This is a temperature reconstruction plot of the Holocene plus data from the Hadley Climate Research Unit. The general shape of the curve is a function of changing incident solar radiation (insolation)
caused by the Milankovitch cycles. The Holocene Climate Optimum occurred from 7000 to 5000 years ago and was caused by the precessional cycle. The decline in temperature after that period is
related to the axial tilt of the earth becoming more upright and the polar regions receiving less insolation. Less insolation at the poles during the summer causes less melting of ice and a cooler planet.
The cooling trend should have continued to the present day if not for humans modifying the earth's radiation balance. Some very small increase in greenhouse gases ,enough to counteract the cooling
trend, is beneficial. This natural cooling trend has about another 8000 years left in its cycle when the earth's axial tilt will be at a minimum. So the earth was on the course for a possible ice age in
about 8000 years but we can prevent that from happening by managing the earth' radiation balance through the addition of greenhouse gases. The figure was taken from Realclimate.org and notice
the very large error bars on the temperature reconstruction in light blue.
Figure 1 Blue curve: Global temperature reconstruction from proxy data of Marcott et al, Science 2013. Shown here is the RegEM version – significant differences between the variants with different averaging methods arise only towards the end, where the number of proxy series decreases. This does not matter since the recent temperature evolution is well known from instrumental measurements, shown in red (global temperature from the instrumental HadCRU data). Graph: Klaus Bitterman.