climate changes of the last 135 years when one considers the contexts of natural variation and natural climate forcing.
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