Humans Are by Far the Dominant Cause of Global Warming: A Comprehensive Review of the Science

The agreement between these studies using a variety of different methods and approaches is quite remarkable. Every study concluded that over the most recent 100-150 year period examined, humans are responsible for at least 50% of the observed warming, and most estimates put the human contribution between 75 and 90% over that period (Figure 2). Over the most recent 25-65 years, every study put the human contribution at a minimum of 98%, and most put it at well above 100%, because natural factors have probably had a small net cooling effect over recent decades (Figures 3 and 4).

Additionally, in every study over every timeframe examined, the two largest factors influencing global temperatures were human-caused: (1) GHGs, followed by (2) human aerosol emissions. This is a dangerous situation because as we clean our air and reduce our SO2 emissions, their cooling effect will dissipate, revealing more of the underlying GHG-caused global warming trend. Note that not all studies broke out the effects the same way (i.e. only examining ‘natural’ and not solar or volcanic effects individually), which is the reason some bars appear to be missing from Figures 2 to 4.

Figure 2: Percent contributions of various effects to the observed global surface warming over the past 100-150 years according to Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, green), Lean and Rind 2008 (LR08, purple), Stott et al. 2010 (S10, gray), and Huber and Knutti 2011 (HR11, light blue).

Figure 3: Percent contributions of various effects to the observed global surface warming over the past 50-65 years according to Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, green), Lean and Rind 2008 (LR08, purple), Huber and Knutti 2011 (HK11, light blue), and Gillett et al. 2012 (G12, orange).

Figure 4: Percent contributions of various effects to the observed global surface warming over the past 100-150 years according to Meehl et al. 2004 (M04, red), Lean and Rind 2008 (LR08, purple), and Foster and Rahmstorf 2011 (FR11, green).

There was a period of warming between 1910 and 1940 which was predominantly caused by increasing solar activity and an extended period of low volcanic activity, with some contribution by human effects. However, since mid-century, solar activity has been flat, there has been moderate volcanic activity, and ENSO has had little net impact on global temperatures. All the while GHGs kept increasing, and became the dominant effect on global temperature changes, as Figures 3 and 4 illustrate.

A wide variety of statistical and physical approaches all arrived at the same conclusion: that humans are the dominant cause of the global warming over the past century, and particularly over the past 50 years. This robust scientific evidence is why there is a consensus among scientific experts that humans are the dominant cause of global warming.

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