In previous articles we have discussed the Milankovitch hypothesis – classically paraphrased as:
Solar insolation at 65ºN in summer determines the start and end of ice ages – with minimum summer insolation preventing snow melt at high latitudes which allows perennial snow cover, positive feedback from reflected solar radiation and the consequent growth of ice sheets.
Conversely maximum solar insolation at high latitudes causes ice sheets to melt and (with the same positive feedback effect) ends the ice age.
And “summer” is usually taken as the insolation on June 21st even if it is a somewhat arbitrary date (we can also average over a month or the season).
So I produced a few contour plots, showing the insolation anomaly by latitude and day of year compared with the present for 8 different years between the start of the last ice age (about 115 kyrs ago) and today.
The challenge for readers is to identify which graph corresponds to the end of the last ice age. And some kind of reason why you chose that graph.
I made them a little smaller so that they could be more easily compared – just click on each set to expand.
The x-axis (left to right) is day of year, and June 21st is about day 200 (actually it is 172, thanks to Climateer for pointing this out!). The y-axis (bottom to top) is the latitude. The colors represent the same in each graph and the contour lines are 10 W/m² apart.
Figures A – D – Click for larger view
Figures E – H – Click for larger view
Articles in the Series
Part One – An introduction
Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz
Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory
Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation
Part Five – Obliquity & Precession Changes - and in a bit more detail
Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name
Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago
Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers
Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover
Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs
Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs
Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2
Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland
Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II