Per the IPCC's gold-standard of global temperature measurements, since the late 1800s, the highest per century warming trend achieved occurred during the 42-year period ending in 1949.
Simply stated, that is when the actual "unprecedented" global warming acceleration was witnessed.
The accompanying graph establishes this as fact, when put in the IPCC context that modern global warming started with the year 1950. This is the decade of the modern era that the newest IPCC report asserts when human CO2-induced climate change began. (See the red circle? More on that fact in a bit.)
Depending on which IPCC spokesperson's claim is to be believed, since 1950 the "accelerating" global warming is not only unprecedented, it's "rapid", "dangerous", "irrefutable", "indisputable", "undeniable", "incontrovertible" and, of course, "irreversible".
However, the empirical evidence does not support any of these claims.
First, the adjacent chart's essentials. The modern period of 1950 to 2013 is 64 years long, which the IPCC characterizes as being dominated by human CO2 emissions with little natural climate influence. The chart's orange curve represents this modern period.
The chart's green plot represents the 64 years ending in 1949 (from 1886 to 1949), the year designated by the IPCC as the end of natural climate change's dominant impacts. Okay, now note the red circle and red dashed line that intersects the green curve: that's when unprecedented warming took place.
Adding some more context, each of the two 64-year periods had human CO2 emissions. For the modern period since 1950, an approximate 1.2 trillion tonnes of human CO2 emissions were released, while the earlier period had some 200 billion tonnes - that's a 6x difference.
Yet, as this charts reveals, the per century warming trends are remarkably similar with the fastest warming acceleration happening in the earlier period. This overall similarity takes place despite the incredible increase of human CO2 emissions after 1949.
Indeed, there are amazing similarities between the two periods but they do have differences. There is a significant divergence of trends at the 24-year mark where the modern warming trend starts to decline while the pre-1950 trend continues to increase for another 7 years. In addition, at the 11-year mark, the modern temperature trend does an abrupt reversal from negative (i.e. cooling trend) to warming for the next 4 years, then it reverses again until it also reaches a cooling trend for the 5-year mark ending 2013.
Ultimately, what is the overall impact of CO2 emissions on modern climate change when using the IPCC's year 1950 start? Over the 64 years ending in 2013, the global warming trend was +0.74°C per century. In contrast, the per century trend was +0.65°C for the earlier 64-year period ending 1949.
That's correct, all that separates these two extended periods of global warming is likely an unmeasurable +0.09°C - nine hundredths of a single degree.
The UK's HadCRUT4 global empirical evidence makes it very clear: modern acceleration of warming temperatures is not unprecedented, nor unusual due to CO2 emissions; nor does the modern period exhibit any warming trend that comes close to even 1.5°C per century. In summary, it is highly probable that any modern warming was just a continuation of rebound warming after the end of the Little Ice Age. In other words, natural climate change still rules.
Additional temperature and climate charts.
Update: Chart's legend previously reversed, now corrected. Thanks to reader Les.