Earth has been cooling, by +10 to +20 degrees C., since the Eocene Thermal Maximum 50 million years ago, when the whole planet was ice free, including the north pole and south pole.
Today, "global warmunists" claim that +2 degrees C. of warming (after 1850) will be a catastrophe, even though we may be half way to +2 degrees C. as of 2015, and the climate seems wonderful !
How do they know +2 degrees C. will be dangerous?
They don't know.
They just pull the number out of thin air, and then try to scare people with it.
Today we are in the interglacial period (in between major ice glaciations) of the Pleistocene Ice Age -- this is one of the coldest climates in Earth’s history.
In the past 800,000 years there have been periods with a lot of ice, followed by interglacial periods with much less ice, in 100,000 year-cycles.
The 100,000 year-cycles seem related to the eccentricity of the Earth’s orbit and its axial tilt.
But the five ice ages in the past 4.5 billion years were not evenly spaced, nor did they last for the same period of time.
There was a strong correlation between average temperature and atmospheric CO2 in the past 800,000 years -- CO2 peaks lagged temperature peaks by an average of 800 years, per ice core studies.
These measurements strongly suggest oceans release some of their dissolved CO2 into the air AFTER they warm from NATURAL causes.
Warming causes CO2 in the air to increase in climate history -- CO2 does NOT cause warming!
The average temperature has slowly increased in Central England since 1700, even as manmade CO2 emissions were tiny before 1850.
The River Thames froze for the last time in 1814 -- since then Earth has warmed slightly -- about one degree C. (+/- 1 degree).
The warming since 1850 was from 1910 to 1940, and from 1975 to 2000.
Here's what the IPCC said about climate change in 2007:
“We should recognize that we are dealing with a coupled nonlinear chaotic system, and therefore the long-term prediction of future climate states is not possible.”
Good advice but the IPCC doesn't listen to their own advice!